Obesity is a known risk factor for many diseases, including postmenopausal breast cancer, endometrial cancer, renal cell cancer, adenocarcinoma of the esophagus, and colon cancer.
For this reason, a working group of experts met earlier this year in Lyon, France, to assess the role of weight control and physical activity in cancer prevention. The group recently released its preliminary conclusions and recommendations, which were based on a review and evaluation of the available scientific evidence. The final report from this group, which was assembled by the International Agency for Research on Cancer, is expected in September.
According to the preliminary report, excess body weight and physical inactivity account for approximately one-fourth to one-third of breast cancer and cancers of the colon, endometrium, kidney, and esophagus. It reported that, in Europe, about half of the adult population is overweight, and obesity is becoming a problem even in urban areas of developing countries.
In the European Union alone, obesity has led to an estimated annual excess of 21,000 cases of colon cancer and 13,000 cases of breast cancer, thanks to "an environment which promotes sedentary lifestyles and over-consumption of highly caloric food," the report states.
What remains to be learned, however, is what the ideal body weight is for an individual person. "We will need to know much more about interactions between genes that confer cancer susceptibility or cancer resistance and their interaction with body weight, physical activity, and dietary intake," said Diane Birt, Ph.D., professor and chairman of the Department of Food Science and Human Nutrition at Iowa State University and a member of the IARC Working Group. "This area of research is in its infancy."
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The BMI is determined by dividing the body weight in kilograms by the square of the height in meters. (Alternatively, the BMI can be calculated by multiplying 704.5 by the result of dividing the body weight in pounds by the square of the height in inches.)
According to current recommendations, a BMI of more than 25 classifies a person as overweight, and a BMI of more than 30 puts a person in the obese category. The desirable range is considered to be from 18.5 to 25.
In terms of physical activity, the group recommended one hour per day of moderate-intensity activity to maintain body weight, and they suggested that more vigorous activity several times a week "may give some additional benefits regarding cancer prevention."
I-Min Lee, Sc.D., associate professor of medicine at the Harvard Medical School and of epidemiology at the Harvard School of Public Health, said that an association between lack of physical activity and cancer has now been made in more than 140 studies involving more than 380,000 cases of cancer, with the data especially strong for colon cancer but less convincing for breast and prostate cancers.
As to what actions should be taken to stem the worldwide obesity problem, the IARC working group had several recommendations for public health action as well as for areas that need more research:
Those already overweight or obese should aim first to avoid additional weight gain, and then to attempt weight reductions of 5% to 10%, although the IARC group cautions that it is not known whether this will reduce future cancer risks. "However, hormonal changes produced by weight loss seem likely to reduce risks of some cancers, and epidemiological evidence suggests that loss of weight even late in life would favorably affect the risks of breast and endometrial cancer," the group wrote.
More study needs to be directed toward body fat and its distribution, because individuals of the same weight may have more or less adipose tissue, depending upon metabolic differences.
More work needs to be done to develop standards for physical activity, body composition, and diet, as well as to further study environmental and hormonal factors involved in the development of obesity. The IARC group added that long-term clinical interventions in subgroups of different ages, sexes, and ethnicities are also needed.
The group called for special studies to determine surrogate markers or biomarkers of energy status as markers for cancer risk and to determine the acute versus chronic effects of physical activity, including the effect of such activity on sex hormones and insulin-like growth factor-I.
Workplaces and schools should provide for physical activity with exercise areas at work and an hour of physical education for students on most days, according to the IARC report, along with education about exercise and nutrition. Schoolchildren should also be provided with healthy meals.
Physicians and other health care providers should allow for education in nutrition and exercise and should serve as role models by maintaining their own proper body weights with proper nutrition and exercise, the report added.
The American Cancer Society and other American groups have echoed the IARCs alarm over the epidemic of obesity and its consequences in terms of cancer, cardiovascular disease, and diabetes. According to the National Center for Health Statistics, more than half of all adults in the United States are overweight. Obesity in children and adolescents has also increased over the last several years.
In a special section on obesity in its 2001 Cancer Facts and Figures, the American Cancer Society noted some other suggestions: Eat less fat and eat more fruits and vegetables. It estimated that only one fourth of adults eat the recommended five or more servings of fruits and vegetables each day.
Michael Thun, M.D., vice president for epidemiology and surveillance research at the American Cancer Society, also noted that obesity is more of a risk factor for cancer in women, because more women than men are obese. The relationship is especially strong for breast cancer, he said, while in men there is a relationship between obesity and colon cancer.
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