Factors determining the preservation of protein status during dietary protein deprivation

Changes in protein content and function were investigated in man Sprague-Dawley Crl:Cd rats that were either fasted, consumed a diet containing 1.5 g fat/100 g initial body weight per day or received ad libitum an 85% carbohydrate, minimal nitrogen diet. Fasted rats became moribund after 9--10 days and were both hypoglycemic and hypoketonemic. In contrast, rats fed the fat-only and high carbohydrate, minimal nitrogen diets survived the 28-day study period. Although rats consuming the high carbohydrate, minimal nitrogen diet lost less body weight and nitrogen than either fasted animals or rats receiving the fat-only diet, they also had a loss of delayed hypersensitivity recall to tuberculin antigen and a 55% decrease in plasma essential amino acid concentrations. Rats fed the fat-only diet maintained delayed hypersensitivity recall and near normal concentrations of plasma essential amino acids despite a greater loss of body nitrogen than was observed in terminally fasted animals. Plasma albumin concentrations and total liver protein content showed comparable decreases in rats consuming the two incomplete diets. We concluded that nonprotein calories prolong survival in the rat and fat or predominantly glucose calories are sufficient at preventing mortality. However, the source or quantity of nonprotein calories consumed during periods of protein deprivation can markedly alter individual tissue protein status and function.