It’s easy to think of breastfeeding as something confined to the early days of parenthood; those hazy, chaotic months when time blurs and your world contracts to the dimensions of a crib, and a body that doesn’t quite feel like your own. You feed your baby, try to sleep, repeat — and eventually, life moves on.
But according to a new study published in Cancer epidemiology, biomarkers & prevention, your body might not. Researchers have found that the act of breastfeeding may leave subtle, long-lasting marks in a woman’s biology. Dr. Naoko Sasamoto and her team from Fred Hutchinson Cancer Center, and other institutions examined how a woman’s history of breastfeeding might influence levels of certain inflammation and metabolism-related substances in her blood even years after she stopped nursing. These substances, known as “biomarkers,” play key roles in our health and risk for chronic diseases like cancer, heart disease, and diabetes.
Researchers explored this question with data from over 16,000 women in their 50s, drawn from the renowned Nurses’ Health Studies. These women had all given birth, some breastfed, others didn’t. On average, more than a decade had passed between a woman’s last birth and the moment her blood was drawn. But when researchers analyzed their blood, a pattern began to emerge.
One of the clearest biomarkers was a hormone called IGF1 (insulin-like growth factor 1). Women who had breastfed—even if only for a short time—had significantly higher levels of IGF1 compared to those who had never breastfed. IGF1 doesn’t get much public airtime, but in the world of medical research into roles of hormones, it’s quietly significant. It plays a role in cell growth and metabolism and has been tied to the risk of several diseases. Higher levels, for instance, have been associated with a lower risk of ovarian cancer—something breastfeeding has also been shown to reduce. So, it’s tempting to think that maybe this hormone is part of the puzzle. But biology rarely offers a simple answer. IGF1 has also been linked to increased breast cancer risk in some studies. That’s the kind of contradiction science often holds: one signal, different stories, depending on the context.
The study also detected a shift in a molecule called IL-10, an anti-inflammatory signal. Longer breastfeeding was linked to lower IL-10 levels. This IL-10 shift wasn’t evenly spread across all women. It was most pronounced in postmenopausal women; a group whose immune and hormonal systems have undergone their own transformation. Among these women, the link between longer breastfeeding and lower IL-10 stood out more clearly. Meanwhile, premenopausal women didn’t show the same pattern, suggesting that the interplay between breastfeeding and inflammation may evolve with time and hormonal change.
But again, when researchers zoomed in on subgroups, they spotted a flicker of something else. Among women with a BMI of 25 or higher (the clinical threshold for overweight), longer breastfeeding was linked to lower levels of the soluble leptin receptor, a molecule involved in fat regulation. That link didn’t hold up in women with lower BMI. Leptin signaling has been connected to both metabolism and cancer risk, including ovarian cancer. So, this may be another breadcrumb in the trail of how breastfeeding imprints on the body in ways that depend on where that body started from.
Taken together, these findings expand how we think about women’s health; not as a linear sequence of events, but as a long, dynamic history written in blood chemistry and biology. We often file breastfeeding into the “early motherhood” chapter. But this study gently asks what if the body keeps turning that page long after the chapter ends? To be clear, this isn’t an argument that everyone should breastfeed. The choice is shaped by emotion, biology, economics, and culture. Not everyone can. Not everyone wants to. And no one’s long-term health should be reduced to one moment in a much bigger story. What the study is saying is more subtle. It’s saying the body remembers. Not just in stretch marks or stories, but in molecules. In echoes that persist decades after the lullabies fade. We don’t yet fully understand these molecular echoes—but their persistence decades later is remarkable.
This study was supported by the National Cancer Institute, the Department of Defense Ovarian Cancer Research Program, the American Cancer Society, and the Rivkin Center for Ovarian Cancer Research. The Nurses’ Health Studies were also supported by institutional funding.
Fred Hutch/UW/Seattle Children’s Cancer Consortium members Dr. Sasamoto contributed to this work.
Lin, N., Vitonis, A. F., Mongiovi, J. M., Farland, L. V., Huang, T., Terry, K. L., Eliassen, A. H., Townsend, M. K., Zhang, C., Hu, F. B., & Sasamoto, N. (2025). History of breastfeeding in relation to circulating inflammatory and metabolic biomarkers. Cancer epidemiology, biomarkers & prevention: a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology, 10.1158/1055-9965.EPI-25-0034. Advance online publication.
Science Spotlight writer Darya Moosavi is a postdoctoral research fellow within Johanna Lampe's research group at Fred Hutch. Darya studies the nuanced connections between diet, gut epithelium, and gut microbiome in relation to colorectal cancer using high-dimensional approaches.
For the Media