Urinary Excretion of N-Nitrosamino Acids and Nitrate by Inhabitants of High- and Low-Risk Areas for Esophageal Cancer in Northern China: Endogenous Formation of Nitrosoproline and Its Inhibition by Vitamin C

A total of 238 samples of 24-h urine were collected from inhabitants of high-risk (Lin-xian) and low-risk (Fan-xian) areas for esophageal cancer in northern China, according to three protocols: ( a ) from undosed subjects; ( b ) from subjects who had ingested 100 mg l-proline three times a day 1 h after each meal; and ( c ) from subjects in Lin-xian who had ingested 100 mg ascorbic acid together with 100 mg l-proline three times a day 1 h after each meal. As an index of individual exposure to N -nitroso compounds or their precursors, ingested in food and/or formed endogenously, the levels of four urinary N -nitrosamino acids and nitrate were determined. The amounts of N -nitrosoproline, N -nitrosothiazolidine 4-carboxylic acid, N -nitrososarcosine, and nitrate excreted in the 24-h urine of undosed subjects in Lin-xian were significantly higher than those in Fan-xian, indicating a higher exposure of the inhabitants in the high-risk area to N -nitroso compounds and their precursors. Ingestion of l-proline resulted in a marked increase in urinary N -nitrosoproline levels in inhabitants from both areas, suggesting that endogenous nitrosation may occur to a larger extent when appropriate amine precursors are ingested in foods. Intake of moderate doses of ascorbic acid by high-risk subjects effectively reduced the urinary levels of N -nitrosamino acids to those found in undosed subjects in the low-risk area. If N -nitroso compounds formed in vivo are among the causative factors for esophageal cancer in this area of northern China, ascorbic acid appears to be effective in lowering the body burden of these carcinogenic compounds, thus offering a rational basis for long-term intervention studies in this area.