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.