Relationship between ascorbic acid dose and N-nitrosoproline excretion in humans on controlled diets

A logarithmic dose-response relationship between ascorbic acid dose and N-nitrosoproline (NPRO) excretion in humans on a controlled diet was established. Seven healthy males were placed on a low-nitrate, low-ascorbic acid diet for 12 consecutive days and given nitrate on days 3-12 and L-proline on days 4-12, after the nitrate dose. Ascorbic acid was given in increasing amounts with the proline on days 5-10. Urine was analysed quantitatively for nitrate, NPRO and ascorbic acid. Ascorbic acid doses as low as 0.05 mmol reduced NPRO excretion by an average of 6 nmol/day; however, as much as 5.68 mmol ascorbic acid did not return NPRO excretion to levels observed before nitrate and proline were administered. Complete inhibition of endogenous NPRO formation from exogenous precursors requires more than the 2:1 molar ratio of ascorbic acid to nitrite that has been demonstrated in vitro. These data may be useful in interpreting epidemiological studies of nitrate exposure and in making dietary recommendations.