Teenagers who eat fruits and vegetables with their regular diet are less prone to mental health problems, a new study shows.

The study, published in the journal PLoS One Thursday, shows that adolescents who had poor diets of junk and processed food were more likely to suffer bouts of depression and anxiety. This is the first study that linked diet quality and mental health among adolescents.

Dr. Felice Jacka from Deakin University's Barwon Psychiatric Research Unit said the results suggest that mental health problems among adolescents can be helped if they eat more fruits and vegetables.

"The results of this study are consistent with what we have seen in adults, but we think it could be more important because three quarters of psychiatric illnesses start before adulthood, and once someone has depression they are likely to get it again," Jacka told AAP.

"If you can prevent it before it starts in childhood and adolescence you, are shutting the gate before the horse bolts," she said. "Having good, nutrition-rich foods is really important for adolescents because it's a time when they are growing rapidly and it's essential they have adequate nutrition."

Jacka analyzed data from 3,000 Victorian adolescent participants aged 11 to 18. The participants filled in questionnaires about their diet and psychological symptoms in 2005 and again in 2007.

The study revealed that those participants who ate healthy diets in 2005 had fewer health problems compared to those who had poor diets. Participants who ate better between 2005 and 2007 had better mental health than those who continued eating junk and processed food.

Although many other factors affect mental health, such as genes and environment, they were found to have no impact on the results of the study.

Jacka said parents should follow national guidelines for eating two servings of fruit and five servings of vegetables to protect their children's mental health.

"We know depression and anxiety have a very early age of onset and they are common in adolescents, and it looks like quality of their diets could be linked to a risk of mental health problems," she said.

"The results suggest we shouldn't just be looking at obesity as a potential outcome of poor diet, we need to look at mental health and physical health as potential outcomes."