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Avoiding Weight Gain Has A Major Role In Cancer Prevention

There is now enough evidence to link adiposity to certain cancers and cardiovascular disease. The remaining uncertainty, however, is the extent of excess weight that constitutes an increase in cancer risk.

Obesity is linked to colon, breast (in postmenopausal women), endometrium, kidney, and oesophagus, point out Dr Franca Bianchini and colleagues at the International Agency for Research on Cancer, Unit of Chemoprevention, Lyon, France. There are plausible mechanistic explanations for most of these associations.

The researchers were strict in selections for a review of the present evidence linking cancers and body mass index (BMI): at least 200 cases for breast and colorectal cancers; 100 cases for colon adenomas, endometrial, and prostate cancers; and 50 cases for cancers at other organ sites. Only those studies where results were reported for categories of BMI were considered.

As they point out, recent decades have seen major changes in lifestyle, leading to steady increases in average body weight and indices of obesity. Many countries have produced surveys on trends in the prevalence of obesity are available for many countries.

There are large differences in the population prevalence of obesity - whether separated by country or by population subgroups. Obesity is more prevalent in high-income countries than in low-income countries like Brazil and China, and it is more widespread in urban populations, and in some racial subgroups.

The MONICA project - a large international study for the monitoring of risk factors for cardiovascular disease, led by the World Health Organization - showed a prevalence of obesity above 20 percent in parts of western Europe and the United States during the 1990s, and it was as high as 35 percent in some eastern European countries.

Dr Bianchini and colleagues said that, in part, associations with cancer risk could be explained by alterations in the metabolism of endogenous hormones, including sex steroids, insulin, and insulin-like growth factors. These can lead to distortion of the normal balance between cell proliferation, differentiation, and apoptosis.

Relative risks measured in the studies showed BMI ranges differed between cancer sites and between studies, but on average the ranges were <22 kg/m and >28.5 kg/m.

In the prevention of diabetes and cardiovascular disease, a BMI in the range 18.5"25 kg/m has been recommended, the researchers said. "The breadth of this range has been justified by the fact that the ratio of adipose tissue to lean body mass can vary substantially between individuals with the same BMI. However, there is increasing evidence that, even within the range of 20-25 kg/m BMI is directly associated with the risk of cardiovascular disease, and some epidemiological studies suggest an increased cancer risk with BMI below 25 kg/m."

Avoiding weight gain seems an important factor for cancer prevention, and for many people to maintain weight within the lower part of this range. For people who are already overweight, avoiding further weight gain should be recommended, and, if possible, they should aim to reduce weight by 5-10 percent.

The best way to achieve a healthy body weight is to balance energy intake with energy expenditure and physical activity, the researchers point out. Calories dense foods should be replaced with foods like vegetables, fruits, whole grains, and beans.

However, they warn, "at the population level, obesity cannot be prevented solely by issuing health recommendations to individuals. Governmental and non-governmental organizations, the food industry, media, worksites, schools, health professionals, and educators all have a responsibility to work together to produce an environment less conducive to weight gain."

Individual efforts to make improvements in nutrition and physical activity are difficult and often ineffective without a supportive community environment.
Lancet Oncol 2002; 3: 565-74.

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