Nitrates in water and food may increase womens' thyroid cancer risks.
Ward, MH, BA Kilfoy, PJ Weyer, KE Anderson, AR Folsom and JR Cerhand. 2010. Nitrate intake and the risk of thyroid cancer and thyroid disease. Epidemiology 21:389-395.
Long-term exposure to nitrates through food and water may increase a woman's risk of thyroid disease, finds a study of older women in Iowa. Public water supplies contaminated with nitrates increased the risk of thyroid cancer in the women. Eating nitrates from certain vegetables was linked to increases in thyroid cancer and hypothyroidism, one type of thyroid disease.
This is the first study to show a link between nitrates and thyroid cancer in people, although nitrates have been shown to cause thyroid tumors in animal studies.
Thyroid cancer is the eighth most common cancer among women. In the United States, the incidence of thyroid cancer has increased steadily since 1980.
Nitrate is a common contaminant of drinking water, particularly in agricultural areas where nitrogen fertilizers are used. High rates of fertilizer application may also increase the natural nitrate levels found in certain vegetables, such as lettuce and root crops.
Researchers from the National Institute of Health studied 21,977 older women in Iowa who had used the same water supply for more than 10 years. They determined cancer incidence using the state health registry. They estimated nitrate intake from public drinking water sources using a public database of nitrate measurements. Dietary intake was measured through questionnaires. Since nitrate levels in private well water were not available, all private well users were combined into one group.
The results show a nearly three-fold increase in thyroid cancer risk for women with more than five year's use of a public water supply that had nitrate levels of 5 milligrams per liter (mg/L) or above. The maximum contaminant level of nitrate in drinking water is currently set at 10 mg/L in the United States. There was no evidence of elevated thyroid cancer risk among private well users.
Women in the highest group of dietary nitrate intake had a three-fold increase in thyroid cancer risk compared to the lowest group of dietary nitrate intake. Thyroid cancer and hypothyroidism risks increased in the women that ate more nitrate-containing vegetables.
Woman were exposed to similar levels of nitrate whether they drank from a public water supply or a private well. Higher dietary nitrate levels were found in women who lived in a larger town, were more educated, exercised more and ate more calories and Vitamin C.
In conjunction with prior studies, the researchers suggest the nitrate inhibits the thyroid glands ability to use iodide. Iodine is a necessary mineral for proper thyroid hormone and gland function.