Niacin, or vitamin B-3, is a water-soluble nutrient belonging to the B complex family. In some respects, niacin is not really a vitamin, because it is not an essential nutrient when you consume a sufficient amount of protein, vitamin B-6, iron and riboflavin. In 1949, scientists proved conclusively that tryptophan, an essential amino acid obtained from protein, is converted to niacin in your body. However, this conversion cannot take place unless you have ample stores of vitamin B-6.
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To synthesize 1 mg of niacin, you must consume approximately 60 mg of tryptophan. The conversion of tryptophan to niacin requires several chemical reactions, some of which are regulated by enzymatic activity. One of these enzymes, called kynureninase, uses vitamin B-6 as a cofactor to shuttle kynurenine to the next step in the niacin pathway. If sufficient amounts of vitamin B-6 are not available, kynurenine is shunted into a different metabolic pathway that does not produce niacin. In addition to vitamin B-6, riboflavin and iron are required to convert tryptophan to niacin.
Niacin deficiency can result from inadequate dietary intake of niacin or tryptophan. The Linus Pauling Institute at Oregon State University reports that niacin deficiency can also result from diseases that interfere with tryptophan absorption or increase its use in non-niacin metabolic pathways. People who take isoniazid for tuberculosis can also develop niacin deficiency, because isoniazid interferes with the activities of both vitamin B-6 and niacin. Severe niacin deficiency results in pellagra, a condition characterized by the “four Ds” -- dementia, diarrhea, dermatitis and – if left untreated – death.
In his book, “Staying Healthy with Nutrition,” Dr. Elson Haas states that only small to moderate amounts of pure niacin occur in foods. Rather, most foods that are considered good niacin sources contain some niacin along with its precursor, tryptophan. Peanuts, poultry, organ meats and fish all contain both niacin and tryptophan. Dried legumes, wheat germ, whole grains, yeast, dates, figs and prunes are plant sources of niacin. Eggs and milk contain high levels of tryptophan, which can be converted to niacin.
Considerations and Recommendations
In healthy individuals, the amino acid tryptophan is converted to niacin in the presence of vitamin B-6, iron and riboflavin. Good protein intake usually ensures adequate levels of tryptophan – hence, niacin – although some diseases can interfere with tryptophan absorption and metabolism. Recommended dietary allowances for niacin vary from 2 mg daily for infants to 18 mg for pregnant women. Your need for tryptophan can usually be met by consuming about 0.4 g of protein for each pound of lean body weight each day. Ask your doctor if you should take extra niacin, vitamin B-6 or tryptophan.