Obesity rates in the UK are soaring. The proportion of obese adults rose from approximately 15% in 1993 to 24% in 2004, and almost two thirds of adults are considered either overweight or obese. Worryingly, around one in four children between the ages of 11 and 15 years is obese.¹

It is widely accepted that overweight and obesity are associated with a number of cancers,² but the extent of the increased risk is only now becoming clear.

The World Cancer Research Fund (WCRF), in a report entitled Food, Nutrition, Physical Activity and the Prevention of Cancer, concludes that there is convincing evidence that being overweight increases the risk of cancers of the colorectum, pancreas, endometrium and kidney, and adenocarcinoma of the oesophagus, and of breast cancer after the menopause.³

As part of the 5-year study that examined 20 cancer sites, the WCRF commissioned a series of systematic literature reviews, which encompassed epidemiological and experimental studies and were carried out to a protocol devised by a methodology task force. A panel of experts then assessed the findings, grading them 'convincing', 'probable' and 'limited', according to the likelihood that risk is modified.

The report, which updates one published in 1997, also considers the role of some components of diet in modifying cancer risk. The study found the evidence for the cancer-protective effect of a diet rich in fruit and vegetables less compelling than previously - it was judged probable rather than convincing. However, the report's recommendation endorses current advice to eat at least five portions of vegetables or fruit each day.

The conclusions about red meats, particularly processed meats, are clearer, and evidence that they cause colorectal cancer is considered convincing. This is a change from the previous report, which concluded that the link was only probable. Consuming 100g of red meat per day could mean an increased risk of colorectal cancer of approximately 30%.

Consumption of alcohol, it is found, raises the risk of cancers of the mouth, pharynx and larynx, oesophagus, colorectum in men, and breast in women both pre- and post-menopause. It is also considered a probable cause of liver cancer and, in women, of colorectal cancer. The report recommends that women should consume no more than one drink per day, while the limit for men is two drinks per day.

The question of how overweight and obesity affects cancer risk in a particular patient group was considered as part of the Million Women Study.⁴ This study recruited 1.2 million women aged 50 to 64 years and followed them up for an average of 5.4 years for cancer incidence and seven years for cancer mortality. Over the period of the study there were 45,037 new diagnoses of cancer and 17,203 deaths from the disease.

Increasing BMI was associated with a significant increased risk for 10 of the 17 cancers examined.

The increased risk per 10 unit increase in BMI for the following cancers was: endometrium, 2.89 (95% CI, 2.62-3.18); oesophagus, 2.38 (1.59-3.56); kidney, 1.53 (1.27-1.84); pancreas, 1.24 (1.03-1.48); ovary, 1.14 (1.03-1.27); leukaemia 1.50 (1.23-1.83); multiple myeloma, 1.31 (1.04-1.65); and non-Hodgkin's lymphoma, 1.17 (1.03-1.34). For breast cancer in postmenopausal women the increased risk is 1.40 (1.31-1.49), and for bowel cancer in premenopausal women 1.61 (1.05-2.48).

In postmenopausal women half of all cases of adenocarcinoma of the oesophagus and endometrial cancer can be attributed to overweight or obesity, while 5% of all cancers (6,000 annually) are attributable to these causes, the researchers conclude.

These reports have helped to clarify the links between diet, obesity and cancer, but further research is needed into the mechanism by which obesity leads to tumour formation and progression, and into the effect that particular foods have on cancer risk. Perhaps the greatest challenge these studies present, however, lies in achieving dietary changes and reducing alcohol consumption in the face of current trends.

The GP's role in meeting this challenge is vital, and patients should be advised to eat a diet rich in vegetables and fruit and to consume less red and processed meat and alcohol. The aim should be a diet that is practical and affordable but that also helps to achieve and maintain an ideal weight. It is particularly important to target parents and children with these messages before dietary habits become established.