Getting high school students to eat healthy and exercise could be as important for reducing mental health problems as it is for reducing obesity.

New research in the BMJ Open Medical Journal shows the wasitlines, eating patterns and exercise routines of high school students could have a major bearing on their mental health.

The research shows a link between teenage girls consuming too many sweet drinks and takeaway foods, and becoming depressed.

It's not just girls who are affected - co-author Professor Boyd Swinburn from Auckland University said there is also a connection between teenage boys staying obese and becoming depressed.

He said healthy habits have positive long-term outcomes.

"When we talk about healthy eating in schools this is not just about trying to prevent obesity - it's also to improve mental health and improve scholastic outcome," he said.

Professor Swinburn said people who were overweight or obese at baseline in the study, went on to have an increased risk of depressive symptoms two years later.