Niacin bioavailability

Cereals are considered one of the best sources of B-vitamins thiamin, riboflavin, niacin, pyridoxine, and folates, but a poor source of fat-soluble vitamins and vitamin Bi2. Most B-vitamins are associated with the aleurone. The niacin associated with cereals is found in free and bound forms. Bound niacin is not well utilized by the human system. Interestingly, the alkaline treatment of maize for production of tortillas and other traditional food products increases niacin bioavailability because the alkali and heat treatment breaks the glycosidic bond that binds free niacin with the other components (Chapters 9 and 17). 

Which amino acid can be converted into niacin Why is niacin bioavailability low in raw maize ... 

Most foods of animal origin contain nicotinamide in the coenzyme form (high bioavialability). Liver and meat are particularly rich in highly bioavailable niacin. Most of the niacin in plants, however, occurs as nicotinic acid in overall lower concentrations and with a lower bioavailability. The major portion of niacin in cereals is found in the outer layer and its bioavailability is as low as 30% because it is bound to protein (niacytin). If the diet contains a surplus of L-tryptophan (Ttp), e.g., more than is necessary for protein synthesis, the liver can synthesize NAD from Trp. Niacin requirements are therefore declared as niacin equivalents (1 NE = 1 mg niacin = 60 mg Trp). 

Milk is an excellent source of calcium, phosphorus, riboflavin (vitamin B2), thiamine (vitamin Bl) and vitamin B12, and a valuable source of folate, niacin, magnesium and zinc (Food Standards Agency, 2002). In particular, dairy products are an important source of calcium, which is vital for maintaining optimal bone health in humans (Prentice, 2004). The vitamins and minerals it provides are all bioavailable (i.e. available for absorption and use by the body) and thus milk consumption in humans increases the chances of achieving nutritional recommendations for daily vitamins and mineral intake (Bellew et al., 2000). 

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. 

The physiologically active forms of niacin are nicotinic acid, nicotinamide, and their coenzymes (93,96). Niacytin and the niacynogens appear to have limited bioavailability, although more work is needed in this area. The absorption and metabolism of niacin has been reviewed (20,93). 

Canola meal is a good source of choline, niacin and riboflavin, but not folic acid or pantothenic acid. It contains one of the highest levels of biotin found typically in North American feed ingredients. Total biotin in canola meal was found to average 1231 pg/kg with a bioavailability for growing broilers of 0.66 compared with 0.17 for wheat, 0.2 for triticale, 0.21 for barley, 0.39 for sorghum, 0.98 for soybean meal and 1.14 for maize (Blair and Misir, 1989). 

Carter EG and Carpenter KJ (1982) The bioavailability for humans of bound niacin from wheat bran. American Journal of Clinical Nutrition 36, 855-61. 

Glycosides of this vitamin (sometimes denoted as vitamin B3) were not described, however, there exists an evidence that bound niacin from wheat bran (termed niac)4in) has a single nicotinic acid moiety at least partially linked to an aromatic amine with glucose, xylose, and arabinose in a 6 3 1 molar ratio per molecule, with approximately three cinnamic acid esters [152,177,178]. It seems that these glycosidic complexes limit the bioavailability of the nicotinamide and for its liberation they must be treated, e. g., by soaking com in a lime solution, traditionally performed in Central America during production of tortillas [179]. 

Deficiencies were common in populations vdiose main calorie source was com (51, 52). Zein is the main protein found in com and it is very low in both niacin and tryptophan. When com is ground with lime water, the small amounts of niacin and tryptophan become more bioavailable. Populations who consume com... 

The determination of "total" (free plus bound) or "free" (bioavailable) niacin is based on well established extraction procedures [1,2,16] Total niacin is released from the food matrix by autoclaving at 121 °C with alkali [17] or 1—2.5 N mineral add [18] free niadn is isolated by boiling with 0.1 N mineral acid for 1 hr [19] or incubating with NAD glycohydrolase (NADase) at 37°C for 18 hr [20]. 

The loss of the vitamin does not usually exceed 10% in baked cereal products. The use of alkahsing baking ingredients, such as baking powders based on ammonium bicarbonate, can increase the bioavailability of niacin due to its release from unavailable forms. 

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. 

Preformed niacin occurs in foods either as nicotinamide (niacinamide) or as the pyridine nucleotide coenzymes derived from it, or as nicotinic acid, without the amide nitrogen, which is the form known as niacin in North America. Both nicotinamide and nicotinic acid are equally effective as the vitamin, but in large doses they exert markedly different pharmacological effects, so it is important, at least in that context, to make and maintain the distinction. In addition to the preformed vitamin, an important in vivo precursor is the amino acid L-tryptophan, obtained from dietary protein. Because the human total niacin supply, and hence niacin status, depends on the dietary tryptophan supply as well as on the amount of preformed dietary niacin and its bioavailability, it has become the accepted practice to express niacin intakes as niacin equivalents, ... 

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