Define Absorption, Storage and Elimination of niacin?
Nicotinic acid and nicotinamide are rapidly absorbed from the intestine rather than the stomach. At low concentrations, absorption in the small intestine occurs as Na+. Dependent facilitated diffusion but at higher concentrations, passive diffusion predominates. Nicotinamide is the major form in the blood stream and arises from the enzymatic hydrolysis of NAD in the intestinal mucosa and liver. NAD and NADP, the main dietary forms of niacin are hydrolyzed by enzymes in the intestinal mucosa to release nicotinamide. The intestinal mucosa is rich in niacin conversion enzymes such as glycohydrolase.
Nicotinamide is released from NAD in the liver and intestines by glychydrolases and transported into tissues as needed. Tissues take up both forms by simple diffusion and erythocytes by facilitated transport. Niacin is methylated in the liver to N' l-methyl nicotinamide (NMN) which is exc eted in the urine along with the oxidation products of NMN. The pattern of niacin products excreted depends on the amount and the form of niacin ingested and the niacin status of the individual. Excess niacin is excreted in the urine primarily as N' l-methyl nicotinamide and N 'l-methyl- 2-pyridone-5-cuboxanlide.