Is Hill's y/d a Nutritious Diet for Hyperthyroid Cats?

As I discussed in my last post, Hill's Pet Nutrition recently released Prescription Diet y/d Feline –Thyroid Health (1), an iodine deficient diet that can result in lowered serum T4 concentrations when fed as the sole source of nutrition to hyperthyroid cats. With all of the recent hype and widespread promotion of this diet, one could be persuaded into believing that feeding y/d is now the treatment of choice for cats with hyperthyroidism.

According to the Hill's website and marketing ads that I'm sure most of you have seen, the y/d diet "improves thyroid health in 3 weeks" when fed to cats with hyperthyroidism. The ads also state that the diet provides "Complete daily nutrition with a taste your cat will love." In addition to reduce T4 hormone production, y/d allegedly helps support kidney health by controlling phosphorus and sodium, and y/d is also said to help support urinary tract health (1).

Overall, this diet seems almost too good to be true. So, in this post, I want evaluate this diet to address the single question — Is Hill's y/d a nutritious diet designed to address the metabolic needs of the older, hyperthyroid cat?

Ideal Composition of Any Diet Fed to Cats with Hyperthyroidism

High Dietary Protein
As discussed in my previous post on "The Best Diet to Feed Hyperthyroid Cats," we have abundant evidence that hyperthyroid cats should be fed a diet that is high in protein content (>50% of total calories).

Cats as obligate carnivores are unique in their need for large amounts of dietary protein (2,3). This absolute requirement for dietary protein intake in cats is critically important when formulating a diet for hyperthyroid cats, in which protein catabolism 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.

This recommendation for higher amounts of dietary protein does not change once euthyroidism is restored. 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 (7,8). 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 (7-10).

When selecting or formulating the ideal diet for a hyperthyroid cat, remember that not all proteins are equal in quality (11). This is especially true for commercial pet food diets. High-quality meat is the best ingredient in a cat food and meat by-products are a close second. Some vegetable and grains are fine, but they supply a less bioavailable form of protein for cats and should not be the primary source of dietary protein for a hyperthyroid cat. Remember that when deprived of protein, cats will continue to break down muscle tissue to create the energy they need (2,3). By feeding only high-quality protein diets, we will help restore the hyperthyroid cat’s muscle mass and improve strength and agility.

Low Dietary Carbohydrates
In addition to high protein requirements, we also have good evidence that hyperthyroid cats will benefit by feeding a diet low in carbohydrates (<10% of total calories). As discussed in my previous post on "The Best Diet to Feed Hyperthyroid Cats," many hyperthyroid cats have subclinical diabetes as evidenced by their mild hyperglycemia, glucose intolerance, and insulin resistance (12,13). Feeding a low-carbohydrate diet will improve insulin sensitivity and help stabilize glucose metabolism in these cats (14). This may also prevent the development of overt diabetes and control long-term obesity after successful control of their hyperthyroid state.

Overall, I believe that it makes sense to feed most hyperthyroid cats (as well as normal and diabetic cats) a diet composition close to what they would be getting in the wild (2,3,15-17). That would be a diet composed of approximately 50-70% protein, 5-10% carbohydrates, and 30-40% fat. Because older cats also lose lean muscle mass in association with the “sarcopenia of aging,” this diet composition needs to be continued even after one treats the cat’s hyperthyroidism and restores a euthyroid state.

What's the Composition of Hill's Prescription y/d Diet?

The most accurate way to evaluate pets foods and their composition is to consider the calories or metabolizable energy (ME) that come from the protein, fat, and carbohydrate fractions (18). This allows us to compare various diets without worrying about their different moisture levels.

Looking at the composition or caloric distribution of Hill's y/d diet in this way reveals that it's a high-carbohydrate, low-protein diet (Table 1). The percent ME from carbohydrates is 23-24%, whereas the percent ME from protein is 27-28% (19). Compared to a cat’s natural diet in the wild, y/d is 2.5 to 5 times higher in carbohydrates and contains only half of the amount of protein normally ingested (Table 1). Feeding y/d for long periods is less than an “ideal” diet for an obligate carnivore, especially in hyperthyroid cats with severe muscle wasting or sarcopenia of aging (3,4,9).

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

Are the Ingredients Found in Hill's Prescription y/d Diet Ideal?

Unfortunately, the ingredients present in y/d are also far less than ideal for cats. In addition to the fact that y/d is a low-protein diet (Table 1), most 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 (Table 2). 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.

Prescription Diet y/d dry also contains "soybean mill run," which is a cheap, by-product filler. Soy may  contain enzyme inhibitors that impede normal protein digestion. Most importantly, soy is a known thyroid goitrogen and is considered to be one factor that may contribute to hyperthyroidism in cats (20).

Table 2: Label Ingredients for Dry y/d Feline Thyroid Diet

For the canned formulation, the ingredients list is more ideal in that the first 3 listed ingredients— liver, meat by-products, and chicken—all contain animal protein (Table 3). 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. (Would you eat liver at every meal for the rest of your life? Would you consider that a healthy diet?)

Table 3: Label Ingredients for Canned y/d Feline Thyroid Diet 

The Bottom Line

Because Hill's y/d diet is a high-carbohydrate, low-protein diet, feeding this diet for long periods is less than ideal for an obligate carnivore, especially in hyperthyroid cats with muscle wasting or older cats that have sarcopenia of aging (which all cats on a protein deficient diet will likely develop). In addition, the protein sources, especially for the dry food, are of inferior quality.  Is it logical to feed vegetable protein to a obligate carnivore?

Finally, it is very unclear why there is such a discrepancy in protein sources between the dry and canned formulations of the y/d diet. If Hill’s can provide animal-based proteins for the canned version, why can’t it be also done for the dry y/d?

References

1. http://www.hillspet.com/products/pd-feline-yd-dry.html
2. MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 1984;4:521-562.
3. Zoran DL. The carnivore connection to nutrition in cats. Journal of the American Veterinary Medical Association 2002;221:1559-1567.
4. Peterson ME, Kintzer PP, Cavanagh PG, Fox PR, Ferguson DC, Johnson GF, Becker DV. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183:103-110.
5. 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.
6. Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
7. 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.
8. Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42.
9. 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.
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. Thompson A. Ingredients: where pet food starts. Topics in Companion Animal Medicine 2008;23:127-132.
12. Hoenig M, Ferguson DC. Impairment of glucose tolerance in hyperthyroid cats. Journal of Endocrinology 1989;121:249-251.
13. Hoenig M, Peterson ME, Ferguson DC. Glucose tolerance and insulin secretion in spontaneously hyperthyroid cats. Research in Veterinary Science 1992;53:338-341.
14. Rucinsky R, Cook A, Haley S, Nelson R, Zoran DL, Poundstone M. AAHA diabetes management guidelines for dogs and cats. Journal of the American Animal Hospital Association 2010;46:215-224.
15. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
16. Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9.
17. 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.
18. Laflamme DP. Determining metabolizable energy content in commercial pet foods. Journal of Animal Physiolology and Animal Nutrition (Berlin). 2001; 85:222-230.
19. 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.
20. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.