Niacin

BACKGROUND

Niacin is a generic descriptor for the closely related compounds, nicotinic acid and its amide nicotinamide, which act similarly as nutrients. The amino acid tryptophan is converted to nicotinamide with an average conversion efficiency of 60:1 and can thus contribute to requirements (Horwitt et al 1981) although this can vary depending on a number of dietary and metabolic factors (McCormick 1988).

Niacin intakes and requirements are often expressed as niacin equivalents where 1 mg niacin equivalent is equal to 1 mg niacin or 60 mg tryptophan.

Niacin functions as a component of the reduced and oxidised forms of the coenzyme nicotinamide adenine dinucleotide (NADH 2 and NAD, respectively), both of which are involved in energy metabolism, and nicotinamide adenine dinucleotide phosphate (NADPH 2 and NADP, respectively). These coenzymes function in dehydrogenase-reductase systems involving the transfer of a hydride ion (McCormick 1988, 1997). NAD is also needed for non-redox adenosine diphosphate-ribose transfer reactions involved in DNA repair and calcium mobilisation. It functions as part of the intracellular respiration system and with enzymes involved in oxidation of fuel substrates. Because of their role in energy metabolism, niacin requirements are, to some extent, related to energy requirements.

Niacin is found in a wide range of foods. Important sources of preformed niacin include beef, pork, wholegrain cereals, eggs and cow's milk. Human milk contains a higher concentration of niacin than cows' milk. In unprepared foods, niacin is present mainly as cellular NAD and NADP. Enzymatic hydrolysis of the coenzymes can occur during the course of food preparation. In mature grains, most of the niacin is bound and is thus only 30% available, although alkali treatment of grain increases availability (Carpenter & Lewin 1985, Carter & Carpenter 1982). The niacin in meats is in the form of NAD and NADP and is more bioavailable. Some foods, such as beans and liver, contain niacin in the free form that is highly available.

The requirement for preformed niacin depends to some extent on the availability of tryptophan. Inadequate iron, riboflavin or vitamin B 6 status decreases the conversion of tryptophan to niacin (McCormick 1989).

Deficiency of niacin causes the disease pellagra which is associated with inflammation of the skin on exposure to sunlight, resembling severe sunburn except that the affected skin is sharply demarcated (McCormick 1988, 1997). These skin lesions progress to pigmentation, cracking and peeling. Often the skin of the neck is involved. Pellagra is the disease of 'three Ds', namely dermatitis, diarrhoea and (in severe cases) delirium or dementia. There is also likely to be an inflamed tongue (glossitis). In mild chronic cases, mental symptoms are not prominent. Pellagra was a major problem in the Southern states of the US in poor Blacks and Whites whose diet consisted of maize (American corn) and little else. Unlike other cereals maize is low in bioavailable niacin and tryptophan is the first limiting amino acid. Pellagra only disappeared after niacin was discovered and mandatory fortification of maize meal was introduced in 1941.

Indicators that have been used to assess niacin requirements include urinary excretion, plasma concentrations, erythrocyte pyridine nucleotides, transfer of adenosine diphosphate ribose and appearance of pellagra. Biochemical changes appear well before overt signs of deficiency. The most reliable and sensitive measures are urinary excretion of N1-methyl nicotinamide and its derivative, N1-methyl-2-pyridone-5-carboxyamide.