Niacin loss from sweat and feces of young women was investigated under the controlled conditions. The average daily niacin intake was 15 mg during the experiment and the tryptophan intake was 900 mg, and so the niacin biosynthesized from tryptophan in vivo would be 15 mg. Namely, the niacin equivalent intake was 30 mg/day. The energy intake was 1800 kcal. During 1 h exercise, the amount of sweat was 600 ml and the niacin output was 20 〜 25μg. This loss was extremely low compared with niacin equivalent intake. This finding indicates that the loss from sweat is ignored. The niacin output in feces was about 2.5 mg/day. Namely, rate of niacin absorption was around 80%.