Cat Water Requirement
Cats require a variety of nutrients in their daily diet including water, protein, fat, vitamins and minerals. In all, a cat needs 40 individual nutrients every day.
Contrary to popular belief, cats are used to "eating" as opposed to drinking water as in their natural habitat they gain their water intake from their prey. Approximately 60% of a cat's body consists of water and needs to be maintained at a constant level. Inadequate amounts of water intake may lead to serious illness. Urinary Tract disease is a growing problem.
Under normal conditions, by ingesting a diet high in moisture, cats can almost completely meet their water requirements and do not need to drink extra water.
Water is the single most important nutrient necessary to sustain normal function of all living cells. Water helps regulate body temperature, cushion the joints and internal organs, digest food, eliminate waste, lubricate tissue and allow salt and other electrolytes to pass through the body. The nonfat component of mammals contains about 73 percent water (about the same amount found in canned food). Cats can lose nearly all their reserves of glycogen and fat, half the body protein stores, and 40 percent of their body weight and survive. However, cats are much less tolerant to losses of body water.
Cats can withstand acute dehydration slightly better than dogs. Although cats can tolerate some depletion of their body water for a short period, they must in the long run remain in water balance. The losses of water from the body must be offset by an equal intake of water. At normal temperatures water is lost from the body via the lungs, skin, urine, milk, and feces. At high temperatures, an additional loss may occur via saliva, which is used to wet the fur and provide evaporative cooling. The body gains water from "free water" present in liquids and solid foods and "oxidation water" arising from the catabolism of carbohydrates, fats, and protein.
The cat has evolved to obtain her water requirements almost entirely on the moisture content in her food - inherited from her desert-dwelling ancestors. Cats can live for long periods without drinking water when receiving food containing 67-73% water but become dehydrated when the water content of the food is 63% or less. Canned diets contain enough water that cats consuming them rarely need to drink. Daily water needs, in milliliters, often are "guesstimated" as equal to the metabolizable energy requirement in kilocalories or approximately 60 ml/kg. Once the diet is consumed, oxidation of nutrients produces an additional 10 to 13 grams of water for each 100 kcal of metabolizable energy. Thus a 4 kg cat consuming a 240 kcal canned diet containing 78% moisture will consume 237 ml or 98% of its daily water need directly from the diet. Thus the cat needs to drink less than 1 oz. of additional water per day whereas a cat consuming a 240 kcal dry diet needs to drink over 7 oz. of water per day. This can be difficult because cats are not naturally big drinkers. Feeding a canned diet containing 78% moisture virtually guarantees homeostatic control of water balance in the cat.
The water content of the commercial foods commonly fed to cats varies from 8% in dry foods to over 75% in canned foods; thus the amount of drinking water required is affected substantially by the water content of the food. When fed canned food (80% moisture) with access to drinking water, cats obtain over 90% of their total water intake from the diet, whereas on dry food, 96% of the total water intake is obtained by drinking. The total free water intake (from food and drinking water) decreases when cats are fed dry food only, so that the water to dry matter intake ratio when fed on commercial dry foods varies from 2.0 to 2.8: 1 whereas on canned foods it varies from 3. 0 to 5.7: 1. Thus for any given dry matter intake cats have a higher water turnover on canned than on dry foods. (National Research Council [National Academy of Science] Nutrient Requirements of Cats).
Diet moisture content is related to the observation that cats fed dry food drink more than six times more water than cats fed canned food but that much of this water contributes to fecal moisture so that urine volume is lower and urine specific gravity higher in cats fed dry food. The urine concentration of all solutes, including potentially calculogenic crystalloids, depends on urine volume.
Cats increase voluntary water intake when fed dry food but not in sufficient amounts to fully compensate for the lower moisture content of the food. In a recent study, cats consuming a diet containing 10% moisture with free access to drinking water had an average daily urine volume of 63 milliliters (ml). This volume increased to 112 ml/day when fed a canned diet with a moisture content of 75%. Urine specific gravity was also higher in cats that were fed the low-moisture food. Decreased urine volume may be an important risk factor for the development of urolithiasis in cats. Diets that cause a decrease in total fluid turnover can result in decreased urine volume and increased urine concentration, both of which may contribute to urinary tract disease in cats. Several studies have shown that dry cat foods contribute to decreased fluid intake and urine volume.
Homeostatic control of water balance in cats differs in some important respects from that of dogs Cats make less precise and rapid compensatory changes in voluntary water intake than dogs in response to changes in the water content of their food. Similarly, their compensatory drinking response to dehydration due to increased environmental temperature is less effective than dogs. This apparent weakness of the cat's thirst drive to respond to changes in her state of hydration has led to the conclusion that feeding canned food assures adequate hydration at all times.
In addition to ensuring adequate hydration, a high water turnover helps eliminate crystallogenic substances before they grow to sufficient size to interfere with normal urinary function. This is a very important consideration for male cats. Cats that cannot urinate for more than 24 hours due to urinary tract obstruction can die from acute renal failure and/or severe damage to the urinary bladder. In addition to the removal of crystals, benefits of increased water intake include dilution of any noxious substances in urine, and more frequent urination to decrease bladder contact time with urine that may reduce the risks of urinary tract disease. For that reason, canned diets are usually prescribed as the first-line therapy for feline lower urinary tract disease.