Breast density clue for cancer risk

Women with a higher body mass index (BMI) in early adulthood may have a lower risk of developing breast cancer after the menopause because of lasting changes in the structure of their breast tissue, according to researchers at The University of Manchester and Manchester University NHS Foundation Trust¹.

An analysis of data from more than 33,000 women found that breast density could partially explain the link between a higher body weight at age 20 and a lower chance of post-menopausal breast cancer.

The findings help explain a long-standing puzzle in breast cancer research: being overweight in middle age is known to increase breast cancer risk, while a higher body weight in childhood, adolescence and early adulthood appears to have the opposite long-term effect.

The researchers followed 33,816 women taking part in the UK Predicting Risk of Cancer at Screening (PROCAS) programme for more than a decade and recorded 1,261 cases of post-menopausal breast cancer.

They examined women's self-reported BMI at age 20 and compared it with measurements of breast density taken during routine mammograms.

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

The team found that the women who had a higher BMI at age 20 generally had lower percentage breast density later in life and were also 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 that lower breast density may account for almost 60 per cent of this effect.

The 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, which estimates 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.

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.”

Senior author Professor Andrew Renehan, 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, added, “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.”

References

1. Jauniaux, B., Harvie, M.N., Sperrin, M. et al. Early-adulthood body mass index, mammographic density and post-menopausal breast cancer risk: a mediation analysis. Br J Cancer (2026). https://doi.org/10.1038/s41416-026-03593-w

Want the latest science news straight to your inbox? Become a SelectScience member for free today>>