Published in Child and Adolescent Mental Health, the University of Sydney study followed 6537 adolescents from 71 secondary schools across NSW, Queensland and Western Australia from Year 7 to Year 10, who were about 12.5 years old on average at the start of the study.

Participants completed annual surveys over four years during high school.

Researchers found that eating one additional serve of any type of fruit each day was associated with a 13 percent lower risk of depression symptoms and a 7 percent lower risk of anxiety symptoms over the study period. 

Average fruit consumption ranged from 2 to 2.5 serves a day across the study period.

Higher consumption of sugar-sweetened drinks, including soft drinks, cordials and sports drinks, was associated with an increased risk of depression and anxiety.

The association with sugary drinks was observed in male and female teenagers, but was stronger in males, particularly for anxiety.

Lead author Dr Lily Davidson, a postdoctoral research associate at the University of Sydney’s Matilda Centre for Research in Mental Health and Substance Use, says adolescence is an important time to investigate the relationship between food and mental health.

“High school is a time when young people start gaining much more independence over what they eat and drink, at the same time as many mental health problems start to emerge,” she says.

“We’re not saying eating an apple will prevent depression, or that having a soft drink will cause anxiety – mental health is far more complex than that. But what you eat is one part of a young person’s life that can potentially be changed, which makes these findings important from a prevention angle.”

Much of the previous research linking adolescent dietary intake and mental health has taken a snapshot in time, making it difficult to establish which came first.

Using data from the Matilda Centre’s Health4Life study, researchers instead followed-up teenagers for three years and excluded adolescents who already met the relevant threshold for depression or anxiety at the beginning of the study.

“One of the big questions in our field is whether poor nutrition comes before mental health problems, or whether young people experiencing poor mental health simply eat differently,” Davidson says.

“By starting with adolescents who weren’t already experiencing elevated symptoms and following them through high school, we could look at whether their dietary intake was associated with the later emergence of those symptoms.”

Participants reported how much fruit and vegetables they usually ate each day and their consumption of sugar-sweetened beverages.

The researchers also accounted for factors including physical activity, age, sex and socioeconomic status.

The findings come amid growing efforts to understand what can be done before young people develop significant mental health problems, rather than intervening only once symptoms appear.

“It’s easy to think of food and nutrition as being about physical health, but our findings add to evidence that what young people eat may also matter for their mental health,” Davidson says.

“We need to understand what makes healthy choices easy or difficult for teenagers and make healthy eating a priority in adolescent mental health prevention.”

Associate Professor Katrina Champion, senior author on the study and a Sydney Horizon Fellow, says accessibility is a key to the future.

“Creating environments that make nutritious foods more accessible and affordable for families and reducing young people’s exposure to sugary drinks moves the conversation away from telling a teenager to eat better and towards addressing the environments that shape their choices,” she says.

“Schools, families and governments all have a role to play in supporting healthy behaviours and better mental health for young people.”