Chapter 3: Nutritional aspects of tryptophan metabolism

Abstract

Niacin is biosynthesized from L -tryptophan (Trp). There is a substantial amount of Trp in the body which can be effi ciently converted to niacin especially during critical situations. Therefore, it is very important to know the factors affecting the conversion ratio of Trp to niacin. In animal experiments, the conversion ratios decreased with increasing dietary protein levels. Feeding of diets containing unsaturated fatty acids increased the conversion ratio, while feeding of diets with saturated fatty acids did not. In the effects of carbohydrate, the conversion ratio was higher in diets containing starch than in diets containing sucrose. Vitamin B 1, vitamin B 2, or vitamin B 6 defi ciency affects the metabolism of Trp to niacin. Mineral defi ciency also affects the metabolism of Trp to niacin. With regard to hormones, thyroxin increased the Trp to niacin conversion ratio, while estrone, progesterone, and adrenaline decreased. Certain chemicals can also impact niacin synthesis; alloxane and streptozotocin decreased the conversion ratio, while pyrazinamide, phthalates, valproic acid, and clofi brate increased the ratio. Moderate food restriction greatly decreased the conversion. In human experiments, the urinary excretion ratio of anthranilic acid formation from Trp was calculated to be approximately 0.06 % against Trp intake, kynurenic acid 0.13 %, xanthurenic acid 0.11 %, 3-hydroxyanthranilic acid 0.13 %, and quinolinic acid 0.34 % in basal metabolic level. Supplementing healthy women with up to 5.0 g/day of Trp had no adverse effects. Urinary excretion of 3-hydroxykynurenine is a good surrogate biomarker for excess Trp ingestion.