Niacin is widely distributed in plant and animal foods, mainly as the pyridine nucleotides NAD and NADP. Good sources are yeast, meats including liver, cereals, legumes, seeds, milk, green leafy vegetables, and fish
Absorption of Niacin
1. Niacin is absorbed in the small intestine. As it is water soluble, excess is excreted in the urine although small amounts may be stored in the liver.
2. The amino acid tryptophan is converted to niacin in the body if sufficient thiamin, riboflavin and B6 are present. More than half the RDA for niacin can be obtained through the conversion of tryptophan.
Transportation of Niacin
1. Unbound acid and amine
2. Passive diffusion and facilitated diffusion
3. Energy-dependent transport system(brain)
4. Change NAD(H) and NADP(H)