Effect of dietary fats on endogenous formation of N-nitrosamines from nitrate in germ-free and conventional rats and rats harbouring a human flora.
Urinary excretion of N-nitrosoproline (NPRO) was measured in groups of four germ-free (GF) and conventional (CV) rats given a purified diet with or without inclusion of 100 g butterfat, coconut oil or maize oil/kg, and drinking water containing 0.235 M-NaNO3. In the CV environment rats given the fat-supplemented diets excreted significantly less NPRO than those on the low-fat diet. No corresponding decrease in NPRO excretion occurred in the GF environment. Nitrate reductase activity was measured in stomach contents and homogenates of stomach tissue from GF and CV rats given the different diets. No activity was detected in any of the contents from GF rats. Nitrate reductase activity was significantly reduced in contents from all the CV rats given the fat-supplemented diets, the effect being most marked in those given butterfat. Activity was much lower in tissue homogenates from GF rats than in those from their CV counterparts, but was not affected by diet in either environment. Groups of four CV rats, or rats harbouring a human faecal flora (HF), were given the purified diet with or without addition of 100 g butterfat or maize oil/kg and drinking water containing 0.235 M-NaNO3. All groups given the fat-supplemented diet with the exception of the HF rats given butterfat. It is concluded that the reduced excretion of NPRO by rats given diets containing fat was mainly due to inhibition of microbial nitrate reductase activity in the foregut. The smaller effect of butterfat on the HF rats accords with earlier findings in human subjects.