The size of a woman's breasts at age 20 may help predict whether she will develop type 2 diabetes later in life, according to new Canadian research.

The study, to be published in this week's issue of the Canadian Medical Association Journal, suggests that women with a bra cup size of D or larger are almost five times more likely to develop diabetes than women with an A cup.

It's well-known that obesity is a risk factor for diabetes and that overweight and obese women tend to have larger cup sizes. However, the researchers found that cup size was not directly linked to overall body fat; the risk was still high even after they adjusted for obesity.

Dr. Joel Ray of St. Michael's Hospital in Toronto led a team of U.S. researchers who studied data from more than 92,000 women who were part of the ongoing Nurses' Health Study II. That study began in 1989 to study risk factors for breast cancer among female nurses in 14 states.

Between 1993 and 2003, a total of 1,844 of the nurses developed adult-onset diabetes.

The researchers found that women who reported having a size A bra cup at age 20 had the lowest risk of developing diabetes later in life. Those with a B cup had more than double the risk of developing diabetes, those with a C cup had more than a four-fold greater risk, and those with D cups or larger were almost five times more likely to develop diabetes.

"As the bra cup size became higher, her risk of getting diabetes in the future went up proportionately," Ray explained to CTV News.

There was still an increased diabetes risk from a larger breast size even if the women were in the normal BMI range (body mass index)."

"When we adjusted for the women's waist circumference or her BMI, the risk estimates were lower, but they remained significantly higher as bra cup size went up," he says.

Ray says the finding is preliminary and needs to be supported through further study. But if validated, the next step would be to search for why larger breasts would be leading to diabetes.

One theory is that breast tissue, which is often 50 to 70 per cent fat (or adipose tissue), is hormonally sensitive and may, like abdominal fat, be influencing insulin resistance, the underlying cause of diabetes.

It's known that girls who are overweight in adolescence tend to experience puberty earlier. Ray theorizes that girls who enter puberty earlier may have more developed breasts at age 20 as well as greater insulin resistance, which hikes their risk of developing diabetes.

Dr. Alexander Sorisky, a senior scientist in the chronic disease program of the Ottawa Health Research Institute, says the finding is intriguing and if further research confirms it, breast size could become a new "anatomic predictor of type 2 diabetes."

"There has to be room to ask innovative questions... but this may give us new ideas on how fat cells are working, and new approaches to the disease," he notes.

Sorisky says there is increasing evidence that fat cells, which store excess energy, can malfunction, causing low-grade inflammation and insulin resistance that can lead to type 2 diabetes.

Ray says he plans to study whether breast reductions lower the markers of insulin resistance, and decrease a woman's risk of diabetes. But that research could take several years. For now, he says the study shouldn't cause larger-breasted women to worry or to consider breast reduction surgery.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip