Abstract
Although the conversion ratio of tryptophan-niacin has been reported to be 1/60 by the weight basis in human adults, there is no data on nursing infants of humans. It is estimated at 0 in 0-5 month-old nursing infants of humans and at 1/120 in 6-11 month-old of humans from extrapolating data obtained in the weaning rats and the adult rats. Then, in order to estimate the conversion ratio of tryptophan-niacin in nursing infants of humans, the rats from newborn to before weaning were used; the enzyme activities involved in the tryptophan-niacin metabolism were measured and compared with those of the adult rats. Consequently, tryptophan-2, 3-dioxygenase (TDO) activity which is the first enzyme in the tryptophan-niacin conversion pathway was very weak in the 1st day immediately after birth, and also in the 7th day. These results mean that niacin is hardly biosynthesized from tryptophan in the early stages of nursing infants in rats. Moreover, since the TDO activity on the 14th and 21st increased with age, when applying these data to humans, it was thought in the latter period of nursing infants of humans i.e., 6-11 month-old, that this conversion pathway begins to operate and it is about 1/120 which is the half of the human adults.
