In our prospective cohort study involving young and middle-aged women, we found that a larger bra cup size at age 20 predicted the onset of type 2 diabetes. Bra cup size, a simple, easily ascertained measure, explained the association beyond the well-established measure of BMI. The association held true within all BMI groups and also appeared to follow a dose–response relation. The association also persisted after we adjusted for other risk factors for type 2 diabetes, such as family history of diabetes mellitus, diet and exercise. Furthermore, by developing 5 separate models to determine the relation between bra cup size and risk of diabetes, each model adjusting for different covariables, we found that the relative risk of diabetes was greater among women with larger bra cup sizes than among those in the reference group (≤ A cup size) in all models (). This finding illustrates that, whatever the approach to adjustment, there may be additional benefit to include breast size in the assessment of risk factors for type 2 diabetes.
Bra cup size, when properly determined, provides a reasonable cross-sectional estimate of overall breast volume.16
Kusano and colleagues17
have successfully studied the relation between breast size and breast cancer risk using cup size divisions similar to ours. Although better methods, including magnetic resonance imaging, exist for measuring breast fat volume, they are expensive and time-consuming, and breast fat volume varies by about 15% across a woman's menstrual cycle.27
If our findings prove to be true, they raise a number of new questions about the pathogenesis of type 2 diabetes. Although it is not known whether adipose tissue in the breast contributes to the pathogenesis of insulin resistance, posterior chest adiposity — marked by a high ratio of subscapular to triceps skinfold thickness — is positively correlated with insulin resistance and type 2 diabetes.28
The storage of steroid hormones within breast tissue, and the behaviour of breast tissue as a paracrine and autocrine organ, including expression of insulin-like growth factor-I by mammary adipocytes, has been widely discussed in relation to breast cancer,29,30
but not in relation to diabetes. Adiponectin and leptin — hormones produced by adipose tissue that are responsible for regulation of glucose and fatty acid metabolism and appetite — have recently been found to be secreted in breast milk.31,32
Whether these hormones are expressed in breast tissue of nonlactating women remains unknown. Thus, while abdominal visceral obesity is known to contribute to the development of insulin resistance,3,12
the additional action of hormonally active breast adipose tissue within this process requires elucidation.
We hypothesized that prepubertal obesity may accelerate and exaggerate the normal state of insulin resistance seen in puberty.33
Although we did not directly test this hypothesis, the observed positive association between self-rated body fat in childhood (especially at age 10) and bra cup size, and the inverse association between age at menarche and bra cup size, is consistent with this concept.5,6
Breast size after puberty may also be a marker of postpubescent excessive insulin secretion and hyperandrogenemia.8,9
It is thus conceivable that breast size in early adulthood (e.g., at age 20) is a marker of childhood adiposity and peripubertal insulin resistance, which may continue into adulthood.
A cross-sectional study of the relation between breast volume determined by magnetic resonance imaging, markers of insulin resistance, and the metabolic syndrome among pre-and postpubertal women could clarify the mechanisms by which breast adiposity may predispose a young woman to type 2 diabetes.34
In addition, an accurate evaluation of chest adiposity in men and women may help decipher the degree to which breast fat tissue28
versus extra-abdominal fat contributes to the overall risk of insulin resistance and type 2 diabetes. Consideration of ethnic background and socioeconomic status in each of these studies is also recommended.
Our study had several strengths: a large sample, near complete prospective long-term follow-up and the use of a standard definition for type 2 diabetes. Other variables potentially associated with the development of diabetes, such as self-reported height, weight and waist circumference, have been previously validated.21,22
Our study has a number of limitations. One limitation was that we relied on the women's recall of their bra cup size at age 20, which was a key variable in the study. If underweight and obese women over-and underestimated their bra cup size at age 20, respectively, then the true relation between breast size and risk of diabetes may have been underestimated. Direct measurement of breast size and adiposity by physical examination or magnetic resonance imaging would have been useful herein. Another potential limitation was that we did not ask about breast augmentation, in part because cosmetic breast surgery was uncommon at the time that the study was initiated. Another limitation relates to the individuals enrolled in the study. The majority of our study participants were white; therefore, we are unsure whether our results would hold true for women of other ethnic backgrounds.28
A fair number of participants did not report bra cup size at age 20. Only about 18% of the Nurses' Health Study II participants included in this study wore a C cup bra at age 20, and only 5% wore a D cup or larger. Moreover, the risk of type 2 diabetes in relation to larger bra cup size was only modest. On the other hand, we found BMI at age 18 to be a strong independent predictor of diabetes, a finding that is consistent with results from other studies.1–3,9
Hence, BMI appears to be a stronger predictor of risk of type 2 diabetes than bra cup size and should remain an established measure in clinical practice. Recent Canadian guidelines also recommend measuring the waist circumference of adults when assessing obesity-related health risks.35
In summary, we documented a statistically significant association between bra cup size and the development of type 2 diabetes. We believe that our findings should be reproduced in other settings, and in studies involving women of different ethnic backgrounds. Finally, the mechanisms underlying the potential risks and health consequences of obesity in the upper and lower torso require additional research.
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