Tryptophan niacin equivalents

In this regard, tryptophan is considered a provitamin and is assigned a niacin equivalent of 1/60. The following fists the vitamin content of many common foodstuffs and in Table 3, values of vitamin B content are compared to niacin potential from tryptophan. 

The RDA for niacin is based on the concept that niacin coen2ymes participate in respiratory en2yme function and 6.6 niacin equivalents (NE) are needed per intake of 239 kj (1000 kcal). One NE is equivalent to 1 mg of niacin. Signs of niacin deficiency have been observed when less than 4.9 NE/239 kj or less than 8.8 NE per day were consumed. Dietary tryptophan is a rich source of niacin and the average diet in the United States contains 500—1000 mg of tryptophan. In addition, the average diet contains approximately 8—17 mg of niacin. In total, these two quantities total 16—34 NE daily. Table 5 Hsts the RDA and U.S. RDA for niacin (69). 

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). 

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 RDA for niacin is expressed in terms of energy intake 6.6 mg niacin equivalent (NE, 1 mg niacin or 60 mg tryptophan) per 1000 kcal (4186 kJ) per day is recommended (13NEday-1 minimum). This is approximately equivalent to 19 and 15 mg NE day -1 for men and women, respectively. The UK RNI value for niacin is 6.6 mg NE per 1000 kcal (4186 kJ) per day for adults. The richest dietary sources of niacin are meat, poultry, fish and whole-grain cereals. 

This value includes niacin equivalents from preformed niacin and from tryptophan. A dietary intake of 60 mg tryptophan is considered equivalent to 1 mg niacin. One niacin equivalent is equal to either to those amounts. 

Niacin, a water-soluble vitamin vital for oxidation by living cells, functions in the body as a component of two important coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). NAD and NADP are involved in the release of energy from carbohydrate, fat, and protein, and in the synthesis of protein, fat, and pentoses for nucleic acid formation. Milk is a poor source of preformed niacin, containing about 0.08 mg per 100 g. However, milk s niacin value is considerably greater than indicated by its niacin content (Horwitt et al. 1981). Not only is the niacin in milk fully available, but the amino acid tryptophan in milk can be used by the body for the synthesis of niacin. For every 60 mg of tryptophan consumed, the body synthesizes 1 mg of niacin. Therefore, the niacin equivalents in 100 g milk equal 0.856 mg including that from pre-... 

Horwitt, M. K., Harper, A. E. and Henderson, L. M. 1981. Niacin-tryptophan relationships for evaluating niacin equivalents. Am. J. Clin. Nutr. 34, 423-427. 

Humans can convert ingested tryptophan to niacin in vivo (19,93,96). Therefore, the niacin activity of food can be expressed in niacin equivalents, which is the sum of the niacin content (nicotinic acid vitamers + nicotinamide vitamers) plus 1/60 of the tryptophan content (in mg). This section will be restricted to analyses for the niacin vitamers. Tryptophan methodology will not be covered. 

The human requirement of niacin is related to the intake of tryptophan. Animal proteins contain approximately 1.4 percent of tryptophan, vegetable proteins about 1 percent. A dietary intake of 60 mg of tryptophan is considered equivalent to 1 mg of niacin. When this is taken into account, average diets in the United States supply 500 to 1,000 mg tryptophan per day and 8 to 17 mg niacin for a total niacin equivalent of 16 to 33 mg. The RDA for adults, expressed as niacin, is 6.6 mg per 1,000 kcal, and not less than 13 mg when caloric intake is less than 2,000 kcal. 

The most extensive such study was that of Horwitt and coworkers (1956). They found that there was a considerable variation between subjects in the response to tryptophan and niacin, and suggested that in order to allow for individual variation, it should be assumed that 60 mg of tryptophan was equivalent to 1 mg of preformed niacin. This ratio has been generally accepted, and is the basis for expressing niacin requirements and intake in terms of niacin equivalents - the sum of preformed niacin and 1 /60 of the tryptophan. 

The depletion/repletion studies of Horwitt et al. (1956) and others have suggested, on the basis of restoration of urinary excretion of -methyl nicotinamide, that the average niacin requirement is 5.5 mg per 1,000 kcal (1.3 mg per MJ). Allowing for individual variation, reference intakes (see Table 8.2) are set at 6.6 mg niacin equivalents (preformed niacin - -1 /60 of the dietary tryptophan) per 1,000 kcal (1.6 mgper MJ). Even when energy intakes are very low, it must be assumed that energy expenditure will not fall below 2,000 kcal, and this is the basis for the calculation of reference intakes for subjects with low energy intakes. 

Nicotinamide-adenine dinucleotide (NAD diphosphopyri-dine nucleotide) and nicotinamide-adenine dinucleotide phosphate (NADP also termed triphosphopyridine nucleotide) represent most of the niacin activity found in good sources that include yeast, lean meats, liver, and poultry. Milk, canned salmon, and several leafy green vegetables contribute lesser amounts but are still sufficient to prevent deficiency. Additionally, some plant foodstuffs, especially cereals such as corn and wheat, contain niacin bound to various peptides and sugars in forms nutritionally not readily available (niacinogens or niacytin). Because tryptophan is a precursor of niacin, protein provides a considerable portion of niacin equivalent. As much as two thirds of niacin required by adults can be derived from tryptophan metaboHsm via nicotinic acid ribonucleotide... 

One of the substances commonly treated as a vitamin is niacin, which is synthesized from the essential amino acid tryptophan. The ratio is approximately 60 mg of tryptophan being required to produce 1 mg of niacin (1). This has led to niacin requirements being expressed as niacin equivalents (NE), based on the amount of tryptophan in the diet. It must be kept in mind that tryptophan is essential and is the precursor to the neurotransmitter serotonin in addition to being part of protein structure. Therefore, niacin can be thought of as tryptophan sparing. 

Dietary Reference Intakes. Many of the DRI units are milligram niacin equivalents (mg NE). These units take into account the fact that approximately 60 mg tryptophan produce 1 mg of niacin. For adult males, the RDA is between 960 mg of tryptophan and 16 mg of niacin, given that 960 mg of tryptophan is equivalent to 16 mg of niacin (56, 57). 

The kynurenine pathway (Fig. 49.1) is the principal pathway for tryptophan metabolism and produces precursors which, together with dietary niacin, are used to synthesise NAD and NADP (Chapter 53). It is generally accepted that 60 mg of dietary tryptophan is equivalent to Img of niacin. 

From the role of tryptophan in the biosynthesis of nicotinic acid, it is obvious that the nutritional studies on nicotinic acid deficiency must take tryptophan intake into account. Indeed, 60 mg of tryptophan in the diet is as effective as 1 mg of nicotinic acid. Since 70 g of protein yields 720 mg of tryptophan, the intake of such an amount of protein corresponds to 12 mg of nicotinic acid in preventing niacin deficiency. Since the requirements for niacin, like those of thiamine, depend essentially on the caloric intake, it is useful to express the requirements in niacin equivalents per 1000 calories. The optimum requirement is 4.4 mg niacin per 1000 calories. 

The vitamin is obtained from the diet in the form of nicotinic acid, nicotinamide, NAD/NADP and tryptophan. The Recommended Dietary Allowance (RDA) for adults is 16mg/day of niacin equivalents for men and 14mg/day for women (Food and Nutrition Board 1998). 

Angyal, G., 1985. Contribution of tryptophan to niacin equivalent of infant formula microbiological method. In Production, Regulation, and Analysis of Infant Formula A Topical Conference. May 14-16, 1985. Association of Official Analytical Chemists, Virginia Beach VA, USA, pp. 152-159. 

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