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How diet affects mental health: What's the evidence?

European College of Neuropsychopharmacology

diet

A new expert review confirms that diet significantly influences mental health and wellbeing, but cautions that the evidence for many diets is comparatively weak.

FAB RESEARCH COMMENT:

This new expert review evaluates the evidence that nutrition and diet can affect mental health, essentially concluding that 'more evidence is still needed' for many nutritional or dietary interventions to become mainstream in psychiatry.  However, they did find some links that are firmly established, such as:

  • the benefit of ketogenic diets for controlling epilepsy
  • the fact that Vitamin B12 deficiency can cause psychological symptoms - including fatigue, depression and memory problems
And they also found good evidence that
  • symptoms of depression, anxiety and related conditions can be reduced by 'Mediterranean-type' diets (high in vegetables, fruits, wholegrains, nuts & seeds; and including fish & seafood, eggs, and some milk and dairy products and unprocessed meats)
  • 'ADHD-type symptoms' can often be reduced by excluding 'trigger' foods (which vary between individuals), and/or artificial additives, as well as an increased intake of the long-chain omega-3 fats found in fish and seafood.    
In other areas, the key issue is the relative lack of randomised, double-blind, placebo-controlled trials involving people with well-defined and specific forms of psychiatric disorders.

(The authors do flag individual differences - including genetics - as an important issue. But that clear findings are hampered by the fact that psychiatric diagnoses are essentially descriptive, with no objective markers, and show very high co-occurence, which further adds to the variability within any population defined by such diagnoses).

The basic fact that nutrition affects brain development and function is well supported by basic scientific evidence, and an abundance of animal studies (as well as some human studies) show not only definitive effects, but actual mechanisms. 

Likewise, 'evidence of association' - from numerous well-designed population studies, case-control studies and other types of research - also point to diet and nutrition as potentially key factors in almost every neurodevelopmental and psychiatric disorder. 

However, the key issue is that within psychiatry (as in other branches of medicine), randomised, double-blind, placebo-controlled trials are still regarded as the only definitive form of evidence for cause and effect. These are very well-suited for studying the (short-term) effects of synthetic drugs or other forms of medical intervention, but have many serious limitations when it comes to studying the effects of nutrition and diet on health and wellbeing. 

This means that a narrow approach - relying only on clinical trials (and systematic reviews and meta-analyses of these) - inevitably limits the firm conclusions that can be drawn. So the equally inevitable conclusion is that 'more research is needed' before nutritional interventions can be adopted into standard clinical practice. 

At the public health level, a similar emphasis on the need for randomised controlled trial evidence continues to prevent action to remedy even well-documented deficiencies of essential nutrients (such as Vitamin D). However, for some issues, such trials are neither possible, nor necessary (as with establishing a causal link between tobacco smoking and lung cancer). 

Given the breadth and depth of the evidence that poor nutrition plays a contributory role in mental - as well as physical - health disorders, and the increasing prevalence and costs of these conditions, there is a strong case already for public health policymakers to place more emphasis at the general population level on improving diet quality, and minimising essential nutrient deficiencies.

Meanwhile, however, this review provides a good summary of both the existing evidence for nutritional approaches to the management of psychiatric disorders, and the additional research needed to progress further the emerging, and much-needed, field of nutritional psychiatry.

For details of this research, see:

3 January 2020 - MedicalXpress

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A new expert review confirms that diet significantly influences mental health and wellbeing, but cautions that the evidence for many diets is comparatively weak. This, the most up to date overview of the new field of Nutritional Psychiatry, is produced, by the Nutrition Network of the ECNP and is published in the peer-reviewed journal European Neuropsychopharmacology.

Lead author, Professor Suzanne Dickson (University of Gothenburg, Sweden) said: "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. However, many common beliefs about the health effects of certain foods are not supported by solid evidence."

The researchers found 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 (a 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 good evidence that

  • a Mediterranean diet, rich in vegetables and olive oil, shows mental health benefits, such as giving some protection against depression and anxiety.


However, for many foods or supplements, the evidence is inconclusive, as for example with the use of vitamin D supplements, or with foods believed to be associated with ADHD or autism.

"With individual conditions, we often found very mixed evidence," said Suzanne 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."

The study confirms that while certain foods can be associated with a mental health condition, this tells us little about why the food causes this effect.

It concludes that the need to link mental health effects with provable dietary causes needs to be the main focus of future research in nutritional psychiatry.

Professor Dickson continued: "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".

The scientists confirmed 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.
Proving the effect of diet on mental health in the general population was more difficult.  Suzanne Dickson 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.

"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. 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."

Commenting, the chair of the ECNP Scientific Programme Committee, Professor Andreas Reif (University Hospital, Frankfurt am Main), said:

"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.

Many high-quality findings (mainly from animal studies) have been published in top notch journals in recent years, but 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 in the new field of Nutritional Psychiatry.

As the potential societal impact of this rapidly developing field is enormous, we must be scientifically sound in making our recommendations. This review is an important and scholarly contribution."