You are what you eat. This is an old saying which many of us hope is not accurate. Nonetheless, we are all aware that we can adversely affect our health by what we eat. The link between cholesterol and saturated fats and heart disease, for example, has been well publicised and is well supported by clinical trials in the medical journals.
There are also associations between certain foods and cancer. Some foods may be a factor in causing cancer and some may have a role in cancer prevention. Unlike the situations described above, it is much harder to show the relationship between food and common cancers, since eating is only one of a number of activities that form part of our lifestyle and contribute to our environmental exposure. As a result, there is some scientific uncertainty about the role of specific foods. Associations that are difficult to prove may also be difficult to definitely rule out. This allows for all manner of theories relating food to cancer and the bookshops are full of such speculation. You will find outrageous claims, such as curing all types of cancer by just altering what you eat.
Before outlining the current scientific facts on diet and cancer it may be helpful to list some points to consider when evaluating popular books on diet and cancer. First, the fact that a vitamin can be shown to alter cancer cells in a laboratory may not translate to what happens in the body. Second, foods that have the potential to prevent cancer should not be assumed to have any role in curing it once it has developed.
Also, if too much of a food is bad for you, that does not mean it should be completely excluded from the diet. This could leave a gap in nutrition that could be harmful, particularly as the rest of the body needs to be as healthy as possible to fight cancer. And, equally, if a food is good for you, it does not follow that ten times as much is ten times as good. The body will just get rid of what it doesn’t need. This is often the rationale of some unorthodox practitioners who advocate high doses of vitamins and other trace elements. When given in this way they are no longer natural products but drugs, where side effects must be balanced with any benefits. There is no doubt that the body prefers to take these nutrients as part of natural foods rather than as purified supplements. In general, a dietary supplement should only be required if your diet is poor, or you can’t eat, and there is a deficiency in that nutrient.
Fat in the diet has been suggested as a risk factor, particularly for breast and bowel cancer. This was based on studies which showed that countries with a high fat intake per person also had these cancers occurring more often. It has been difficult to show a strong relationship between fat and breast cancer in studies where one group of people is matched to another in all characteristics, except for their fat intake, and the incidence of breast cancer compared in the two groups with different fat intakes. It has also been difficult to show any big impact of fat intake on the behaviour of breast cancer once it has developed. Another confounding factor is whether we are talking of polyunsaturated or saturated fats. It seems that the polyunsaturated fats are more likely to be the culprits. A tantalising observation is that southern Europeans have a high intake of mono-unsaturated fats in the form of olive oil. Their cancer rates are less than other groups with similar or lower overall fat intakes.
Stronger associations have been shown between high fat intake and red meat consumption and the development of cancer of the large bowel. An overall analysis of thirteen studies, though, showed a relationship between overall calorie intake and bowel cancer but was not convincing that the relationship was specifically with fats. Even the association with meat may not be due to the fat content but other substances in meat.
Although less work has been done in prostate cancer, the consumption of animal fat correlates strongly with the death rate from prostate cancer. Again, the type of fat is important since there is no correlation with vegetable fats.
In the 1970s, Burkitt made the observation that diseases such as large bowel cancer were common in Western countries where a lot of refined food was eaten and low in African nations where high-fibre diets were common. It was suggested that fibre protects against cancer of the large bowel. There are simple mechanisms by which this may occur. Fibre traps fluids which would dilute carcinogens in the bowel, or the amount of carcinogen produced may be decreased by the presence of the fibre. Fibre would also increase the bulk in the large bowel anddecrease the transit time through the bowel. Dietary fibre may also be protective against breast cancer by decreasing the processing and re-uptake of oestrogens in the bowel. This may also apply to other hormone-sensitive tumours.
It is difficult to be absolutely certain of the role of fibre because fruit and vegetables, which are a major source of fibre, also contain other substances that may protect against cancer. These are often referred to as micronutrients and include vitamins and trace elements.
The micronutrients from fruit and vegetables can help prevent cancer at all stages of its development. At the initiation stage, where DNA damage occurs, folate and B vitamins participate in DNA repair. Cancer promotion, which means producing greater numbers of the cell with the mutated DNA, can be slowed by agents that affect growth rates. These agents include vitamin A, retinoids and folate. Retinoids and antioxidants also inhibit growth in the progression of a tumour. It is difficult to demonstrate the cancer-preventive effects of micronutrients in fruit and vegetables because there is no way of telling when and if a cancer would have, developed. Studies that monitor people until the development of cancer take many years to arrive at an answer.
Cancer-causing agents in food
Food contaminants or additives, or even the way the food is prepared, may result in taking in carcinogens with the food. Aflatoxins produced by a fungus can contaminate peanuts or maize, resulting in a higher llisk of liver cancer. Fish preserved by salting develops high concentrations of nitrosammes which are associated with causing nasopharyngeal cancer. Cooking practices, such as frying food in hot oil, may lead to the production of carcinogens which are then ingested with food.