A poor diet may worsen mood disorders like anxiety and depression, according to a study which is the most up to date overview confirming that certain foods can influence mental health condition.
According to the researchers, including those from the University of Gothenburg in Sweden, there are some areas where this link between diet and mental health is firmly established.
Citing examples, they said, the ability of a high fat and low carbohydrate diet to help children with epilepsy, and the effect of vitamin B12 deficiency on fatigue, poor memory, and depression, are well established in previous research.
The current study, published in the journal European Neuropsychopharmacology, also found found that a Mediterranean diet, rich in vegetables and olive oil, shows mental health benefits by offering protection against depression and anxiety.
However, for many foods such as vitamin D supplements, it noted that the evidence is inconclusive.
Foods believed to be associated with autism also do not have any evidence of being associated with improving mental health conditions, the researchers noted.
"With individual conditions, we often found very mixed evidence," said Suzanne Dickson, study co-author from the University of Gothenburg.
"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," Dickson said.
While certain foods are linked to specific mental health conditions, the researchers said little is known about why a diet involving them causes this effect.
"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," Dickson said.
Citing an example of how some food may specifically improve mental health, the researchers said nutrition in the womb and in early life can have significant effects on brain function in later life.
However, they said proving the effect of diet on mental health in the general population was more difficult.
Dickson added that the dietary effects on mental health in adults are fairly small, making it difficult to detect these effects.
The scientists speculated that dietary supplementation may only work if there are deficiencies due to a poor diet, with genetics also playing a part.
Subtle differences in metabolism between people may mean that some respond better to diet changes that others, they added.
Practical difficulties also exist which need to be overcome while testing diets, which set them apart from the routine clinical trial process.
The researchers explained with an example that participants in a clinical trial can be given dummy pills to see if there is an improvement due to the placebo effect, but they can't be easily given 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," Dickson said.