Higher BMI in early adulthood linked to lower breast cancer risk after menopause

Higher BMI at 20 was linked to a lower risk of post-menopausal breast cancer in an analysis of more than 33,000 women.

Lower breast density later in life may partly explain the link between higher body mass index, or BMI, in early adulthood and lower breast cancer risk after menopause.

The findings could help explain why higher body weight in childhood, adolescence and early adulthood appears linked to lower long-term risk, while being overweight in middle age is associated with increased breast cancer risk.

 

First author Dr Benoit Jauniaux, a surgical trainee in the North-West Deanery, said: “Researchers in previous studies have observed that women with a higher BMI in childhood or their early adulthood appear to have a lower risk of breast cancer after the menopause, but we have not fully understood why.

“This study suggests that downstream changes to breast density may explain a large part of this effect.”

The study was published in the British Journal of Cancer on 17 September 2026 and was supported by the National Institute for Health and Care Research (NIHR) Biomedical Research Centre (BRC): Manchester.

Researchers at The University of Manchester and Manchester University NHS Foundation Trust followed 33,816 women taking part in the UK Predicting Risk of Cancer at Screening (PROCAS) programme for more than a decade.

They recorded 1,261 cases of post-menopausal breast cancer and compared women’s self-reported BMI at age 20 with breast density measurements from routine mammograms.

Breast density refers to the amount of fibrous and glandular tissue compared with fatty tissue in the breast. Women with denser breasts are known to have a higher risk of breast cancer.

Women with a higher BMI at age 20 generally had lower percentage breast density later in life and were less likely to develop post-menopausal breast cancer.

For every five-point increase in BMI at age 20, the risk of developing post-menopausal breast cancer fell by around 15 per cent. Further analysis suggested lower breast density may account for almost 60 per cent of this effect.

Researchers believe the timing of body weight gains may be crucial because women’s breasts are still developing during adolescence and early adulthood, potentially leading to long-term changes in tissue structure.

The study also found that different measures of breast density may reflect different biological pathways linked to breast cancer risk.

One measure, estimating the percentage of dense tissue within the breast, appeared to capture some of the lasting effects associated with higher body weight in early adulthood. Another measure, based on the total volume of glandular tissue, appeared to be more strongly influenced by body weight later in life.

Senior author Professor Andrew Renehan is professor of cancer studies and surgery at The University of Manchester and programme co-lead in the Cancer Prevention and Early Detection Theme at the NIHR BRC: Manchester/

The researcher said: “These findings do not suggest that gaining weight is protective.

“What they offer is further insight into how breast tissue and consequent breast cancer risk may be shaped across a woman’s lifetime, and we want to understand these mechanisms further.

“Maintaining a healthy weight remains important, because excess weight in later adulthood is linked to a higher risk of breast cancer and many other serious diseases.

“But this study adds to growing evidence that exposures during early life can have lasting effects on health decades later.”