New studies at Johns Hopkins University and Tsukuba University in Japan show that inducing special body enzymes, which neutralise and dispose of cancer-causing substances is likely to be an effective way to lower risk of cancer.
Researchers will be trying to develop drugs that could maximise this effect, but the necessary testing for long-term effectiveness and safety means such plans lie in the future.
Meanwhile, research shows that eating vegetables is one simple way to boost these protective enzymes.
Several years ago, scientists discovered that special body enzymes, called phase II enzymes, could detoxify cancer-causing substances before this can occur.
Dr. Paul Talalay made headlines with his discovery that a nutritional substance in broccoli, called sulforaphane, could raise levels of these protective enzymes, and other research has identified many more such substances.
In their new research, published in the Proceedings of the National Academy of Sciences, Dr. Talalay and colleagues focused on how cells control activity of the phase II enzymes.
The scientists found a body protein working like a switch to regulate enzyme levels.
When that "switch" was turned off, mice exposed to a cancer-causing substance developed far more tumours than those with a functioning "switch."
When mice were given a drug that has been shown to raise protective phase II enzymes, as long as the control "switch" was functioning to allow increased enzyme production, the number of tumours that developed after exposure to the carcinogen was cut in half.
Scientists believe that protective substances in food work by sparking cells to release this controlling protein (essentially turning on the "switch"), prompting the production of phase II enzymes, which then detoxify carcinogens and prevent cell damage that could have led to cancer.
Although broccoli got the initial publicity, the entire family of cruciferous vegetables contains a variety of related substances that stimulate phase II enzymes.
Other cruciferous vegetables include cauliflower, Brussels sprouts, cabbage, kale, chard, bok choy, collards and radishes.
The protective substances they contain exist whether the vegetables are eaten cooked or raw.
The health-promoting benefits of garlic seem to be due to phytochemicals called allyl sulfides, another group that can boost phase II enzymes.
Onions are also a source of these substances.
Vegetables and fruits supply a whole range of nutrients and phytochemicals that seem to help protect against cancer.
Ellagic acid from berries, grapes and nuts boosts phase II enzymes, as do phenols, which are found in berries and citrus fruits as well as tea.
Scientists say that most likely, protective effects come from vitamins, enzyme-boosting phytochemicals and perhaps other not-yet-identified substances in fruits and vegetables, all working together.
Source: American Institute for Cancer Research