April 25 (UPI) -- Researchers have figured out why exercise is good for the heart, at least in mice: It helps the organ generate more new heart muscle cells.
Exercise can even deliver the benefit after a heart attack, according to researchers from the Harvard Department of Stem Cell and Regenerative Biology, Massachusetts General Hospital, Harvard Medical School and the Harvard Stem Cell Institute. The researchers, who published their findings Wednesday in the journal Nature Communications, noted that public health, physical education and the rehabilitation of cardiac patients can help prevent heart failure.
"We wanted to know whether there is a natural way to enhance the regenerative capacity of heart muscle cells," Dr. Ana Vujic, a researcher at Harvard, said in a press release. "So we decided to test the one intervention we already know to be safe and inexpensive: exercise."
Normally, young adults can renew around 1 percent of their heart muscle cells each year -- a percentage that decreases with age.
Among mice, the death of as few as 23 per 10,000 cells is enough to induce fatal heart failure, according to research published in 2003 in The Journal of Clinical Investigation.
"Our study shows that you might be able to make your heart younger by exercising more every day," said Dr. Richard Lee, a Harvard professor of stem cell and regenerative biology.
Researchers divided the mice into two categories: one group was allowed to run roughly 5 kilometers a day for eight weeks, and the other group stayed sedentary.
After administering a chemical to the mice that labeled newly made DNA as cells prepared to divide, the researchers were able to see where new cells were being produced. They found that mice that exercised had 4 1/2 times the number of new heart muscle cells than mice without treadmill access.
In mice that had sustained heart attacks, the researchers reported exercise still led to increased production of heart muscle cells.
"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," said Dr. Anthony Rosenzweig, a professor of medicine at Harvard. "Our study suggests exercise can help tip the balance in favor of regeneration."
Researchers plan to find out why exercise leads to increased regenerative activity in the heart.
"Now we need to find the signals that are sufficient to turn this pathway on," Rosenzweig said.