The availability of dietary nitrate for the endogenous nitrosation of L-proline

Urinary excretion of N-nitrosoproline (NPRO) following ingestion of a high-nitrate salad meal, without then with an oral dose of 500 mg L-proline, was investigated in 16 healthy volunteers. The mean excretion rate following consumption of the high-nitrate meal alone was significantly lower than that measured after ingestion of the salad plus proline. Supplementation of the same meal (plus proline) with vitamin C from dietary sources resulted in a significant decrease in mean urinary NPRO levels in healthy subjects. The nitrosation-inhibiting effect of vitamin C was not affected by an increase in the fat content of the meal. Supplementation of the high-nitrate salad with alcohol or coffee did not affect subsequent urinary NPRO levels. No significant difference was observed in the urinary NPRO concentrations of smokers and nonsmokers after ingestion of high-nitrate salad (with or without vitamin C) plus proline.