Niacin (Vitamin B3): Dietary Source, RDA, and Functions

Niacin (vitamin B3), also known as nicotinic acid, is an essential water-soluble vitamin. Niacin occurs naturally in food such as meat, poultry, and fish. Our liver can make niacin from the amino acid tryptophan. Riboflavin, vitamin B6 and iron are required to catalyze some reactions, and one mg of niacin is formed from 60 mg of tryptophan.

Alternate name	Niacin
Year of Discovery	1937
Discovered by	Conrad Elvehjem

The term nicotinic acid should not be confused with nicotine. Nicotinic acid is a vitamin, but nicotine is a potent poison from tobacco. Otto Warburg (Nobel Prize, 1931) elucidated the structure of NAD+, and Alexander Todd (Nobel prize, 1957) demonstrated its function.

Chemistry

Niacin is pyridine-3-carboxylic acid (pyridine ring with a substituted carboxylic acid group). Niacinamide is the acid amide (pyridine ring with a substituted amide group). Niacinamide is the active form of vitamin present in tissues.

The main coenzyme forms of niacin are nicotinamide adenine dinucleotide or NAD and nicotinamide adenine dinucleotide phosphate or NADP.

Dietary Sources

Niacin is present in enriched and whole grains and high-protein foods like meat, milk, and eggs. You can choose cereals (whole grains), white bread, and pasta fortified with niacin as a rich source.

The richest natural sources of niacin are;

• Dried yeast
• Liver
• Peanut and peanut butter
• Whole cereals
• Legumes
• Mushroom
• Potatoes
• Meat
• Poultry
• Fish

About half of the requirement of niacin is met by the conversion of tryptophan to niacin. About 60 mg of tryptophan will yield 1 mg of niacin.

Recommended Dietary Allowances (RDA)

The niacin requirement is expressed in niacin equivalents (NEs), allowing some conversion of the amino acid tryptophan to niacin.

The RDA of niacin for adults is 16 mg/day of NEs for men and 14 mg/day of NEs (niacin equivalents) for women. The tolerable upper intake level for niacin for adults is 35mg/day. [2]

Life Stage Group	Niacin
Infants
0-6 months	2 mg/day of NEs
7-12 months	4 mg/day of NEs
Children
1-3 years	6 mg/day of NEs
4-8 years	8 mg/day of NEs
Males
9-13 years	12 mg/day of NEs
14-18 years	16 mg/day of NEs
>19 years	16 mg/day of NEs
Females
9-13 years	12 mg/day of NEs
14-18 years	14 mg/day of NEs
>19 years	14 mg/day of NEs
Pregnant girl or women	18 mg/day of NEs
Lactating mother	17 mg/day of NEs

Absorption

Niacin is absorbed in the small intestine and is transported freely in the blood or the forms of nicotinic acid or nicotinamide. Niacin is excreted in urine as 1-methyl nicotinamide or NAM. Urinary 1-methyl nicotinamide is the most reliable and sensitive measure of niacin status.

Functions

Niacin is essential for healthy skin, blood cells, the brain, and the nervous system. Niacin helps to convert food into energy.

Niacin functions as a component of the metabolically essential coenzymes NAD and NADP. These coenzymes act as hydride ion acceptors or donors in biological redox reactions. They also serve as coenzymes for dehydrogenases.

Niacin is used to lower serum cholesterol. Nicotinic acid inhibits the flux of free fatty acids from adipose tissue, reducing the acetyl CoA pool, which lowers the serum cholesterol level. In high doses, niacin reduces Lipoprotein(a) levels.

Deficiency

The niacin deficiency led to a classical clinical condition called “pellagra.” Pellagra was common in the United States and Europe in populations dependent upon corn-based diets. Pellagra has disappeared from developed countries except for cases of chronic alcoholism. It persists in parts of India, China, and Africa.