Niacin, absorption cereals

Bioavailability of Niacin. Factors which cause a decrease in macm availability include (1) Cooking losses (2) bound form in corn (maize), greens, and seeds is only partially available (3) presence of oral antibiotics (4) diseases which may cause decreased absorption (5) decrease in tiyptophan conveision as in a vitamin B deficiency. Fac.tois that increase availability include (1) alkali treatment of cereals (2) storage in bver and possibly in muscle and kidney tissue and (3) increased intestinal synthesis. 

Niacin (nicotinic acid pyridine-3-carboxylic acid) and nicotinamide are precursors of NAD+ and NADP+ (Figure 38-19). Niacin occurs in meat, eggs, yeast, and whole-grain cereals in conjunction with other members of the vitamin B group. Little is known about absorption, transport, and excretion of niacin and its coenzyme forms. A limited amount of niacin can be synthesized in the body from tryptophan, but it is not adequate to meet metabolic needs. 

Niacin is categorized as a vitamin because its precursor, tryptophan, is an essential amino acid, so the human synthesis of niacin is dependent upon diets. Preformed niacin is widely distributed in plant and animal foods. The typical preformed niacin sources in diets are meat and meat products, cereals, dairy products, beverages, and eggs. However, cereals with esterified niacin in complexes have this vitamin imavail-able for absorption, but its bioavailability can be increased by treatment with alkali to hydrolyze the esters. Coffee can be a source of niacin, as nicotinic acid is liberated in coffee by roasting. 

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