Protein metabolism in rats during long-term dietary restriction: influence of aging

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.

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.

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.

In terms of protein metabolism, aged rats adapted less efficiently than adult rats to a long-term dietary restriction.