Watching a funny film boosts the way blood vessels function, a study says.
Maybe laughter really is the best medicine
Researchers from the University of Maryland School of Medicine in Baltimore compared the effects of watching funny and stressful films.
Stress caused blood flow to slow by around 35%, but laughter increased it by around 22%, they told the American College of Cardiology.
UK heart experts said there was increasing interest in the idea that positive emotions benefited health.
The US team studied 20 healthy, non-smoking volunteers, with an average age of 33.
They watched either a segment of a film which would cause mental stress, such as the opening part of Saving Private Ryan, or a segment of a film which would make them laugh, such as King Pin.
At least 48 hours later, they were shown a film which would cause the opposite effect to the first.
Before watching each film, the volunteers fasted overnight and were tested to see how well blood vessels in the brachial artery in the arm responded to a sudden increase in blood flow, in a test called a flow-mediated vasodilation.
Volunteers watched a 15-minute segment of the film while lying down in a temperature-controlled room.
After the film was shown, their blood vessels were tested again.
'Laughing cuts heart risk'
No difference was seen in blood vessel dilation between the two groups before they watched the films.
But brachial artery flow was reduced in 14 of the 20 volunteers following the film clips that caused mental stress.
In contrast, beneficial blood vessel relaxation, or vasodilation, was increased in 19 of the 20 volunteers after they watched the film segments that generated laughter.
The researchers suggested laughter caused the tissue that forms the inner lining of blood vessels, the endothelium, to expand in order to increase blood flow.
The endothelium is known to have a powerful effect on blood vessel tone and regulates blood flow, adjusts coagulation and blood thickening, and secretes chemicals and other substances in response to wounds, infections or irritation.
It also plays an important role in the development of cardiovascular disease.
Impairment of the function of the lining of blood vessels is an early sign of cardiovascular problems.
Michael Miller, director of preventive cardiology at the University of Maryland Medical Center, who led the research, said: "The endothelium is the first line in the development of atherosclerosis or hardening of the arteries, so, given the results of our study, it is conceivable that laughing may be important to maintain a healthy endothelium, and reduce the risk of cardiovascular disease.
"At the very least, laughter offsets the impact of mental stress, which is harmful to the endothelium."
He added: "Thirty minutes of exercise three times a week, and 15 minutes of laughter on a daily basis is probably good for the vascular system."
Professor Andrew Steptoe, British Heart Foundation Professor of Psychology at University College London, said: "The results of this study are interesting but not unexpected.
"There is increasing scientific interest in the possibility that positive emotional states are beneficial to health. If this is the case, we need to understand the biological processes involved.
"We have shown associations between positive emotional states such as happiness and low levels of the stress hormone cortisol, and people with a more positive outlook also appear to be less affected by stressful events.
"Whether these effects are significant for health remains to be seen."