Women who exercise more and keep their weight under control may dramatically reduce their odds of developing breast cancer, a population-based study by a team of investigators at Meharry Medical College and the Vanderbilt-Ingram Cancer Center suggests.
The researchers, along with colleagues at the Shanghai Cancer Institute in China, report a strong link between "energy balance" and breast cancer risk in the journal Cancer Epidemiology, Biomarkers & Prevention. Energy balance represents the difference between energy intake (eating) and energy expenditure (activity).
While certain factors are known to increase breast cancer risk - including family history and age of first menstruation and onset of menopause - there is a need for more information about those risk factors that can be modified.
Women with low levels of physical activity and higher body mass index levels (weight divided by height) were at more than twice the risk of developing breast cancer than women who undertook approximately three metabolic equivalent hours (MET) per day, per year, of exercise, and had lower BMI levels, the researchers found. This level of exercise is equivalent to about 45 minutes of brisk walking or 20 minutes of vigorous exercise per day.
"Given the substantial level of weight gain in industrialized countries in the last two decades," said lead author Alecia S. Malin, DrPH, CHES, assistant professor of Surgery at Meharry and assistant professor of Medicine at Vanderbilt, "there is great interest in understanding the influence of energy balance on cancer risk, and in developing preventive measures that can effectively minimize excess risk. Our study suggests that the promotion of behavior patterns that optimize energy balance - weight control and increased physical activity - may be a viable option for breast cancer prevention."
She further points out that the anti-cancer effect of lowering caloric intake alone, demonstrated in animals, is not generally considered to be a feasible strategy for cancer prevention in humans. Indeed, her team's results show that greater energy intake alone was not associated with an increased risk of breast cancer among physically inactive women, suggesting that it is the combination of exercise and weight control that is important.
Data were derived from the Shanghai Breast Cancer Study, in which women 25-65 years old who were newly diagnosed with breast cancer, along with a random sample of healthy controls, were enrolled between August 1996 and March 1998.
Information was gleaned from in-person interviews of 1,459 breast cancer cases and 1,556 controls. The body mass index of these women was calculated based on measurements taken by the interviewers of their weight, circumference of waist and hips, and height.
"This direct approach enabled us to overcome the primary problem affecting the accuracy of energy balance assessments," said Malin. "Self-reporting leads to under-reporting, particularly when overweight people account for their own energy intake. They have a tendency to give socially acceptable answers, or to respond based on their desire to lose weight and improve their eating habits."
Extrapolation of the results for Westerners, Malin noted, should take into account the inherent differences in the relationship between BMI levels and disease risk that appear to exist between Western and Asian women. A 25 kg/m2 body mass index among Western women is considered to be normal weight, while a BMI of 25 kg/m2 among Asian women is considered to be in the overweight category and was associated with an increased breast cancer risk in this study.
SOURCES:
Cancer Epidemiology, Biomarkers & Prevention, June 2005
Vanderbilt University Medical Center (http://www.vanderbilt.edu)
