nitrate or nitrite is unlikely to be the rate-limiting factor. However, people with a low intake of fruits and vegetables, who are at increased risk for many epithelial tumors (including stomach cancer) will receive a relatively greater proportion of nitrate from drinking water than those with high intakes of fruits and vegetables (Steinmetz and Potter, 1991). People with low fruit and vegetable intakes will also have a relatively lower intake of ascorbic acid and other vegetable-derived antioxidants, the proportion of Americans consuming the recommended five servings of fruits and vegetables per day is less than 20% (Lanza et al., 1987), so some overlap between low dietary intake and high water-nitrate concentration can occur. Diets low in fruits and vegetables might or might not be high in nitrosatable amines, but this possibility contributes some uncertainty to the conclusion that exposure to nitrate or nitrite from drinking water is unlikely to be associated with increased human cancer incidence.
Numerous studies have been conducted in laboratory animals to evaluate the reproductive and developmental toxicity of nitrate and nitrite. In general, little evidence of toxicity has been found except at relatively high doses, which also can produce maternal methemoglobinemia. A single study in rats has reported developmental effects of exposures encountered by humans (Markel et al. 1989). But the few studies that have been conducted in humans have yielded no evidence of any reproductive or developmental effects of nitrate or nitrite. EPA (1990a) has reviewed the studies of the reproductive and developmental toxicity of nitrate and nitrite. Several studies reported since the EPA publication are also described here.