Agency for Toxic Substances and Disease Registry
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Nitrate and nitrite levels in the body are the result of internal nitrate production and external sources. Intake of some amount of nitrates is a normal part of the nitrogen cycle in humans. The mean intake of nitrate per person in the United States is about 40–100 mg/day (in Europe it is about 50–140 mg/day). In the proximal small intestine, nitrate is rapidly and almost completely absorbed (bioavailability at least 92%) (31). Approximately 60% to 70% of an ingested nitrate dose is excreted in urine within the first 24 hours. About 25% is excreted in saliva through an active blood nitrate transport system and potentially is reabsorbed. Half-lives of parent nitrate compounds are usually less than 1 hour; half-lives of metabolites range from 1 hour to 8 hours (35, 36). Nitrate may also be synthesized endogenously from nitric oxide (especially in the case of inflammation), which reacts to form nitrite (22, 31). | |
In vivo conversion of nitrates to nitrites significantly enhances nitrates’ toxic potency. The major metabolic pathway for nitrate is conversion to nitrite, and then to ammonia. Approximately 5%–10% of the total nitrate intake is converted to nitrite by bacteria in the saliva, stomach, and small intestine. This reaction is pH dependent, with no nitrate reduction occurring below pH 4 or above pH 9. This is the main reason why infants are more susceptible to nitrite toxicity from elevated nitrate/nitrite ingestion. Another potential metabolic pathway is the reaction of nitrite with endogenous molecules to form N-nitroso compounds, which may have toxic or carcinogenic effects (22). | |
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