Protein deprivation in primates. I. Nonpregnant adult rhesus monkeys

Young adult female rhesus monkeys, after quarantine and stabilization in the laboratory, were fed a semisynthetic diet containing 15.3% casein (13.4% protein) or 0.5, 0.25. or 0.125 that amount for a 6-month period, after which they were returned to the 15.3% diet. The diets, made isocaloric by the addition of carbohydrates to replace the missing casein, were calculated to supply 4, 2, 1, or 0.5 g protein/kg body wt per day if the animals ate 120 kcal/kg per day, an assumption that proved approximately true for the three highest protein groups. Although there was a temporary loss of appetite when the protein-deficient diets were introduced, the animals eventually recovered their appetites and ate their customary amounts regardless of protein concentration. The graded deficiencies in protein produced weight losses (up to 35% in animals fed the 0.5-g diet), but the animals regained their initial weight within 6 weeks after returning to the 4-g diet. Plasma albumin concentration and free nonessential:essential amino acid ratio adequately reflected depletion and repletion. Some values, especially for the less deprived animals, returned toward normal within the period of restriction. Plasma globulin and hair bulb morphology were unaffected. Hepatic changes were moderate. Several of the most deprived animals became pregnant at the first opportunity after 3-months repletion.