We’ve long been told “you are what you eat”, with diet influencing our heart health, weight and even cancer risk.
Yet how our food choices impact our mental wellbeing was less understood.
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Scientists from the University of Gothenburg looked at an array of studies investigating the link between diet and a range of mental-health disorders.
They found “reasonable evidence” a Mediterranean diet rich in fish, olive oil and vegetables could ward off anxiety and depression.
Lacking vitamin B12 - an essential nutrient found in animal products - may also trigger fatigue, a foggy memory and low mood, the team added.
“We found there is increasing evidence of a link between a poor diet and the worsening of mood disorders, including anxiety and depression,” lead author Professor Suzanne Dickson said.
“The message is the effects of diet on mental health are real, but we need to be careful about jumping to conclusions on the base of provisional evidence.”
It is increasingly accepted that “what is good for the heart is good for the brain”.
Yet the scientists felt when it comes to our mental health, the so-called benefits of certain foods are exaggerated and lack evidence.
The structure and function of the brain requires a healthy intake of fat, protein, vitamins and minerals, they wrote in the European Neuropsychopharmacology journal.
It is therefore “logical food intake and quality would have an impact on brain function, which makes diet a modifiable variable to target mental health”.
After analysing a range of studies, the scientists concluded there is “reasonable evidence” a Mediterranean diet minimises the risk of certain mental-health issues, warranting further investigation.
They also found a “strong link” vitamin B12 deficiency in childhood increases the risk of depression in later life. The vitamin is thought to play a key role in neurodevelopment.
Results also support the low-carb, high-fat Keto diet - popular with celebrities like Gwyneth Paltrow and Halle Berry - eases seizures in epileptic children.
When it comes to other claims, like diets improve autism or ADHD symptoms, the scientists stress evidence is inconclusive.
“With individual conditions, we often found very mixed evidence”, Professor Dickson said.
“We can see an increase in the quantity of refined sugar in the diet seems to increase ADHD and hyperactivity, whereas eating more fresh fruit and vegetables seems to protect against these conditions.
“But there are comparatively few studies and many don't last long enough to show long-term effects”.
While further research is required, the scientists admit such studies are difficult to carry out.
“In healthy adults dietary effects on mental health are fairly small and that makes detecting these effects difficult,” Professor Dickson said.
“It may be dietary supplementation only works if there are deficiencies due to a poor diet.
“We also need to consider genetics; subtle differences in metabolism may mean some people respond better to changes in diet that others.
“There are also practical difficulties which need to be overcome in testing diets.
“We can give someone a dummy pill to see if there is an improvement due to the placebo effect, but you can't easily give people dummy food.”
With the microbiome also a growing area of research, some believe gut health may influence mental wellbeing.
“Many high-quality findings (mainly from animal studies) have been published in top-notch journals in recent years,” Professor Andreas Reif, chair of the European College of Neuropsychopharmacology scientific programme committee, said.
“This contrasts with the comparative shortage of hard evidence on how nutrition and mental health are connected in humans.
“This leaves room for speculation and flawed science.
“This comprehensive review is therefore much-needed as it sheds light on hypes and hopes, facts and fiction.”