Abstract
We have postulated that bacteria can produce carcinogens from the diet or from intestinal secretions. The gut bacterial flora are dependent on the diet and the diet therefore controls both the supply of substrate and the flora acting on it; thus this would explain the correlation between diet and the incidence of colon cancer. The most likely substrates are the bile acids. The fecal concentration of bile acids is dependent on the amount of dietary fat; people living in areas with a high incidence of colon cancer have higher fecal concentrations of bile acids than do people living in low incidence areas. Their fecal bile acids are also more highly degraded. We have postulated four types of desaturation reaction needed to produce a polycyclic aromatic hydrocarbon from the bile acid nucleus; all four have been demonstrated using human gut bacteria. The organisms carrying out the reactions are much more numerous in feces of high incidence populations than that of low incidence populations. Studies on individuals show that cancer patients have both high fecal bile acid concentrations and bacteria able to desaturate the bile acid nucleus; controls had either or neither of these properties but very rarely both.
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