Firstly, can it really be said with confidence that if you eat much less, or much more, of a particular foodstuff - or combination of foodstuffs - you will become immune or less at risk to this or that cancer? To begin with, it has been proved beyond doubt that cancer can be prevented in experimental animals through dietary changes. In particular, reduced energy (kilojoule) and fat intakes and increased consumption of fibre- rich foods and certain vitamins have been found to inhibit the development of cancer. The question is, can these findings be appliedto human beings?
Here there are two very important questions: Firstly, to what extent is there a link between human diet and cancer, and secondly, how drastic are the changes we have to make to our eating patterns in order to bring about a significant drop in cancer development?
The first question is possibly easier to answer than the second. Researchers are now convinced that, in dietary terms, humans share the same cancer risks as animals. Simply put, these are:
- eating too much food in general and too much fat in particular
- not eating enough of the fibre contained in grains, legumes (beans, peas, lentils), fruit and vegetables
- low intakes of certain vitamins
The fat, vitamin and fibre factors
A high fat intake leads to a high concentration of faecal bile acids which can, under certain circumstances, act as carcinogens (cancer- producing substances). A low fibre intake results in a high faecal pH value, which favours the development of carcinogens; it also causes waste to take longer to move through the bowel, allowing time for cancer-causing substances to develop and take effect. On the other hand, a low intake of certain vitamins has a negative effect on the mechanisms which combat damaging reactions in the body's cells.
High fat diets - especially those high in saturated fats - have been linked to cancers of the breast, prostate and colon-rectum and low fibre diets to the latter in particular. A diet low in vitamin C is associated with gastric cancer, while one that is deficient in vitamin A seems to create favourable conditions for prostate cancer; a lack of vitamin D is linked to cancer of the colon. The mechanisms involved in all these processes are far from clear and much research still has to be done. (See also The Cabbage Patch Diet on page 187.)
The challenge to change
Authorities on cancer such as the US National Cancer Institute, the 1988 US Surgeon-General's Report and the 1989 Report of the UK COMA (Committee on Medical Aspects of Food Policy) have laid down guidelines for dietary changes which they regard as vital in the fight against cancer. Firstly, they maintain that fat should supply only 25-30% - instead of the present 400/a - of our total food intake. In effect, this reduction by a third or so would bring our fat consumption to the level of the poorest people in the big cities in the 1920s! No fall in fat intake has occurred recently in Western populations.
For a marked fall, there would have to be a big increase in the intake of carbohydrate, or starchy, foods. But as mentioned above, our bread consumption is low - 120-150g or 3 to 4 slices daily - and possibly decreasing. Why has it fallen? Firstly, there has been a marked increase in the consumption of more 'palatable' foods such as dairy produce, meat, fish and eggs and secondly, the modern era's obsession with slimness has led to the (erroneous) belief that bread is specially fattening. It is invariably one of the first foods to be restricted or cut out totally in any attempts at weight loss.
In practical terms, to reduce our fat intake to the recommended levels, the average person would have to eat a good 50% less of all visible fat - butter, margarine, spreads and obvious fat on meat - than at present. The realization that changes in this area must be large, not trivial, has been underlined by recent studies which show that in the case of breast cancer, only a very low fat intake which supplies as little as 25% of energy (50-60g fat daily), is likely to give protection.
What about fibre? The experts recommend that it be increased from the present 10-15g to 25-30g or more daily, through a higher consumption of fibre-containing foods, cereal products (wholewheat bread, brown rice and unrefined breakfast cereals), legumes, vegetables and fruit. This means adopting a diet similar to that of strict vegetarians. In the USA, surveys show that only half of the population eat one serving of fruit and one serving of 'garden' vegetables on any one day - yet four or five helpings arerecommended. The fact of the matter is that in most countries, there have been no significant increases in the amounts of plant foods eaten daily.
The picture is much the same when it comes to intakes of vitamins and mineral salts. Admittedly, pill manufacturers are doing a roaring trade, but as any doctor will tell you, that is not the way to get your vitamins. Nature has provided us with an abundance of fruit and vegetables which, apart from being excellent sources of vitamins and fibre, are a good deal lighter on the pocket than the packaged variety. In addition, some vegetables have been found to be particularly effective in fighting diet-related cancers because of their high concentrations of certain vitamins.