Background: Protein depletion is frequent in the elderly, but the underlyin
g mechanisms are not yet fully understood. In particular, it is unknown whe
ther there is a defect of adaptation to a restriction of food intake in the
elderly. This study was performed to compare the effects of B-week dietary
restriction (DR) on protein metabolism in both adult and aged rats, Method
s: 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 in
take 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 eva
luated. Results: C-Adult rats gained 30.4% of initial weight, whereas the b
ody weight (BW) of C-Aged rats was maintained. DR induced a rapid decrease
in BW during the first 2 weeks in RAdult rats, but afterward BW remained st
able. In R-Aged rats, BW loss was linear during the 6 weeks and significant
ly higher than for R-Adult rats (p < .01). In both restricted groups, muscl
e protein content was moderately affected by DR, whereas DR induced a marke
d decrease in visceral protein content. Nitrogen balance was decreased by D
R but stayed positive in R-Adult rats, whereas it became null in R-Aged rat
s. Conclusions: In terms of protein metabolism, aged rats adapted less effi
ciently than adult rats to a long-term dietary restriction.