Explain the Bioavailability of Nicotinic Acid?

We have already learnt earlier that niacin is provided in the diet primarily as the pyridine nucleotides-NAD and NADP. In addition to its synthesis from dietary niacin, NAD may also be synthesized in the liver from the dietary amino acid, tryptophan. The synthesis of niacin from tryptophan also depends on enzymes that require vitamin B6 and riboflavin, as well as, an enzyme containing haeme (iron). On an average, 1 mg of niacin can be synthesized from the ingestion of 60 mg of tryptophan. Hence, the recommended allowance for niacin is expressed as mg NE (Niacin equivalents) where 1 mg NE = 1mg niacin or 60 mg tryptophan. There are several dietary drug and disease factors that reduce the conversion of tryptophan to niacin, such as the use of oral contraceptives.