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 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.
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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: http://bit.ly/PtkQGB
Cancer Causes & Control, online April 9, 2014.
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