Various health practitioners agree that exercise is good for the heart — but the reasons why are still not well understood.

In a new study researchers from the Harvard Department of Stem Cell and Regenerative Biology (HSCRB), Massachusetts General Hospital (MGH) and Harvard Medical School (HMS), and the Harvard Stem Cell Institute (HSCI) uncovered one reason why exercise might be beneficial: it stimulates the heart to make new muscle cells, both under normal conditions and after a heart attack.

Published in Nature Communications, the findings have implications for public health, physical education and the rehabilitation of cardiac patients.

The human heart has a relatively low capacity to regenerate itself. Young adults can renew around 1% of their heart muscle cells every year, and that rate decreases with age. Considering that losing heart cells is linked to heart failure, interventions that increase new heart cell formation have potential to prevent heart failure.

The two senior authors behind the study were Richard Lee, M.D., Harvard Professor of Stem Cell and Regenerative Biology, and Principal Faculty member of HSCI, and Anthony Rosenzweig, M.D., Paul Dudley White Professor of Medicine at HMS, Chief of the Cardiology Division at MGH, and Principal Faculty member of HSCI.

“Maintaining a healthy heart requires balancing the loss of heart muscle cells due to injury or aging with the regeneration or birth of new heart muscle cells. Our study suggests exercise can help tip the balance in favor of regeneration,” said Rosenzweig.

“Our study shows that you might be able to make your heart younger by exercising more every day,” said Lee.