:: Avoid Obesity

In this section, the adverse effect of obesity (or being overweight) and the protective effect of exercise on cancer risk are summarised. It is based on the evidence from a comprehensive review on weight control and physical activity published by the International Agency for Research on Cancer. Because of the relationship between obesity and physical activity it is important to separate the effects of the two.

Obesity.
Obesity is an established and major cause of morbidity and mortality. It is the largest risk factor for chronic disease in Western countries after smoking, increasing in particular the risk for diabetes, cardiovascular disease and cancer. Most countries in Europe have seen the prevalence of obesity (defined as a body mass index, BMI, of ³30 kg/m2) rapidly increase over the years figure 1. The prevalence can range from less than 10% in France to about 20% in the UK and Germany and higher in some Central European countries (>30%). It is associated with an increased risk of cancer at several sites and the evidence is clear for cancer of the colon, breast (post-menopausal), endometrium, kidney and oesophagus (adenocarcinoma). There is still an excess risk after allowing for several factors such as physical activity. Overweight (BMI of 25-29 kg/m2) is similarly associated with these cancers though the effect on risk will be less.

The risk of colon cancer increases approximately linearly with increasing BMI between 23 and 30 kg/m2. Compared to having a BMI of <23 kg/m2 there is a about a 50-100% increase in risk in people with a BMI ³30 kg/m2. The association appears to be greater in men than in women. For example, in the American Cancer Society cohort study of about 1.2 million people, the mortality ratio for colon cancer in those with a BMI of ³30 kg/m2 was 1.75 in men and 1.25 in women compared to those with a BMI of <25 kg/m2. The evidence also suggests that the risk does not depend on whether the person had been overweight in early adulthood or later in life.

Over 100 studies have consistently shown a modest increased risk of breast cancer in postmenopausal women with a high body weight. On average, epidemiological studies have shown an increase in breast cancer risk above a BMI of 24 kg/m2. A pooled analysis of 8 cohort studies of about 340.000 women showed an increase in risk of 30% in women with a BMI ³28 kg/m2 compared to those with a BMI of <21 kg/m. Factors that have been shown to attenuate the association between obesity and breast cancer include family history (heavier women with a family history have a higher risk than similar women without a family history) and the use of hormone replacement therapy (the risk of breast cancer associated with obesity is greater in women who had never used HRT). In contrast, among premenopausal women obesity is not associated with an increase in risk.

There is consistent evidence that being overweight is associated with increased risk for endometrial cancer. Women with a BMI of >25 kg/m2 have a two to three fold increase in risk. Although limited, the evidence suggests that the risk is similar in pre- and post-menopausal women. There is evidence that the risk is greater for upper-body obesity.

The association between kidney (renal cell) cancer and BMI is also well established and is independent of blood pressure. Individuals with a BMI of ³30 kg/m2 have a two to three-fold increase in risk compared to those below 25 kg/m2. The effect is similar in men and women. There is a similarly strong association between being overweight and adenocarcinoma of the lower oesophagus and the gastric cardia; about two-fold increase in risk in individuals with a BMI of >25 kg/m2. A modest association has been reported in a pooled analysis of BMI and thyroid cancer (relative risk in those in the highest tertile of BMI was 1.2 in women and 1.5 in men). The evidence on obesity and gallbladder cancer is limited but there is a suggestion of almost a two-fold increase in risk, especially in women.

In Western Europe, it has been estimated that being overweight or obese accounts for approximately 11% of all colon cancers, 9% of breast cancers, 39% of endometrial cancers, 37% of oesophageal adenocarcinomas, 25% of renal cell cancer and 24% of gallbladder cancer.