Formation of N-(Nitrosomethyl)urea in stomachs of experimental pigs and human volunteers given fish sauce in vivo.

N-(Nitrosomethyl)urea (NMU) was characterized in carcinogenic nitrosated fish sauce recently (Deng et al. J. Agric. Food Chem. 1998, 46, 202-205; Biomed. Environ. Sci. 1999, 12, 54-61). To study the possibility of intragastric synthesis of NMU, experimental mini-pigs surgically fitted with a flexible stomach cannula and human volunteers were used. Fish sauce samples (20-30 mL) and nitrite were injected into the gastric lumen through the cannula for pigs or taken orally for human volunteers. Gastric juice samples were taken out 30 min later. Concentration of NMU in condensed extracts of these samples was analyzed with HPLC-photohydrolysis-pyrolysis-thermal energy analyzer. Results showed that there was formation of NMU in the gastric lumen of both models in vivo and that the formation of NMU was nitrite- and pH-dependent. NMU was also detectable in the condensed extract of 100 mL of a mixture of pooled fasting human gastric juice samples and fish sauce sample (9:1, v/v) after treatment with 500 micromol/L of nitrite in vitro. In conclusion, there is intragastric formation of NMU, even at natural amounts of nitrite.