Watching a good comedy could be as effective as going for a run in boosting health, according to groundbreaking research that links laughter to the healthy function of the blood vessels.

Scientists found that a spell of hearty laughter causes the tissue that forms the inner lining of the blood vessels, the endothelium, to dilate, increasing the blood flow in the same way as a bout of aerobic exercise.

The finding adds to evidence that a laugh a day may help keep a heart attack away. Michael Miller, the director of the Centre for Preventive Cardiology at the University of Maryland, who has studied the healing power of laughter for a decade, said: "The old saying that laughter is the best medicine definitely appears to be true when it comes to protecting your heart."

For his study, 20 young healthy volunteers were assigned to watch 15-minute segments of distressing films, such as Stephen Spielberg's film about the Normandy landings, Saving Private Ryan, and funny ones such as the comedy There's Something About Mary. Researchers monitored blood flow in the brachial artery in the arm of each volunteer before and after each film segment. They found blood flow was reduced in 14 of the 20 participants after watching distressing clips and increased in 19 out of the 20 after funny clips.

The difference in flow between happy and sad responses exceeded 50 per cent. The findings are published in the journal Heart.

Dr Miller said: "The magnitude of change we saw in the endothelium is similar to the benefit we might see with aerobic activity, but without the aches, pains and muscle tension associated with exercise. We don't recommend that you laugh and don't exercise but we do recommend that you try to laugh on a regular basis. Thirty minutes of exercise three times a week and 15 minutes of laughter daily is probably good for the cardiovascular system."

Previous research has shown that laughter lowers blood pressure, boosts immune function and triggers the release of endorphins in the brain, the body's natural opiates, which produce a sense of wellbeing.

Judy Goldblum-Carlton, a humour therapist at Maryland University, said: "When you laugh heartily, every organ is being massaged including your heart, lungs and digestive system. Headaches can just go away"

Mirth and its medical benefits

* Lowers blood pressure

* Reduces stress hormones

* Provides a work-out for the diaphragm

* Reduces pain

* Increases immune function and aids healing

* Promotes muscle relaxation

* Exercises the heart

* Boosts the circulation

* Improves breathing

* Has no known negative side-effects