Q & A: Hypothyroidism in a 17-year-old Chihuahua

My patient is an older 17-year old male Chihuahua. His weight is normal at 6.8 pounds. He is having episodes of pain at time of defecation and appears painful in his back.

We ran routine blood work which was all completely normal. However, a screening serum total T4 concentration was low at 0.4 μg/dl (reference range, 0.38-4.0 μg/dl). I added on a free T4 concentration and that was also low at  0.3 ng/ml (0.6 to 3.7 ng/ml). 

Would you recommend starting a thyroid supplementation in this dog?

My Response:

I doubt if this dog is truely hypothyroid for a number of reasons (1-3).

• First of all, the clinical signs the dog is showing are not at all characteristic of hypothyroidism.
• Secondly, the dog has neither anemia or hypercholesterolemia, which would make a diagnosis of hypothyroidism more likely.
• Finally, would be very unusual for a 17-year old dog to develop hypothyroidism; most dogs with hypothyroidism are young to middle age adult dogs, not geriatric animals.

In this dog, it's more likely that the serum thyroid concentrations are low either because of 1) a non-thyroidal illness, or 2) administration of a drug that is causing "suppression" of the circulating thyroid values. Such thyroid-lowering drugs include the following (3-5):

• Phenobarbital
• Sulfur antibiotics
• Clomipramine (Clomicalm) 
• Aspirin and other anti-inflammatory drugs
• Glucocorticoids

The steroids involved can be oral, parenteral, or topical, applied to the eyes, ears or skin. We need to get a good drug history in these dogs not only by reviewing the record to see what has been dispensed, but also by asking the owner what they're actually doing, as they may have bags of medications at home that we know nothing about.

Of course, if you do believe that the dog has hypothyroidism, then the next step would be to verify the low serum T4 concentration, repeat the serum free T4 using an equilibrium dialysis technique if possible, and to measure a serum cTSH concentration (2,3). If the total and free T4 remain low and the cTSH value is high, then treatment would be indicated.

The most accurate way to make a diagnosis (or rule out) a diagnosis of hypothyroidism is by use of thyroid scintigraphy (6,7). Although not widely used or available as a diagnostic test for dogs with suspected hypothyroidism, the diagnosis of hypothyroidism is easily made by the lack of finding any visible cervial thyroid tissue with thyroid imaging (see Figure below).

Thyroid Image (Scintigraphy) in a Boston Terrier with Hypothyroidism.
Notice the uptake of the radioactive tracer by the salivary glands, but the complete lack of uptake by the thyroid gland.  The normal location for the 2 thyroid lobes is indicated by yellow ovals.

Thyroid imaging typically reveals decreased or even absent radionuclide uptake in dogs with hypothyroidism (thyroid gland is not visible on the scan). In contrast, dogs that have falsely low serum thyroid hormone concentrations secondary to illness or drug therapy have a normal thyroid image. In one study of comparing thyroid imaging to other diagnostic tests for hypothyroidism in dogs, there was no overlap between dogs with primary hypothyroidism and dogs with nonthyroidal illness when thyroid scintigraphy was employed (6).   Of all of the current thyroid imaging techniques (CT, ultrasound), nuclear imaging is considered to be the best test for dogs with suspected hypothyroidism.

References:

1. Chastain CB. Canine pseudohypothyroidism and covert hypothyroidism. Problems in Veterinary Medicine 1990;2:693-716.  
2. Mooney, CT. Canine hypothyroidism: A review of aetiology and diagnosis. New Zealand Veterinary Journal 2011;59:105-114.
3. Peterson ME, Melián C, Nichols R. Measurement of serum total thyroxine, triiodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. Journal of the American Veterinary Medical Association 1997;211:1396-1402. 
4. Kantrowitz LB, Peterson ME, Trepanier LA, et al. Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in epileptic dogs treated with anticonvulsants. Journal of the American Veterinary Medical Association 1999;214:1804-1808. 
5. Williamson NL, Frank LA, Hnilica KA. Effects of short-term trimethoprim-sulfamethoxazole administration on thyroid function in dogs. Journal of the American Veterinary Medical Association 2002;221:802-806. 
6. Diaz Espineira MM, Mol JA, Peeters ME, et al. Assessment of thyroid function in dogs with low plasma thyroxine concentration. J Vet Intern Med 2007;21:25-32. http://www.ncbi.nlm.nih.gov/pubmed/17338146
7. Taeymans O, Peremans K, Saunders JH. Thyroid imaging in the dog: current status and future directions.J Vet Intern Med 2007;21:673-684. http://www.ncbi.nlm.nih.gov/pubmed/17708386

Can Increasing the Amount of Fat or Carbohydrate in a Cat's Diet Compensate for Low Protein Intake?

As I discussed in a recent post on Optimal Protein Requirements for Older Cats and Cats with Hyperthyroidism, energy requirements sharply and progressively increase in older cats starting at 10 to 12 years of age (1-3). If daily caloric intake is not increased, progressive weight loss will result, due in large part to the loss of lean body mass (i.e., muscle mass), a phenomenon referred to “sarcopenia” of aging (4-7).

In addition to this increased caloric intake, older cats also require higher amounts of protein to maintain protein reserves compared with younger adult cats (3, 8-11). As cats age, they absorb and metabolize protein less efficiently (10). Therefore, it’s extremely important to feed high-quality protein (i.e., animal source rather than grain-based), as well as an adequate quantity of protein to aging cats.

Animals derive energy from the oxidation of the macronutrients carbohydrate, fat, and protein (12). Most animals, including rats and humans, generally adapt to the diet being fed and can oxidize whatever fuel mixture is contained in their prevailing diet (13,14). This enables most animals (especially omnivores) to use widely differing diet compositions to satisfy their energy requirements.

A number of veterinarians have contacted me over the last few weeks with concerns about the potential problems associated with feeding a high protein diet to the geriatric cat, especially in those cats with kidney disease. Basically, the questions come down to the following:

• If animals can adapt their energy intake based on a wide variety of feeding regimens, why can’t we just increase the amounts of fat and carbohydrate fed to compensate for the increasing energy requirements in our senior (and hyperthyroid) cats? Won't that help prevent loss of muscle mass?
• But can cats as obligate carnivores adapt to the same extent?

In this post, my mission is to review energy and protein metabolism in cats. I will attempt to explain why, unfortunately, increasing the amount of fat or carbohydrate fed to an older or hyperthyroid cat generally cannot compensate for a diet deficient in an optimal protein content.

Overview of the Body's Energy Production (Nutritional Biochemistry 101)

Fig. 1: Pathways of
intermediary metabolism

Let's start with a brief review of nutritional biochemistry. If you are a practicing veterinarian like myself, it's highly likely that you may not remember all of the metabolic pathways an animal uses for energy production.

The initial biochemical reactions by which energy is derived from carbohydrates, fats, and proteins are different. However, all 3 macronutrients eventually go through a final common pathway for energy generation called the citric acid cycle — also known as the tricarboxylic acid cycle (TCA) cycle or the Krebs cycle (Figure 1) (12,15).

Carbohydrates: Glucose derived from dietary carbohydrates is first oxidized through the glycolysis pathways to yield pyruvate and then acetyl-CoA. The acetyl-CoA is then oxidized in the citric acid cycle (12,15). High-energy electrons produced in the citric acid cycle enter the electron transport chain to generate adenosine triphosphate (ATP), which transfers its energy from chemical bonds to energy-absorbing chemical reactions within the cell (Figure 1).

Fats: Fatty acids from dietary fats are initially oxidized to acetyl-CoA by the beta-oxidation pathway, which then enter the citric acid cycle and subsequently generate ATP via oxidative phosphorylation in the electron transport system (Figure 1).

Figure 2: Protein catabolism

Protein: Once protein synthesis is maximized, excess amino acids can be used for energy by undergoing transamination and deamination. In oxidative deamination, an amino group is removed from the amino acid and converted to ammonia.

The remaining carbon skeleton from these deaminated amino acids (organic ketoacids) can be recycled to make nonessential amino acids, or they can be oxidized for energy (12,15). If used for energy, the catabolized amino acids enter either the glycolysis pathway or the citric acid cycle to ultimately form ATP in the electron transport system (Figure 1). The carbon skeletons of catabolized amino acids can also be converted to glucose or ketone bodies in the liver.

Ammonia is toxic to the body, so enzymes convert it to urea by addition of carbon dioxide in the urea cycle, which takes place in the liver. Urea can safely diffuse into the blood and then be excreted into the urine (Figure 2).

Interconversion of nutrient molecules: In metabolism, there commonly is interconversion of nutrient molecules. Excess glucose is stored as glycogen (glycogenesis); when glycogen stores are filled, glucose and amino acids are used to synthesize lipids (lipogenesis). Therefore, glycogen and adipose tissue both serve as long-term forms of energy storage in the body.

In contrast, amino acids are not "stored" in the body, other than that found in muscle and other structural proteins. Although amino acids can be converted to either fat or glucose, the opposite does not occur — fat and carbohydrates cannot be directly converted to amino acids to be made into protein (12-15).

Protein Structure, Functions & Metabolism

Proteins, from the Greek proteios meaning "first" (16), are important biological molecules that consist of string of amino acids linked together in sequence as polypeptide chains. Proteins vary in shape and size, some consisting of only 20-30 amino acids and others of several thousands (12,15).

Proteins are present in every living cell. In the skin, hair, cartilage, muscles, tendons, and ligaments, proteins hold together, protect, and provide structure to the body. As enzymes, hormones, antibodies, and globulins, they catalyze and regulate body chemistry. In addition, protein is required for growth and tissue repair, as well as for maintaining muscle mass and tone.

Essential and Nonessential Amino Acids
More than 20 amino acids are involved in the synthesis of protein in the body. Essential amino acids are those that cannot be formed in sufficient amounts to meet the requirements for growth and maintenance and must be supplied in the diet. Nonessential amino acids are those that the body can produce in sufficient amounts from other nutrients and metabolites and, thus, do not need to be supplied in the diet.

Even with the nonessential amino acids, however, fat and carbohydrates cannot be directly converted to amino acids to be made into protein (12,15). Although all mammals can synthesize 10 nonessential amino acids from precursor carbon skeletons (if adequate nitrogen and energy are available), there is no direct way that amino acids can be synthesized directly from either fat or carbohydrates.

Amino Acid Catabolism & Synthesis—Protein Turnover
Although essential amino acids are not stored as such in the body for any significant period of time, all body proteins are continuously being broken down and resynthesized in a process known as protein turnover (12,15,17). During protein turnover, some amino acids enter catabolic pathways and are permanently lost. Therefore, adequate amounts of high-quality dietary protein (containing all the essential amino acids) must be consumed each day to replace the both the essential and nonessential amino acids lost to catabolism.

Hypermetabolic states, such as hyperthyroidism or other illness, increase both protein turnover and nitrogen losses and therefore increase the daily protein requirements (18,19).

Protein as an Energy Source
Unlike fat or carbohydrate, protein cannot be stored as such in the body (other than as muscle protein itself).  In animals fed diets containing more protein than is needed, the extra protein is metabolized and used for energy (Figures 1 & 2).

In all species, but most importantly in cats (see below), ingested amino acids are converted to carbohydrates via gluconeogenesis (12,15,17, 20-22). This pathway is also used under starvation conditions to generate glucose and energy from the body's own proteins, particularly those found in muscle.

Factors That Determine How Dietary Proteins are Used

• All or None Rule: To make a certain protein, the necessary amino acids must be present in the cell in the right amounts, or the protein will not be made. Essential amino acids that are not used to make proteins are not stored.
• Adequacy of Caloric Intake:  If the diet fails to provide sufficient calories as carbohydrates and fats, proteins will not be synthesized; instead the ingested amino acids will be used as a source of energy.
• Nitrogen Balance:  Normally, the amount of protein synthesis is equal to the amount of protein breakdown (12,17). If there is more protein synthesis than breakdown, then we have a positive nitrogen balance (e.g., recovery from injury). If there is greater protein breakdown than synthesis, then there is a negative nitrogen balance (e.g, starvation, illness, hyperthyroidism).
• Hormonal Controls:  Anabolic hormones (e.g., insulin, growth hormone, sex hormones) stimulate the production or maintenance of proteins.  Catabolic hormones (e.g, glucocorticoids, thyroid hormone) stimulate the breakdown of proteins (18,19).

Cats Do Not Need Carbohydrate But Have High Protein Requirements

The cat, as a strict carnivore, has evolved to to depend on protein as a major energy source. The "natural" diet of cats in the wild is based upon the consumption of small mammals, birds, and insects — the composition of this diet is high in protein and fat but low in carbohydrate (20-22). This natural diet high in animal protein contains all of a cat's essential amino acids, which is not the case for plant-based proteins.

Cats have no dietary requirement or need for carbohydrate but are adapted physiologically and metabolically for high protein intake (23). In support of that, normal cats do well when fed diets containing 70% protein (24-27). The lack of a need for dietary carbohydrates is related to the fact that cats have developed a tremendous ability to synthesize their needed glucose from protein catabolism via hepatic gluconeogenesis (24,25,28).

Cats have a much higher protein requirement than other species, such as dogs and humans. The protein requirement for the adult cat is 2-3 times as much as the adult dog (23), and their protein requirement increases even further as cats reach old age (3,8-11). This high protein requirement of cats is primary related to the fact that cats have a limited ability to decrease the hepatic enzymes responsible for amino acid catabolism, even when fed a lower than optimal protein intake (24,29,30).

The Domestic Cat: A Metabolically Inflexible Carnivore

When most omnivores (e.g., humans, rats, pigs, dogs), ingest a diet high in protein, the activities of the amino acid-catabolizing enzymes in the liver increase to cope with the higher flux of amino acids (31-33). The activity of the urea cycle enzymes also increase to metabolize the increased ammonia generated upon the catabolism of the amino acids. On the other hand, when fed a diet low in protein, omnivores accommodate by lowering the hepatic activity of these catabolic enzymes of amino acid metabolism, as well as decreasing hepatic urea production (31-33).

By contrast, cats have a very limited ability to down-regulate these hepatic catabolic enzymes when fed a low-protein diet. In a classic study, Rogers et al (29) compared the activity of several catabolic enzymes of amino acid metabolism in adult cats fed either a high- or low-protein diet or fasted for 5 days. Results showed little changes in the hepatic enzyme activities between the 3 groups of cats, with hepatic enzyme activities remaining set at high levels to cope with a high protein diet (even when they weren’t being fed!).

Why would cats be so different?
Well, compared to other carnivores, cats may not be so strange after all. A similar degree of metabolic "inflexibility" has also been reported in other carnivores such as barn owls and the rainbow trout (34,35).

This limited metabolic flexibility in cats and other obligate carnivores likely represents an evolutionary adaptation to a consistently abundant supply of dietary protein. The moderately high and fixed activity of the urea cycle provides a safeguard against ammonia toxicity after ingestion of a high-protein meal. In addition, the high rate of amino acid catabolism allows for a readily available source of energy via direct oxidation or as a substrate for hepatic gluconeogenesis.

It is only when a cat is fed a lower protein diet, a condition that would never happen in the wild, that the high rate of protein catabolism becomes a disadvantage.

Bottom Line

• Unlike omnivores (dogs, pigs, rats, humans), cats have a limited ability to decrease the activity of the hepatic enzymes responsible for removing amino groups from the amino acids when fed a low protein diet. 
• Because these feline hepatic enzyme systems are constantly active, a fixed amount of dietary (or muscle) protein will always be catabolized for energy no matter how much energy in the form of carbohydrate or fat the cat ingests. 
• In addition, neither fat nor carbohydrates can be directly converted to amino acids to be made into protein. In this regard, the carnivorous cat is similar to omnivores.
• Overall, this explains why muscle wasting can occur so quickly in the older, geriatric cat, which becomes ill or develops a poor appetite or is fed a low-protein diet.

References:

1. Perez-Camargo G: Cat nutrition: What is new in the old? Compendium for Continuing Education for the Practicing Veterinarian 2004;26 (Suppl 2A):5-10.
2. Laflamme D. Nutrition for aging cats and dogs and the importance of body condition. Veterinary Clinics of North America: Small Animal Practice 2005;35:713-742.
3. Pérez-Camargo G. Feline decline in key physiological reserves: implication for mortality. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 6-13.
4. Fujita S, Volpi E. Nutrition and sarcopenia of ageing. Nutrition Research Reviews 2004;17:69-76.
5. Paddon-Jones D, Short KR, Campbell WW, et al. Role of dietary protein in the sarcopenia of aging. The American Journal of Clinical Nutrition 2008;87:1562S-1566S. 
6. Short KR, Nair KS. Mechanisms of sarcopenia of aging. Journal of Endocrinological Investigation 1999;22(5 Suppl):95-105.
7. Wolfe RR. Sarcopenia of aging: Implications of the age-related loss of lean body mass. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 12-17.
8. Little SE: Evaluation of the senior cat with weight loss, In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;1176-1181.
9. Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42.
10. Patil AR, Cupp C, Pérez-Camargo G. Incidence of impaired nutrient digestibility in aging cats. Nestlé Purina Nutrition Forum Proceedings. 2003;26,2(A):72.
11. Wakshlag JJ. Dietary protein consumption in the healthy aging companion animal. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 32-39.
12. Gross KL, Yamka RM, Khoo C, et al. Macronutrients. In: Hand MS, Thatcher CD, Remillard RL, Roudebush R, Novotny, BJ (eds), Small Animal Clinical Nutrition. Mark Morris Institute. 2010; 49-105. 
13. Davy KP, Horton T, Davy BM, et al. Regulation of macronutrient balance in healthy young and older men. International Journal of Obesity and Related Metabolic Disorders 2001;25:1497-1502.
14. Galgani J, Ravussin E. Energy metabolism, fuel selection and body weight regulation. International Journal of Obesity 2008;32 (Suppl 7):S109-119.  
15. Brody T. Nutritional Biochemistry. Second Edition. Academic Press. 1998.
16. New Oxford Dictionary of English. Oxford University Press, 2001.
17. Berg JM, Tymoczko JL, Stryer L. Protein turnover and amino acid catabolism. Biochemistry. 5th edition ed. New York: W H Freeman, 2002. 
18. Mitch WE. Mechanisms accelerating muscle atrophy in catabolic diseases. Transactions of the American Clinical and Climatological Association 2000;111:258-269. 
19. Riis AL, Jørgensen JO, Gjedde S, et al. Whole body and forearm substrate metabolism in hyperthyroidism: evidence of increased basal muscle protein breakdown. American Journal of Physiology: Endocrinology and Metabolism 2005; 288:E1067-1073. 
20. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
21. Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9.
22. Crissey SD, Slifka KA, Lintzenich BA. Whole body cholesterol, fat, and fatty acid concentrations of mice (Mus domesticus) used as a food source. Journal of Zoo and Wildlife Medicine 1999;30:222-227. 
23. National Research Council. Proteins and amino acids. In: Nutrient Requirements of Dogs and Cats. Washington, DC: National Academies Press. 2006; 111-145.
24. MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 1984;4:521-562. 
25. Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Nutrition Research Reviews 2002;15:153-168. 
26. Hendriks WH. Protein metabolism in the adult domestic cat (Felis catus). PhD Thesis, 1996. 
27. Hamper B, Bartges J, Kirk C, et al. The unique nutritional requirements of the cat: a strict carnivore In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders, 2012;236-242.
28. Kettelhut IC, Foss MC, Migliorini RH. Glucose homeostasis in a carnivorous animal (cat) and in rats fed a high-protein diet. American Journal of Physiology 1980; 239:R437-440.
29. Rogers QR, Morris JG, Freedland RA. Lack of hepatic enzymatic adaptation to low and high levels of dietary protein in the adult cat. Enzyme 1977;22:348-356.
30. Green AS, Ramsey JJ, Villaverde C, et al. Cats are able to adapt protein oxidation to protein intake provided their requirement for dietary protein is met. The Journal of Nutrition 2008;1053-1060. 
31. Waterlow JC. The mysteries of nitrogen balance. Nutrition Research Reviews 1999;12:25-54. 
32. Rosebrough RW, Steele NC, McMurtry JP. Effect of protein level and supplemental lysine on growth and urea cycle enzyme activity in the pig. Growth 1983;47:348-260.
33. Schimke RT. Adaptive characteristics of urea cycle enzymes in the rat. Journal of Biological Chemistry  1962;237:459-68. 
34. Walton MJ. Metabolic effects of feeding a high protein/low carbohydrate diet as compared to a low protein/high carbohydrate diet to rainbow trout Salmo gairdneri. Fish Physiology and Biochemisty 1986;1;7-15. 
35. Meyers MR, Klasing KC. Low glucokinase activity and high rates of gluconeogenesis contribute to hyperglycemia in barn owls (Tyto alba) after a glucose challenge. Journal of Nutrition 1999; 129:1896-1904.  

Is It Feasible for Older Cats to Ingest the Optimal Amounts of Protein They Need Each Day?

As I discussed in my recent post on the “Optimal Protein Requirements for Older Cats and Cats with Hyperthyroidism,” the optimal daily protein intake for normal, young to middle-aged cats fed appears to be at least ~5.5 g/kg, whereas older cats and cats with hyperthyroidism need at least ~6.0-7.0 g/kg to prevent loss of muscle mass. If these cats already have loss of lean body mass (i.e., muscle wasting), they may require even higher amounts of daily protein to help restore lost muscle mass.

The major issue we face is to identify and feed a diet that will provide adequate amounts of protein to these older cats and cats with hyperthyroidism. Over the past couple of weeks, I’ve received a number of questions from veterinarians and concerned owners that I’d like to address. Here are the main ones:

• How do we calculate how much protein these foods actually provide?
• Is it even possible or practical to attempt to feed those amounts of protein?
• What if our cats won’t eat as much protein as we would like them to ingest?
• Why can’t we just increase the amounts of fat and carbohydrate fed to compensate for the senior (and hyperthyroid) cats’ increasing energy requirements?
• What about older cats that have chronic renal disease? Shouldn’t these cats all be on a low protein diet?

In this post, I plan to address the first three questions. I’ll follow-up with the last two questions in separate posts because of their importance in clinical practice.

How to Calculate the Protein Content of a Food: Dry Matter Basis vs. Metabolizable Energy

Cats in the wild typically ingest diets containing > 50% of these daily calories as protein (1-3), so that may be a good goal to shoot for when selecting the composition of a diet. When analyzing a diet, I like to examine or calculate the amount of the calories each nutrient (i.e., protein, fat or carbohydrates) provides — this is called the “metabolizable energy,” abbreviated ME (4,5). This measure disregards any part of the food that does not provide any energy (kcal) such as water, ash, or fiber. It only considers the 3 nutrients that provide the needed calories and nothing else.

Let’s take an example using 5 commercial foods containing a wide range of protein content. In each of these diets, I’ve set the fat content and calories (kcal) to be equal, but I’ve varied the protein and carbohydrate amounts. Remember, when we increase or decrease the levels of protein, we must always adjust the levels of carbohydrates, fat, or both to compensate. In other words, the percentage of calories that come protein, fat, and carbohydrates must equal 100% (% protein calories + % fat calories + % carbohydrate calories = 100% of the calories ingested).

So let’s take our first hypothetical diet, which on a dry matter (DM) basis contains 65% protein, 25% fat, and 4% carbohydrate (Table 1). To convert the nutrient composition from a DM to a ME basis, we must remember that protein and carbohydrate both provide approximately 3.5 kcal/g of food, whereas fat provides much more — approximately 8.5 kcal/g of food. Thus, in this diet, the energy provided by protein is 65% times 3.5 kcal/g, or 228 kcal, and the energy provided by fat and carbohydrate are 213 kcal and 14 kcal, respectively, for a total of 454 kcal. The percentage of metabolizable energy that is provided from protein is then calculated (by dividing 228 kcal by 454 kcal and multiplying by 100), which gives us a protein content (ME) of 50%.

Doing the same calculations for diets that contain protein levels (DM basis) of 55%, 45%, 35% and 30% are also included in Table 1. As you can see, looking at nutrients on a dry matter basis “overestimates” the calories provided by protein (ME basis) by ~30%, while greatly underestimating the calories provided by fat (by ~50% in this example).

Table 1. Comparison of Nutrient Content:
Dry Matter (DM) Basis vs. Metabolizable Energy (ME) Basis

Bottom Line: When analyzing the composition of a cat food, one can look at the protein content on a DM or ME basis. In the end, it doesn’t make that much of a difference, but one must realize that the percent protein on a DM basis “overestimates” the calories provided by protein (ME basis). Cats in the wild ingest at least 50% of their calories as protein (ME). As you can see, that equates to ~65% of their diet being composed of protein on a DM basis.

Calculating the Amount of Protein that Commercial Diets Provide

Once we select our diet, we can do the calculations for protein content of a can or cup of food, as I described in detail in my post on “Optimal Protein Requirements for Older Cats and Cats with Hyperthyroidism.” Again, for this calculation, we need to know the protein content of the diet (DM basis), as well as the moisture content of the diet (generally 75% for canned food and 10% for dry food diets).

Let’s take another example by examining 5 commercial foods containing a wide range of protein content (Table 2). All of these diets are 5.5 oz cans (156 g) containing 75% moisture, so this leaves us with 39 g of food in each can on a DM basis (156 times 25% = 39 g). From the information on the label or the company’s website, we will hopefully find the protein content (DM) of each diet (note that if the protein content on a DM basis is not listed, you will have to call the company for that information). The guaranteed analysis (GA) information cannot be used for this calculation, since it listed only the minimum percentage of the crude protein contained the product and is therefore highly inaccurate.

Once the protein content (DM) is know, we multiply that value by total dry matter weight of the can. This provides us with the total protein content in each can of food (e.g., 39 g times 65% = 25.4 g of protein).

In this example, I’ve then used the weight of an average older cat (4.5 kg) to calculate the amount of protein ingested on a body weight basis (g/kg).

Table 2: Calculation of Daily Protein Ingested Based on Protein Content of Food.
*Each 5.5 oz can contains 156 g of food. If moisture content of diet is 75%, that leaves 25% dry matter and 39 g of food (156 times 25% = 39 g).

Bottom Line: As you can see in Tables 1 & 2, we need to feed a diet containing at >50% protein (DM basis) or >40% protein on a ME basis to even come close to providing the needed amounts of protein per kg.

The pet food companies may tell you that these protein levels are unnecessary and that the Association of American Feed Control Officials (AAFCO) requires that feline diets contain a minimum of 26% DM protein for maintenance (4,6). But remember: those are “minimal,” not optimal values. And most importantly, those recommendations are for normal adult cats, not hyperthyroid cats or geriatric cats prone to muscle wasting associated with sarcopenia of aging (7,8).

Is It Even Possible to Feed the Optimal Amounts of Protein to these Geriatic Cats?

The average older cat weighs 10 lbs (4.5 kg); if our goal is to provide 6.0-7.0 g of protein/kg/day, that calculates into an optimal daily protein intake of ~27-31 g. To consume those amounts of protein, that 4.5 cat would have to eat almost three 5.5-oz cans of food per day containing 30% protein (DM), almost two cans of food per day containing 45% protein (DM), or just over one can of a food containing 65% protein (DM) (Table 2).

In general, older cats tend not to eat those large amounts of food each day, unless they have uncontrolled hyperthyroidism and are polyphagic (9,11). Most older euthyroid cats (again weighing 4.5 kg) would eat only one 5.5-oz can per day, which provides only ~2.6 g/kg/day to 5.6 g/kg/day (Table 2).

So what can we do to help prevent "sarcopenia of aging" with its associated muscle wasting in these older cats? We want to feed an energy dense food that contains a highly digestible, high-quality source of protein.

Feeding an Energy-Dense Diet to Older Cats & Cats with Hyperthyroidism

What if our cats won’t eat as much protein as we would like them to ingest?

1. In addition to providing a diet with an adequate amount of protein, we need to make sure that the cat’s energy requirement is fulfilled. Most of the cat foods marketed as “Senior” diets are too low in caloric content. Remember as cats age (>12 years), their energy requirements actually increase (8, 12-14); these cats will lose weight (and muscle mass) on many of these commercial senior diets.
2. When calorie intake is inadequate to meet energy needs, body proteins are catabolized and used for energy (4). Again, feeding frequent small meals of an energy-dense, highly digestible diet that meets the senior cat’s increasing energy requirements will minimize protein degradation and avoid protein:calorie malnutrition (8, 12-14).
3. Identify diets that are palatable for the cat. Many older cats may partially lose their sense of taste or smell. Feeding a variety of different flavors or types of food helps maintain the appetite in many of these older cats.
4. Warming the food to body temperature or moistening the food may increase help to increase appetite in some cats.
5. Feeding frequent small meals of energy-dense, fresh food may help increase daily food intake.
6. Cats are solitary feeders by nature and elderly cats often do not cope well with competition and stressors. Older cats in multi-cat homes may benefit from being fed separately or being offered supplemental meals.
7. One must also identify underlying medical problems that can lead to decrease in appetite (8,9).

Feeding Highly Digestible, High Quality Protein to Geriatric Cat & Cats with Hyperthyroidism

Again, how can we help compensate for the fact that many older cats will not eat enough food to fulfill their “optimal” daily protein needs? Again, the answer may lie in the source of the protein used in the cat food diet. Feeding a highly digestible, high-quality source of protein would likely allow us to feed lower amounts of protein but still prevent loss of lean body mass and muscle wasting. Improving protein quality without increasing protein intake may help fulfill of these additional protein needs of older cats (4,15).

It is generally accepted that animal protein has a higher digestibility and is of higher quality than that of plant protein sources (16,17). Proteins of plant origin usually have a lower digestibility than animal proteins because plant fiber and the carbohydrates found in plants lower digestion, due to a reduced degradation rate of nutrients in the gut and increased bacterial activity (18).

The biological value (or quality) of a protein is a measure of that protein's ability to supply amino acids (especially the 11 essential amino acids) and to supply these amino acids in the proper proportions (4,15,17). The higher the biological value of a protein, the less would be needed in the diet to meet all of an animal’s essential amino acid requirements.

Figure 1: Protein Biologic Value of Common Pet Food Ingredients. 
Data from reference no. 15.

As shown in Figure 1, it is well-established that animal proteins (e.g., meat, meat by-products) have a higher biological values than vegetable proteins (e.g., corn gluten meal, soybean meal, soy protein isolate) (15-17,19). To meet a cat’s optimal daily protein needs, less protein intake would be required when its biological value is high (e.g., when animal protein is fed as the main source of protein).

What About Hill’s y/d? Will Cats Really Eat the Recommended Amounts of y/d Diet Every Day for the Rest of Their Lives ?

Again, one must realize that the amount of protein a cat ingests per day is highly dependent upon the total amount of food consumed. Eating a diet with less-than-optimal quantities of protein might be adequate if that cat ingests a large enough quantity of the food.

The recommended amounts of y/d to feed, as listed on the Hill’s website (20,21), seem reasonable for an uncontrolled hyperthyroid cat. Once euthyroid, however, will the cats continue to consume those same amounts of food? No, that's highly unlikely.  In general, the amount of food hyperthyroid cats ingest decreases as euthyroidism is restored. But remember, older cats still need higher amounts of protein, even when not hyperthyroid, to maintain their muscle mass as they age.

Let’s again take our average older cat weighing 4.5 kg. For that cat to consume protein at our “optimal” amount of 6.0-7.0 g/kg/day, the cat would have to eat almost two 5.5-oz cans of canned Hill's y/d per day or ¾ cup of dry Hill’s y/d per day. Those amounts exceed the amounts listed on the Hill’s feeding guide (18,19). Again, most older euthyroid cats (again weighing 4.5 kg) would eat only one 5.5-oz can per day, which provides ~3.3 g/kg/day, about half of what they actually need.

What about the sources of protein for Hill's y/d diets? Do they include a high-quality, easily digestible source of protein? Are the ingredients of high biological value? Unfortunately, the answer to both questions is no —the ingredients present in y/d are far less than ideal for cats. In addition to the fact that y/d is a low-protein diet, much of the diet’s protein is derived from plant sources. This is especially true for the dry formulation, in which the only listed animal protein on the label is "dried egg product," and this is the fifth ingredient. In other words, this diet does not contain any meat. The primary protein source in dry y/d is corn gluten meal, commonly used in many pet foods because of its low cost.

For the canned formulation, the ingredients list is better in that the first 3 listed ingredients— liver, meat by-products, and chicken—all contain animal protein. Liver is a very nutritious organ meat and a good source of animal protein, but the daily feeding of a pet food containing liver as the first ingredient might be questioned. Do you want to feed your cats a liver diet at every meal for the rest of their lives? Would you consider that a healthy diet?

References

1. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
2. Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9.
3. Crissey SD, Slifka KA, Lintzenich BA. Whole body cholesterol, fat, and fatty acid concentrations of mice (Mus domesticus) used as a food source. Journal of Zoo and Wildlife Medicine 1999;30:222-227. 
4. Gross KL, Yamka RM, Khoo C, et al. Macronutrients. In: Hand MS, Thatcher CD, Remillard RL, Roudebush R, Novotny, BJ (eds), Small Animal Clinical Nutrition. Mark Morris Institute. 2010; 49-105. 
5. Schenck PA. Nutrients. In: Home-Prepared Dog and Cat Diets.  Wiley-Blackwell. Ames IA, 2010; 23-49.
6. AAFCO. (Association of American Feed Control Officials). Official Publication, 2007. 
7. Wolfe RR. Sarcopenia of aging: Implications of the age-related loss of lean body mass. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 12-17. 
8. Little S: Evaluation of the senior cat with weight loss, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press. 
9. The Special Needs of the Senior Cat. Information brochure, Cornell University College of Veterinary Medicine.   
10. Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183:103-110. 
11. Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. Journal of the American Veterinary Medical Association 1995;206:302-305. 
12. Perez-Camargo G: Cat nutrition: What is new in the old? Compendium for Continuing Education for the Practicing Veterinarian 2004;26 (Suppl 2A):5-10.
13. Pérez-Camargo G. Feline decline in key physiological reserves: implication for mortality. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 6-13. 
14. Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42. 
15. Lewis LD, Morris ML, Hand MS. Nutrients. In: Small Animal Clinical Nutrition III. Mark Morris Associates. Topeka, 1987; 1-25.
16. Funaba M, Matsumoto C, Matsuki K, et al. Comparison of corngluten meal and meat meal as a protein source in dry foods formulated for cats. American Journal of Veterinary Research 2002;63:1247-1251.
17. Funaba M, Oka Y, Kobayashi S, et al. Evaluation of meat meal,chicken meal, and corn gluten meal as dietary sources of protein in dry catfood. Canadian Journal of Veterinary Research 2005;69:299-304.
18. Murray SM, Patil AR, Fahey GC, et al. Raw and rendered animal by-products as ingredients in dog diets. Journal of Animal Science 1997; 75:2497-2505. 
19. Feline Nutrition. http://maxshouse.com/feline_nutrition.htm
20. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health (Dry).
21. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health (Canned).

Is the Protein Content of Hill's y/d Too Low to Restore and Maintain Muscle Mass in Cats with Hyperthyroidism?

Unless you have been vacationing on a deserted island for the last 3 months, you have been inundated with ads and mailings about Hill’s new y/d diet (1,2), the iodine-deficient food being marketed as the “Safe, effective and easy way to manage thyroid health…. Clinically proven nutrition to restore thyroid health in 3 weeks.”

The corporate team at Hill’s Pet Nutrition has spent a “ton of money” on the marketing of this new diet. They certainly deserve a gold star for doing their best to convince practicing veterinarians and cat owners that this diet, with its "breakthrough nutrition," is an accepted, first-line treatment option for cats with hyperthyroidism (3-6).

However, before we all become engulfed in this marketing hype, let’s step back and review the actual data provided by Hill’s concerning this diet. Is this diet really the best treatment option, especially when we consider that Hill’s is recommending that y/d be fed to the hyperthyroid cat for the remainder of his life?

Hyperthyroid cat
with muscle wasting

As I have discussed in a previous post on the nutritional value of Hill’s y/d, evaluation of the diet's composition reveals that it is a low-protein, high-carbohydrate diet. That fact suggests that y/d may be a less than an “ideal” diet for an obligate carnivore, especially in hyperthyroid cats with severe muscle wasting or an older cat prone to sarcopenia of aging (7-10). As I've discussed in my previous post on the Best Diet to Feed Hyperthyroid Cats, sarcopenia (from the Greek meaning poverty of flesh) is the degenerative loss of skeletal muscle mass and strength that commonly occurs with aging in both humans, as well as animals (10-15).

As I discussed in my last post on Optimal Protein Requirements for Older Cats and Cats with Hyperthyroidism, aging cats need increasing amounts of both energy and protein as they age in order to maintain their lean body mass and prevent muscle wasting associated with sarcopenia.

Therefore, in this review, my aim is to address two important questions:

• Can y/d, fed at the recommended amounts, provide enough protein for a hyperthyroid cat to promote weight gain and restore lost muscle mass? Or will cats continue to lose lean body mass while being fed this iodine-deficient diet?
• Once euthyroidism is established, can y/d maintain muscle mass in these older, geriatric cats? Or will they develop progressive loss of lean body mass associated with sarcopenia of aging, a common phenomenon in aging cats fed senior diets?

What’s the Diet Composition of the Hill’s y/d Diets?

Remember that cats as obligate carnivores need proportionally more protein in their diet compared to other mammals (16,17). Cats do not have a dietary requirement for carbohydrates (18). Therefore, cats are adapted to eat a protein-rich, carbohydrate-poor diet. The composition of a cat’s diet in the wild (as a percentage of calories or metabolizable energy ingested) is approximately 50-60% protein, 30-50% fat, and 5-10% carbohydrates (19-21).

When the diet composition or caloric distribution of Hill’s y/d diet is examined (Table 1), it is clear that this is a less than optimal-protein, higher than desired-carbohydrate diet. This diet provides only 27-28% of calories or metabolizable energy as protein, whereas y/d provides 23-24% of its calories from carbohydrates (1,2,22). Compared to a cat’s natural diet in the wild, y/d contains only half of the amount of protein normally ingested and is 2.5 to 5 times higher in carbohydrates.

Table 1: Diet composition of Hill's y/d vs. Natural Cat Diet

How Much Protein Is Ingested by Cats Eating Canned Hill’s y/d?

So how much protein (g/kg/day) does this diet actually provide? Here are my calculations:

• Each 5.5-oz can of y/d contains 156 grams (g) of food and provides 188 kcals (34 kcal/oz or 1.2 kcal/g) of energy (2).
• The moisture content for canned y/d is 71%, so that leaves us with 29% dry matter. So out of the 156 g of canned food, there are 45 g of dry matter.
• Hill’s canned y/d is 34% protein on a dry matter basis (DMB) (i.e., there are 34 g of protein per 100 g of dry matter).
• From this information, we can calculate that each 5.5-oz can of yd diet contains 15 g of protein/can of food (45 g X 34% = 15 g).

Based on the feeding guide provided on the website, cats should be fed 0.75 can (117 g; 141 kcal) to 2.5 cans (390 g; 470 kcal) per day based upon their optimal body weight (~47-63 kcal/kg/day). On a body weight basis, cats eating the recommended amount of canned y/d would be fed 39-53 g of canned food/kg/day.

As far as protein consumption goes, feeding a cat those amounts of y/d translates into 11 g to 37.5 g of protein ingested per day. On a body weight basis, cats eating canned y/d would be fed an average of 4.5 g protein/kg/day. Again, this calculation is based on the feeding guide provided on the Hill’s website (2).

How Much Protein Is Ingested by Cats Eating Dry Hill’s y/d?

Again, here are the similar calculations for the protein content (g/kg/day) of the dry y/d formulation:

• Each 8-oz measuring cup of dry y/d contains 4 oz (113 g) of food by weight and provides 519 kcals of energy (4.6 kcal/g).
• Since the moisture content of dry y/d is 6%, that leaves us with 94% dry matter. So for each cup of dry y/d, we have 106 g of dry mater.
• Hill’s y/d dry is 36% protein DMB (i.e., there are 36 g of protein per 100 grams of dry matter).
• From this information, we can calculate that each cup of y/d diet contains 38 g of protein/113 g of food fed (106 g X 36% = 38 g).

Based on the feeding guide provided on the Hill’s website, cats should be fed 1/4 cup (~28 g; 130 kcal) to 7/8 cup (~100 g; 455 kcal) per day based upon their body weight (~43-65 kcal/kg/day). On a kg basis, the recommended amount of dry y/d to be fed ranges from 9.5-14.2 g/kg/day.

As far as protein consumption goes, feeding a cat those amounts of dry y/d translate into ~10 g to 36 g of protein ingested per day. On a body weight basis, cats eating dry y/d would be fed an average of 4.0 g protein/kg/day. Again, this calculation is based on the feeding guide provided on the Hill’s website (1).

The Bottom Line

So, let’s turn back to our original questions: Is the amount of protein provided by the Hill’s Prescription Diet y/d diets optimal or even sufficient for hyperthyroid cats? And just as important, will the amount of protein fed be high enough to prevent progressive muscle wasting in the older senior cat once euthyroidism has be reestablished?

As I discussed in my last post, the optimal daily protein intake in aged cats (older than 10-12 years of age) appears to be at least 6-8 g/kg/day to help maintain lean body mass and promote optimum health. Because virtually all cats with hyperthyroidism develop moderate to severe weight loss due largely to muscle wasting (7,8), most untreated cats need even more, probably about 7-10 g of high quality protein/kg each day.

So getting back to our questions, put in a more direct way:

• Will feeding y/d and only providing ~4-4.5 g of protein/kg/day be enough for the hyperthyroid patient? 
• How about the geriatric, euthyroid cat? Will feeding y/d provide enough protein to prevent sarcopenia and progressive muscle wasting?

For both questions, the answer is clear: No, Hill's y/d will not provide adequate protein to meet the needs of the older cat or the cat with hyperthyroidism for the reasons outlined below:

• Hill's y/d is a low-protein diet, providing only 27-28% of its calories or metabolizable energy as protein.
• Feeding y/d will provide only 50-75% of the protein needed for older cats or cats with hyperthyroidism. 
• Even euthyroid cats fed this low-protein diet for prolonged periods will likely continue to lose muscle mass and develop complications associated with “sarcopenia of aging.” 
• If the cat’s appetite ever diminishes to the point that they no longer eat the recommended amounts to be fed (remember that older cats tend to eat less as they age), their endogenous protein catabolism would be accelerated; these senior cats could rapidly become severely protein malnourished.

Hyperthyroid cat with severe muscle wasting

With hyperthyroidism, and all older cats prone to sarcopenia of aging, I'd like to hope that practicing veterinarians will use some common sense and remember that our goal is to always treat the whole cat, and not concentrate on a single aspect of their disease.

Certainly lowering the serum T4 concentration in cats with hyperthyroidism is important, but can y/d diet really ever be recommended or touted as the "treatment of choice" when progressive loss of lean body mass and muscle wasting can virtually be guaranteed?

References

1. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health (Dry). 
2. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health (Canned).  
3. Hill's Pet Nutrition website. As Easy as Feeding. Manage Thyroid Health through Breakthrough Nutrition. 
4. MultiVi Multimedia & Broadcast PR website. Hill's Pet Nutrition Launches New Pet Food for Hyperthyroid Cats. 
5. Heflon M. Managing feline hyperthyroidism with nutrition. October 11, 2011. p. 26. 
6. MultiVi Multimedia & Broadcast PR website. Hill's Pet Nutrition Informational Brochure on Prescription Diet y/d Thyroid Feline Health. 
7. Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183:103-110. 
8. Joseph RJ, Peterson ME. Review and comparison of neuromuscular and central nervous system manifestations of hyperthyroidism in cats and humans. Progress in Veterinary Neurology 1992;3:114-119.
9. Perez-Camargo G: Cat nutrition: What is new in the old? Compendium for Continuing Education for the Practicing Veterinarian 2004;26 (Suppl 2A):5-10.
10. Wakshlag JJ. Dietary protein consumption in the healthy aging companion animal. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 32-39. 
11. Fujita S, Volpi E. Nutrition and sarcopenia of ageing. Nutrition Research Reviews 2004;17:69-76. 
12. Short KR, Nair KS. Mechanisms of sarcopenia of aging. Journal of Endocrinological Investigation 1999;22(5 Suppl):95-105. 
13. Laflamme D. Nutrition for aging cats and dogs and the importance of body condition. Veterinary Clinics of North America: Small Animal Practice 2005;35:713-742. 
14. Wolfe RR. Sarcopenia of aging: Implications of the age-related loss of lean body mass. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 12-17. 
15. Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42. 
16. MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 1984;4:521-562. 
17. Zoran DL. The carnivore connection to nutrition in cats. Journal of the American Veterinary Medical Association 2002;221:1559-1567. 
18. Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Nutrition Research Reviews 2002;15:153-168. 
19. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
20. Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9.
21. Crissey SD, Slifka KA, Lintzenich BA. Whole body cholesterol, fat, and fatty acid concentrations of mice (Mus domesticus) used as a food source. Journal of Zoo and Wildlife Medicine 1999;30:222-227. 
22. Veterinary Information Service. Endocrinology Message Boards — Y/D Prescription Diet for Hyperthyroid Cats. Dietary data posted on July 18, 2011 by Rosalie Behnke, Hill's Veterinary Consultation Service.

Optimal Protein Requirements for Older Cats and Cats with Hyperthyroidism

Cats, as obligate carnivores, are unique among companion animals in their need for large amounts of dietary protein (1-3). As I’ve discussed in my previous post on "The Best Diet to Feed Hyperthyroid Cats," we have abundant evidence that most cats, but especially those with hyperthyroidism, should be fed a high-protein diet.

An adequate intake of dietary protein is critical when formulating a diet for hyperthyroid cats, in which loss of lean body mass and muscle wasting is universally present (4-6). Remember that protein is the primary macronutrient responsible for maintenance of muscle mass. Restoring and preserving any remaining muscle tissue in cats treated for hyperthyroidism depends upon the cat consuming a diet with sufficient amounts of high-quality protein.

I’ve received a number of inquires both from veterinarians and cat owners asking about the daily protein requirements for cats. Specifically, how many grams of protein do hyperthyroid cats need to eat every day to maintain their muscle mass? How about clinically normal geriatric cats or senior cats with a nonthyroidal illness? These are excellent questions, given the fact that all cats will need higher amounts of protein as they age to prevent a loss in lean body mass and associated muscle wasting.

Once we determine how much protein we need to feed our cats, the next question is: how do we calculate the exact amount of protein a cat is actually ingesting from the cat’s commercially prepared diet?

Protein Requirement of Normal Cats: AAFCO and NRC Recommendations

In the United States, the primary sources for “minimum” nutritional requirements in healthy cats are the National Research Council (NRC) and the Association of American Feed Control Officials (AAFCO) (7-10). Over the last few years, AAFCO's nutrition profiles have replaced the NRC recommendations as the official source for nutritional information for commercial pet foods in the United States (10). Although AAFCO establishes standards on which States base their feed laws and regulations, it has no direct regulatory authority. As with the recommended dietary allowances (RDAs) for people, AAFCO pet food nutrient profiles are not necessarily optimal, but rather act as guidelines to fulfill the minimal requirements (see How Much Protein Do Normal Cats "Want" to Eat, below).

More importantly for this discussion, neither AAFCO nor the NRC has ever established nutrient profiles for senior or geriatric “normal” cats (7-10). As could be expected, these groups do not have specific nutritional recommendations for cats with specific disease processes, such as hyperthyroidism. Therefore, the AAFCO guidelines provide us with little assistance in determining the optimal daily protein needs of cats, especially older cats or cats with hyperthyroidism.

How Much Protein Do Normal Cats “Want” to Eat?

Remember that cats as obligate carnivores need proportionally more protein in its diet compared to other mammals (1-3). Cats do not have a dietary requirement for carbohydrates. Therefore, cats are adapted to eat a protein-rich, carbohydrate-poor diet. The composition of a cat’s diet in the wild (as a percentage of calories or metabolizable energy ingested) is approximately 50-60% protein, 30-50% fat, and 5-10% carbohydrates (2,11-13).

First, let’s do some calculations about the likely daily protein intake for the living in the wild:

• First, we know that in the wild where cats can choose what prey they eat, they would normally ingest at least 50% of their daily calorie needs as protein.
• The daily energy requirements for a typical, inactive adult cat is about 40-45 kilocalories (kcal)/kg/day, whereas the energy requirement for active or underweight cats is up to 80 kcal/kg/day (9,14,15).
• So, if 50% of a cat’s calories are derived from ingested protein, and protein provides 3.5 kcal of energy per gram (g), that would calculate into 5.7 g/kg/day of protein (for a cat consuming 40 kcal/kg/day) up to 11.4 g/kg/day (for a cat eating 80 kcal/kg/day).

In support of these calculations is a recent study (16) that examined the diet composition of 42 clinically normal colony cats (aged 2.9-9.1 years) that were allowed to voluntarily choose the composition of their diet in order to fulfill their daily caloric need. In other words, this study asked the question: When cats can choose their own proportion of protein, fats, and carbohydrates to fulfill their daily energy requirements, what did they actually eat?

In that study (16), investigators found that the “intake target was close to 26 g/day of protein, 9 g/day of fat, and 8 g/day of carbohydrate, yielding a macronutrient energy composition of 52% protein, 36% fat and 12% carbohydrate.” Since the mean body of weight of these 42 cats was 4.9 kg, that would calculate to a daily protein intake of approximately 5.3 g/kg/day.

The daily protein value of 5.3 g/kg/day is close to the lower end of my calculated range of 5.7 to 11.4 g/kg/day for cats in the wild. Why lower? The likely reason relates to the fact that these research cats were inactive. Therefore, their calculated energy requirement was only about 45 kcal/kg/day (9,14,15). Remember that these cats were allowed to voluntarily choose to eat protein over carbohydrates, but they were not allowed to eat as much as they wanted. The more calories cats ingest, of course, the higher their daily protein intake.

Summary: The optimal daily protein intake in clinically normal, young to middle-aged cats appears to range from approximately 5.5 g/kg up to 11.5 g/kg.

Dietary Protein Requirements for the Senior or Geriatric Cat: What Do We Know?

Up to this point, everything I’ve discussed as been in clinically normal, young to middle-aged adult cats (less than 10 years of age). What happens to daily energy and protein requirements for cats as they age? Well, it turns out that maintenance energy requirements decrease as cats mature and become middle-aged (4-9 years of age), explaining the tendency of many cats to gain weight during this time (17,18).

However, energy requirement sharply and progressively increase again in these cat when they become older, starting at 10 to 12 years of age (17-19). If daily caloric intake is not increased, progressive weight loss will result, due in large part to the loss of lean body mass (i.e., muscle mass), a phenomenon referred to the “sarcopenia” of aging (20-22).

The term age-related sarcopenia is derived from Greek (meaning "poverty of flesh") and is characterized by a degenerative loss of skeletal muscle mass and strength, as well as increased muscle fatigability that occurs in both humans and companion animals. For more information on the sarcopenia of aging, also see my previous post on "The Best Diet to Feed Hyperthyroid Cats," in which I discuss this phenomenon in more detail.

In addition to an increased caloric intake, older cats also require higher amounts of protein to maintain protein reserves compared with younger adult cats (19, 22-26). As cats age, they absorb and metabolize protein less efficiently — therefore, it’s extremely important to feed high-quality protein (i.e., animal source rather than grain-based), as well as an adequate quantity of protein to aging cats.

It's very clear that not all proteins are created equal, especially when feeding a obligate carnivore, such as the cat (9,27).  The biological value of a protein is a measure of that protein's ability to supply amino acids (especially the 11 essential amino acids) and to supply these amino acids in the proper proportions. It is well-established that animal proteins (e.g., meat, meat by-products) have a higher biological values than vegetable proteins (e.g., corn gluten meal, soybean meal, soy protein isolate).

In addition to biological value, protein digestibility is key — what good is a food with a higher protein content if the protein isn't also easy to digest? In the short digestive tract of cats, plant proteins are far less digestible than meat proteins.  These issues are important when selecting a food for any cat, but they become of utmost importance when selecting a diet for the geriatric cat. Therefore, grain-based proteins should never be used as the primary protein source in geriatric cats.

The dogma that all older cats be fed reduced energy “senior” diets must be questioned based on what is now known about the increasing energy requirements and nutritional needs of older cats (23,24). The higher maintenance energy requirements of geriatric cats, in combination with their impaired ability to digest protein, will lead to loss of muscle mass if their overall energy and protein needs are not met (22-26).

There have been two studies examining the protein requirements needed to maintain lean body mass (i.e., muscle protein) in adult, colony cats (28,29). None of these study cats were geriatric or had sarcopenia of aging, but both studies emphasize the importance of adequate high-quality dietary protein in maintaining lean body (i.e., muscle) mass. 

In the first study (28), cats were fed 3 isocaloric diets with protein levels of 22%, 28% or 36% on a dry matter (DM) basis. In this study, the protein sources for these diets included poultry, soy, fish, and crystalline amino acids to meet amino acid requirements. Only the cats on 36% DM protein were able to maintain their lean body mass, whereas the cats on the 28% and 22% protein diets lost lean body mass. When using maintenance of lean body mass as an index of daily protein needs in these young adult cats, the calculated protein requirement was 5.28 g/kg/day (28). This is identical to the calculated protein intake (5.3 g/kg/day) in the study reported above that looked at what dietary composition cats want to eat (16).

The second study examined the effects of two isocaloric diets containing a low or high amount of  proteins (29). In this study, the protein sources for these diets included corn gluten, soy protein isolate, poultry and poultry by-product meal. These cats were previously fed a 36% (on a dry mater basis) protein diet and were switched to diets at 30% DM and 53% DM protein in a crossover design. Interestingly, when cats were on the 30% protein diet, they lost about 1.2% of their lean body mass, but when the cats were fed 53% protein diet, they averaged an accumulation of 4.2% of lean body mass (29). This change in muscle mass is not surprising, as cats can oxidize protein for energy as dietary protein is increased (30).  The third study by the same investigators calculated a protein requirement of 5.28 g/kg/day was needed for maintenance of lean body mass in cats.

Therefore, the amount of protein intake in cats for maximal retention of muscle mass has yet to be defined, but it is clear that some cats may benefit from high protein diets well beyond the NRC requirement and feeding normal healthy adult cats at least 5.5-6 g/kg BW (or more) may actually be ideal. As cats age and reach 11 to 12 years of age, both caloric and protein intake should be progressively increased by a factor of 1.1 to 1.6 (10-60%) to maintain muscle mass and prevent the sarcopenia of aging (10, 19). For generic cats, this increased protein need calculates to at least 6.0-8.5 g/kg/day.

Summary: To help maintain lean body mass and promote optimum health, geriatric cats should receive diets that will provide 6.0-8.5 g of high quality protein/kg body weight per day, or diets that provide at least 50% of calories or metabolizable energy (ME) as protein.

Dietary Protein Requirements for the Hyperthyroid Cat: What Do We Know?

We now know that older cats will develop progressive muscle wasting associated with by “sarcopenia of aging” unless their protein intake is increased (22-24). To make this loss of muscle mass even worse, older, geriatric cats the commonly develop diseases such as including hyperthyroidism, renal failure, diabetes, or neoplasia. All these maladies will accelerate the catabolism of skeletal muscle, potentiating the muscle wasting associated with sarcopenia of aging (31).

So how much protein does a hyperthyroid cat need to eat each day? All cats with untreated hyperthyroidism have an increased metabolic rate (3). So obviously, cats with hyperthyroid have a greater energy requirement. Because excess thyroid hormone is catabolic and breaks down muscle tissue, daily protein needs also go up in cats with hyperthyroidism. But how much protein should be fed to these cats? The short answer is that we don’t know, but likely, it will vary tremendously depending on the degree of hyperthyroidism. Obviously, untreated hyperthyroid cats need at least 60-80 kcal/kg per day for energy (and probably much more) and most need at least 7.0-10 g (and preferably more) of high quality protein/kg each day.

This recommendation for higher amounts of dietary protein does not change once euthyroidism is restored. Remember that these are elderly cats prone to sarcopenia of aging and therefore will continue to break down muscle mass if deprived of adequate protein in order to create the energy they need (22-24). By feeding only high-quality protein diets, we will help restore the hyperthyroid/euthyroid cat’s muscle mass and improve strength and agility.

Summary: To help restore muscle mass and promote optimum health, untreated hyperthyroid cats should receive diets that will provide 7.0-10 g of high quality protein/kg body weight per day, or diets that provide at least 50% of metabolizable energy (ME) as protein.

How to Calculate the Protein Content of Dry and Canned Commercial Cat Foods?

So how do we calculate how much protein a commercial cat food diet actually contains? Here is how I do the calculation:

1. First determine the grams of food in a can or cup of food feed.
• For canned diets, there are 28.3 g per oz of canned food, so a 5.5-oz can of food contains ~156 grams of food.
• For dry food diets, remember that a cup is a measurement for volume, whereas a gram is measurement of weight, so there is not a general conversion of cups to grams. Depending on size and density of the dry kibble, one 8-oz. cup may hold under 3 oz (~85 g) to over 4 oz (~113 g) of food. To determine the weight (oz or g) in a cup of dry food, look for the information listed on the product bag or manufacturer's website; otherwise, email or call the company to find the information.
2. Second, look up the moisture content for the canned or dry formulations.
• Most canned foods contain about 75% water, so that leaves us with 25% dry matter. In a can containing 156 g of food, that leaves us with 39 g of dry matter (i.e., 156 g X .25 = 39 g).
• Most dry foods contain about 10% moisture, leaving 90% of dry matter. In a cup of dry food (113 g), that leaves us with ~102 g of dry matter (i.e., 113 g X 0.9 = 101.7 g).
3. Third, look up the diet’s percent protein on a dry matter basis (DMB), again from the information on the can or bag or from the information on the company’s website.
• If the food contains 30% protein (DMB), that means the diet contains 30 g of protein per 100 g of dry matter. This is true for either a canned or dry diet.
4. Finally, multiple the grams of food (DMB) contained in a can or cup of dry food by the percent of protein (DMB) in each can or cup.
• If a 5.5-oz can of diet has 75% moisture and 30% protein on a DMB, then that food contains 11.7 g protein/can of food (39 g X 30% = 11.7 g). 
• If the same can has 75% moisture and 50% protein on a DMB, then that that diet contains 19.5 g protein/can of food (39 g X 50% = 19.5 g). 
• If a cup of dry food has 10% moisture and 30% protein on a DMB, then that food contains 30.6 g protein/cup of food (102 g X 30% = 30.6 grams).

My Bottom Line

The optimal daily protein intake in clinically normal, young to middle-aged cats appears to range from approximately 5.5 g/kg up to 11.5 g/kg. Both energy needs and protein requirements progressively increase as cats age, starting at age 10-12 years. Therefore, to help maintain lean body mass and promote optimum health, geriatric cats should receive diets that will provide at least 6-8 g (and preferably more) of high quality protein/kg body weight per day. Hyperthyroidism induces a marked increase in a cat’s metabolic rate, as well as muscle catabolism and loss of lean body mass. Therefore, untreated hyperthyroid cats have even a higher protein needs to restore lost muscle mass —about 7-10 g (and preferably more) of high quality protein/kg each day.

To provide these amounts of protein, the key is to feed highly nutritious diets that provide at least 50% of calories or metabolizable energy as protein from animals, not plants or grains. Attaining these amounts of daily protein will be next to impossible feeding diets that contain less than 35-40% of calories as protein, unless the cat can eat large amounts of food (and therefore calories) each day.

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