Association of Body Mass Index and Age with Subsequent Breast Cancer Risk in Premenopausal Women

Authors: Schoemaker, M.J. et. al.
Source: JAMA Oncol doi: 10.1001/jamaoncol.2018.1771
jamanetwork.com/journals/jamaoncology/fullarticle/2685650

It is understood that a higher BMI is associated with increased breast cancer risk among postmenopausal women. Interestingly, the inverse has been observed among premenopausal women: a higher BMI has been associated with a lower risk of disease in this population. The investigators note, however, that this trend has not been adequately studied. The number of women with premenopausal breast cancer is relatively low, and study designs have not been entirely adequate.

The group pooled women diagnosed with breast cancer prior to age 55 from Australia, Asia, Europe and North America. The height and weight data was recorded to calculate BMI for participants within determined age groups: 18-24, 25-34, 35-44 and 45-54 years. The rates of premenopausal breast cancer (in situ and invasive) were recorded.

More than 570,000 women participated. Obesity, defined as a BMI greater than 30, was more common among women who were older than 45, nulliparous, had early onset of menses, had a family history of breast cancer or were black. For consistency, women with BMI under 18.5 were excluded.

The risk of breast cancer consistently declined with increasing BMI in each age group. Increased BMI was associated with a 4-fold decrease in risk among women aged 18-24 and a 1.9 to 2.5-fold decreased risk among women in the older age groups. This protective effect was greater for in situ versus invasive disease in groups aged 25-34 and 35-44. For women aged 18-24, increased BMI appeared to be more strongly protective against ER and/or PR positive breast cancer, but it still appeared protective against ER and/or PR negative disease.

The reasons for these outcomes are not yet clear, but there are many theories. Childhood adiposity may reduce the susceptibility of developing breast tissue to carcinogens. It is known to slow the process of puberty, which may decrease breast cancer risk. It is associated with reduced breast density, which may also be protective. Furthermore, as adipose tissue is a source of estrogen, increased levels of the hormone in obese premenopausal women may decrease ovarian estrogen production and thereby protect breast tissue from cancer.

It is important to note that this study is not advocating that premenopausal women gain weight in an effort to reduce breast cancer risk. These results provide insight into the physiology of premenopausal breast cancer and suggest new areas to study and understand.