N° 18 | December 2007

Eat mosty foods of a plant origin

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The power of the WCRF/AICR Second Expert Report comes not only from its rigorous methodology and comprehensive evidence base, but also from the integrated approach taken in coming to judgement and making personal recommendations and public health goals.

The recommendation to eat mostly foods of plant origin is one example of this approach. Judgement of the evidence for plant-based foods shows that eating more of particular types protects against cancers of various sites.

Foods containing dietary fibre are all plant foods and these probably protect against colorectal cancer. Non-starchy vegetables and also fruits probably protect against cancers of the mouth, pharynx and larynx, the oesophagus, and the stomach. Fruits also probably protect against lung cancer. Allium vegetables in general probably protect against stomach cancer, although garlic specifically was judged to probably protect against colorectal cancer.

Foods containing folate, most but not all of plant origin, probably protect against cancer of the pancreas. Foods of plant origin containing carotenoids probably protect against cancers of the mouth, pharynx and larynx, and lung cancer. Foods containing two specific carotenoids probably have protective effects against cancers of two sites: those containing beta-carotene and the oesophagus; and those containing lycopene and the prostate. Foods containing vitamin C probably protect against oesophageal cancer, and those containing selenium, some but not all of which are of plant origin, against prostate cancer.

Vegetables and fruits contain a wide range of vitamins, minerals, and biologically active compounds such as phytochemicals that may protect against cancer. Vitamins C and E can donate electrons to free radicals and block their damaging activity. And compounds such as isothiocyantes and polyphenols can activate the signalling pathways that influence the antioxidant response element, and upregulation of the expression of detoxifying enzymes. Fibre in plant-based foods is thought to have many benefits, including helping to speed up the passage of food through the gut. But it is clear that evidence of benefit from foods containing particular nutrients, cannot reliably predict the effects of supplements of those nutrients.

While there is evidence that fruits and vegetables probably protect against cancer, the evidence does not appear as strong as it did 10 years ago when the first WCRF/AIRC Expert Report rated the protective effect of these foods as convincing. But because of the integrated approach taken in making the recommendations, the evidence overall for increasing the amounts of plant-based foods in diets is just as compelling.

‘Plant-based diets’ give emphasis to those plant foods that are high in nutrients, high in dietary fibre (and so in nonstarch polysaccharides) and low in energy density. Low energy-dense foods probably protect against weight gain, overweight, and obesity. They are high in water and fibre, and provide more bulk in diets. So compared to energy-dense foods, people can eat larger quantities while consuming fewer calories. And by filling up on low energy-dense foods there’s a better chance that a person’s diet will have less high energy-dense foods.

The importance of the link between obesity and cancer is discussed in detail in the next section. It is clear that achieving and maintaining a normal level of body fatness (usually measured as body mass index) throughout life lowers the risk of a number of cancers.

The WCRF/AICR public health goal for the population average consumption of non-starchy vegetables and of fruits is at least 600g daily. This is best made up from a range of various amounts of non-starchy vegetables and fruits of different colours, including tomato-based products and allium vegetables. Relatively unprocessed cereals (grains) and/or pulses (legumes), and other foods that are a natural source of dietary fibre, should contribute to a population average of at least 25g non-starch polysaccharides daily.

The personal recommendations – those for individuals to follow to protect themselves against various cancers – are: eat at least five portions/servings (at least 400g) of a variety of non-starchy vegetables and of fruits every day; eat relatively unprocessed cereals (grains) and/or pulses (legumes) with every meal; limit refined starchy foods; and people who consume starchy roots or tubers as staples should also ensure intake of sufficient non-starchy vegetables, fruits, and pulses.

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