Sunday, October 9, 2011

Water Content of Hill’s y/d Thyroid Diets: Lower Than Most Other Commercial Cat Foods

Drinking a healthy amount of water is vital to a cat’s health. Although water is certainly important for clinically normal cats, it is absolutely essential in maintaining normal metabolic processes in cats with hyperthyroidism. In addition to their increased metabolic rate, hyperthyroid cats have an elevated body temperature and show increased moisture loss through the respiratory and gastrointestinal routes (1-3)

As I discussed in my recent posts, Hill's Pet Nutrition recently released Prescription Diet y/d Feline –Thyroid Health (4,5), an iodine deficient diet that can result in lowered serum T4 concentrations when fed as the sole source of nutrition to hyperthyroid cats. So, in this post, I want evaluate this diet to address the questions:
  • What’s the moisture content of the dry and canned Hill's y/d diets?
  • How much of the hyperthyroid cat’s daily water needs are met by feeding this diet?

Daily Water Recommendations for Cats

So how much should a cat drink? The normal daily water requirement (from all sources) averages 60 ml/kg/day, with a range of approximately 50 to 65 ml/kg/day (6,7). So a 4 kg cat requires 200 to 250 ml per day to function at peak levels. Of course, these requirements increase if the water loss goes up for physiological or pathological reasons including high temperature, lactation, diarrhea, vomiting, or polyuria.

Origin of Water for Cats

The daily water needs of cats are derived from 3 available sources: water the cat drinks, water contained in food, and the water produced when carbohydrates, proteins, and fats are metabolized for energy. This "metabolic water" represents only 10-15% of the total required water (8).

Cats eating canned food will receive much of their daily water needs from its food, since most canned food is about 75-80% water. In contrast, dry food is only 7-10% water. Normal cats eating canned food may need to drink less than 30 ml of additional water per day, whereas a cat consuming only a dry diet may need to drink over 200 ml per day to stay hydrated (7,9).

Table 1: Water intake and urine volume in cats fed dry or canned foods.
From data by Burger and Smith (7).
This higher amount of drinking water needed can be difficult to achieve because cats are not prone by nature to drink large amounts of water. In support of that, studies show that cats fed canned food ingest a larger total volume of water per day (i.e., drinking water plus water in food) compared with cats fed dry food diets (Table 1). In addition, the cats fed moist food excrete more water in their urine, which may help prevent many lower urinary tract problems (7,9).

Hyperthyroid Cats, Water Balance, and Dehydration

As cats age, they develop a number of important changes in water metabolism that increase their daily water requirements and predispose them to dehydration. First of all, even healthy geriatric cats have higher water losses than younger cats, possibly due to reduced urine concentrating ability even without obvious evidence of overt kidney disease (10,11). In addition, aging impairs thirst sensitivity, which is already low in cats compared with other species (11-13).

Together, these characteristics predispose older cats to dehydration. Chronic dehydration can impair normal metabolic processes and exacerbate subclinical disease. Dehydration also reduces a cat’s ability to thermoregulate.

The potential for dehydration will be exacerbated in cats with concurrent diseases that cause polydipsia and polyuria, such as diabetes mellitus, chronic kidney disease, or hyperthyroidism. Thyroid hormones have a diuretic action, an effect that was reported in cats almost 70 years ago (14). Although concurrent primary kidney disease con­tributes to polyuria and polydipsia in up to a third of cats with hyperthyroidism (3), these signs also occur in many cats with­out any evidence for kidney disease.

Because of all of these factors, many hyperthyroid cats will develop mild to moderate degrees of chronic dehydration. Keeping a hyperthyroid cat well hydrated helps ensure that adequate oxygen and vital nutrients will reach all the tissues of the body — this allows the cat to metabolize its nutrients and sustain or restore normal body function. Maintaining adequate hydration also helps absorb the excess body heat typically generated by hyperthyroid cats because of their “revved up” metabolic state (1-3).

In addition, it has been suggested that diseases that promote polyuria (such as hyperthyroidism or renal disease) predispose to urinary tract infections (15,16). In one study, over 20 percent of cats with untreated hyperthyroidism has a positive urine culture, diagnostic for a urinary tract infection (16).

Calculating the Volume of Water Provided by the y/d and other Cat Foods

The moisture content of all cat food (both canned and dry) is listed the product label as the guaranteed analysis (16). The average canned diet contain 78% moisture, so for every 100 grams of food fed, 78 grams (which equals 78 mls) is water. Dry foods, on the other hand, contain only about 8-10% moisture, so for every 100 grams of food fed, 8 to 10 ml is water.

For the Hill’s canned Prescription Diet y/d (5), the maximum moisture content is lower (71%) than the average canned food (i.e., most contain 78%). Therefore, if a cat ate a entire 55 oz (156 g) can of Hill’s y/d diet, the cat would be getting approximately 110 ml of water from the food (156 g X 0.71 = 110.76 ml), compared with 120 ml for most other canned foods. Again, this is not a big difference, but it is important to realize that the canned diet does not contain as much water as other foods.

For the dry y/d diet (4), the maximum moisture content is also lower (6%) than the average dry food (i.e., most contain 8-10%). For an average size (4.5 kg) cat ingesting the recommended amount of dry y/d per day (i.e., 80 g), that amount of dry y/d would provide only 4.8 ml of water per day!

The Bottom Line

Since up to a third of hyperthyroid cats have concurrent kidney disease, and up to 20% have concurrent urinary tract infections (3,16), keeping our hyperthyroid cats well hydrated is of utmost importance. Feeding a canned diet containing 75-80% moisture together with plenty of fresh water freely available helps guarantee control of water balance in both normal cats and cats with hyperthyroidism.

If Hill’s y/d is fed as treatment for cats with hyperthyroidism, I would strongly recommend that the canned version is used, rather than the dry food in order to help maintain the cat’s hydration. Clean fresh water should be available at all times and readily accessible to further encourage water intake. Additional methods for increasing water intake such as adding water or broth to the can food, placing ice cubes in the cat’s water, using unique water bowls, and providing water fountains also may help in some cats.

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  2. Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. Journal of the American Veterinary Medical Association 1995;206:302-305. 
  3. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press.
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  11. Little S. Managing the senior cat. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
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  13. Markham RW, Hodgkins EM. Geriatric nutrition. Veterinary Clinics of North America: Small Animal Practice. 1989; 19:165-185. 
  14. Radcliffe CE: Observations on the relationship of the thyroid to the polyuria of experimental diabetes insipidus. Endocrinology 1943;32:415-421.
  15. Mayer-Roenne B, Goldstein RE, Erb HN: Urinary tract infections in cats with hyperthyroidism, diabetes mellitus and chronic kidney disease. Journal of Feline Medicine and Surgery 2007;9:124-132.
  16. Bailiff NT, Westropp JL, Nelson RW, et al. Evaluation of urine specific gravity and urine sediment as risk factors for urinary tract infections in cats. Veterinary Clinical Pathology 2008;37:317–322.
  17. Roudebuch P, Dzanis, Debraekeleer J, et al. Pet food labels. In: Hand MS, Thatcher CD, Remillard RL, Roudebush R, Novotny, BJ (eds), Small Animal Clinical Nutrition. Mark Morris Institute. 2010; 191-206. 

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