The protein-sparing effect of carbohydrate. 1. Nitrogen retention of growing pigs in relation to diet.

1. Measurements were made of the losses of nitrogen in the faeces and urine of sixty pigs of approximately 33 kg, given dietary regimens comprising twenty-nine combinations of fish flour (0-800 g/d) and maize starch (0-1200 g/d). 2. The results were used to develop a generalized equation describing N retention as a joint function of N intake and starch intake. 3. The protein-sparing effect of starch was exerted in all circumstances. It was greatest when protein intake exceeded 220 g/d but some effect persisted with protein-free diets. With a high protein supply, the increase in N retention per unit increase in dietary starch decreased from 36 mg/g with the first increment to 3 mg/g with the highest attainable starch intake. 4. From the generalized equation the relationships between net protein utilization and protein concentration and food intake could be described as continuous functions. The equations may be of use in comparing the protein values of diets measured under non-standardized conditions.