Vitamin B3 (niacin)
Niacin, or vitamin B3, is required in many cellular metabolic processes.
Niacin functions as a co-enzyme in electron-transfer reactions such as the catabolism of carbohydrates, fats, proteins, and alcohol, as a substrate for NAD-consuming enzymes, and as ligand for inhibitory neurotransmitters, pro-inflammatory cytokines and calcium mobilisation. Date of preparation: December 2018.
Importance of niacin for health
Niacin is used as a collective term for all vitamin B3 vitamers, including nicotinamide, nicotinic acid, nicotinamide adenine dinucleotide (NAD), or nicotinamide adenine dinucleotide phosphate (NADP) (1). NAD and NADP are required in many metabolic processes in cells (2, 3):
- as a co-enzyme in electron-transfer reactions (catabolism of carbohydrates, fats, proteins, and alcohol)
- as a substrate for NAD-consuming enzymes (ADP-ribosylation, NAD-dependent deacetylation, calcium mobilization)
- as ligand (inhibitory neurotransmitter, pro-inflammatory cytokine, calcium mobilization)
Nicotinamide and nicotinic acid absorption occurs rapidly in the stomach and intestine. In liver and intestines, nicotinamide is released from NAD and transported to tissues for NAD synthesis when required (1). Additionally, nicotinic acid or nicotinamide riboside can also be utilized for NAD production as well as de novosynthesis from tryptophan (4). All pathways form the intermediary nicotinamide or nicotinic acid mononucleotides (3).
Various animal-source foods such as poultry, beef, and fish are a good source for dietary niacin, primarily as highly bioavailable NAD and NADP. Nuts, legumes, and grains also contain niacin, which may be bound to polysaccharides and glycopeptides resulting in reduced bioavailability (2).
The amino acid tryptophan can be used for niacin production via the kynurenine pathway, producing quinolinic acid, which then is converted to nicotinic acid mononucleotide, which enters the Preiss-Handler pathway for NAD synthesis (3). This multi-step metabolic pathway involves enzymes dependent on zinc, iron, riboflavin, and vitamin B6 (5). Niacin can be used to reduce levels of cholesterol, low density lipoprotein (LDL), triglycerides, and lipoprotein while it increases high density lipoprotein (HDL) (6).
Pellagra occurs in the late stage of severe niacin deficiency (7). Niacin deficiency can occur due to inadequate intake of NAD precursors from diet, which includes tryptophan. Other conditions that may result in niacin deficiency include Crohn’s disease, megaduodenum, Hartnup’s disease, or carcinoid syndrome. Skin, the digestive system, and the nervous systems are commonly affected by niacin deficiency (3).
High intakes of nicotinic acid and nicotinamide as dietary supplement or medicinal treatment may cause adverse effects, such as flushing, burning, tingling, and itching sensations. Up to 3000mg nicotinic acid per day has been shown to cause hypotension, fatigue, impaired glucose tolerance or insulin resistance, gastrointestinal effects, or hepatitis. Thus tolerable Upper Limits (UL) have been established (2).
Niacin interacts with certain types of medications, and several types of drugs might affect niacin levels (2):
- Isoniazid and pyrazinamide (Rifater®, tuberculosis treatment)
- anti-diabetes mediation
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