By Allison Bond
NEW YORK (Reuters Health) - Overall body size, rather than shape, is a better indicator of breast cancer risk after menopause, according to a recent study.
The analysis of U.S. women contradicts past research suggesting that having an apple shape with a large midriff measurement, regardless of weight or body mass index (BMI), might signal greater breast cancer risk.
"When we looked at both BMI and waist size, we found that BMI explained the relationship (with breast cancer risk), and that the waist circumference had little effect," said Mia Gaudet, an American Cancer Society epidemiologist who led the new study.
BMI, a measure of weight relative to height, is used to gauge obesity. Having a BMI in the obese range (30 or greater) has also been linked to breast cancer risk up to twice that of women in the normal weight range (BMIs of 25 or less, in this study).
Fat around the waistline that contributes to the apple-shaped body - versus a pear shape, where fat settles around the hips - is associated with extra inflammation and growth signals that have been linked to both heart disease and cancer risk.
To see whether excess abdominal fat contributes to breast cancer risk independent of overall BMI, Gaudet and her team analyzed data about nearly 29,000 postmenopausal women over an average of 11.6 years.
The women were participants in the Cancer Prevention Study-II Nutrition Cohort, a long-term study that began in the early 1990s. Starting in 1997, participants in Gaudet's study had filled out questionnaires every two years to evaluate their cancer risk and outcomes.
The survey asked for the women's weight, which was used to calculate BMI, and also provided specific instructions for the participants to measure and record their waist circumference. Women in the study were predominantly white and those who had a prior cancer diagnosis or had taken menopausal hormones were excluded from the analysis.
Researchers examined just the group's breast cancer diagnoses between 1997 and June 30, 2009, and found that without adjusting for BMI, a larger waist was linked to a higher breast cancer risk. For every 10-centimeter increase in waist size, the risk of breast cancer increased by 13 percent.
After including BMI in the calculation, however, waist size did not change cancer risk, but BMI did: For every one-point increase in BMI, there was a 4 percent rise in breast cancer risk.
The results were published in the journal Cancer Causes & Control.
The study may not be the last word when it comes to BMI and breast cancer, according to Victoria Seewaldt, a professor of medicine at the Duke Cancer Institute in Durham, North Carolina.
"To date, the data has been conflicting," Seewaldt told Reuters Health in an email.
For example, a 2012 analysis published in the journal PLoS One found that although a high BMI was linked to an increased risk of breast cancer after menopause, obesity did not confer a higher risk among premenopausal women.
Seewaldt, who was not involved in the new study, said she was surprised by its results since waist size is linked to other diseases including insulin resistance - a key player in type 2 diabetes - and the so-called metabolic syndrome, which increases the risk of stroke, heart disease and other conditions.
Importantly, the study didn't determine whether a higher BMI causes a higher breast cancer risk or whether another factor predisposed the women in the study to both obesity and breast cancer.
The researchers did take into account many lifestyle factors, such as exercise, diabetes and whether participants smoked, however.
Although the study was well-designed, it's not clear whether BMI is an accurate indicator of breast cancer risk among women who are not Caucasian, Seewaldt noted.
"BMI is not a good measure of obesity across racial and ethnic groups. I would be hesitant to change clinical practice and switch focus from abdominal circumference to BMI only, particularly for Asian- and African-American women," Seewaldt said.
No matter what, the age-old advice to maintain a healthy weight still stands, the study's authors said.
"The results of our study are very consistent with the American Cancer Society's recommendation with regards to physical activity, and to achieve and maintain a healthy weight and be physically active throughout life," Gaudet said.
SOURCE: bit.ly/PtkQGB Cancer Causes & Control, online April 9, 2014.