Nitrate in public water supplies and the risk of colon and rectum cancers.
BACKGROUND
Nitrate is a widespread contaminant of drinking water, but its potential health effects are unclear. In the body, nitrate is reduced to nitrite, which can react with amines and amides by nitrosation to form N-nitroso compounds, known animal carcinogens. N-nitroso compound formation is inhibited by certain nutrients, such as vitamin C, and increased by meat intake.


METHODS
We investigated the association of nitrate in public water supplies with incident colon and rectum cancers in a case-control study conducted in Iowa from 1986 to 1989. Nitrate levels in Iowa towns were linked to the participants' water source histories. We focused our analyses on the period from 1960 onward, during which nitrate measurements were more frequent, and we restricted analyses to those persons with public water supplies that had nitrate data (actual or imputed) for greater than 70% of this time period (376 colon cancer cases, 338 rectum cancer cases, and 1244 controls).


RESULTS
There were negligible overall associations of colon or rectum cancers with measures of nitrate in public water supplies, including average nitrate and the number of years with elevated average nitrate levels. For more than 10 years with average nitrate greater than 5 mg/L, the odds ratio (OR) for colon cancer was 1.2 (95% confidence interval [CI] = 0.9-1.6) and for rectum the OR was 1.1 (CI = 0.7-1.5). However, nitrate exposure (>10 years with average nitrate >5 mg/L) was associated with increased colon cancer risk among subgroups with low vitamin C intake (OR = 2.0; CI = 1.2-3.3) and high meat intake (OR = 2.2; CI = 1.4-3.6). These patterns were not observed for rectum cancer.


CONCLUSIONS
Our analyses suggest that any increased risk of colon cancer associated with nitrate in public water supplies might occur only among susceptible subpopulations.
De Roos
Anneclaire J
AJ
Occupational and Environmental Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Rockville, MD, USA. aderoos@fhcrc.org
Ward
Mary H
MH
Lynch
Charles F
CF
Cantor
Kenneth P
KP
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003110:Colonic Neoplasms / Q000139:chemically induced* / Q000453:epidemiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009566:Nitrates / Q000506:poisoning*; D012004:Rectal Neoplasms / Q000139:chemically induced* / Q000453:epidemiology; D012307:Risk Factors; D014481:United States / Q000453:epidemiology; D014876:Water Pollution; D014881:Water Supply / Q000032:analysis*
D009566:Nitrates