"There's lots of hype about the Mediterranean diet [fruits, vegetables, whole grains, olive oil, nuts, fish] but the traditional Norwegian diet [fish, shellfish, game, root vegetables, dairy products, whole-wheat bread] and the traditional Japanese diet [fish, tofu, rice] appear to be just as protective" of mental health, he said.
The association between diet and mental well-being may start even before birth. A 2013 study of more than 23,000 mothers and their children, led by Berk's frequent collaborator and Deakin colleague Felice Jacka, suggests a link between a mother's consumption of sweets and processed foods during pregnancy and behavioral and mental health issues in her child at age 5.
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It's unclear how diet relates to mental health, said Rif El-Mallakh, a professor of psychiatry at the University of Louisville School of Medicine. "There seems to be a clear link, but it's an association — it doesn't tell you cause and effect," he said. "We don't know which is the chicken and which is the egg."
It could be, he said, that mood disorders change how and what people choose to eat.
But an alternate theory is that the relationship works the other way: Certain foods, or their absence, may contribute to poor mental health. For example, studies in people and rats have linked zinc deficiency to depression. Also, illnesses that cause deficiencies — including celiac disease, an autoimmune disease in which the body reacts to gluten — have shown associations with mood disorders.
"There's a two-way street between what's going on in the gut and what's going on in the brain," said Linda A. Lee, director of the Johns Hopkins Integrative Medicine and Digestive Center — and recent research points to bacteria as possible middlemen in this back-and-forth. Gut bacteria are known to make most of the body's serotonin, one of several chemicals that regulate mood, and the bugs may even have a hand in shaping behavior. A 2011 study in mice for example, showed that swapping the gut bacteria of two strains of mice — one known for its daring behavior, the other for its fearfulness and shyness — could make the timid mice more willing to explore and the bold mice more anxious and hesitant.