Effect of ascorbic acid and green tea on endogenous formation of N-nitrosodimethylamine and N-nitrosopiperidine in humans

Many constituents present in the human diet may inhibit endogenous formatio n of N-nitroso compounds (NOC). Studies with human volunteers showed inhibi ting effects of intake of ascorbic acid and green tea consumption on nitros ation using the N-nitrosoproline test. The aim of the present study was to evaluate the effects of ascorbic acid and green tea on urinary excretion of carcinogenic N-nitrosodimethylamine (NDMA) and N-nitrosopiperidine (NPIP) in humans. Twenty-five healthy female volunteers consumed a fish meal rich in amines as nitrosatable precursors in combination with intake of nitrate- containing drinking water at the Acceptable Daily Intake level during 7 con secutive days. During 1 week before and after nitrate intake a diet low in nitrate was consumed. Using the same protocol, the effect of two different doses of ascorbic acid (250 mg and 1 g/day) and two different doses of gree n tea (2 g and 4 g/day) on formation of NDMA and NPIP was studied. Mean nit rate excretion in urine significantly increased from control (76 +/- 24) to 167 +/- 25 mg/24 h. Intake of nitrate and fish resulted in a significant i ncrease in mean urinary excretion of NDMA compared with the control weeks: 871 +/- 430 and 640 +/- 277 ng/24 h during days 1-3 and 4-7, respectively, compared with 385 +/- 196 ng/24 h(p < 0.0002). Excretion of NPIP in urine w as not related to nitrate intake and composition of the diet. Intake of 250 mg and 1 g of ascorbic acid per day resulted in a significant decrease in urinary NDMA excretion during days 4-7 (p = 0.0001), but not during days 1- 3, Also, consumption of four cups of green tea per day (2 g) significantly decreased excretion of NDMA during days 4-7 (p = 0.0035), but not during da ys 1-3. Surprisingly, consumption of eight cups of green tea per day (4 g) significantly increased NDMA excretion during days 4-7 (p = 0.0001), again not during days 1-3. This increase is probably a result of catalytic effect s of tea polyphenols on nitrosation, or of another, yet unknown, mechanism. These results suggest that intake of ascorbic acid and moderate consumptio n of green tea can reduce endogenous NDMA formation. (C) 1999 Elsevier Scie nce B.V. All rights reserved.