Laughter affects the body much in the same way that exercise does. Laughter has been found to decrease stress hormones, enhance immune activity, and lower both systolic blood pressure and bad cholesterol while raising good cholesterol.

Drs. Lee Berk and Stanley Tan have been studying the effects of laughter on the human body since the 1980s. They recently completed a study showing that laughter lowers the level of the hormone leptin while increasing the level of the hormone ghrelin, suggesting that it increases people's short-term appetite, just like moderate exercise does.

They've coined the term "eustress," the opposite of distress, to describe the body's typical reaction to laughter.

In the recent study of laughter's effect on appetite hormones, 14 healthy volunteers watched both a distressing and a humorous video, one week apart. Leptin and ghrelin levels were measured one week before the study's start and both before and after subjects watched the videos.

The distressing video clip was the opening 20 minutes of the movie Saving Private Ryan, a clip which most viewers find highly upsetting. For the eustress video, subjects got to choose from a variety of stand-up comedians and movie comedies, since what people find funny varies.

The subjects showed no change in leptin and ghrelin after watching the distressing video. But after watching the humorous video, the subjects' leptin decreased and ghrelin increased, responses normally seen when appetite increases.

Berk cautions that the study doesn't show that humor increases appetite. But the responses seen are those that would be expected if the subjects' appetite did increase.

If laughter does actually increase appetite, it opens new treatment options for several conditions, such as elderly patients who suffer from "wasting disease" and patients with chronic pain. Both usually suffer from appetite loss and are poor candidates for exercise therapy.

Despite researching the effects of laughter for over 30 years, Berk continues to be amazed at how many different biological processes it seems to optimize. It all adds up to the idea that laughter is good medicine for everyone.

The results of the study were presented at the 2010 Experimental Biology Conference, which was held from April 24-28 in Anaheim.

Lee Berk, DrPH, MPH, is an associate professor of allied health studies, physical therapy and pathology at Loma Linda University in California, as well as director of the molecular research laboratory at the University's Schools of Allied Health.

Stanley Tan, MD, PhD, is an endocrinologist and diabetes specialist at Oak Crest Health Research Institute in California.