Postmenopausal women who want to significantly decrease their breast-cancer risk would be wise to exercise regularly and keep their weight within a normal range for their height, according to new findings from the Women's Health Initiative to be published in the journal Obesity.
The multicenter team of researchers, led by Anne McTiernan, M.D., Ph.D., of Fred Hutchinson Cancer Research Center, found that women who had the lowest body-mass index, or BMI, and the highest physical-activity levels had the lowest levels of circulating estrogens, sex hormones that can fuel breast-cancer growth.
Specifically, they found a significant decrease in the two most common, biologically active forms of estrogen, estrone and estradiol, among the most active, lean women studied. The researchers found that women with high BMI and low physical-activity had mean estrogen concentrations that were 50 percent to 100 percent higher than that of women with low BMI and high activity levels.
"Women with high levels of estrogens have a two-to-four-times-higher risk of breast cancer than women with very low levels," said McTiernan, a member of the Hutchinson Center's Public Health Sciences Division and co-investigator of the Women's Health Initiative Clinical Coordinating Center, which is based at the Center. "If a woman can keep her own natural estrogens lower after menopause, it is probably going to be beneficial in terms of reducing her risk of breast cancer."
The study, based on a random sample of 267 postmenopausal women nationwide selected from the WHI Dietary Modification Trial, is the first of its kind to examine the dual impact of body weight and physical activity on levels of various circulating sex hormones thought to impact cancer risk.
"Other studies have looked at the impact of body weight by itself or physical activity by itself but this is the first to look at both together regarding their influence on hormone levels," McTiernan said. "This gives us a new understanding that combining weight control with high levels of physical activity is necessary for keeping estrogens at a healthy level in postmenopausal women." Exercising vigorously for 30 to 60 minutes a day, five days a week would achieve this benefit, McTiernan said.
BMI is a measure of body fat based on height and weight. The World Health Organization divides BMI into the following categories for both men and women: underweight (18.5 or lower), normal (18.5 to 24.9), overweight (25 to 29.9) and obese (30 or greater). A BMI calculator is available on the National Institutes of Health Web site at http://nhlbisupport.com/bmi/bmicalc.htm
The National Heart, Lung and Blood Institute, U.S. Department of Health and Human Services, funded the study, which also involved investigators from Harbor-UCLA Research and Education Institute, Albert Einstein College of Medicine, University of Pittsburgh, University of Florida, University of Southern California and Northwestern University.
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