Eating healthily is good for your brain and mental health, scientists have confirmed.
A review of studies on different diets found there was a 'direct relationship' between eating well and stress, mental health and brain function.
Experts from across Europe, led by the University of Gothenburg in Sweden, said this relationship was firmly established in some areas but vaguer in others.
The ketogenic diet, for example, can help children with epilepsy, while a vitamin B12 deficiency could contribute to poor memory or exhaustion.
And the Mediterranean diet – which scales back on sugar, red meat and saturated fat – can protect against anxiety and depression.
The researchers said there wasn't enough evidence explaining why foods had certain effects and that new discoveries were 'urgently needed'.
A healthy diet should contain a lot of fruits and vegetables, as well as nuts and beans and fish. Unhealthier foods like red meat and products high in sugar and salt should be reduced (stock image)
'We have found 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.
'However, many common beliefs about the health effects of certain foods are not supported by solid evidence.'
The researchers said claims such as that eating more sugar could worsen ADHD were not backed up by long-term studies.
And there have also been 'diverse' results on the effects of vitamin D supplements on autism or memory power.
The study, which involved scientists from University Medical Centre Groningen in Germany and Radboud University, University College Cork in Ireland and the University of Birmingham in England, considered findings from dozens of studies.
It found fat and sugar can worsen ADHD while fruit, veg and vitamin supplements reduced aggression – but said the proof of these links was weak.
The researchers found the Mediterranean diet could reduce depression and anxiety in people with the conditions.
But it also said that supplements of the nutrients found in the olive oil-abundant diet did not reduce symptoms in another study.
Despite finding opposing things in different studies, the researchers said it was still clear that what people eat could affect their mental health.
They wrote in the paper: 'It is becoming clear the negative consequences of a poor-quality diet can impair mental health and cognitive function, which is likely to [get worse] with age.
'Interestingly, nutrition and, in particular, malnutrition and obesity, are closely intertwined with mood regulation and stress sensitivity, suggesting a strong link between diet, metabolism and mental wellbeing.'
WHAT SHOULD A BALANCED DIET LOOK LIKE?
Meals should be based on potatoes, bread, rice, pasta or other starchy carbohydrates, ideally wholegrain, according to the NHS
• Eat at least 5 portions of a variety of fruit and vegetables every day. All fresh, frozen, dried and canned fruit and vegetables count
• Base meals on potatoes, bread, rice, pasta or other starchy carbohydrates, ideally wholegrain
• 30 grams of fibre a day: This is the same as eating all of the following: 5 portions of fruit and vegetables, 2 whole-wheat cereal biscuits, 2 thick slices of wholemeal bread and large baked potato with the skin on
• Have some dairy or dairy alternatives (such as soya drinks) choosing lower fat and lower sugar options
• Eat some beans, pulses, fish, eggs, meat and other proteins (including 2 portions of fish every week, one of which should be oily)
• Choose unsaturated oils and spreads and consuming in small amounts
• Drink 6-8 cups/glasses of water a day
• Adults should have less than 6g of salt and 20g of saturated fat for women or 30g for men a day
Source: NHS Eatwell Guide
The team suggested not getting the proper nutrients from food could mean certain chemicals may not be available for the brain to grow healthily, or that eating unhealthy food could release hormones which change people's moods.
Professor Dickson added: '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.
'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 research was published in the medical journal European Neuropsychopharmacology.
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