The question, “What’s the best diet to feed my hyperthyroid cat?” is an extremely common one that I get from concerned cat owners. I’m certain that many of you get the same question.
With the recent introduction of the Hill’s y/d diet (2), the iodine deficient diet which, according to the company website “restores thyroid health,” I thought that my opinion of what diets should be fed to cats with hyperthyroidism might be of interest.
In this post, I’m not going to talk specifically about y/d or any other diet, but which types of food and nutrients hyperthyroid cats need in general. I’ll specifically address the pros and cons of y/d in my upcoming blog posts.
The Many Metabolic Problems Facing the Hyperthyroid Cat
When secreted in excess, thyroid hormones have profound metabolic effects on the whole body, and dysfunction of multiple organ systems (CNS, cardiac, gastrointestinal, hepatic, and renal) is common in hyperthyroid cats (3-5).
Weight Loss and Muscle Wasting
Weight loss, despite a normal to increased appetite, is the classic and most common signs seen in cats with hyperthyroidism (3-5). These cats lose weight because their hyperthyroidism accelerates their metabolic rate and body’s energy expenditure; they are burning up their food calories faster than they can consume their daily meals.
When hyperthyroid cats first lose weight, the case can usually be first noticed as a loss of muscle mass in the cat’s lumbar paravertebral area. Despite this loss of muscle mass, most mildly hyperthyroid cats retain their “belly” during the initial stages of their thyroid disease and may even have a higher than ideal body condition score.
With time, severe muscle wasting, emaciation, cachexia, and death from starvation can occur if the cat’s hyperthyroidism is left untreated (3-5). In hyperthyroidism, the cat’s body consumes its own muscle tissue to get the protein it needs to sustain its carnivorous life.
Even with treatment of hyperthyroidism, recovery of muscle mass and function may be prolonged, lasting several weeks to months. This is especially true if these cats are not provided with enough protein in their diet to rebuild and maintain their lost muscle mass.
Hyperglycemia, Glucose Intolerance, Insulin Resistance, and Overt Diabetes
Hyperthyroid cats commonly develop profound changes in glucose and insulin metabolism. Mild to moderate hyperglycemia is common in hyperthyroid cats, which is generally attributed to a “stress” reaction (3,5).
However, the actual metabolic changes are actually much more complicated: hyperthyroidism frequently causes moderate to severe “endogenous” insulin resistance, as demonstrated by high resting serum insulin concentrations and an exaggerated insulin response during an IV glucose tolerance test (8,9). This insulin resistance is associated with a decreased glucose clearance (impaired glucose tolerance), which is indicative of a prediabetic state (See Figures below).
Surprisingly, the insulin resistance and prediabetic state so common in hyperthyroid cats does not always improve and may even worsen despite successful treatment of hyperthyroidism (9): see Figure above. This indicates that hyperthyroid cats may have long-lasting alterations of glucose tolerance and insulin secretion that cannot always be reversed by treatment. In accord with that, some of these hyperthyroid cats (not diabetic at time of diagnosis) will go on to develop overt diabetes mellitus in the months to years after treatment of hyperthyroidism.
Sarcopenia of Aging
In addition to loss of muscle mass from the catabolic effects of thyroid hormone excess, cats also tend to loss muscle mass they age, independent of their thyroid status. This phenomenon, referred to as sarcopenia of aging, is also common in elderly human beings (10-12). 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.
In adult cats, maintenance energy requirements decrease by about 3% per year up until the age of 11 years, and then actually start to increase again (13). This contributes to a tendency of senior cats to lose muscle mass if their energy needs are not met. Lean body mass of aging cats drops dramatically after 12 years of age, and by age 15, cats may have a mean lean tissue mass that is a third less than cats aged 7 years or less (13, 14). Body fat also tends to progressively decrease in cats after the age of 12 years; this combination of reduced lean mass and body fat contributes to weight loss experienced by many elderly cats.
The ability to digest protein is also compromised in many geriatric cats. After the age of 14 years, one-fifth of geriatric cats have reduced ability to digest protein (13-15). Reduced protein digestibility in geriatric cats seems to occur in parallel with reduction of lean tissue and it might predispose them to negative nitrogen balance. (16).
Although moderation of calorie intake might be suitable for some mature cats, it does not appear to match the needs of most geriatric cats. In contrast, it seems more logical to use highly digestible, energy-dense food for geriatric cats in order to prevent or slow their decline in body weight and lean body tissue (13,16,17).
Reducing protein intake in geriatric cats, at a time when lean tissue has been lost, is contraindicated. Geriatric cats seem to have nutritional requirements closer to kittens than to mature adult cat.
Diet Recommendations for Hyperthyroid Cats
High Dietary Protein
As discussed in my last post about what to feed normal cats, obligate carnivores, such as the cat, are unique in their need for large amounts of dietary protein (specifically, dispensable nitrogen) that separates them from omnivores and herbivore species (18-20). 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.
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 has been restored. The dogma that all older cats should 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 (12,13).
In most geriatric cats, logic dictates the use highly digestible, energy-dense food mitigate the decline in body weight and lean body tissue and to avoid protein:calorie malnutrition (12,16,17). Protein reduction for this geriatric life stage, at a time when lean tissue is being lost, is contraindicated. Geriatric cats seem to have nutritional requirements closer to kittens than to mature adult cats.
Low Dietary Carbohydrates
Since most of these cats also have subclinical diabetes —as evidenced by their mild hyperglycemia, glucose intolerance, and insulin resistance— feeding a low carbohydrate diet (<10% of total calories) also is strongly recommended (21).
Feeding a low carbohydrate diet will improve insulin sensitivity, reduce the need for exogenous insulin, and help stabilize glucose metabolism in these cats (21-23). This may prevent the development of overt diabetes and control long-term obesity in these cats after successful control of the hyperthyroidism.
Feeding Your Hyperthyroid Cat: The Bottom Line
Like normal and diabetic cats, I believe that it makes sense to feed most hyperthyroid cats a diet composition close to what they would be getting in the wild. That would be a diet composed of approximately 50-60% protein, 5-10% carbohydrates, and 30-50% fat.
Because older cats also loss 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 euthyroidism.
Check out this website (http://binkyspage.tripod.com/foodfaq.html), which gives you a breakdown of the composition of the various prescription and over-the-counter diets. It turns out that many of the over-the-counter diets have a better composition of protein and carbohydrates than you might have thought — even better than many of the more expensive prescription diets. Very few of my hyperthyroid cat patients require a prescription diet to fulfill their nutritional needs.
As I discussed in my last post, the composition of almost all dry food cat diets are much too high in carbohydrates and most are too low in protein content. That is why I believe it's best to limit the amount of dry food that is fed to cats, or even better, not feed dry food at all.
Cats with advanced renal disease (IRIS Stage 3 or 4) may need lower amounts of dietary protein to lessen uremic episodes (24). However, at least in early to mid-stage renal disease, lowering of the serum phosphate concentration is much more important in management than dietary protein restriction, and this can be easily accomplished with phosphate binders without lowering the protein content of the diet (25,26). It may seem impossible, but no studies have conclusively demonstrated that severe restriction protein alone will prevent further deterioration of kidney function in cats (27).
The major problem that I have with some of the prescription kidney diets is that they restrict protein to the point that some cats – especially those with concurrent hyperthyroidism - will continue to catabolize their own muscle mass despite adequate control of the thyroid condition.
Once we have selected a few diets with the required composition breakdown of carbohydrates, protein, and fat, we next have to look at the ingredient list. Not all of the proteins in cat foods are equal in quality. Remember that quality meat is the best ingredient in a food and that meat by-products are a close second. Some vegetable and grains are fine, but they may supply a less bioavailable form of protein for cats and should not be the primary source of dietary protein.
Remember that when deprived of protein, carnivores will continue to break down muscle tissue to create the energy they need (18-20). By feeding only high-quality protein diets, we will help restore the cat’s muscle mass and improve strength and agility.
Let's use some common sense and not perpetuate the muscle wasting in these older cats by feeding diets that are too low in poor-quality protein.
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