Nitrates from a surprising source may be increasing thyroid cancer rates in women, according to a recent report from a group of researchers from the National Cancer Institute, University of Iowa, University of Minnesota, and Mayo Clinic. The authors note that thyroid cancer diagnoses increased at an annual rate of 6 percent per year during the period 1997-2003. Thyroid cancer is now in the top ten for cancers most common in women.
Nitrate came under suspicion because it inhibits thyroid uptake of iodine and thus inhibits the production of the thyroid hormones triiodothyronine (T3) and thyroxin (T4). This causes an increase in thyroid stimulating hormone, TSH, as the body tries to maintain healthy levels of T3 and T4. Chronically high levels of TSH are associated with thyroid disease, including cancer.
I usually hear about nitrates in terms of processed meats such as bacon and hot dogs. Nitrate is used as a preservative, and generally considered harmless until it is converted to nitrite and nitroso compounds. Reference 2 suggests looking for nitrate-free meat products and/or consuming vitamin C to inhibit the conversion to nitrosamines.
Reference 1, however, reports that the nitrate sources of concern are green leafy vegetables, certain root vegetables, and drinking water (especially in agricultural areas). Nitrate has been widely used as a fertilizer since the 1950's , and the concentration of nitrate in vegetables may have been increasing during this time. The authors cite data showing that organically grown lettuce has lower nitrate levels than conventionally grown lettuce.
The relation between nitrate and thyroid cancer was studied in a group of 21,977 women in Iowa over the period 1986 to 2004. Higher nitrate levels in both drinking water and diet were associated with higher rates of thyroid cancer, by factors of 2.6 to 2.9. Higher intake of vitamin C showed some protective effect in women with high nitrate intake.
Surprisingly, women in the higher nitrate intake group had healthier lifestyles overall: they exercised more, smoked less, and had more education. High nitrate vegetables appear to fit into the healthy lifestyle.