Niacin unavailable

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. 

Either acid or alkaline hydrolysis can be applied, converting nicotinamide to nicotinic acid. Alkaline hydrolysis releases also the unavailable vitamers providing the estimation of the total niacin content. Acid hydrolysis, instead, is slower than alkaline hydrolysis therefore the former is usually coupled with enzymatic digestion by using takadiastase, papain, and clarase. Extraction with water and dilute sulfuric or hydrochloride acid has been applied to release the vitamers from the matrix without degrading nicotinamide [598]. 

Due to the relative stability of the niacin vitamers, either acid or alkaline hydrolysis can be used to convert nicotinamide to nicotinic acid for quantitation of both vitamers as nicotinic acid (9,44). Acid hydrolysis is used to quantitate biologically available niacin. Alkaline hydrolysis releases both the biologically available and the unavailable vitamers and provides an estimate of the total niacin content. Because alkaline hydrolysis is much faster than acid hydrolysis, the latter is usually supplemented with enzymatic hydrolysis. The most common enzymes are takadiastase, papain, and clarase. On occasion, organic solvents such as methanol have been used to extract free nicotinic acid. 

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. 

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. ... 

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. 

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. 

RECOMMENDED DAILY ALLOWANCE OF NIACIN. Estimation of niacin requirements are complicated (1) by the fact that some tryptophan is converted to niacin in man, (2) by the paucity of people of different ages receiving diets varying in niacin and tryptophan content, and (3) by the possible unavailability of niacin in some foods (such as corn). 

Amount available for absorption. In the case of bread, rice, and maize, the total amounts present are 1.7, 1.5, and 1.2 mg per lOOg, but apart from the niacin added in the fortification of white flour, 90% of this is unavailable for utilization by humans. Assuming that 60 mg tryptophan yields 1 mg niacin equivalent. 

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