Fat people are not more jolly, according to a study that instead found obesity is strongly linked with depression and other mood disorders.

Whether obesity might cause these problems or is the result of them is not certain, and the research does not provide an answer, but there are theories to support both arguments.

Depression often causes people to abandon activities, and some medications used to treat mental illness can cause weight gain. On the other hand, obesity is often seen as a stigma and overweight people often are subject to teasing and other hurtful behavior.

The study of more than 9,000 adults found that mood and anxiety disorders including depression were about 25 percent more common in the obese people studied than in the non-obese. Substance abuse was an exception - obese people were about 25 percent less likely to abuse drugs or alcohol than slimmer participants.

The results appear in the July issue of Archives of General Psychiatry, being released Monday. The lead author was Dr. Gregory Simon, a researcher with Group Health Cooperative in Seattle, a large nonprofit health plan in the Pacific Northwest.

The results "suggest that the cultural stereotype of the jolly fat person is more a figment of our imagination than a reality," said Dr. Wayne Fenton of the National Institute of Mental Health, which funded the study.

"The take-home message for doctors is to be on the lookout for depression among their patients who are overweight," Fenton said.

Both conditions are quite common. About one-third of U.S. adults are obese, and depression affects about 10 percent of the population, or nearly 21 million U.S. adults in a given year.

Previous studies produced conflicting results on whether obesity is linked with mental illness including depression, although a growing body of research suggests there is an association.

This latest study helps resolve the question, said Dr. Susan McElroy, a psychiatry professor at the University of Cincinnati and editor of a textbook on obesity and mental disorders.

"This is a state-of-the-art psychiatric epidemiology study that really confirms that there is, in fact, a relationship," she said.

The study was based on an analysis of a national survey of 9,125 adults who were interviewed to assess mental state. Obesity status was determined using participants' self-reported weight and height measurements.

About one-fourth of all participants were obese. Some 22 percent of obese participants had experienced a mood disorder including depression, compared with 18 percent of the nonobese.

McElroy said the study bolsters previous research suggesting that drug and alcohol abuse are less common in the obese. One reason might be that good-tasting food and substances of abuse both affect the same reward-seeking areas of the brain, McElroy said. Why some people choose food as a mood-regulator and others drugs or alcohol is uncertain, she said.

The study found the relationship between obesity and mental illness was equally strong in men and women, contrasting with some previous research that found a more robust link in women.