The economics of water and salt balance.

Two environmental features often associated are a shortage of water and an excess of electrolytes. We explored the economics of this situation by jointly manipulating the instrumental cost of consuming water and the amount of salt in the diet of rats. As the dietary salt increased, water intake increased; and as water cost increased, water intake fell. Food intake also declined as water cost increased, and the rats maintained a minimum ratio of water: salt consumed across all conditions. For all diets, as water intake fell, food intake and body weight also declined, perhaps defending the ratio of body water to lean body mass. There was no evidence that the slope of the demand curve for water changed as a function of dietary salt.