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 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 nonobese. Substance abuse was an exception: Obese people were about 25 percent less likely to abuse drugs or alcohol than the slimmer participants, the AP reports.
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.
Previous studies produced conflicting results on whether obesity is linked to 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 their mental states.
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.