Nitrate in water tied to colon cancer risk in some

Moderate levels of nitrates in drinking water may raise the risk of colon cancer in certain vulnerable groups of people, a study in Iowa suggests.

Nitrates can get into public water supplies from nitrogen-containing fertilizers used in agriculture, or from human or animal waste. U.S. regulations set a cap of 10 milligrams per liter (mg/L) on the amount of nitrate public water systems may allow in drinking water. In the new study, federal researchers found that for certain individuals, long-term exposure to drinking-water nitrate levels higher than 5 mg/L was associated with an increased risk of colon cancer. The at-risk groups were people with a relatively low vitamin C intake, a diet relatively high in meat, or a history of inflammatory bowel disease. However, the question of whether drinking-water nitrate can cause cancer is not yet answered, according to the lead investigator, Dr. Anneclaire J. De Roos. She noted that a previous study in Iowa found no association between drinking-water nitrate and colon cancer. Other studies, in several regions of the world, have produced mixed results. “Although our study results are intriguing and biologically plausible,” Dr. De Roos told Reuters Health, “taken together with past studies, there is not enough evidence yet to lean one way or another toward a conclusion about whether or not drinking-water nitrate causes colon cancer.”Dr. De Roos, at the Fred Hutchinson Cancer Research Center in Seattle, and colleagues at the National Cancer Institute report the findings in the November issue of Epidemiology.Ingested nitrates are converted into N-nitroso compounds, which have been shown to cause cancer in animals. Certain individuals may be more likely than others to produce these compounds. For example, because vitamin C appears to inhibit the formation of N-nitroso compounds, low intake of the vitamin might promote their formation. In addition, some research has tied higher meat intake to greater levels of N-nitroso compounds in feces. The new study included 376 Iowa residents diagnosed with colon cancer in the late 1980s, 338 with rectal cancer, and 1,244 healthy controls subjects. All were long-time residents in homes serviced by municipal water systems, as opposed to private wells. Dr. De Roos and her colleagues found no overall link between nitrate levels and the risks of colon or rectal cancer. There was, however, a connection between nitrate exposure and colon cancer in some groups.People with a relatively lower vitamin C intake — less than 132 mg per day — had a two-fold increase in their risk of colon cancer when they were exposed to more than 5 mg/L of nitrate for more than 10 years. A similar increase was seen among people who ate a couple of servings of meat each day and had the same nitrate exposure. In addition, participants with a history of inflammatory bowel disease — an established risk factor for colon cancer — faced an increased risk of the disease. Among these individuals, there was a greater risk for those exposed to nitrate levels above 5 mg/L for up to 10 years. According to the researchers, the possible link between drinking-water nitrate and colon cancer warrants further study — including research looking at private wells, which in agricultural areas may have particularly high nitrate levels. (Source: Epidemiology 2003;14:640-649: Reuters Health: Amy Norton: November 27, 2003: Oncolink)