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.