The effect of varying levels of dietary protein on carcass composition of eleven- and eighteen-month-old male rats

Abstract

Carcass composition of male Sprague-Dawley rats, aged 11 and 18 months, in response to varying levels of dietary protein was determined. Groups varying of ten rats of each age were fed diets containing from 1.53 to 8.05 percent protein as casein supplemented with d-1-methionine for five weeks. The 8.05% protein groups were used as controls. Carcasses were analyzed for total nitrogen and percent protein, fat, and water. Liver composition and total serum protein values were also determined. Two-way analysis of variance and Student's t-tests were used to determine significant age and diet effects. Differences in the response of the two age groups of rats were evident. Eighteen-month-old rats required more protein than the younger animals for the maintenance of body weight. When compared to control values, older rats also needed a higher level of dietary protein to maintain normal total carcass nitrogen. Fatty livers in older rats persisted at higher dietary protein levels than fatty livers in 11-month-old rats, indicating that 18-month-old rats required more protein to support adequate liver lipoprotein synthesis. These findings suggest that 18-month-old rats have a higher dietary protein requirement than 11-month-old rats. High serum protein values for older rats at lower protein levels, however, do not support this conclusion. The increased body weight and proportionally greater fat mass of older animals was a complicating factor in this study. Further research is needed to more clearly define changes in protein requirements during aging. For future studies, using rats of a more advanced age and three, rather than two, different age groups is recommended.