A new expert review confirms a link between a poor diet and mood disorders.
However, researchers in the new field of nutritional psychiatry caution that the evidence for many diets is comparatively weak.
“We have found that there is increasing evidence of a link between a poor diet and the worsening of mood disorders, including anxiety and depression,” said Professor Suzanne Dickson of the University of Gothenburg in Sweden and lead author of the new paper. “However, many common beliefs about the health effects of certain foods are not supported by solid evidence.”
The researchers did find that there are some areas where this link between diet and mental health is firmly established, such as the ability of a high fat and low carbohydrate diet — the ketogenic diet — to help children with epilepsy, and the effect of vitamin B12 deficiency on fatigue, poor memory, and depression.
They also found that there is evidence that a Mediterranean diet, rich in vegetables and olive oil, shows mental health benefits, such as giving some protection against depression and anxiety.
“With individual conditions, we often found very mixed evidence,” said Dickson. “With ADHD, for example, 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 of them don’t last long enough to show long-term effects.”
She adds that while the study confirms that certain foods can be associated with a mental health condition, this tells us little about why the food causes this effect. Researchers note that the need to link mental health effects with provable dietary causes needs to be the main focus of future research in nutritional psychiatry.
“There is a general belief that dietary advice for mental health is based on solid scientific evidence. In reality, it is very difficult to prove that specific diets or specific dietary components contribute to mental health,” she said.
The scientists did confirm that some foods had readily provable links to mental health, for example, that nutrition in the womb and in early life can have significant effects on brain function in later life. However, proving the effect of diet on mental health in the general population was more difficult, they said.
“In healthy adults dietary effects on mental health are fairly small, and that makes detecting these effects difficult — it may be that dietary supplementation only works if there are deficiencies due to a poor diet,” Dickson said. “We also need to consider genetics. Subtle differences in metabolism may mean that some people respond better to changes in diet that others.”
“There are also practical difficulties which need to be overcome in testing diets,” she continued. “A food is not a drug, so it needs to be tested differently to a drug. 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. Nutritional psychiatry is a new field. The message of this paper is that the effects of diet on mental health are real, but that we need to be careful about jumping to conclusions on the base of provisional evidence. We need more studies on the long-term effects of everyday diets.”
The study was published in the journal European Neuropsychopharmacology.