A new report published this week by the World Cancer Research Fund (WCRF) will show how much our diet needs to change if we are to reduce the risk of cancer. In this week's BMJ, Professor Tim Key from the Cancer Research UK unit at the University of Oxford discusses what would be needed to achieve the report's goals.
The report concludes that obesity increases the risk of cancer of the oesophagus, colorectum, pancreas, breast, endometrium, and kidney. Its goal is for the average body mass index of the population to be between 21 and 23. Yet, mean body mass index in adults in the UK is now about 27 and has not fallen into the target range since the 1940s, says Professor Key. Enormous efforts by individuals, society, and government will therefore be needed to reverse the current trend.
The report also shows that alcohol increases the risk of cancers of the mouth, pharynx, larynx, oesophagus, colorectum, and breast and also causes cirrhosis, which predisposes to liver cancer. It recommends much lower drinking limits than currently advised in Britain, so a substantial shift in drinking habits would be needed to achieve these goals.
Evidence for a protective effect of fruit and vegetables is less convincing. Nevertheless, the report recommends that people should eat at least five portions of vegetables and fruits each day. To meet this individual recommendation, the average consumption in the population would need to be about 7.5 portions a day. The average consumption in Britain is currently about three portions a day, says Professor Key, so the consumption of fruit and vegetables would need to be more than doubled.
The major new conclusion in the report is that red and processed meat convincingly cause colorectal cancer. This is based on results from several studies showing that, on average, people who ate the most red or processed meat had about a 30% increased risk compared with those who ate the least. The report therefore recommends that the average intake of red and processed meat in the population should be no more than 300g each week.
In most regions of the world, total meat consumption is well above this goal. In Britain, the current mean intake is about 970g a week in men and about 550g a week in women, so reaching this goal would require a large reduction in meat consumption.
The report identified several foods and nutrients for which some evidence exists of a beneficial effect on the risk of cancer. For example, foods rich in folate may reduce the risk for cancer of the pancreas, and diets rich in calcium may reduce the risk for colorectal cancer.
Professor Key believes that, while the effects of obesity and excessive alcohol consumption are clear, the goal for intake of fruit and vegetables has financial and environmental implications and requires careful consideration.
It may be better to concentrate efforts on increasing the consumption of plant foods such as cereals and beans, which supply energy and protein and can therefore partially replace meat, he concludes.
These wider questions will be dealt with in the WCRF's policy report, due to be published next year.