Endogenous nitrosation in relation to nitrate exposure from drinking water and diet in a Danish rural population

Increasing levels of nitrate in drinking water is of concern due to the possibility of an associated increase in long-term exposure to endogenously formed carcinogenic N-nitroso compounds. Excretion of N-nitrosoproline in 12-h overnight urine after intake of 500-mg L-proline was used to quantify the rate of endogenous nitrosation in 285 individuals in an area in northern Denmark with large variation in nitrate concentration of the drinking water. Nitrate intake was measured in a 24-h duplicate diet sample. The crude association between nitrate concentration in drinking water and rate of endogenous nitrosation in individuals is only weakly positive and not quite statistically significant (P = 0.08). The risk of having detectable nitrosation increases significantly with total nitrate intake and tobacco smoking. In nonsmokers, nitrosation is determined by nitrate intake. Smokers have increased nitrosation which does not depend on nitrate intake. Effect modification through dietary factors was evaluated and indicated a protective effect of tea consumption, while the effect of eating vegetables was not clear-cut. The experimental design (12-h urine sample; proline dose taken in the evening) is likely to underestimate the effect of nitrate in drinking water relatively to nitrate in the diet.