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

Abstract

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)