Diet and Mental Health: The Evidence to Date

Deborah Brauser

January 15, 2020

Although diet can influence mental health and cognitive function, evidence of benefit for many specific diets is actually quite weak, a new review suggests.

In the "most up-to-date overview of the new field of nutritional psychiatry," investigators with the Nutrition Network of the European College of Neuropsychopharmacology (ECNP) found strong evidence that following a ketogenic diet of high fat and low carbohydrates may reduce seizures in children with epilepsy. They also confirmed that the Mediterranean diet guards against depression and anxiety.

In addition, there is a strong link between vitamin B12 deficiency and an increased risk of fatigue, depression, and memory problems.

However, evidence of any efficacy of vitamin D supplements or any nutrient in mitigating symptoms of autism or attention deficit hyperactivity disorder (ADHD) is "inconclusive," the researchers note.

"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," principal investigator Suzanne L. Dickson, PhD, Institute of Neuroscience and Physiology, University of Gothenburg, Sweden, said in a press release.

"However, many common beliefs about the health effects of certain foods are not supported by solid evidence...[and] with individual conditions, we often found very mixed evidence," she added.

The findings were published in the December issue of European Neuropsychopharmacology.

"Reason for Optimism"

As previously reported by Medscape Medical News, there have been numerous studies linking diet and mental health, including the Dietary Approaches to Stop Hypertension (DASH) diet that has been tied to lower depression risk.

Large population-based studies have also suggested links between mental health and specific nutrients, but they often "do not demonstrate cause, benefit, or remedy," the current investigators note. However, they add, there is "reason for optimism."

The review showed there are "strong correlations" between a healthy diet and better mental health, especially with increased consumption of fresh fruits and vegetables.

However, although several recent studies and analyses have shown a link between the Mediterranean diet and reduced risk for depression or even a protective effect against the disease, a meta-analysis of 14 cohort studies published in 2019 and consisting of more than 56,000 participants showed no significant association.

Still, "when cross-sectional studies were analyzed an inverse significant association was found between depression odds and the adherence to the Mediterranean diet," the researchers write.

"Together, these studies provide a reasonable evidence base to further investigate the effect of specific dietary interventions on mental health," they add.

Take Your Vitamins?

When assessing the effect of vitamins, investigators found that "the link is strongest" between deficiencies in B12 and fatigue, lethargy, depression, mania, psychosis, and depression; in thiamine and CNS symptoms; in folic acid and neurodevelopment in utero and infancy and depression in adults; and niacin and dementia.

"Yet even for these deficiencies, the role of mild 'subclinical' or multiple mild deficiencies in the genesis of mental dysfunction is unclear," the researchers write.

For example, research into the role of vitamin D supplementation has shown conflicting results.

Overall, the investigators note that a diet rich in polyphenols, polyunsaturated fatty acids (PUFAs), and nutritional supplements "has been reported to exert favorable effects on mental health, including on cognitive performance, mood, stress, reactivity, and neuroinflammation."

As for ADHD and autism, several studies have suggested a benefit from particular dietary elements or risk from a poor diet.

"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," Dickson said.

However, "there are comparatively few studies, and many of them don't last long enough to show long-term effects," she added.

"Results of nutritional autism are also very diverse and have not been subjected to rigorous meta-analyses," the investigators note.

Don't Jump to Conclusions

On the other hand, there is strong evidence that nutritional decisions in early life can affect later-life brain function. A high-quality diet is strongly linked to a reduced risk for cognitive decline and improved cognitive function in older adults, whereas a poor diet appears to increase cognitive decline and other health problems.

However, "at present we lack a detailed understanding of the metabolic and cellular mechanisms that underpin these associations," the researchers note.

This is also true of gut microbiota. Although "it is clear" that it can potentially affect mental health, the mechanism "has yet to be elucidated."

Future research will need to focus on "provable dietary causes" associated with psychiatric conditions, the investigators note in the release.

"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," said Dickson.

She noted that some individuals may respond better than others to dietary changes because of subtle differences in their metabolism.

In addition, "a food is not a drug, so it needs to be tested differently to a drug." For example, there are no "placebo" foods that can be used in a randomized controlled trial, she said.  

"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 basis of provisional evidence," Dickson said.

Routine Clinical Care?

Commenting on the findings for Medscape Medical News, Wolfgang Marx, PhD, head of the Nutraceutical Research stream at the Food & Mood Centre, Deakin University, Geelong, Australia, said the investigators provided a "clear and objective overview" of the current state of nutritional psychiatry.

This includes the "largely consistent epidemiological, clinical, and meta-analytic data supporting the link between diet quality and depression," he said. "Crucially, the authors also identify key research questions that are required to be addressed to advance the field, including the need for large clinical trials to further test efficacy and mechanistic studies to improve our understanding of how diet, and dietary components, may improve symptoms."

Marx, who was not involved with the research, noted that the researchers "rightly highlight" some of the challenges of developing evidence in this area, such as the difficulty of blinding dietary interventions.

"It is important to note that in addition to the emerging data supporting the role of diet in reducing risk of mental illness, dietary interventions can be effective in reducing the elevated risk of cardiometabolic diseases common in people with mental illness," he said.

"While more research is required to support the role of dietary interventions as a therapy for mental disorders, there is already sufficient evidence to suggest that dietary interventions, as well as other lifestyle interventions such as physical activity, should be incorporated into routine clinical care from the outset to address the physical comorbidities associated with mental illness," Marx said.

A "Much Needed" Review

"This review is an important and scholarly contribution," Andreas Reif, MD, PhD, University Hospital Frankfurt, Germany, said in the release.

Reif, who was not involved with the research, is also chair of the ECNP Scientific Program Committee.

"The interface between gut and the brain on the one side and diet and mental health on the other side is one of the most debated issues in biological psychiatry at the moment, and is an exciting development which has gained momentum in the last decade," he noted.

He added that the high-quality evidence from mostly animal studies published in recent years contrasts with the "comparative shortage of hard evidence" in humans.

"This leaves room for speculation and flawed science," Reif said. "This comprehensive review is therefore much needed as it sheds light on hypes and hopes, facts and fiction."

"As the potential societal impact of this rapidly developing field is enormous, we must be scientifically sound in making our recommendations," he concluded.

The study was supported by the ECNP. Dickson and Marx have reported no relevant financial relationships.

Eur Neuropsychopharmacol. 2019;29:1321-1332. Full article

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