A newly discovered hormone that keeps the bones of breastfeeding women strong could also help bone fractures heal and treat osteoporosis in the broader population. Researchers at UC San Francisco and UC Davis showed that in mice, the hormone known as Maternal Brain Hormone (CCN3) increases bone density and strength.

Their results, published July 10 in Nature, solve a long-standing puzzle about how women’s bones remain relatively robust during breastfeeding, even as calcium is stripped from bones to support milk production.

“One of the remarkable things about these findings is that if we hadn’t been studying female mice, which unfortunately is the norm in biomedical research, then we could have completely missed out on this finding,” said Holly Ingraham, PhD, the senior author of the new paper and a professor cellular molecular pharmacology at UCSF. “It underscores just how important it is to look at both male and female animals across the lifespan to get a full understanding of biology.”

More than 200 million people worldwide suffer from osteoporosis, a severe weakening of the bones that can cause frequent fractures. Women are at particularly high risk of osteoporosis after menopause because of declining levels of the sex hormone estrogen, which normally promotes bone formation. Estrogen levels are also low during breastfeeding, yet osteoporosis and bone fractures are much rarer during this time, suggesting that something other than estrogen promotes bone growth.