The pharmacology of dietary nitrate and the origin of urinary nitrate

From results of nitrate balance studies and analysis of ileostomy fluid it is concluded that, in man, ingested nitrate is absorbed from the upper digestive tract and is secreted in saliva and sweat. The nitrate concentrations in saliva and sweat reach maximum values within 1 hr of nitrate ingestion and return to almost zero within a further 5 hr.

Urinary nitrate reaches a maximum concentration 4-6 hr after nitrate challenge and returns to the baseline value within 24 hr. Approximately 65-70% of a challenge dose of nitrate is excreted in urine in the following 24 hr and less than 1% is excreted in faeces; the remainder of the challenge dose is secreted in sweat or is degraded in the saliva or other digestive secretions by bacterial action.

It is concluded that the urinary nitrate found when volunteers eat a 'nitrate-free' diet is likely to be due to undetected dietary nitrate rather than to endogenous nitrate synthesis.