In many regions of the world, overweight has become increasingly common over the last two decades and in some western countries it has reached epidemic dimensions. Overweight and insufficient physical activity are associated with several illnesses, including cardiovascular disease and adult-onset diabetes, and cancer. Limiting weight gain reduces the risk of postmenopausal breast cancer and cancer of the colon, endometrium of the uterus, kidney and adenocarcinoma of the oesophagus. Regular physical activity reduces the risk of breast and colon cancer, and possibly that of endometrial cancer and prostate cancer. Up to one third of tumours of the colon, breast and kidney can be attributed to overweight and insufficient physical activity. These were the main conclusions of a panel of international experts convened at the International Agency for Research on Cancer of the World Health Organization (WHO) in Lyon on 13–20 February 2001. Their full review and evaluation of the relevant scientific information will be published later this year in Volume 6 of the IARC Handbooks of Cancer Prevention.

The widely accepted measure of overweight is provided by the body mass index (BMI), calculated by dividing the body weight (in kg) by the height squared (m2). Thus, an adult who weighs 100 kg and is 2 m tall has a BMI of 25. The World Health Organization considers a BMI of less than 18.5 to be an indicator of underweight, whereas a BMI >25 indicates overweight and >30 obesity. The normal, desirable range is from 18.5 to 25. For an adult of 1 m 75, this weight range would be from 57 to 77 kg.

In Europe, about half the adult population is currently overweight, and urban areas of many developing countries have a similar prevalence. The fundamental causes of the obesity and overweight epidemic are societal, resulting from an environment that promotes sedentary lifestyles and over-consumption of highly-caloric food.

Scientists have estimated that in the European Union, 21 000 cases of colon cancer and 13 000 cases of breast cancer could be avoided annually by maintaining a normal body weight. Moderate endurance-type activity such as brisk walking and cycling, for at least 30 min several days a week is recommended by the IARC group. In addition to reducing the risk of chronic diseases such as coronary heart disease and diabetes, such physical activity would also protect against some cancers.