These compounds have been linked to colon and rectal cancers. The substance in soy, genistein, is an isoflavone that appears to protect against cancer. You can increase your isoflavone intake with soy milk, tofu, and the textured soy protein used in veggie burgers. Garlic extract has slowed the growth of breast, skin and colon cancers in mice. It seems the raw version of onions and garlic is most useful in cancer protection.
More research is needed to determine if supplements are as effective as the food source.
They contain a potent anti-oxidant, capsaicin, which interferes with the union of nitrites and amines. These nitrosamines are linked to stomach cancer. Also, capsaicin may keep the carcinogens in cigarette smoke from causing the genetic damage that can lead to lung cancer. Though not a diet recommendation, it can't go unsaid that tobacco is the chief cause of lung cancer. This information is not intended to be a substitute for professional medical advice.