Diet should be highlighted because it is a preventive strategy.
Strong associations have been found at an international level between the incidence of breast, colon, prostate and endometrial cancer and the intake per individual of fat. This, together with results from animal studies, led to the suggestion that fat should only provide 30, per cent of our energy. There are, however, many other factors, such as affluence, that could be associated with a country having a high fat intake. When case-control studies were done in breast cancer to look at this specifically, only a weak association was found and even this was not found in cohort studies. In animal studies, polyunsaturated fats were the culprits and it has even been suggested that mono-unsaturated fats are protective.
With large bowel cancer, any association seems to be with animal fat such as that found in red meat. This has been shown in case-controlled studies where there is also an association with total energy intake, that is, general overeating. This is coupled with the suggestion that a sedentary lifestyle may also be to blame.
The association reported in some case-controlled studies between fat and prostate cancer seems to be related to animal fats as well. There seems little association with vegetable fats.
A recommendation to people wanting to reduce their chance of developing cancer would be to limit their intake of animal fats through red meats. Also, they should not be overweight and should exercise regularly.
The association between fibre intake and cancer risk also needs further study. International studies comparing Africa with the West suggest increased dietary fibre is associated with decreased large bowel cancer, breast cancer and several other cancers including oesophageal, head and neck, stomach, prostate, endometrial and ovarian cancers. There are several difficulties with understanding this association.
First, fruit and vegetables may reduce your cancer risk but they contain other substances that may reduce cancer as well as fibre. Second, there are many different types of fibre and not all may be associated with decreased cancer risk. Bran cereal has been found to reduce the development of polyps in individuals with familial polyposis. Wheat bran is associated with decreased risk of bowel cancer in animals.
There may be a difference in the association between fibre and cancers at the top of the large bowel compared with those at the bottom. Large studies looking at bowel cancer risk and dietary fibre intake are still in progress. Dietary fibre appears to reduce the risk of breast cancer independently of fat intake. This may be secondary to the fact that fibre decreases production of some of the sex hormones but soy extracts can inhibit some of the signalling pathways in cancer cells.
In case-controlled studies the association with ovarian cancer is with vegetable fibres but not cereals. A vegetarian diet is also associated with a decreased risk of prostate cancer.
The recommendation for prevention at the current level of knowledge, therefore, would be to include a variety of different sources of fibre in the diet including fruits, vegetables and whole grains.
Vitamins and minerals
Vitamins and minerals also appear to have a part to play in preventing cancer. Part of the goodness of fruit and vegetables can be attributed to them, but it can be difficult to separate from other factors such as vegetable fibre. Specific studies have suggested a link between carotenoids and the prevention of smoking-related cancers, vitamin C and protection from stomach cancer, folate and cancer of the cervix, and calcium and large bowel cancer. Specific clinical trials to test these associations need to be very large and many are ongoing. Trials, to date, have shown mixed results/rln lung cancer, for example, beta carotene and retinol have-not been shown to reduce the incidence of lung cancer in smokers, but high-dose vitamin A has been shown to reduce the occurrence of second primaries after successful treatment of the first lung cancer.
There are certainly good reasonable theories about how vitamins and minerals could protect against cancer. They have been shown to block the action of cancer-causing chemicals. Vitamin A, beta carotene, vitamin C and selenium are antioxidants which can mop up cancer-causing agents. Vitamin A and derivatives, the retinoids, can slow growth and cause cells to differentiate—that is, to change to more mature forms. Finally, it is suggested that vitamins and minerals may affect the immune system but evidence for this is sparse and the subject of ongoing research.
A further difficulty with trials is that the process of developing cancer requires multiple steps over a long time and thus exposure to the dietary factor should be over a long time. It is also very difficult to judge what chemical action the nutrient has had and whether a step in the development of cancer has been blocked.
If vitamins and minerals are important to the body in the doses extracted from food, it does not follow that large doses as tablets or injections will have the same beneficial effects. They will certainly have different side effects. It would seem that the best preventive strategy at present is to eat foods containing the important vitamins and minerals. Although it may seem self-evident, vitamins and minerals used to prevent cancer may not be useful as a treatment for cancer, although much is written in the literature of alternative medicine advocating this. In fact, a developed cancer has progressed beyond the steps where these nutrients are thought to work, so you may be preventing the next cancer but not treating the established cancer.