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Proving a preventive role for anything in human cancer is very difficult as cancer can take so long to develop in man. It is also very difficult to rule out all the other varied factors which we know may also have a protective role, in human subjects. So to date most of the evidence indicating that fasting may prevent cancer, is from animals.
Prevention of cancer in animals
There seems to be plenty of evidence in a variety of different animal studies that intermittent fasting, with or without calorie restriction, can prevent cancer.
These data show increased resistance to a variety of cancers after intermittent fasting for only a matter of weeks. The cancers prevented include lymphoma (cancer of the immune system), liver and breast cancer. Intermittent fasting not only prevented animals from developing cancer, but those with cancer survived longer than the animals which were not fasted.
Free radicals and cancer
Free radicals have been thought to be associated with the development of various types of cancer for a long time. All kinds of substances have been linked to the production of free radicals in our bodies – from environmental pollutants to radiation and chemicals. The antidote to free radicals is antioxidants which ‘mop’ them up and stop them being harmful. We produce enzymes which are natural antioxidants and some foods and vitamins (such as vitamin E) are thought to have antioxidant properties. But as we age we produce increasing quantities of free radicals and become less able to neutralize them. This may partly account for why cancer is more common in older people. In the animal fasting studies there was an increase in antioxidant enzymes as well as reduction in free radicals.
Is there likely to be a benefit in humans?
This gives hope that fasting may be able to prevent cancer in humans. Aside from reduction in free radicals and antioxidants in animals, fasting has been shown to have other anti-cancer effects in humans. It has been shown to have an effect on levels of a hormone called Insulin-like Growth Factor (IGF-1) which at high levels has been linked with a number of cancers.