Both the measured and calculated estimates of nitrate and nitrite intake discussed above are averages. The primary source of nitrate, for example, is vegetables; both daily vegetable intake and the nitrate content of vegetables vary considerably. Other limitations of the estimates include those inherent in analytic techniques and inaccuracies in food-consumption tables. Nonetheless, the estimates indicate the relative importance of different sources of intake to overall exposure. In particular, they indicate that drinking water is not an important contributor to nitrite exposure and that it contributes substantially to nitrate exposure only in areas of notable contamination.
Infants constitute a special case. Breast-fed infants are exposed to very little nitrate or nitrite, but formula-fed infants can be exposed to nitrate from the water used to prepare their formula. Daily fluid intake of newborn infants has been estimated at 850 mL (ICRP 1975) and 150 mL/kg (Hull and Johnstone 1987).
Ingested nitrate is rapidly absorbed through the small intestine and distributed throughout the body. It is excreted in saliva, sweat, feces, and urine (ECETOC 1988; EPA 1990a). Nitrate excreted in feces is used by fecal microorganisms as a source of nitrogen. Nitrate is reduced to nitrite in areas of the gastrointestinal tract where large numbers of bacteria are found—the mouth, the achlorhydric stomach, the small intestine, and the colon. The amount of nitrite formed depends on the amount of nitrate ingested, the person's nitrate reductase activity, pH, and the number and type of bacteria present and their nitrate reductase activity; but in general, about 5% of ingested nitrate is thought to be converted to nitrite (Stephany and Schuller 1980; Eisenbrand et al.