SYDNEY - Eating lots of fruit and vegetables could help protect teenagers from mental health problems, according to an Australian study published on Thursday in the scientific journal PLoS One.

The study of 3,000 adolescents aged 11 to 18 found that those who had poor diets filled with junk and processed foods were more likely to suffer mental health problems such as depression and anxiety.

The new research, carried out by Felic Jack from Deakin University's Barwon Psychiatric Research Unit in Victoria, is the first to demonstrate the link between diet quality and mental health disorder in adolescents.

The participants filled in questionnaires about their diets and psychological symptoms in 2005 and again in 2007.

The study found that teenagers who ate healthy diets had fewer mental health problems than those with poor diets.

The study also found that improvements in diet quality were mirrored by improvements in mental health, while deteriorating diet quality was associated with poorer psychological functioning.

Jacka said the finding suggested it could be possible to stop some mental health problems developing in adolescents by ensuring they ate healthy diets.

"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 said.

Studies show one in five Australian adolescents has some forms of mental health problems, caused by genes and environmental factors such as stressful events in early childhood.

Jacka said parents could protect children against mental health problems by eating two serves of fruit and five serves of vegetables a day, as well as sticking to wholegrain food and lean meats while avoiding junk food.