So say University of Iowa scientists who studied runner's high in rats.
"We have known for a long time that exercise is great for the heart. This study helps us better understand why," Eric Dickson, MD, says in a news release.
Dickson's team split rats into two groups.
One group of rats ran on a treadmill for 15 minutes per day for four days. On the fifth day, the rats ran for a longer time -- 25 minutes -- at a faster pace.
For comparison, the other group of rats spent the same amount of time on a treadmill that was switched off, letting the rats loll there without exercising.
On the study's sixth day, the researchers removed the rats' hearts and induced heart attacks in those hearts.
The hearts of the exercising rats had less heart attack damage than the hearts of the rats that didn't exercise on the treadmill.
The researchers repeated the experiment with another group of rats.
But this time, the rats got a shot of a chemical called naltrexone on the last two days of treadmill time. Naltrexone blocks opioid receptors.
After the shot, the researchers removed the rats' hearts and induced heart attacks in those isolated hearts. All of the rats' hearts showed similar amounts of heart attack damage.
The findings suggest that opioids play a role in the heart-healthy aspects of exercise, note Dickson and colleagues.
The findings appear in the American Journal of Physiology -- Heart and Circulatory Physiology.