Nitrate metabolism was investigated in longterm metabolic balance studies on healthy young men. Under conditions of constant low ingestion of nitrate (<180 pumol/day per subject), the amount of nitrate excreted in urine was an average of 4-fold greater than the amount ingested. Balance studies with '5N03-showed that the source of the excess nitrate in urine was the endogenous biosynthesis of nitrate, rather than the emptying of a body pool. Nitrate biosynthesis occurred when nitrate ingestion was high as well as low, and the amounts synthesized appeared to be independent of intake and comparable to the amounts ingested from normal diets. Analysis of the '5N03-data also revealed that half of ingested nitrate was recovered as urinary nitrate. Because nitrate in urine is the net result of (i) intake, (ii) endogenous synthesis, and (iii) metabolic losses, the magnitude of the losses is such that, despite ongoing synthesis, the amount of nitrate in the urine ofpeople consuming most diets will be less than the amount ingested.Nitrosamines are potent animal carcinogens (1). Small amounts ofthese compounds are formed in some foods and in the animal and human body from the reaction ofamines and nitrite. A common precursor to nitrite is nitrate; higher than average levels of nitrate in some environments have been linked to elevated incidences of human cancer (2, 3). The environment is not the only source of nitrate: metabolic balance studies performed early in this century (4) and preliminary studies from our laboratory (5) corn starch dessert, and protein-free cookies (7). Mean energy intake was 46 kcal (1 cal = 4.184 J) per kg of body weight, with an average of-total calories from protein of 7.0%, from carbohydrates of 53%, and from fat of 40%.Another group of eight young men (mean age, 22; mean weight, 72 kg) participated in a second study lasting 56 days. During wk 1 of the study, the subjects consumed their freechoice diet. For the next 4 wk, they consumed the basal formula to which milk protein had been added. In addition, twice daily they consumed an omelette made of dried egg, water, and salt. Carbonated and sucrose beverages, a cornstarch dessert, and protein-free cookies provided the remainder of calories in the diet.For the final 3 wk of the study, 25 g of standardized wheat bran preparation (American Association ofCereal Chemists certified food grade wheat bran) was added to the basal formula. The diet otherwise was identical to that of the previous 4 wk. Daily protein intake was higher in this second study-1.5 g of protein per kg of body weight. Mean daily calories consisted of 14% from protein, 45% from carbohydrate, and 41% from fat.A third study involved another six young men (mean age, 24; mean weight, 79 kg) and lasted for 1 wk. The subjects consumed the high protein soy, milk, and egg diet utilized in the second study, with no addition of bran. On the penultimate morning of this study, after an overnight 10-hr fast, each subject was given a 300-mg (3500 jmol) dose of Na'5NO3 (.99% N = 15N) in ...