Protease inhibitors: possible anticarcinogens in edible seeds

Protease inhibitors, which are common constituents of seeds (rice, beans, and maize) have been shown to inhibit breast, colon, and skin cancers in animal experiments. Epidemiological studies have shown that diets rich in these components (ie seed proteins) decrease the occurrence of prostatic, breast, and colon cancers in man. We have demonstrated that a typical purified inhibitor from soybeans, the Bowman-Birk inhibitor, is excreted in the feces complexed with digestive proteases from the duodenum. This results in greater excretion of proteins, thus effectively decreasing the amount of protein available for digestion. Excess protein in the diet in addition to fats may contribute to increased cancer. Thus, the addition of seeds (chick-peas, beans, etc) to the diet rather than removal of meat or fat offers the opportunity of preventing these cancers, because the protease inhibitors present would effectively remove a portion of the proteins being consumed. Protease inhibitors also act directly as antioxidants in relation to tumor promoters and ionizing radiation. To what extent this contributes to the nutritional effects of eating seeds containing protease inhibitors remains to be determined.