Thursday, October 6, 2011

Daily Water Requirements and Needs for Cats

Running water is naturally enticing
to many cats and is one way to get
cats to drink more water
Drinking a healthy amount of water is vital to a cat’s health. Most people don’t think of water as a nutrient. But considering that water accounts for about two-thirds of a cat’s body weight and serves as the hub of all chemical processes in the body, it's actually the king of all nutrients (1,2).

Water serves many physiological functions: it transports nutrients and oxygen through the blood stream and into the cells, moisturizes the air in the lungs, regulates body temperature, protects and moisturizes the joints and internal organs, and helps eliminate waste products of metabolism through the kidneys and gastrointestinal tract.

Cats Do Not Naturally Drink Much Water

Cats do not normally need to drink very much water. Cats have evolved to obtain their water requirements almost entirely on the moisture content in their food. In the wild, cats obtain most of their water from freshly killed prey (e.g., small rodents, birds, amphibians, and insects), all of which contain about 70 to 75 percent water (3-7). Cats can do well for long periods without drinking any water when receiving canned food containing 67 to 73 percent water (8, 9); however, they will become dehydrated when the water content of the food drops to less than 61 percent (9). Therefore, normal cats eating rodents or birds or house cats eating canned-only foods may obtain enough water in their diets so that extra drinking water may not always be needed (2,10).

It's very important to realize that domestic cats have a diminished "thirst drive" and ideally will derive most of their daily water intake from the moisture contained in their food. In this respect, control of water balance in cats differs markedly from that of dogs and most other animals. When dehydrated, cats are slower to initiate drinking or to drink enough for complete rehydration — one study found that dehydrated dogs will drink enough to replenish 6 percent of their body weight in an hour compared to the 24 hours it takes for dehydrated cats (11).

In response to changes in the water content of food, cats adjust their voluntary water intake less precisely and less rapidly than do dogs. Similarly, their compensatory drinking response to dehydration induced by higher environmental temperatures or concurrent disease is less effective than it is in dogs.

How Much Water Does a Cat Need to Drink?

The amount of water drunk by an individual cat depends on a variety of factors, including the cat's size and activity, the season, and whether the cat's diet includes wet food or dry cat food only (2,10,12). Factors such as high heat, exercise, or lactation can double or triple the amount a cat drinks. And, of course, diseases such as hyperthyroidism, kidney disease, and diabetes will all greatly increase a cat's daily water requirements (I'll talk more about water and hydration in these cats in my next post).

So how much should a cat drink? A normal cat’s daily water requirement ranges from 5 to 10 fluid ounces per day (or an average of 60 ml/kg/day). Cats eating canned food will receive much of their daily water needs from its food, since canned food is about 70 to 80 percent water. In contrast, dry food is only 7 to 10 percent water. Normal cats eating canned food may need to drink less than 1 ounce of additional water per day, whereas a cat consuming only a dry diet may need to drink over 7 ounces per day to stay hydrated. This higher amount of water can be difficult to achieve because cats are not prone by nature to drink large amounts of water.

Will Type of Food (Dry vs. Canned) Affect the Amount of Water a Cat Drinks? 

A cat consuming a predominantly dry food diet will drink more water than a cat consuming a canned food diet. But in the end, when water from all sources is added together (moisture in their diet plus the water they drink), the cat on dry food consumes about half the amount of water required for adequate hydration compared to a cat eating canned food (2, 13).

Put another way, when cats are fed only dry food, they do increase the amount of water drunk but not nearly enough to fully compensate. In one study (12), cats consuming a dry food diet containing 10% moisture with free access to drinking water had an average daily urine volume of 60 milliliters (or 2 fluid ounces). This urine volume almost doubled when the cats were then fed a canned diet containing 75 percent moisture.

Calculating the Volume of Water Provided by the Cat’s Diet

The moisture content of all cat food (both canned and dry) is listed on the product label as part of the Guaranteed Analysis (14). Canned diets usually contain about 75% moisture, so for every 100 grams of food fed, 75 grams (which equals 75 ml) is water. Dry foods, on the other hand, contain only about 10% moisture, so for every 100 grams of food fed, 10 ml if water.

The weight of a standard, larger can of cat food is 5.5 ounces (156 grams). If the moisture content of the food is listed as 78 percent max and the cat eats the entire can of food, he or she would be ingesting approximately 120 ml of water from the food (156 gm X 0.78 = 121.68 ml). Again, if the same cat at 156 grams of a dry food containing 10% moisture, that would provide only 15.6 ml of water.

The weight of a standard, smaller can of cat food is 3 ounces (85 grams). If the moisture content of the food is again listed as 78 percent max and the cat eats the entire can of food, he or she would be ingesting approximately 65 ml of water from the food (85 gm X 0.78 = 66.3 ml). Again, if the same cat ate 85 grams of a dry food containing 10% moisture, that would provide only 8.5 ml of water.

Water Requirements in Cats with Hyperthyroidism

Although water is certainly important for clinically normal cats, it is absolutely essential in maintaining and restoring health in sick cats, and that certainly includes cats with hyperthyroidism. To that end, I'll address the special water needs of hyperthyroid cats in my next post.

References
  1. Kohn CW, DiBartola SP. Composition and distribution of body fluids in dogs and cats. In: DiBartola SP, ed. Fluid therapy in small animal practice. Philadelphia: WB Saunders Co, 1992;1–34.
  2. Case LP. Nutrition: feeding cats for health and longevity. In: The Cat: Its Behavior, Nutrition and Health. Iowa State Press, Ames, IA 2003;289-327.
  3. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
  4. Mutze GJ, Green B, Newgrain K. Water flux and energy use in wild house mice and the impact of seasonal aridity on breeding and population levels. Oecologia 1991;88:529–538. 12.
  5. Angilletta MJ. Estimating body composition of lizards from total body electrical conductivity and total body water. Copeia 1999;3:587–595.
  6. Austad S, Kristan D. Does caloric restriction of laboratory mice mimic natural food intake of wild mice? Gerontology 2002;42:8–18.
  7. Hall JM, Hung F, Zurich MW. The influence of diet on the body condition of the house cricket and consequences for their use in zoo animal nutrition. Zoologische Garten 2003;73:238–244. 
  8. Caldwell FT. Studies in water metabolism of the cat. The influence of dehydration on blood concentration, thermoregulation, respiratory exchange, and metabolic-water production. Physiological Zoology 1931; 4:324-355.
  9. Prentiss PG. Wolf AV. Eddy HE. Hypopenia in cat and dog: ability of the cat to meet its water requirements solely from a diet of fish or meat. American Journal of Physiology 1959; 196:625-632. 
  10. National Research Council. Feeding behavior of dogs and cats. In: Nutritional Requirements of Dogs and Cats. National Academies Press, Washington, D.C. 2006; pp 22-27
  11. Adolph EF. Tolerance to heat and dehydration in several species of mammals. American Journal of Physiology 1947;151:564–575. 
  12. Seefeldt SL, Chapman TE. Body water content and turnover in cats fed dry and canned rations. Am J Vet Res 1979;40:183–185. 
  13. Kirk CA, Debraekeleer J, Armstrong PJ. Normal cats. In: Hand MS, Thatcher CD, Remillard RL, et al, eds. Small animal clinical nutrition. 4th ed. Philadelphia: WB Saunders Co, 2000;291–351.
  14. Food and Drug Administration website. Pet food labels—general

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