Both breast density and BMI are well-established risk factors for breast cancer—so what happens to the first when you drastically change the second? This is what Nasreen Vohra and researchers from East Carolina University set out to examine. They looked at 80 women who underwent weight loss surgery (78% of these were a laparoscopic gastric bypass procedure). These women also had a pre- and post-operative mammogram, which was used to make Volpara measurements. This is the first instance of the effect of weight reduction surgery being examined in relation to breast density.
Following surgery, the average body mass index (BMI) decreased from 46 kg/m2 to 34 kg/m2. All women had a significant concomitant reduction in breast volume (-580 cm3 on average) and fibroglandular volume (-10.9 cm3). Interestingly, the reduction in fibroglandular volume (FGV) only reached statistical significance in peri/postmenopausal women. This may be because the group of premenopausal women in the study was small. Furthermore, the effect of BMI on breast tissue may differ according to menopausal status. Fat cells secrete estrogen, which has a proliferative effect on surrounding cells, including those in dense breast tissue. However, this effect would be more prominent in post-menopausal women in whom fat tissue is the main source of estrogen . In pre-menopausal women most estrogen comes from the ovaries and the influence of BMI on FGV is thus expected to be less prominent.
Nearly half of the women (35 patients) had type II diabetes prior to the weight loss surgery. In 21 of these women, the disease was resolved following the operation. Curiously, FGV only reduced in women who were not diabetic after the surgery. One possibility is the fact that type II diabetes is associated with increased levels of insulin, which causes cells to proliferate . The increased level of insulin in diabetes-2 sufferers could cause dense tissue to increase (or at least fail to decrease).
The ratio of FGV to the total breast volume gives the volumetric breast density (VBD). Despite the two components of VBD undergoing a decrease, the average VBD increased from 5.2% to 7.9%. What’s the explanation for this? This is because although the FGV decreases, this drop is quite small (-15.5%). However, the breast volume shrinks much more drastically due to the loss of fat in the breast (women lost 41% of their breast fat volume after surgery).
If lower BMI corresponds to lower breast cancer risk, how does one reconcile this with the fact that VBD increases after a reduction in BMI? Although this study did not look at this directly, it is hypothesized that FGV (being the site where breast cancer arises) may be more indicative of risk that VBD. Furthermore, breast fat produces estrogen and other proliferative molecules that encourage carcinogenesis. As both of these components decrease upon the reduction of BMI, the overall effect on risk may turn out favourable despite the small increase in VBD. This is something that this group plans to explore in the future.
1. Simpson, E.R., Sources of estrogen and their importance. J Steroid Biochem Mol Biol, 2003. 86(3-5): p. 225-30.
2. Lorincz, A.M. and S. Sukumar, Molecular links between obesity and breast cancer. Endocr Relat Cancer, 2006. 13(2): p. 279-92.