Effect of Dietary Protein Levels on Protein Nutritional Status in Growing Female Rats Kept under Constant Darkness

The purpose of this study was to clarify the effects of dietary protein levels on protein nutritional status in rats kept under constant darkness. Thirty-six 4-wk-old female rats (F344 strain) were divided into six groups. Each group was given a diet with one of three different protein levels and kept under normal light and dark cycles (7:00-19:00 light period/19:00-7:00 dark period, N group) or under constant darkness (D group) for 4 wk. The protein levels of the diets were 10%, 20%, and 30% casein. The six groups are referred to as the N10%, N20%, N30%, D10%, D20%, and D30% groups. Body weight gain was low in the D groups, and that in the D30% group was much lower than that in the N30% group. The D30% group retained less nitrogen than the N30% group. As for the amount of urinary nitrogen excreted every 4 h, the values for the D-groups were higher than those for the N-groups in the 11:00-15:00 periods, and that for the D30% group was higher than that for the N30% group in the 15:00-19:00 periods, which means that protein catabolism was higher in the D30% group. It was shown that when rats kept under constant darkness were fed a high-protein diet for 4 wk, their nitrogen retention decreased and their protein nutritional state dropped.