Influence of fruit and vegetable juices on the endogenous formation of N-nitrosoproline and N-nitrosothiazolidine-4-carboxylic acid in humans on controlled diets.

Water, green pepper, pineapple, tomato, strawberry, carrot, and celery juices were made 46 mg/100 ml in ascorbic acid by the addition of distilled water or ascorbate. The ability of each juice to inhibit endogenous formation of N-nitrosoamino acids (NAA) in humans was determined in controlled experiments. Sixteen men consumed a standard diet low in nitrate and ascorbic acid for 18 consecutive days. Nitrate (5.24 mmol) and L-proline (4.35 mmol) were given orally on days 3, 5, 7, 9, 11, 13, 15, 17 and 18. On days 3 and 18, L-proline was immediately followed by 100 ml distilled water (positive control). On days 5, 7, 9, 11, 13, 15 and 17, L-proline was immediately followed by 100 ml juice or 46 mg ascorbate in 100 ml distilled water (treatment). Only diet was given in between dosing days to ensure baseline levels of NAA excretion. Urine was collected for 24 h following treatments and analyzed for NAA. Green pepper, pineapple, tomato, strawberry and carrot treatments significantly inhibited N-nitrosoproline (NPRO) formation relative to the positive control. Also, green pepper, pineapple and tomato juices significantly inhibited NPRO formation relative to ascorbic acid alone. Green pepper significantly inhibited N-nitrosothiazolidine-carboxylic acid formation relative to ascorbic acid alone. These data demonstrate that green pepper, tomato, pineapple, strawberry and carrot juice have greater ability to inhibit endogenous nitrosation than would be expected based solely on their ascorbate content.