Assessment of the efficiency of protein utilization in young squirrel and macaque monkeys

The efficiency of protein utilization was assessed in three groups of infant squirrel monkeys fed diets containing restricted amounts of lactalbumin, casein, and soy protein isolate as the sole nitrogen source for 3-week periods. Lactalbumin was fed in similar conditions to a group of macaques for comparison. The regression of weight gain on protein intake was calculated for each dietary protein yielding an estimate of protein needs for weight maintenance and an estimate of the efficiency of utilization of protein for growth (slope X percentage protein in tissue). Mean requirements for weight maintenance of infant squirrel monkeys for lactalbumin, casein and soy protein isolate were 3.43, 3.63, and 7.96 g/kg per day and growth was accomplished with an efficiency of 27.8, 32.1, and 22.2%, respectively. In contrast, the macaques utilized lactalbumin with an efficiency of 65%, similar to that of the infant cebus monkey and adult man. The low efficiency found in young squirrel monkeys was altered neither when the habitual feeding schedule was prolonged from 16 to 24 hr per day in a group of casein-fed animals nor when sulfur amino acids were added to casein.