Scientists believe the mood of teenagers could be determined by the food they eat - and are about to put their theory to the test.
A group of Australian scientists believe too few omega-3 fatty acids, found in fish and seafood, and too many omega-6 fatty acids, found in processed oils and nuts, raises the risk of depression in adolescents.
Up to 1000 Sydneysiders aged 14 to 17 will be recruited for the biggest study of its kind on whether adolescents truly are what they eat.
Lower seafood consumption has been linked to higher rates of depression in adults. The study will attempt to prove it affects teenagers in the same way.
The How Food Affects Mood study, by the Australasian Research Institute at Sydney Adventist Hospital and the University of NSW, will use DNA testing from cheek swabs and dietary surveys.
Researchers want to know if an imbalance in levels of omega-3 and omega-6 - due to increases in processed foods - heightens the probability of depression.
Humans evolved with a diet equal in both essential fatty acids but current Western diets have up to 15 times more omega-6 than omega-3.
Omega-3 fatty acids are critical to brain development and function, but because our bodies cannot effectively make them we rely on dietary sources.
However, most Australians consume less than a quarter of the optimal recommended intake because it is only found in a few foods, including oily fish such as salmon and sardines.
Previous NSW research showed children need to eat five times more fish and seafood than they do.
The study will be the first to measure which variant a teenager has of the serotonin transporter gene, which is responsible for our uptake of "feel-good" serotonin and mood control.
In 2006 Australian research found people who carry a "short" version of the gene tend to be more susceptible to depression, while those with the "long" version are more resilient against negative life events. The institute's Dr Ross Grant said the results would be used to give teenagers positive messages about healthy eating. "Often kids who are physically unhealthy are emotionally unhealthy as well," he said.
Co-researcher Margaret Morris said dietary intervention could ultimately be used to prevent and treat mood disorders.
"If there is a causal link between omega-3, serotonin transporter genotype and depression, we can develop better strategies to deal with it," she said.
Beaumont Hills parents Tracy and Martin McCoy say it's a challenge to get their three daughters to eat enough seafood. While Courtney, 13, doesn't mind prawns, Emilly, 16, and Amy, 9, won't go near fish.
"I don't like the fishy taste, and it stinks," Emilly said.
Amy added: "I'll have the chips."
American research suggests fish oil capsules and fatty fish do an equally good job of enriching the blood and other body tissues with omega-3 fatty acids.
Researcher Dr Williams Harris, of the University of South Dakota,said: "We went into the project assuming that fish would be better, based on some previous literature [but] it doesn't make any difference whether you get your omega-3 fatty acids from a concentrate in a capsule or in fish - they have the same effect on enriching the tissues with omega-3."
Nevertheless, Dr Harris said he would encourage people to eat fish.
"Fish of course brings with it proteins and minerals and other factors that are good for our health that the capsules don't bring, but we weren't able to measure any of those things," he said.
Fatty Acid Facts
* Omega-3s: cannot be made by the body. They are obtained from oily fish, cod liver oil and egg. Emerging evidence shows links between low levels of omega-3 and mood, behaviour, learning and neurological disorders.
* Omega-6s: come from industrial oils used in takeaways, ready meals, chips, biscuits and ice-cream.
* Western diets have up to 15 times more omega-6 than omega-3.