Nitrate in water linked to colon cancer risk

A contaminant found in drinking water may raise the risk of colon cancer in certain vulnerable groups of people, a study suggests. The contaminant is nitrate, which can get into public water supplies from nitrogen-containing fertilizers used in agriculture, or from human or animal waste. The question whether drinking-water nitrate can cause cancer, however, is not yet answered, according to the researchers at the Fred Hutchinson Cancer Research Centre in Seattle, USA. They 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. Many people regularly consume nitrate, most often through vegetables, which contain nutrients such as fibre and folate that may protect against colon cancer. But drinking water can be a person's major source of nitrate if the local water supply has levels above the limit. Once in the body, nitrate can eventually be converted by stomach substances into N-nitroso compounds, which has 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 linked higher meat intake to greater levels of N-nitroso compounds in faeces. The researchers included in the new study 376 Iowa residents diagnosed with colon cancer in the late 1980s; 338 with rectal cancer; and 1,244 healthy "controls" for comparison. All were long-time residents in homes serviced by municipal water systems, as opposed to private wells.

They 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 i.e. 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.

COMMENT