Sources of nitrate exposure in residents of rural areas in Quebec, Canada

Nitrate exposure was investigated in a group of 187 people using well water and living in four areas of rural Quebec (Canada) with intensive agricultural activities. Nitrate intake was evaluated using a 24-h dietary recall and a food frequency questionnaire, in conjunction with a validated food database and measurements of nitrate concentrations in private wells. The total internal dose was estimated by means of the 24-h urinary nitrate excretion, while taking into account risk factors for endogenous nitrate formation. Mean (geometric) 24-h urinary nitrate excretion was 16.9 mg N for the 100 people with low groundwater contamination (mean nitrate concentration=0.18 mg N/l) and 23.3 mg N in the 87 individuals with moderate groundwater contamination (mean nitrate concentration=7.1 mg N/l). A multivariate analysis revealed that dietary nitrate intake during the last 24 h was the principal source of exposure, followed by water intake during the last 24 h. The Quetelet index was also a significant predictor of urinary excretion. The total predictive model explained only 29% of the variability in urinary nitrate excretion (R2=0.286). Neither the inflammatory status as indicated by elevated C reactive protein, the presence of Helicobacter pylori antibodies nor the occurrence of diarrhea during the last 24 h prior to urine collection were associated with urinary nitrate excretion. In conclusion, food and to a lesser extent water contribute to nitrate exposure in this rural setting with moderate water contamination. Better predictors of endogenous nitrate production are needed to improve our ability to model nitrate body burden and estimate associated health risks.