A new study explores the surprising interaction between body mass index and breast cancer. The researchers hope that the results will lead to a better understanding of the mechanisms and risk factors involved.

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According to a recent study, having a higher BMI could protect against breast cancer.

In the United States, breast cancer is the most commonly diagnosed cancer in women.

Also, compared with other cancers, this affects younger women more often.

Understanding why it happens in some people and not others is, of course, a priority.

A number of risk factors for breast cancer are now known, and one of these is adiposity. This is the amount of fat an individual carries on their body.

However, the influence of adiposity — as measured by body mass index (BMI) — is not as clear-cut.

Before menopause, a higher BMI seems to be protective against breast cancer, and the opposite is true after the menopause, at which point a higher BMI starts to increase risk.

This relationship has been studied previously, but because cases of premenopausal breast cancer are less common than postmenopausal breast cancer, individual studies have often lacked the participants required to draw solid conclusions.

The latest study, published this week in JAMA Oncology, attempts to fill the gaps. Led by Minouk J. Schoemaker, Ph.D. — from the Institute of Cancer Research in London, United Kingdom — the scientists combined data from 19 studies.

This data pool included 758,592 premenopausal women, and, among them, 13,082 new cases of breast cancer were diagnosed across an average follow-up period of 9 years.

The team wanted to understand how BMI at different ages affected risk. As the study authors write, “We aimed to estimate the relative risk associated with BMI at different ages, age at breast cancer diagnosis, and breast cancer characteristics, and to explore whether [the] associations were modified by other risk factors for breast cancer.”

As expected, the authors identified an inverse association between BMI and breast cancer risk. In particular, the risk of breast cancer in women aged 18–54 was reduced as BMI increased. This association was most pronounced in the 18–24 age group.

The authors are aware of the study’s limitations. Firstly, the study is based on the participants’ self-reported weight, which could have been under- or over-reported. Also, this was an observational study, so cause and effect cannot be identified. And, women with the same BMI measurements might have varying levels of fat and different types of fat distribution.

The authors are quick to add a caution to their findings, saying:

Study authors are not advocating weight gain as a way to reduce premenopausal breast cancer risk.”

They hope that the results will be useful in teasing apart the factors involved in the development of breast cancer. They write that “[u]nderstanding the reasons for the association seen between BMI and premenopausal breast cancer risk could potentially help to identify risk factors that might be modified.”

Additional work will surely follow, but the unexpected association is now backed up by a decent amount of data. We will have to wait patiently for the mechanisms behind the interaction to be unraveled.