Young women with high body fat have a decreased chance of developing breast cancer before menopause, according to a new study published in JAMA Oncology. The finding may help researchers better understand the role obesity plays in breast cancer risk.

An international team of researchers pooled data from 19 different studies, comprising 758,592 women aged 18–54 years from around the world. The approach allowed the team to identify risk factors and patterns that would be difficult to detect with a smaller number of women.

Using patient-reported height, weight, and other health-
related factors, researchers evaluated the risk of developing breast cancer in relation to body mass index (BMI) in the following age ranges: 18–24, 25–34, 35–44, and 45–54. Overall, 13,082 participants (1.7%) developed breast cancer during the observed time periods.

Relative risk of premenopausal breast cancer was reduced 12%–23% for each five-unit increase in BMI, depending on age. Very obese women aged 18–24 saw the strongest effect and were 4.2 times less likely to develop premenopausal breast cancer compared to women with low BMI at the same age.

Additional studies are needed to determine why the effect is occurring, but the researchers cautioned that young women should not intentionally gain weight to lower their breast cancer risk.