:: Eat a variety of vegetables and fruits every day: eat at least five portions daily. Limit your intake of foods containing fats from animal sources

Diet and nutritional factors commenced to be the focus of serious attention in the aetiology of cancer from the 1940s onwards. Initially dealing with the effect of feeding specific diets to animals receiving chemical carcinogens, research turned to the potential of associations with human cancer risk. Initially this was conducted through international comparisons of estimated national per capita food intake data with cancer mortality rates. It was consistently found that there were very strong correlations in these data, particularly with dietary fat intake and breast cancer. As dietary assessment methods became better, and certain methodological difficulties were identified and overcome, the science of Nutritional Epidemiology emerged.

Doll and Peto estimated that somewhere between 10% and 70% of all cancer deaths were associated with dietary and nutritional practices, with the best estimate around 30%. In 1983, the United States Academy of Science concluded that after tobacco smoking, diet and nutrition was the single most important cause of cancer. Since then, the epidemiological search has been to improve knowledge of the exact relationships between food and nutrition and cancer risk and to identify associations with particular components of diet and determine the best intervention strategy.

Initially much attention focused on intake of fat in the diet, particularly from animal sources. Although the results from ecological studies and data from animal experiments were very strong regarding this association, findings from retrospective and prospective epidemiological studies have been null particularly regarding the association with the breast cancer and colorectal cancer.

A number of epidemiological studies indicate a protective effect of higher intakes of vegetables and fruit on the risk of a wide variety of cancers, in particular oesophagus, stomach, colon, rectum and pancreas. A higher consumption of vegetables and fruits has been associated with a reduced risk of cancer at various sites in several studies from Europe, mostly using a case-control design. The relation is however less consistent in data of several cohort studies from North America. If any, the association was apparently most marked for epithelial cancers, in particular those of the alimentary and respiratory tract, although such an association is weak to non-existent for hormone-related cancers.

Cereals with high fiber content and whole-grain cereals have been associated to lower risk of colorectal cancer and other digestive tract in a few European studies. However, recent large cohort and intervention studies are not supporting this association.

Lower rates of many forms of cancer reported in southern European regions, like in Southern Europe, have been attributed to a diet lower in fats from animal sources, and meats, and higher in fish, olive oil, vegetables and fruits, grains, and moderate alcohol consumption. While a link has been suggested, this has not yet been proved convincingly.

The association with reduced risk of cancer exists for a wide variety of vegetables and fruits. There also exists increasing evidence that consumption of higher levels is also beneficial for other chronic diseases. Vegetables and fruits contain a large number of potentially anticarcinogenic agents, with complementary and overlapping mechanisms of action. However, the exact molecule(s) in vegetables and fruits which confers this protection is unknown and the exact mechanism of action is unknown. Insight into the mechanisms of action is only incomplete, but this is not required for public health recommendations. It is in any case not possible to recommend dietary supplementation with vitamins and minerals to reduce cancer risk based on the evidence currently available.

Nonetheless, it is difficult to be precise about the advisable quantity of fruits and vegetables and it is difficult to imagine the successful implementation of a randomised trial of increased consumption of fruits and vegetables. The best available evidence comes from observational studies and the search continues for the molecule(s) in fruits and vegetables responsible for the apparent protection.

Fruits and vegetables should be taken with each meal whenever possible, and systematically replace snacks in between meals. In line with World Health Organisation (WHO) and United States recommendations, ‘Five-a-day' (minimum 400 gr/day, i.e. 2 pieces of fruit and 200 gr of vegetables) is advocated in this campaign, which could lead to a reduction in cancer risk. Particular attention regarding changing nutritional practices needs to be paid to the countries of central and Eastern Europe, where rapid changes in dietary patterns have been shown to have had a rapid, and positive influence, on death rates from chronic disease.