Whereas various books announce with great media support the protective virtues of such or such diet or of such or such food with respect to cancers, reports resulting from various institutions state their growing reserve about the intake of certain foods which were until now recognised as protective. Which one is it?
Can we understand these divergences of opinion from which contradictions result, either because some observations are too general or on the contrary because of a too compartmentalised vision of the issue?
A good understanding of the relationship between nutrition and cancer not only requires a knowledge of the functionality of the considered nutrients, but also the integration of their possible function in "the natural history of cancer".
Nevertheless, let us recall that carcinogenesis proceeds in several stages, namely the successive phases of initiation, promotion, tumour growth and progression.
Benefits of fruit and vegetables
- Do fruit and vegetables protect against cancers? Yes. Very convincing studies justifie the nutritional recommendations in the case of the higher air-digestive ways, stomach and lung cancers.
- On the other hand, regarding other cancers, the observations are vague and inconclusive.
- Thus, the results of a meta-analysis (Riboli and Norat, 2003) show that while fruit highly reduce the risk of gastric, larynx, bladder, lung and oesophagus cancers, they have a lesser impact on the risk of breast and colon cancers.
- These last two cancers seem prevented much better by the consumption of vegetables.
- Thus, there are specific effects: cancers, the risks of which are reduced by fruit consumption are cancers where environmental genotoxic carcinogens (smoking, alcohol acetaldhyde) or infectious (pylori helicobacter) attack directly the epithelial cells and where various antioxidants (rich in the raw eaten fruits) find their best role.
These results were confirmed in a multicentric prospective study, EPIC (Gonzales and Riboli, 2006), and through experimentation based on the results of the SUVIMAX study where supplementation in antioxidants only profited to men who, certainly, presented antioxidant plasma levels weaker than those of women, but were more affected than the women by cancers whose risks were modified by antioxidants, namely those related to smoking and alcohol.
- In vegetables which we usually consume cooked, certain thermolabile antioxidants will lose some of their effectiveness.
Garlic, onions, cabbages, chicory
- However, vegetables contain other molecules, in particular sulfur molecules of allyls, allium (garlic and onion), isothiocyanates of the cruciferea (the entire family of cabbages and chicory) implied in more complex metabolic ways of detoxification, of antibiotic and/or anti-proliferation activity.
- These molecules appear to play a part in the risk reduction of colon and stomach cancers.
Fruit and vegetables fibres
- The fibres seem to take part in the reduction of risk of colon cancer.
- The implied mechanisms are multiple: elimination of carcinogens using physical methods (adsorption), influence on the metabolism of the bile acids (inhibition of the production of mutagen secondary bile acids), effect on the colic flora.
- An abundant consumption of fruit and vegetables characterises a rather poor calorific intake, therefore less likely to cause overweight or obesity, major risk factors of many cancers.
Avoid being overweight
Putting on weight represents a risk factor, but food is not the only cause of overweight and plays rather an indirect part.
In colorectal cancer
- In colorectal cancer, the increase in fat tissue, and in particular visceral obesity, represents a risk factor by the constitution of a metabolic syndrome , which, in addition to the deterioration of the lipidic parameters, is accompanied by the synthesis of a growth factor acting on the proliferation of tumoral cells, the IGF-1.
- The fibres of wholemeal cereals take part in the reduction of the risk while conflicting with the appearance of insulin-resistance.
- Concerning hormone-dependant cancers ( breat, endometrium, ovary for woman, prostate for man), the initiating genotoxic factors remain not well known.
- The most convincing risk factors for these cancers rest on anthropometric characteristics.
- Thus, putting on weight, overweight and obesity increase the risk of endometrial cancer and of breast cancer for menopausal woman (WCRF, 2007).
- The mechanistic assumption rests on the endogenous synthesis of estrogens starting from the steroids and the aromatase of adipose tissue, estrogens which support the proliferation of the hormone-dependant female tumours.
- The constitution of a metabolic syndrome characterized by a visceral obesity is also accompanied by the synthesis of IGF-1. In the same way, the SHBG (sex hormone binding globulin, less active binding protein) is decreased in the event of a metabolic syndrome, releasing consequently the sex hormones which act like as many growth factors.
- An effect which is related to the phase of promotion and tumour growth.
Other implied variable: height and weight
Quite clearly, food cannot be held as the only cause of overweight and obesity, since a genetic component and physical activity are important too.
- So the modification of the risk by food or nutrients likely to take part in putting on weight, such as lipids, was more difficult to highlight and their influence on the risk seemed modest.
- However, being tall in an adult seems to constitute a realistic risk factor for breast, ovary (WCRF, 2007), and prostate cancer.
- Frequent cancer incidence among women and men is weaker in people of average weight and height.
- However, adult height reflects the effect of a whole of determinants (genetic, environmental, hormonal), among which nutritional factors (for example, a contribution rich in energy and/or proteins stimulate the hormone growth and this induces the hepatic synthesis of IGF-1). This effect of height suggests that these factors intervene early in life.
In the same way, we know that food will stimulate in the young adolescent an early menstruation, which represents a risk factor for breast cancer.
We also thinks that an in-utero programming/impregnation can induce a childhood obesity.
This is to say that to establish in an early way dietary habits making it possible to maintain the BMI (between 18 and 25) is important and compromises the health of the adult. However, even in adulthood, a not very dense energy supply and a sufficient physical activity remain important to avoid putting on weight, in particular for menopausal woman, to avoid a risk of breast cancer and of endometrium.
Errors to avoid
If there no food is to be banished - because frustration and the loss of cultural identity can also be detrimental, there are foods which one should eat with moderation.
Red meat and cured meats
Thus, because red meat and cured meats increase in a noticeable way the risk of colon cancer, it is appropriate to have instead sea products, since epidemiologic studies concluded to a reduction of the risk of cancers of the colon and prostate associated with fish consumption.
High energy density food
We must also moderate the food intake of high energy density (fats, and mainly, fats and sweets) which will facilitate putting on weight and thus will take part in the risk of many cancers.
- Sweet drinks are also accused, in particular in the development of childhood obesity, because they are often over-consumed in large amounts.
- These practices of moderation are to be adopted at an early age, within the family, through education, in order to create favourable food habits which will last.
What about food supplements?
A balanced and diversified diet proves to be sufficient to ensure a viable cancer prevention. Food supplements are thus not necessary.
- On the contrary, a supplement could create an imbalance by bringing an additional and excessive quantity of only one molecule.
- For example, the studies of intervention which used only one molecule (B-carotene) with supra-nutritional amount had a contrary effect than the one expected: an increase in lung cancer for smokers or workmen having been exposed to asbestos; when the aim was to protect them.
- Several results of the studies on supplements may be worrying since antioxidants used on subjects suspected of hosting already initiated cells, on their way to carcinogenesis, can push these cells towards the stage of a tumour.
- In certain cases, intervention studies could demonstrate that one micronutriment was protective. It is the case of calcium in the case of colorectal cancer: several studies show a reduction in the risk of progression of a small adenoma towards a larger adenoma which can be regarded as a pre-cancer status in colonic cancer.
- Nevertheless, food supply, in particular in the form of dairy products, appears to be sufficient. However, moderation is also essential since dairy products are risk factors for prostate cancer.
- Food represents only one of the factors of carcinogenesis and cannot be the only cause in the appearance of cancers. Even if there are certain specificities in the nutritional prevention of cancers (calcium and colorectal cancer, alliaceous and gastric cancer), it would be illusory to be focused on only one nutrient or food.
All eating habits must be taken into account, to privilege food of plant origin and to maintain one's weight within normal guidelines.
- Beyond dietary habits, having a regular physical activity avoids overweight and the constitution of a metabolic syndrome.
- If these practices are present from early childhood, they will be maintained more easily during adulthood, and will be more effective.
- However, to adopt them at adulthood remains useful because they can always delay the beginning and the progression of a tumour.
- These same preventive measures will help the cancer patient to prolong and/or maintain recovery periods.
World cancer research fund/American Institute for cancer Research. Food, nutriton, physical activity and the prevention of cancer: A global perspective, Washington DC, AICR, 2007.