Breast Size Not All It's Stacked up to Be
Abstract & Commentary
By Allan J. Wilke, MD, Residency Program Director, Associate Professor of Family Medicine, University of Alabama at Birmingham School of MedicineHuntsville Regional Medical Campus, Huntsville. Dr. Wilke reports no financial relationship to this field of study.
Synopsis: Bra cup size at age 20 predicts risk of diabetes mellitus in later life.
Source: Ray JG, et al. Breast size and risk of type 2 diabetes mellitus. CMAJ. 2008;178:289-295.
The Nurses' Health Study 2 began prospectively gathering data on 116,609 women in 1989. Along with providing the usual information about demographics, anthropomorphic data, health and lifestyle, these women were asked for their heights and weights at age 18 (to calculate body mass index [BMI]) and for their brassiere cup size (BCS) at age 20. They were also asked to estimate their body fat in childhood (BFC) by selecting from drawings that pictured what they looked like between the ages of 5 and 10 years. After excluding women who were already known to be diabetic or who developed gestational diabetes or who did not report BCS or BMI, the researchers had 92,106 women with complete datasets. Most of the women were white and at baseline the average age was 38. They were similar with respect to cumulative months of lactation, diet, multivitamin use, and amount of vigorous exercise. The women were placed in BMI quintiles (≥ 18.8, 18.9-20.1, 20.2-21.2, 21.3-23.0, and ≤ 23.1) and four bra cup size groups (≥ A, B, C, ≤ D). As BCS increased, so did BMI, waist circumference (WC), and BFC. Age of menarche was inversely related to BCS. Family history of diabetes mellitus (DM) and personal history of smoking were more frequent as BCS increased. Regarding the primary endpoint of this study, during 886,443 person-years of follow up (9.6 years on average), 1844 (2%) women developed DM. Compared to women in the first quintile, women in the fifth quintile had an age-adjust hazard ratio of 5.05 (95% confidence interval [CI] 4.29-5.95). Women who wore a D-cup at age 20 had worse diabetic outcomes than women who wore an A-cup. DM occurred 2.1 years earlier and the age-adjusted hazard ratio was 4.99 (CI 4.12-6.05). Adjusting for BMI at age 18, current BMI, and WC attenuated the risk to 1.58 (CI 1.29-1.94), which was still statistically significant. Although for several of the twenty BCS/ BMI combinations there were too few cases of DM to be statistically significant, there was a general trend within each BMI quintile for an increase in the hazard ratio with increasing BCS.
Commentary
The conclusions of this study are biologically plausible. Elevated BMI1 and abdominal obesity2 are risk factors for type 2 diabetes mellitus. Obesity in childhood predicts age of thelarche and breast size.3 The interaction of BCS and BMI suggests a dose-response relation and indicates that the two measures are independent of one another. Using the same database, BCS is also associated with premenopausal breast cancer.4 Insulin-like growth factor is expressed in breast tissue.5 Since this is a cohort study and it doesn't show cause and effect, the question remains, "Do large breasts (primarily composed of endocrino-active adipose tissue) predispose to the development of diabetes or is there some underlying metabolic defect that leads to obesity, large breasts, and diabetes?"
A more practical question is, "Should bra cup size at age 20 be a vital sign?" This has its attractions. It doesn't involve any actual measurements (although the article spends a paragraph describing the official procedure); your patient can just tell you. I performed an informal survey of my wife, adult daughters, and some women coworkers. None were embarrassed and all but one claimed to remember. Recall bias is an obvious concern, but recording the data prospectively should reduce that.
The correlation between BCS and smoking puzzles me. Is the reasoning that women with larger BCS are more likely to have larger BMIs (which is true according to this study), and that these women were smoking in an attempt to lose weight? It may be a follow-up question to ask your female patients. The combination of diabetes and smoking is particularly dangerous.
This study's chief strengths are its large size and its prospective design. Recall bias is its main limitation. The BMI quintiles seem overweighed to the lower end; the fifth quintile includes people who are not obese (BMI < 25). Further division may have demonstrated that women with a BMI of 26 fare better than women with a BMI of 30, but the differences in the hazard ratios for the fourth and fifth quintile were already great.
In a commentary in the same issue,6 Dr. Sorisky reviews the current state of knowledge about adipose tissue and suggests that we should call it FAT (functional adipose tissue). He then discusses dysfunctional adipose tissue. I recommend it as an easy and informative read. Our understanding of FAT has advanced way beyond white and brown!
References
1. Peterson K, et al. Management of type 2 diabetes in youth: an update. Am Fam Physician. 2007;76:658-664.
2. McCarthy HD. Body fat measurements in children as predictors for the metabolic syndrome: focus on waist circumference. Proc Nutr Soc. 2006;65:385-392.
3. Biro FM, et al. Pubertal maturation in girls and the relationship to anthropometric changes: pathways through puberty. J Pediatr. 2003;142:643-646.
4. Kusano AS, et al. A prospective study of breast size and premenopausal breast cancer incidence. Int J Cancer. 2006;118:2031-2034.
5. Chong YM, et al. Breast Cancer Res Treat. 2006;99:275-288.
6. Sorisky A. A new predictor for type 2 diabetes? CMAJ. 2008;178:313-315.
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