The metabolism of dietary nitrites and nitrates.

Abstract

Abstract Nitrites and nitrates occur as dietary constituents being particularly high in certain vegetables. Endogenous oxidation of nitric acid also produces nitrate. Nitrate is readily absorbed and rapidly excreted in urine (60-70%) with an elimination half life of about five hours. Nitrate is secreted by an active transport mechanism (25%) into saliva; and by passive diffusion into breast milk. About 3% nitrate appears in urine as urea and ammonia in humans; in rats this is about 11%. Nitrate is reduced to nitrite by both mammalian enzymes and nitrate reductases present in microorganisms residing in the gastrointestinal tract. In rats approximately 50% of the nitrate reduced to nitrite is produced by mammalian enzymes, the remainder by microorganisms; this ratio is species dependent. Nitrite is absorbed by the gastrointestinal tract and is rapidly oxidised to nitrate. The plasma half life is less than an hour in most species and consequently nitrite is not normally detected in body tissues and fluids after oral administration. Nitrite is oxidised via a coupled reaction with oxyhaemoglobin producing methaemoglobin (ferrihaemoglobin). It appears in certain cases that endogenous nitrite may be a major factor in this process rather than exogenous material. Nitrite or a chemical species derived from it may be involved in the generation of nitrosamines and nitrosamides of toxicological importance, although neither nitrite or nitrate are carcinogenic.