Thursday, November 10, 2011

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?

  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 NestlĂ© 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 NestlĂ© 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.


Anonymous said...

Thank You for this article! Let common sense prevail. Cats are carnivores!

Unknown said...

Thanks for your honesty Doc. My old cat is currently diagnosed with the thyoid issue. My vet rec'd the y/d diet, both canned and dry.

Do you think that the canned mixed with 50% chicken meat would be a good choice?

thanks in advance,
Rick Bowman

Workaholic Meg said...

This article is excellent in that it looks at y/d from the other side and reminds veterinary healthcare providers that they should always be treating the patient, not the condition.

Rather than jump on the bandwagon of Hills bashing, I think something to consider in y/d's case is that it is a treatment option. The first line of treatment isn't always the one that is found to manage the patient well, so proper assessments on thyroid levels and body condition should be made at each visit. In the beginning of starting such a diet, this should be done monthly.

Another aspect to consider is the frequency of kidney disease that is found in hyperthyroid cats once euthyroid is achieved. While I have no data or studies to refer to, in my experience approximately a third of the cats we treat for hyperthyroidism have concurrent renal insufficiency or failure. This makes a higher protein diet much less desirable.

Since y/d has been released, we have had several clients adopt it and be able to manage their cats on diet alone. Body condition scores are good and they report improvement overall from being managed with methimazole. (However, the majority of these clients were having difficulties with methimazole compliance and tolerance.)

My bottom line:
I have seen too many hyperthyroid cats come to us untreated after many years, wasting away unnecessarily because owners weren't able to medicate their cats and couldn't afford I-121. Keep your head up and out of the hype and view y/d as what it is: another tool in your arsenal to manage a chronic condition.

Dr. Mark E. Peterson said...

Rick: Unfortunately, the chicken is going to contain enough iodine to counteract the effects of the y/d. It cannot be combined with any other food.

Dr. Mark E. Peterson said...

Meg: I agree that 25 - 30% of hyperthyroid cats have concurrent renal disease. In some of those cats, a lower-protein diet may be a benefit. However, this diet is being marketed as a cure-all for all hyperthyroid cats, even those that are young and without any kidney disease. In those cats (the majority), the other treatment options are superior.

JL said...

I have two cats (both 12 yrs old) with hyperthyroidism - one seems to be doing well and the other wasting away before my eyes. My vet had recommended they both be fed exclusively y/d. The one who is wasting away was formerly on Tapazol - could he be fed y/d and remain on Tapazol? He has an appointment in one week ( the soonest they could do) but I am becoming very concerned for him.

Dr. Mark E. Peterson said...

Yes, you need to have your cat checked as soon as possible if severe muscle wasting is occurring. We don't want to give both methimazole and y/d together because it can be a dangerous combination and lead to severe hypothyroidism.

As far as the y/d is concerned in general, both of your cats are relatively young. I'd recommend that you at least consider definitive therapy (radioiodine or surgery) to ablate or remove the thyroid tumors that your cats have... with either y/d or methimazole, the thyroid tumor will continue to grow and may eventually become malignant. Good luck!

Lisa Adler-Golden said...

I opened up my house to an adult female stray on Friday. When I took her into the vet to get her checked, they gave her T4 level as close to 24 and put her on y/d and methanizole (5mg, 2x day). I'm due to return next Tuesday to have her T4 checked again. What would be a good canned food for her later and what can I give to her now to boost the amount of protein she's eating? Right now I'm mostly worried about dumping calories into her and getting her up to a healthy weight (5 lbs when I took her in, I estimate she should be be 8-10 lbs). Thanks.

Dr. Mark E. Peterson said...

First of all, it can be dangerous to give both methimazole with the y/d diet. We have seen severe hypothyroidism develop with this combination. If you are going to continue the y/d diet, then the methimazole should be stopped.

But it sounds like you want to feed a better diet to your cat -- one that will help restore lost weight and muscle mass. There are a number of OTC diets that can be used to do that (see

But then your cat's hyperthyroidism needs to be managed with one of the other treatment options (ie, methimazole, surgery or radioiodine), all which would be better choices in my opinion.

Music Mombo said...

YD has worked for my cat. Vomiting and excessive thirst is gone. Coat is restored. Levels are now testing acceptable. But still underweight. Is there ANY Protein that can supplement the YD that does NOT contain iodine?

Dr. Mark E. Peterson said...

No, the only way to feed a low-iodine meat or plant-source of protein would be to grow plants in iodine deficient soil. Or feed chickens or beef an iodine deficient diet. Iodine is an essential nutrient.

It's totally unnatural to feed a diet low in iodine (can you tell I'm not a big fan of y/d?).

Simsam said...

My cat has been on Felimazole pills which affected her stomach, then transdermal gel which caused ear allergic reaction so am now trying y/d as last resort. Surely better than nothing.

Dr. Mark E. Peterson said...

I agree that use of y/d is better than nothing. But what about surgery or radioiodine?

Pet Guardian said...

Just wondering iff there is a reference for the natural cat dietary intake. Thanks in advance.

Dr. Mark E. Peterson said...

Here's a few paper to look at:

1. Eisert R. Hypercarnivory and the brain: protein requirements of cats reconsidered. Journal of comparative physiology B, Biochemical, systemic, and environmental physiology 2011;181:1-17.

2. Hendriks WH. Feline paleolithic nutrition: a consideration of its nature and its implications for nutrition of domesticated cats. The Waltham International Nutritional Sciences Symposium Pet nutrition – Art or Science? Cambridge, UK, 2010;22-23.

3. Hewson-Hughes AK, Hewson-Hughes VL, Colyer A, et al. Consistent proportional macronutrient intake selected by adult domestic cats (Felis catus) despite variations in macronutrient and moisture content of foods offered. Journal of comparative physiology B, Biochemical, systemic, and environmental physiology 2013;183:525-536.

4. Laflamme D, Gunn-Moore D. Nutrition of aging cats. Vet Clin North Am Small Anim Pract 2014;44:761-774, vi.

5. Laflamme DP. Protein requirements of aging cats based on preservation of lean body mass. 13th Annual AAVN Clinical Nutrition & Research Symposium 2013;17.

6. Laflamme DP. Determining protein requirements: Nitrogen balance versus lean body mass. Companion Animal Nutrition Summit: Tackling Myths about Pet Nutrition 2013;37-40.

7. Laflamme DP, Hannah SS. Discrepancy between use of lean body mass or nitrogen balance to determine protein requirements for adult cats. J Feline Med Surg 2013;15:691-697.

8. Verbrugghe A, Hesta M, Daminet S, et al. Nutritional modulation of insulin resistance in the true carnivorous cat: a review. Critical reviews in food science and nutrition 2012;52:172-182.

Babettesmom said...

I have a 15 year old cat with a T4 value of 4.6 ug/dL. She has lost a substantial amount of weight and I am contemplating radioiodine treatment. Before I do that however, I would like to first experiment with a dietary adjustment especially since her T4 lever is so close to the high normal range. I have a suspicion that the fertilizers we use in the lawn may have an effect on her thyroid since she likes to drink water from the sprinkler. I noticed that she has a tendency to lose weight in the fall which coincides with abundant fertilizer treatment. In the winter she usually gains her weight back. This fall however, she lost a lot more weight (3lbs) compared to previous years and that is why we took her to the Vet. My question is this: Is it ok to feed her gerber baby food (beef, chicken) and supplement with the YD cat food so that she can get the proper vitamins? On the baby food label, I read that it contains 0% iodine. I don't know however, if the iodine level is just too low to report on the label but is still high enough to stimulate the cat's thyroid. If it is OK to give her baby food, and if I want to bypass the YD cat food, are there vitamin supplements that I can give her since I am sure the baby food may not contain all the nutrition a cat may need. My goal is to help her gain some weight and lower her T4 level in the hopes that this is a transient ailment. (With regard to the fertilizer, I am wondering if it contains a lot of fish byproducts and thus copious amounts of iodine??)

Dr. Mark E. Peterson said...

There is no conclusive evidence that fertilizers cause feline hyperthyroidism.

Baby food is low is iodine, but it likely contains too much if you are going to feed y/d. You could call Hill's and ask them, but I doubt if it would work.

The severity of hyperthyroidism must be based on more than just a T4 value. If your cat is has lost a substantial amount of weight (3 pounds), that doesn't sound like mild disease to me. We (both humans and cats) have our own normal range for T4 and other thyroid hormones.. it could be that her normal range only goes up to 2.5-3 ug/dL; that would make T4 value compatible with the degree of weight loss.