More than a third of children in the U.S. are overweight or obese, according to the Centers for Disease Control and Prevention.

Rutgers-Camden psychology professor Naomi Marmorstein studies depression and obesity in young people. She tries to answer the question - which came first?

Well, one can predict the other.

"What we found was that for girls, the onset of depression in early adolescence – so by age 14 – very strongly predicted the onset of obesity during the next development stage," said Marmorstein.

Marmorstein did not find that association among boys.

This was the first study to look at gender and age of onset for both diseases.

The director of research at the University of Pennsylvania's Weight Loss and Eating Disorder clinic, Lucy Faulconbridge, studies the link between obesity and depression as well.

The study makes a strong case for more cross-cutting care, she said. "It's highly predictive, and that means that if a teenager has depression, then one of the things that a doctor or clinician should be monitoring them for is obesity later on."

Lack of awareness, treatment options

"Both depression and obesity are extremely common and extremely disabling disorders for many young people, and they tend to co-occur," says Marmorstein.

At her clinic, Faulconbridge often sees them being treated in isolation. "So often, people think about treating one disorder, but actually if that person is struggling with depression they may not be able to lose weight," she said. "They may need extra help."

However, treatment options are few and far between, she said."That's because there's a history of depressed people being ruled out of weight loss trials."

She is in the process of creating her own, behavior-based, weight loss treatments that take depression into account.

Both Faulconbridge and Marmorstein say they hoped this research would lead to better, earlier treatment of both diseases.