Diet and the risk of cancerBMJ 2007; 335 doi: http://dx.doi.org/10.1136/bmj.39373.676204.BE (Published 01 November 2007) Cite this as: BMJ 2007;335:897
This week, the World Cancer Research Fund (WCRF) launches its second report on diet and cancer through simultaneous conferences in London and Washington. Entitled Food, Nutrition, Physical Activity, and the Prevention of Cancer, the report updates the previous publication from the charity. It is the culmination of five years' work by scientists in nine universities from four countries who have assessed the original research according to a specially developed standardised review protocol.1
The possible influence of diet on the risk of cancer is constantly topical. The subject is important because people can change their diets, and even a moderate effect on risk could prevent several thousand cancers each year in a country the size of the United Kingdom. However, apart from the confirmed adverse effects of alcohol and obesity on the risk for some types of cancer,2 3 4 5 progress in understanding has been slow and the evidence remains confusing.
The conclusions in the WCRF report about the effects of obesity and alcohol are similar to those reached by other expert consultations.1 2 3 4 5 Obesity increases the risk of cancer of the oesophagus, colorectum, pancreas, breast, endometrium, and kidney. The report's public health goal for obesity is for the median body mass index of the population to be between 21 and 23. Mean body mass index in adults in the UK is now about 27.6 Enormous efforts by individuals, society, and government will be needed to reverse the current trend and approach the goal proposed—mean body mass index in the UK has not fallen into the target range since the 1940s. The mean body mass index is also in the overweight range in most other Western countries and is rising rapidly from previously low levels in many developing countries.7
Alcohol increases the risk for cancers of the mouth, pharynx, larynx, oesophagus, colorectum, and breast and also causes cirrhosis, which predisposes to liver cancer. Globally, alcohol consumption has increased in recent decades, with the highest consumption in Europe and North America.7 The WCRF panel made recommendations that men should not drink more than two units of alcohol a day and that women should not consume more than one unit a day. These recommendations are much lower than current government advice in Britain of up to three or four drinks a day for men and two or three for women.8 A substantial shift in drinking habits would be needed to achieve these goals.
The role of fruit and vegetables remains poorly understood. The panel concluded that the evidence that diets rich in vegetables and fruits protect against cancer is overall less compelling than it was in the mid-1990s. They judged that vegetables, fruits, and other foods containing dietary fibre probably protect against several cancers, but that the evidence was not convincing for any specific cancer. Despite the absence of convincing evidence for a protective effect, the panel reiterated the previous recommendation that people should eat at least five portions of vegetables and fruits each day, and pointed out that, to meet this individual recommendation, the average consumption in the population would need to be about 7.5 portions a day. Currently, only a minority of the world's population consumes the recommended five portions of fruit and vegetables a day, and consumption is low in many regions of the developing world.5 The average consumption in Britain is currently about three portions a day,9 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. The previous WCRF report concluded that the evidence for this link was “probable,” and the 1998 Department of Health report concluded that the evidence was “moderate.”2 3 This new conclusion is based mainly on results from prospective observational studies, supported by results from case-control studies. 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 expert panel concluded that substantial evidence existed of a dose-response relation and of plausible mechanisms, thus fulfilling their criteria for the association to be causal. Further epidemiological evidence that may confirm this conclusion should become available within the next few years. On the basis of their conclusion, the panel proposed a public health goal that the average intake of red and processed meat in the population should be no more than 300 g each week. Consumption of meat has been rising in most of the world, and total meat consumption is well above the 300 g per week goal in most regions except Africa and South Asia.5 The current mean intake of red and processed meat in Britain is about 970 g a week in men and about 550 g a week in women,9 so reaching this goal would require a large reduction in meat consumption.
Many challenges remain. Obesity and excessive alcohol intake definitely increase the risk of cancer but are hard to control, while our understanding of whether and how particular foods affect risk is still rudimentary. The report identified several foods and nutrients for which some evidence exists of an 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. Future research should clarify these possible associations. The goal for intake of vegetables and fruits requires careful consideration—achieving the goal may not reduce rates of cancer, whereas the financial and environmental costs could be large. Vegetables and fruits provide only small amounts of energy and protein, so an increase in intake may be largely on top of the rest of the diet. This could be expensive for people, and it has implications for the optimal use of agricultural land, water, pesticides, transport, etc. 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. The production of staple plant foods is usually more efficient than the production of meat, so replacing some meat with plant foods should reduce the demands on the environment.10 These wider questions will be dealt with in the WCRF's policy report, due to be published next year.
Competing interests: TK is a member of the Vegan Society.
Provenance and peer review: Commissioned based on an idea from the author; not externally peer reviewed.