Protein Metabolism in Rats During Long‐Term Dietary Restriction: Influence of Aging

Background: Protein depletion is frequent in the elderly, but the underlying mechanisms are not yet fully understood. In particular, it is unknown whether there is a defect of adaptation to a restriction of food intake in the elderly. This study was performed to compare the effects of 6‐week dietary restriction (DR) on protein metabolism in both adult and aged rats. Methods: Adult (3‐month‐old) and aged (22‐month‐old) rats were acclimatized for 2 weeks and then fed a standard diet for 6 weeks, either ad libitum (control adult [ C Adult ] and aged [ C Aged ] rats) or with only 50% of the average intake of the second week of acclimatization (restricted adult [ R Adult ] and aged [ R Aged ] rats). Protein metabolism, in terms of tissue protein content, nitrogen balance, and 3‐methylhistidine (3‐MH) urinary excretion, was evaluated. Results: C Adult rats gained 30.4% of initial weight, whereas the body weight (BW) of C Aged rats was maintained. DR induced a rapid decrease in BW during the first 2 weeks in R Adult rats, but afterward BW remained stable. In R Aged rats, BW loss was linear during the 6 weeks and significantly higher than for R Adult rats (p <.01). In both restricted groups, muscle protein content was moderately affected by DR, whereas DR induced a marked decrease in visceral protein content. Nitrogen balance was decreased by DR but stayed positive in R Adult rats, whereas it became null in R Aged rats. Conclusions: In terms of protein metabolism, aged rats adapted less efficiently than adult rats to a long‐term dietary restriction. (Journal of Parenteral and Enteral Nutrition 23: 32–37,1999)