Effect of nicotinamide intake on urinary excretion of N1-methylnicotinamide and oxidation of [7a-14C]tryptophan in the rat.

Oxidation of tryptophan and urinary excretion of N1-methylnicotinamide were studied in weanling rats fed ad libitum for 10 days 12% casein diets containing from 0 to 500 mg of nicotinamide per kilogram of diet. N1-methylnicotinamide was measured in urine collected during the last 3 days of the experiment. After the feeding period, rats fed the niacin-free and 500 mg nicotinamide diets were placed in metabolic cages, and each rat was given 4 g (dry weight) of diet containing a tracer dose of DL-[7a-14C]tryptophan. Expired CO2 was collected hourly for 10 hours. From rats consuming levels of nicotinamide ranging between 20 and 150 mg/kg of diet, N1-methylnicotinamide excretion increased linearly with increasing nicotinamide intake. From these results and others from this laboratory (4), it was calculated that an additional 10--11 mg of dietary tryptophan above the approximately 20 mg/day essential for growth are needed by the growing rat for the formation of 1 mg of nicotinamide. The addition of 500 mg of nicotinamide per kilogram of diet to a niacin-free diet had no effect on the oxidation of DL-[7a-14C]tryptophan.