For the study the participants' food intake was assessed using a 212-item food frequency questionnaire and the Child Behaviour Checklist was used to assess internalising mental health problems, such as withdrawn and depressed behaviours, and externalising mental health problems, such as delinquent and aggressive behaviours.

Dr. Oddy says previous studies have shown that one in five children can be expected to develop some form of mental health problem by the time they reach adulthood, and 50% of all adult mental health problems develop during adolescence.

She says it has been known since 1985 that children and teenagers have been increasing their energy intake by consuming more soft drinks and processed foods and the number of overweight adolescents has doubled and obesity has tripled in that age group and at the same time there have been marked increases in sedentary behaviours such as TV viewing and computer use.

Dr. Oddy believes research into factors that influence mental health in young people must have a much higher priority and says their findings show that there is a need to look at the overall diet, rather than concentrate on individual nutrients.

The research is published online in the international journal Preventive Medicine.