Bra size link to diabetes

“Diabetes is linked to breast size,” is the headline in The Sun . The report below goes on to say that “women who wear a large bra size are much more likely to develop diabetes than women with an A cup”. Type 2 diabetes is often linked to lifestyle factors, such as obesity and a lack of exercise but “even after adjusting for such factors and any family history, researchers found that the risk was still high”, the newspaper adds.

The newspaper story is based on a study involving data from more than 90,000 women in Canada. Researchers looked at women’s cup size and the rates of diabetes developing over 20 years. A link between breast size and diabetes was seen, but the researchers are unable to say from this study if the relationship is simply due to the overall weight or waist circumference increase you might expect in women who had larger than average breast size, as the link between obesity and diabetes is well known.

Where did the story come from?

Dr Joel Ray from the Li Ka Shing Knowledge Institute, University of Toronto, Canada and colleagues from the Harvard School of Public Health, Harvard Medical School and the Institute for Health Sciences in the Netherlands carried out this study. This study, the analysis and the Nurses’ Health Study II were supported by the Canadian Institutes of Health Research, the Research Division at St Michael’s Hospital, Toronto and the US National Institutes of Health. It was published in the peer-reviewed: The Journal of the Canadian Medical Association.

What kind of scientific study was this?

This was a secondary analysis of data collected from a prospective cohort study, the Nurses’ Health Study II, which aimed to study risk factors for breast cancer among women and began in 1989.

In this study, researchers looked at data collected from about 92,000 women (average age 38 years) and used the answers to questionnaires (which were completed every two years) to detect the cases of type 2 diabetes. The women were asked whether they had been diagnosed with diabetes, what their blood test results were, and what medications they were receiving for their diabetes.

The women’s bra cup sizes at the age of 20 was taken from the answers given in the 1993 questionnaire and categorized as A or less, B, C and D or more. The researchers excluded women who had a diagnosis of diabetes at the start of the study or who had had diabetes during pregnancy. They also excluded over 20,000 more women who had no information recorded about breast size, or other details that the researchers required for the study.

The researchers used statistical models to adjust for other factors which can affect diabetes including the age when periods started, the number of children the women had had, the degree of physical activity, current body mass index (BMI) and their BMI at aged 18 and details of smoking, diet, multivitamin use and any family history of diabetes.

What were the results of the study?

A total of 1,844 new cases of type 2 diabetes arose during the study, at an average age of 44.9 years. When the researchers adjusted for age alone, the chances of developing diabetes increased in women with larger cup sizes compared with those with bra cup size of A or less; the increase in risk was proportional to the cup size (about double for B cup, four times for C and five times for women with D cup or more).

All these increases were reduced to less than a doubling in risk when the researchers adjusted for the other factor for which they had available information. such as the age when periods started, the number of children, the degree of physical activity, current BMI, BMI at age 18, and details of smoking, diet, multivitamin use and any family history of diabetes. These adjustments left the increase in risk of developing diabetes at between 30% and 80%, depending on which cup size was being looked at.

What interpretations did the researchers draw from these results?

The researchers say, “a large bra cup size at age 20 may be a predictor of type 2 diabetes in middle-aged women”. However, they add that the question as to whether this link is independent of traditional indicators of obesity remains to be determined.

What does the NHS Knowledge Service make of this study?

Conclusions from this research are limited by the very strong association shown between body mass index (BMI) and the risk of developing diabetes. This is illustrated by the large fall in the risk of developing diabetes when the researchers added an adjustment for the known risk factors for type 2 diabetes into their statistical model.

Asking women their breast cup size may be a useful alternative to measuring their weight, BMI or waist circumference, but it remains to be seen if the link shown here is anything other than the well-researched link between being overweight and diabetes.