Do Big Breasts Up Diabetes Risk?

While new research links breast size to diabetes risk, doctors remain skeptical.


Jan. 28, 2008— -- It might be seen as a vindication for small-breasted women everywhere.

After suffering through decades of bra-cup envy, those with diminutive décolletages in their late teens and early 20s may enjoy a lower risk of developing type 2 diabetes later in life than their fuller-chested counterparts — or at least, that's what a new study suggests.

But many doctors caution that the conclusions of this new research may have more to do with obesity — and therefore bigger breasts — than they do with breast size alone.

Researchers at Harvard University and the University of Toronto surveyed 92,106 women who participated in the Nurses' Health Study, asking each of the participants to recall her bra size at the age of 20. The average age of the women participating in the study was 38.

Results showed that, compared with women who recalled having an A cup or smaller at the age of 20, women who recalled having a D cup or larger had about three times higher risk for developing type 2 diabetes.

The results also showed that women who reported wearing B cup and C cup bras experienced a higher risk for the development of diabetes than women who wore an A cup — even after figuring in age, body mass index (BMI), waist circumference, eating habits, family history of diabetes, physical activity level and pregnancies.

The findings were published Monday in the Canadian Medical Association Journal.

Although researchers involved with the study said it's still unclear how breast size could be tied to diabetes risk, lead study investigator Dr. Joel Ray, an assistant professor and clinician scientist in the division of endocrinology and metabolism at the University of Toronto, believes the correlation has something to do with how breasts develop during puberty.

"Breast development is a hallmark of puberty, and we know the onset of puberty occurs earlier in obese girls," Ray explained. "But in addition to puberty and breast development beginning earlier [in obese girls], we believe the overall effect of it may be more pronounced as well, in the sense that overall net production of breast is greater."

Supporting this idea is the fact that puberty, in addition to being a time for breast development, is also a period marked by raised insulin resistance, which corresponds with the infamous middle school growth spurt.

Ray said that just as breast development is both accelerated and more pronounced in obese girls, so may be their levels of insulin resistance — a cornerstone for the development of type 2 diabetes.

However, many experts remain unconvinced of the study's findings and question why they should be asked to abandon the tried-and-true methods of evaluating patients' type 2 diabetes risk simply by calculating their BMIs and evaluating their lifestyles.

"In reality, obesity is a major determinant of risk for diabetes," said Dr. Lewis Kuller, professor and chair in the department of epidemiology at the University of Pittsburgh. "Breast size, generally, is highly correlated with body size and body fatness, since what you're storing in the breast is mainly fat tissue."

And in light of this, Kuller said, breast size may simply tell us something we already know about diabetes risk.

"It's not like we don't already have a measure for how fat someone is," he added. "You can tell just by looking in the mirror. You're not gaining some new knowledge from evaluating your breast size 20 years ago that you couldn't already predict just by looking in the mirror."

Other diabetes researchers agreed. Body fat "is strongly associated with diabetes. If breast size is simply measuring fatness, this finding is not particularly interesting," said Dr. Roberta Ness, chair of the department of epidemiology at the University of Pittsburgh.

"My first impression is that this [study] is silly and likely to become a punch line on the Tonight Show," said Dr. David Nathan, director of the diabetes center and general clinical research center at Massachusetts General Hospital. "However, after more serious consideration, I would note that there are likely... other factors that may explain the association [between breast size and diabetes risk]."

The first problem identified by Nathan was the fact that although researchers attempted to control for the body mass and waist size of the participants, most experts believe these risk factors are so central to predicting diabetes risk that no amount of statistical adjusting could account for their impact.

Dr. Charles Clark, director of the diabetes collaborating center for continuing health professional education at Indiana University in Indianapolis, Ill., said that in comparison to BMI and waist circumference — two of the most important risk factors for diabetes, according to the American Diabetes Association (ADA) — breast size as a factor in diabetes risk is "not very important."

"Factors to consider that are much more predictive [of diabetes risk] can be found at the ADA web page," said Clark.

Nathan also said that responses of study participants in recalling their bra sizes from decades ago and in estimating their body fat at the ages of five and 10 were bound to be "soft and not very reliable."

Even Ray echoed Nathan's concerns about how data was collected and interpreted in the study.

"Some of these risk factors, like BMI and waist circumference, we made adjustments for, but does that really mean that breast size is an independent predictor of diabetes risk? I think we can still be challenged on that even though we showed it in these models," Ray said.

Because breast size is also determined by many factors unrelated to diet and metabolic rate — such as genetic and environmental factors — some doctors believe that examining bra size is a clinical waste of time.

"Breast size is determined by a number of genetic and environmental factors," Clark said. "Because it is related to age at [first menstruation] and pre-pubertal growth rates that are also important determinates of obesity, breast size is probably a surrogate for the presence of obesity or the predilection to become obese."

However, lead study author Karin Michels, co-director of the obstetrics and gynecology epidemiology center at Brigham and Women's Hospital in Boston, Ma., said that because they controlled for the participants' BMI levels as accurately as possible, their results reveal an "additional marker over and above obesity to predict risk for diabetes."

But Michels stopped short of recommending that doctors begin to examine a young woman's breast size as a possible indicator of future diabetes risk.

"Whether this may translate into a practical or clinical measure, I would be cautious in making that conclusion," Michels said.

Moreover, Ray expressed concern that female patients might get unnecessarily worked up about the study's findings.

"I'm concerned about overemphasis of this study or a physician applying it in practice," Ray said.

"Women might get very worked up about the relation of their breast size to diabetes risk when we don't have a substantial body of evidence to show that link."

ABC News Live

ABC News Live

24/7 coverage of breaking news and live events