Comparisons of urinary creatinine, skeletal muscle mass, and indices of muscle protein catabolism in rats fed ad libitum, with restricted food intake, and deprived of food.

Urinary excretions of creatinine (CR) and 3-methylhistidine (3-MeHis), and skeletal muscle mass (SKM) were measured in rats (male, Wistar, weighing about 290 g) fed a 20% casein diet ad libitum, rats restricted to 70% of the food intake of the ad libitum group, and rats deprived of food for 20 days. At the same time, catabolic rates of muscle protein derived from 3-MeHis/CR, 3-MeHis/BW, and 3-MeHis/SKM were compared. The CR/SKM ratio in the energy-restricted group was similar to the ad libitum group, while the ratio was higher in the food-deprived group than in the other two groups. This means that CR excretion was promoted per unit of skeletal muscle mass by food deprivation. These results indicate that CR excretion can be an index for SKM under the condition of energy restriction, but not under the condition of food deprivation. The present data also suggest that it is possible to use the 3-MeHis/CR ratio instead of 3-MeHis/SKM which is the most reliable index of the three indices, under the condition of mild energy restriction. In food deprivation, however, the 3-MeHis/CR data tended to underestimate the catabolic rate of muscle protein compared with the 3-MeHis/SKM data. The 3-MeHis/BW data was similar to the 3-MeHis/SKM with all dietary conditions. The catabolic rate of muscle protein was constant or decreased very slightly under ad libitum feeding, decreased slightly in energy restriction, and increased drastically with food deprivation.