Moderate food restriction suppresses the conversion of L-tryptophan to nicotinamide in weaning rats

Calorie restriction leads to a change in the metabolism of nutrients. Nicotinamide is biosynthesized from L-tryptophan. We attempted to determine the effects of food restriction on the biosynthesis of nicotinamide from L-tryptophan. Weaning male rats were fed a conventional chemically defined diet without preformed niacin for 63 d. However, the food intake was restricted to 80 and 65% of the intake of the ad libitum-fed control group of rats. The 24-h urine samples were periodically collected, and the urinary excretion of nicotinamide and its catabolites was measured. The conversion percentages were lower in both restricted groups than in the ad libitum-fed control group during the experimental period (control group, 1.37 +/- 0.24%; 80%-restricted group, 0.20 +/- 0.04%; 65%-restricted group, 0.15 +/- 0.02%; control vs. restricted groups, p < 0.01). Food restriction, even at mild level, suppressed the conversion of L-tryptophan to nicotinamide when compared to the ad libitum-fed control group.