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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