"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."