Unavailable Niacin in Cereals

A small fraction of the niacin in niacytin may be biologically avtdlable as a result of hydrolysis by gastric acid. About 10% of the total is released as free nicotinic acid after extraction of maize or sorghum meal with 0.1 mol per L of hydrochloric acid, and Carter and Carpenter (1982) have shown that about 10% of the total niacin content of maize is biologically available to humans beings. 

1 Unavailable Niacin in Cereals Chemical analysis reveals niacin in cereals (largely in the bran), but this is biologically unavailable, because it is bound as niacytin - nicotinoyl esters to a variety of macromolecules ranging between Mr 1,500 to 17,000. In wheat bran, 60% is esterified to polysaccharides, and the remainder to polypeptides and glycopeptides (Mason et al., 1973). In calculation of niacin intakes, it is conventional to ignore the niacin content of cereals completely. 

In the liver, there is litde utilization of preformed niacin for nucleotide synthesis. Although isolated hepatocytes will take up both vitamers from the incubation medium, they seem not to be used for NAD synthesis and cannot prevent the fall in intracellular NAD(P), which occurs during incubation. The enzymes for nicotinic acid and nicotinamide utilization are more or less saturated with their substrates at normal concentrations in the liver, and hence are unlikely to be able to use additional niacin for nucleotide synthesis. By contrast, incubation of isolated hepatocytes with tryptophan results in a considerable increase in the rate of synthesis of NAD(P) and accumulation of nicotinamide and nicotinic acid in the incubation medium. Similarly, feeding experimental animals on diets providing high intakes of nicotinic acid or nicotinamide has relatively little effect on the concentration of NAD (P) in the liver, whereas high intakes of tryptophan lead to a considerable increase. It thus seems likely that the major role of the liver is to synthesize NAD(P) from tryptophan, followed by hydrolysis to release niacin for use by extrahepatic tissues (Bender et al., 1982 McCreanor and Bender, 1986 Bender and Olufunwa, 1988). 

In most extrahepatic tissues, nicotinic acid is a better precursor of nucleotides than is nicotinamide. However, muscle, brain, and to a lesser extent the testis are able to take up nicotinamide from the bloodstream effectively, and apparently utilize it without prior deamidation (Gerber and Deroo, 1970). 

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Niacin was discovered as a nutrient during studies of pellagra. It is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Two compounds, nicotinic acid and nicotinamide, have the biologic activity of niacin its metabolic function is as the nicotinamide ring of the coenzymes NAD and NADP in oxidation-reduction reactions (Figure 45-11). About 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. The niacin content of foods is expressed as mg niacin equivalents = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable, this is discounted. 

The niacin content of foods is generally expressed as mg niacin equivalents 1 mg niacin equivalent = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable (section 11.8.1.1), it is conventional to ignore preformed niacin in cereal products. 

Chemical analysis reveals niacin in cereals (largely in the bran), but this is biologically unavailable, as it is bound as niacytin - nicotinoyl esters to polysaccharides, polypeptides and glycopeptides. 

The role of corn diets in the production of pellagra may be explained in part by the low tryptophan content of corn. Recent experiments in rats suggest another possible explanation. Some of the niacin in com, and in certain other cereals, appears to be present in bound form which is unavailable to the organism, unless previously hydrolyzed by alkali. ... 

Two forms of vitamin B3, also known as niacin, are found in food [1,2] nicotinic acid and nicotinamide. In living tissues, nicotinamide is a moiety of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) in meat, it is found free because of the postmortem hydrolysis of NAD [1]. Nicotinamide is also a form used for food fortification [4]. Nicotinic acid is the prevalent vitamer in mature cereal grains nevertheless, it is unavailable due to its linkage to a number of polysaccharides (niacytin) and polypeptides (niacinogen) [1]. 

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. 

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