Niacin is a term that refers to several related compounds, namely nicotinic acid, nicotinamide, and certain derivatives that exhibit the biochemical effects of nicotinamide. It is an essential, water-soluble vitamin that is involved in numerous biochemical processes in the body and, like vitamins B1 and B2, plays a significant role in the production of the body’s energy, as well as nervous system and digestive system function.
      More specifically, the niacin compounds are essential to the synthesis of the important coenzymes NAD (nicotinamide adenine dinucleotide) and NADP (nicotinamide adenine dinucleotide phosphate) which in turn are integral to numerous reduction-oxidation reactions.

      Niacin is found in food sources in a number of biochemical forms, and is also synthesized in the body from tryptophan, which is also obtained from dietary sources. In the latter case, niacin is synthesized from tryptophan in a ratio of approximately 60:1 (60 mg of tryptophan is converted to 1 mg of niacin).
      Niacin and its related compounds are measured in milligrams of niacin equivalents (mg NE), where 1 mg NE is equal to 1 mg niacin, and 1 mg NE is equal to 60 mg of dietary tryptophan.
      Figures for the recent National Academy of Sciences Dietary Reference Intakes study are based upon slightly different age ranges than those previously evaluated, accounting for the overlapping groups in the following table.

      The various forms in which the related compounds of niacin are ingested occur in many types of foods. Some of the best sources include whole and enriched grains (such as fortified cereals), milk products, legumes (beans and peas), liver, various meats, fish and poultry.
      Niacin is also synthesized in the body by tryptophan, so dietary sources of tryptophan also contribute as a source of this vitamin, though to a much lesser degree.

      The most common symptom of severe niacin deficiency is a disease known as pellagra, which is manifested by digestive tract disorders (vomiting, constipation or diarrhea), swelling in the mouth (especially the tongue), skin rash, particularly in areas exposed to sunlight, and neurological disorders such as hallucinations and delirium that can lead to mental impairment.
      Previously a much larger problem in the developed world than today, niacin deficiency occurred primarily in populations where the dietary intake of the vitamin was insufficient. Pellagra is still a concern in many parts of the third world, and among individuals with intestinal malabsorption, disorders that disrupt the normal biochemistry of tryptophan, and chronic alcoholism.

      Supplemental niacin is used most commonly to offset inadequate dietary intake. There are numerous, complicated biochemical pathways involved in the absorption of niacin, some of which are enhanced by the interaction of this vitamin with other vitamins of the B complex. It is for this reason that other B vitamins, riboflavin and vitamin B6 in particular, and certain minerals (iron for example) are often combined in dietary supplements, and in multivitamins containing the B complex.

      The health risks reported have generally, though not exclusively, been the result of massive, clinical doses of niacin administered by health care providers as a therapeutic remedy for a number of disorders. The manifestations of these very high doses include a flushing effect (itching, tingling or burning sensation), nausea, vomiting and signs of liver toxicity.
      Some of these symptoms have also been reported in a few individuals from large supplemental doses of niacin, though to a much lesser extent. Notwithstanding, individuals who consume large supplemental amounts of niacin for reasons other than dietary deficiency should be aware of a number of additional potential dangers, particularly blurred vision, peptic ulcers, high blood sugar and the possibility of serious liver damage.
      It is for these reasons that the recent National Academy of Sciences Dietary Reference Intakes study on niacin has recommended a Tolerable Upper Intake Level (UL) of 35 mg/day for adults 19 years and older for this vitamin. Anyone with disorders relating to the manifestations mentioned above, as well as individuals with a history of gout, cardiac arrhythmia, migraines or alcoholism, should be particularly cautious, even at the recommended UL for niacin.

      The apparent ability of supplemental niacin to lower blood cholesterol levels has been widely reported. High cholesterol, particularly the low-density lipoprotein (LDL) fraction, has been long associated with heart disease and should not go untreated. The consumption of large doses of niacin in this regard, however, can be unproductive and dangerous, and individuals who do so are practicing medicine on themselves at their own peril. A proper diet, particularly with regard to fiber intake, has been demonstrated to be a much safer and effective non-clinical approach to the management of blood cholesterol. In any case, individuals with high cholesterol should be under the advice of a health care professional.
      The use of supplemental niacin as an energy booster has also been widely reported. This application of the vitamin, however, requires considerable more research to be substantiated at any level.