Feeding Hill's y/d and Feline Immune Function

Is there a potential problem with the low level of iodine in y/d possibly reducing a cat's immune response?

I asked our Hill's representative who told me that the iodine content of y/d is not that low and it's not even deficient in iodine content — it's just lower than other commercial diets. The Hill's rep told me that they wouldn't worry at all about immune function or deficiencies in cats fed this diet, even for years.

I had heard that iodine played a role in immune function. What are your thoughts on this matter?

My Response:

Is Hill's y/d an iodine deficient diet?
The 2009 study by Wedekind, et al (1), in a study supported by Hill's Pet Nutrition, clearly showed that the iodine requirement in normal cats fed a corn gluten meal-corn based diet (similar to the ingredients in Hill's y/d) was 0.46 mg I/kg of diet. This amount of dietary iodine supplies about 20-25 μg of daily iodine to the average-sized cat.

This iodine requirement of 0.46 mg/kg of diet is more than 2-fold higher than the iodine content of Hill's y/d, which contains only 0.2 mg I/kg of diet (2,3). The y/d diet provides only about 10 μg of iodine to the average sized cat each day; again, less than half of the minimum daily iodine requirement published in the recent study by Wedekind (1).

So the Hill's folks and I can debate whether y/d is truly iodine deficient or not, but the published study that they supported (1) provides strong evidence that y/d clearly is an iodine-deficient diet. The Hill's folks get around this by calling it an iodine-restricted diet (2,3), but I would rather be completely transparent and label the diet for it what it is!

Iodine deficiency and immunity
Based on a number of studies in man and other species, it's clear that iodine has many extra-thyroidal functions in the body. Iodine plays an important role in the immune response (4-7). It also has anti-inflammatory and anti-oxidative actions (8-12).

Iodine deficiency in humans is associated with an increased prevalence of infectious diseases. In one study of adolescent girls and women in Bangladesh, where iodine deficiency is commonplace, the presence of iodine deficiency was associated with an increased risk of eye infections, ear infections, pneumonia, and diarrhea/dysentery (13). Childhood survival is also threatened by iodine deficiency, in part due to infectious complications (14).

Remember that the biggest concern in these iodine deficient populations is not infection, however, but rather the goiter, hypothyroidism, cretinism and other developmental defects that develop. Inasmuch as one third of the world' population are iodine deficient, correcting this publish health problem has become the goal of a massive global campaign (15,16).

Iodine supplementation as a therapy for infectious diseases
Before the discovery of antibiotics, iodine was used for a wide variety of ailments in the 19th and early part of the 20th centuries, especially for syphilis and chronic lung disease (17,18). The Nobel laureate Albert Szent Györgi (1893–1986), the physician who discovered vitamin C, wrote in his book (19): “When I was a medical student, iodine in the form of potassium iodide was the universal medicine. Nobody knew what it did, but it did something and did something good.”

In accord with that, it was recently suggested that iodine therapy might play a useful role in treating AIDS infections in both humans, as well as cats (20).

Hill's y/d and immune function in cats
So based on what we know in other species, I am indeed worried about y/d resulting in a decreased immune response in cats fed this diet for prolonged periods. Remember that in people, world authorities are doing all that they can do to prevent and correct iodine deficiency (15,16). No one would ever try to induce iodine deficiency in any human patients, even if they had thyroid disease.

So Hill's has succeeded in doing something that has never done before— that is, to make a diet, that fed chronically to our patients, will induce an iodine-deficient state. Even if thyroid function remains normal, that does not mean that the cat's whole body iodine levels are not deficient and that their immune function is not compromised.

Remember that Hill's has not provided us with any research data on immune function of any of the cats fed y/d in their colony for years. I have heard of a few hyperthyroid cats, however, that developed upper respiratory tract or urinary tract infections while being fed this diet — is this due to a suppressed immune function or just coincidence that they were eating a diet low (or deficient) in iodine? We just don't know. But I find it strange that these older cats would suddenly develop URI when they have not had a history of respiratory disease.

In my opinion, well-designed long-term studies of cats fed this diet need to be done to address this issue before any conclusive statements can be made. Again, Hill's claims that such studies would be difficult and expensive to do — but remember, if this diet had required FDA approval as a drug (and y/d really is a powerful drug if you stop and think about it), that is exactly what the FDA would require the company to do before release of this therapeutic diet to the market.

Bottom Line
If you decide to use this diet for management of your hyperthyroid cats, you should be on the look-out for signs of a suppressed immune response. If signs of infection do develop, the diet should be stopped and iodine supplementation given, and the hyperthyroidism should be treated with one of the other standard treatment methods (i.e, radioiodine, methimazole, or surgical thyroidectomy).

Because of this potential issue with immune function and iodine deficiency, I feel strongly that this is another reason that the Hill's y/d diet should never be fed chronically to a euthyroid cat that does not have hyperthyroidism.  The suggestion that feeding an iodine deficient diet to clinically normal cats is "safe" is simply contrary to all reason or common sense, considering what we know about iodine deficiency in man.

References:

1. Wedekind KJ, Blumer ME, Huntington CE, et al. The feline iodine requirement is lower than the 2006 NRC recommended allowance. Journal of Animal Physiology and Animal Nutrition 2010;94:527-539. 
2. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health (Dry). 
3. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health (Canned).
4. Marani L, Venturi S, Masala R. Role of iodine in delayed immune response. Israeli Journal of Medical Science 1985;21:864. 
5. Simmons SR, Karnovsky ML. Iodinating ability of various leukocytes and their bactericidal activity. Journal of Experimental Medicine 1973;138:44-63.
6. Weetman AP, McGregor AM, Campbell H, et al. Iodide enhances IgG synthesis by human peripheral blood lymphocytes in vitro. Acta Endocrinologia 1983;103:210-215. 
7. Venturi S, Venturi M. Iodine, thymus, and immunity. Nutrition 2009;25:977-979.
8. Miller DW. Extrathyroidal benefits  of iodine. Journal of American Physicians and Surgeons 2006;11:106-110. 
9. Ware CM, Wishner LA. The lipid antioxidant properties of iodine compounds. Lipids 1968;3:182-183.
10. Smyth PP. Role of iodine in antioxidant defense in thyroid and breast disease. Biofactors 2003;19:121-130.  
11. Patrick L. Iodine: deficiency and therapeutic considerations. Alternative Medicine Review 2008;13:116-127. 
12. Soriguer F, Gutierre-Repiso C, Rubio-Martin E, et al. Iodine intakes of 100-300 μg/d do not modify thyroid function and have modest anti-inflammatory effects. British Journal of Nutrition 2011;25:1-8.
13. Harun-Or-Rashid M, Khatun UF, Yoshida Y, et al. Iron and iodine deficiencies among under-2 children, adolescent girls, and pregnant women of Bangladesh: association with common diseases. Nagoya Journal of Medical Science 2009;71:39-49.
14. Cobra C, Muhilal, Rusmil K, et al. Infant survival is improved by oral iodine supplementation. Journal of Nutrition 1997;127:574-578.
15. Zimmermann MB. Iodine deficiency. Endocrine Reviews 2009;30:376-408.
16. International Council for the Control of Iodine Deficiency Disorders. www.iccidd.org
17. The Encyclopaedia Britannica. Vol 14. 11th ed. New York. Encyclopaedia Britannica Company; 1911:726.
18. Iodine. Manual of Therapeutics. Parke, Davis & Company. Detroit, Michigan 1919;360-361.
19. Szent-Györgyi A. Bioenergetics. New York, Academic Press; 1957:112.
20. Mamo JCL, Naissides M. Could iodine be effective in the treatment of human immunodeficiency virus and AIDS-associated opportunistic infections. International Journal of Infectious Diseases 2005;9:292-293.