N-nitrosoproline excretion by rural Nebraskans drinking water of varied nitrate content

The N-nitrosoproline (NPRO) test for in vivo nitrosation was applied in a study of 44 rural Nebraska men drinking high- or low-nitrate water from private wells. The subjects followed diets low in NPRO and nitrate for 5 days. On days 4 and 5 they avoided ascorbate-rich foods. Urine was collected for 24 h on day 4 while the subjects followed normal activities and on day 5 after an overnight fast and taking 500 mg L-proline. We determined NPRO, nitrate, creatinine, and specific gravity in the urines, and nitrite and nitrate in single saliva specimens collected on days 4 and 5. Results for all variables were separated into those above and below the median values and were analyzed by univariate and multivariate consideration of the contingency tables. Nitrate concentration in drinking water (> or = or < 10 ppm nitrate-nitrogen) was significantly associated with both day 4 and day 5 NPRO (> or = or < 1.5 micrograms/day; P < 0.04); and with urine nitrate (> or = or < 1.5 mmol/day), saliva nitrite (> or = or < 5 mg/liter), and saliva nitrate (> or = or < 25 mg/liter) (P < or = 0.002). Urine nitrate was significantly (P < or = 0.03) associated with both day 4 and day 5 NPRO, with odds ratios of 4.2 and 5.4, respectively. Creatinine was positively associated with NPRO on day 4 (P = 0.04). These findings, like those of a recent study in Denmark, showed an association between nitrate intake in water and NPRO formation. Their significance for people drinking high-nitrate water remains to be determined.