Aging: a barrier to renutrition? Nutritional and immunologic evidence in rats

Background: Previous reports suggest that correcting the malnourished state is more difficult in elderly people than in younger ones and that protein requirements may be higher in elderly than in younger adults. Objective: The aim of this study was to establish whether malnourished old rats respond to protein-supplemented nutritional repletion as do young adul t rats. Design: Adult (3 mo old) and old (22 mo old) rats were submitted to dietary restriction programs that induced similar metabolic and nutritional altera tions. Malnourished adult and old rats were then killed (R groups) or refed for 1 wk with a high-protein diet (HPD; 23% protein) or a very-high-protei n diet (VHPD; 27% protein). Control groups at both ages were fed ad libitum throughout the experiment. Effects of food repletion were evaluated in ter ms of protein metabolism intestinal histomorphometry, and nonspecific immun e status. Results: In adult rats, HPD sufficed to increase body weight and restore ba sal values of liver weight and protein content (P < 0.01 compared with the R adult group), nitrogen balance (P < 0.01 compared with the R adult group) , and hydrogen peroxide production by polymorphonuclear neutrophils and mon ocytes (P < 0.01 compared with the R group); VHPD had no supplementary effe ct except on nitrogen balance. In old rats, HPD was less effective and grea ter benefit was observed with VHPD in terms of body weight gain (10%; P < 0 .01 compared with the old group fed HPD), albuminemia, muscle weight and pr otein content, plasma arginine concentration, and hydrogen peroxide product ion by stimulated polymorphonuclear neutrophils and monocytes compared with the old R group (P < 0.01). Conclusion: Aging is a significant variable affecting the response to nutri tional support.