A new psychology paper research finds that adolescent females who are either obese or depressed are more likely to develop the other.

By assessing a statewide sample of more than 1,500 males and females in Minnesota over a period of more than 10 years, the authors found that depression occurring by early adolescence in females predicts obesity by late adolescence. Meanwhile, obesity that occurs by late adolescence in females predicts the onset of depression by early adulthood. No significant associations between the two disorders across time were found in males during the study.

 The analysis was not designed to investigate the reasons for these associations, but various hypotheses speak to possible explanations. Some believe depression can lead to obesity through an increased appetite, poor sleep patterns, and lethargy, while obesity can cause depression due to weight stigma, poor self-esteem, and reduced mobility. 

"Adolescence is a key developmental period for both obesity and depression, so we thought it significant to look at the onset of these disorders at an early age," says Naomi Marmorstein, an associate professor of psychology at Rutgers University–Camden, who co-authored the study with University of Minnesota psychology professor William Iacono and research associate Lisa Legrand. 

"When researchers looked at this connection over time, data had been mixed," Marmorstein says. "Some found that depression and obesity go hand-in-hand, while others did not see that connection. We tried to take the next step in clarifying this link by looking at a sample of youth that we followed from ages 11 to 24."

They believe their method improves on past research that included recurrence or persistence of depression and obesity rather than focusing on the onset of each disorder. Participants were assessed at ages 11, 14, 17, 20, and 24 by using height and weight measurements and clinical, interview-based diagnosis of major depressive disorder. The researchers looked specifically for onsets of either disorder by age 14, between the ages of 14 and 20, and between ages 20 and 24.

"When a person is young, she is still developing eating and activity patterns, as well as coping mechanisms," Marmorstein explains. "So if she experiences a depressive episode at age 14, she may be more at risk for having an onset of unhealthy patterns that persist."

Thus, a child who is obese may be more susceptible to negative societal messages about obesity or teasing, which could contribute to depression.

"At this age, adolescents are starting to establish relationships becoming self-conscious, so teasing can be particularly painful," Marmorstein says.

She says prevention efforts aimed at both of these disorders at the same time when one of them is diagnosed in adolescents might help in decreasing their prevalence and comorbidity.

"When an adolescent girl receives treatment for depression, the clinician might consider incorporating something relating to healthy eating and activity," she says. "Exercise can assist in the treatment of depression to begin with, so it seems like a good reason to combine prevention efforts for both depression and obesity."

Marmorstein says it is unknown why no associations across time between the two disorders were found in male adolescents, but hypothesizes that it could be a result of different developmental processes leading to obesity and depression in males and females.