These values, which vary by age and sex, include: DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. Intake recommendations for niacin and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by an expert committee of the Food and Nutrition Board (FNB) at the National Academies of Sciences, Engineering, and Medicine. No functional biochemical tests that reflect total body stores of niacin are available. A 'niacin index' (the ratio of erythrocyte NAD to NADP concentrations) below 1 suggests that an individual is at risk of developing niacin deficiency. A 'niacin number' (the ratio of NAD to NADP concentrations in whole blood x 100) below 130 suggests niacin deficiency. Another measure of niacin status takes into account the fact that NAD levels decline as niacin status deteriorates, whereas NADP levels remain relatively constant.

Indicators of inadequacy such as this and other biochemical signs (e.g., a 2-pyridone oxidation product of N1-methyl-nicotinamide below detection limits in plasma or low erythrocyte NAD concentrations) occur well before overt clinical signs of deficiency. An adult has deficient niacin status when urinary-excretion rates are less than 5.8 micromol/day. Excretion rates in adults of more than 17.5 micromol/day of these two metabolites reflect adequate niacin status, while excretion rates between 5.8 and 17.5 micromol/day reflect low niacin status. The most sensitive and reliable measure of niacin status is the urinary excretion of its two major methylated metabolites, N1-methyl-nicotinamide and N1-methyl-2-pyridone-5-carboxamide. Levels of niacin in the blood are not reliable indicators of niacin status. Unmetabolized nicotinic acid and nicotinamide might be present in the urine as well when niacin intakes are very high. The liver methylates any remaining excess to N1-methyl-nicotinamide, N1-methyl-2-pyridone-5-carboxamide, and other pyridone oxidation products, which are then excreted in the urine. Some excess niacin is taken up by red blood cells to form a circulating reserve pool. Once absorbed, physiologic amounts of niacin are metabolized to NAD. Įven when taken in very high doses of 3–4 g, niacin is almost completely absorbed. Ingested niacin is absorbed primarily in the small intestine, but some is absorbed in the stomach. When NAD and NADP are consumed in foods, they are converted to nicotinamide in the gut and then absorbed. The body also converts some tryptophan, an amino acid in protein, to NAD, so tryptophan is considered a dietary source of niacin. Most dietary niacin is in the form of nicotinic acid and nicotinamide, but some foods contain small amounts of NAD and NADP. NADP, in contrast, enables anabolic reactions, such as the synthesis of cholesterol and fatty acids, and plays a citical role in maintaining cellular antioxidant function. NAD is also required for enzymes involved in critical cellular functions, such as the maintenance of genome integrity, control of gene expression, and cellular communication. NAD is primarily involved in catabolic reactions that transfer the potential energy in carbohydrates, fats, and proteins to adenosine triphosphate (ATP), the cell's primary energy currency. NAD and NADP are required in most metabolic redox processes in cells where substrates are oxidized or reduced. NAD is also converted into another active form, the coenzyme nicotinamide adenine dinucleotide phosphate (NADP), in all tissues except skeletal muscle. More than 400 enzymes require NAD to catalyze reactions in the body, which is more than for any other vitamin-derived coenzyme.

Niacin is naturally present in many foods, added to some food products, and available as a dietary supplement.Īll tissues in the body convert absorbed niacin into its main metabolically active form, the coenzyme nicotinamide adenine dinucleotide (NAD). Niacin is the generic name for nicotinic acid (pyridine-3-carboxylic acid), nicotinamide (niacinamide or pyridine-3-carboxamide), and related derivatives, such as nicotinamide riboside. Niacin (also known as vitamin B3) is one of the water-soluble B vitamins. For a general overview of Niacin, see our consumer fact sheet on Niacin. This is a fact sheet intended for health professionals.

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