Healthy guts for a healthy brain

Healthy and safe food

Every human has their own personal bacterial population, also known as the microbiome. Photo: Shutterstock

Estimated reading time: 8 minutes

Can the symptoms of ADHD or Parkinson’s disease be combated by changing your diet? And do gut bacteria play a role in stress or the appetite of people with obesity? More and more studies point to a link between these bacteria, nutrition and behaviour. Now Wageningen scientists and their European colleagues are trying to get a better understanding of this link in a large research project.

While you read this article, millions of bacteria are hard at work in your intestines, helping you digest your food, releasing vitamins and making sure you can get more energy from your meals. In fact, every human being walks around with their own personal bacterial population. This unique collection of intestinal bacteria is termed the microbiome. That microbiome could easily be a key factor not just for your digestive system but also for your behaviour and mental health. The mental health link has to do with what is known as the ‘gut-brain axis’, explains Wageningen microbiologist and associate professor Clara Belzer. She has been studying the human gut microbiome for more than ten years. “Our brain and guts are in direct contact with one another. That is why scientists suspect that what happens in your intestines has a big effect on your behaviour. Research shows that people with ADHD, Alzheimer’s or obesity have distinctly different microbiomes. There are also studies showing behavioural improvements in people in these categories when they are put on certain diets.”

Research shows that people with ADHD, Alzheimer’s or obesity have distinctly different microbiomes

Researchers are looking into nutrition and behaviours of children and the elderly. Photo: Shutterstock

Belzer represents Wageningen University & Research (WUR) in Eat2beNICE, a large-scale European research project that examines the link between nutrition and behaviour. Nineteen research groups from eleven different countries are taking part. As Belzer explains, Eat2beNICE is a broad study. “The research teams are looking at children and the elderly, at healthy people and the sick. In Wageningen we’re focusing specifically on the microbiomes of all those groups.”

Stool samples

Part of the research project consists of experiments in which subjects are asked to eat more vegetables, fruit, nuts or olive oil. This Mediterranean diet is known to be good for your health. Belzer’s task is to analyse stool samples from the people taking part in the experiment. “That shows us the relationship between their lifestyle and behaviour on the one hand and their microbiome on the other.” The Wageningen researchers are also collaborating with Radboud University in Nijmegen in a long-term study following children into puberty. Every two to three years, the scientists collect new faecal samples. The children and their parents also have to fill in questionnaires about their lifestyle and behaviour. Belzer: “In principle these are healthy children without behavioural problems. But you still see some of them scoring slightly higher for certain kinds of behaviour, such as characteristics that are typical of ADHD. We’re looking for links with the bacteria in their intestines.”

New field

The field Belzer and her colleagues are working in is quite new. Willem de Vos, a professor at WUR, was one of the first people to publish on the human microbiome some fifteen years ago. There are now various centres around the world specialising in microbiome research. “It was not that long ago that each and every bacterium in our body was seen as pathogenic and a bad thing”, says Belzer. “Nowadays, scientists advocate cutting the use of antibiotics and say it’s actually good for children to play in the mud.”

Healthy nutrition is important from a young age and partially defines the behaviour. Photo: Shutterstock

Research into the role the microbiome plays in our health is important, says Belzer, because autoimmune diseases, obesity and behavioural disorders are becoming increasingly common in the Western world. “They could be related to the change in our diet. For example, we see that the composition of the microbiome is much less diverse in Western countries: we have far fewer different species of bacteria in our intestines.” The interaction between the bacteria and their host – in this case humans – is finely tuned, she stresses. “The same bacteria have slightly different effects in humans than in mice. That is the result of many years of evolution. But now a lot of changes are taking place in short order in the West. Our diets are different, we are using antibiotics and more children are being born by caesarean, which means they get different bacteria at birth. As a result, you see things going wrong now in that interaction between humans and bacteria. Perhaps that is one explanation for all those diseases of affluence.”


If we find out what the various gut bacteria do exactly, we can steer that development in the right direction, for example by adding ‘beneficial’ bacteria during a treatment. “Imagine you could give children with ADHD a probiotic drink that made them less likely to exhibit impulsive behaviour. Or take the example of overweight people, who sometimes have a deficiency in their sense of satiation. Giving them a bacterium that counteracts that can help them lose weight, and might avoid the need for an operation to reduce the stomach.”

Scientists say it’s actually good for children to play in the mud

Belzer mentions a study on faecal transplants in mice. “If you insert the faeces of an obese mouse in a thin mouse, the thin mouse starts to gain weight even though it doesn’t eat more or exercise less! That suggests that the gut bacteria in fat mice have adapted to extract as much energy as possible from food.”

Chicken-and-egg case

Belzer stresses that she and her partners have not yet finished their project. The study is due to continue until the end of 2023, but the initial results are very promising. “We see that certain kinds of human behaviour are indeed commonly associated with certain bacteria. The next question of course is how that effect works. We don’t really have any idea at present. Anyway, it is also a chicken-and-egg case: which is causing which? After all, many people with a disease that affects their behaviour also start eating different foods.”

In addition to vitamins, bacteria are able to produce hormones and neurotransmitters

Adding beneficial bacteria to the nutrition of overweight people can help them with any disturbances in their sense of satiation. Photo: Shutterstock

The scientists do have some theories. “We all agree that bacteria have a big influence on the layer of mucus in the intestines. You often find that healthy people have a thicker layer of mucus in their intestines. A thinner layer means more substances can pass through the gut wall. We see an association there with behavioural problems.” What is more, it is known that in addition to vitamins, bacteria are able to produce hormones and neurotransmitters. “These substances can also affect our behaviour. But we don’t yet know which bacterium is responsible for what exactly, or whether those substances produced in the intestines actually reach the brain. We’re still researching that.”

The role of policy

One thing is certain: Eat2beNICE is generating a great deal of new knowledge for the Netherlands and Europe as a whole. That knowledge is useful for both policymakers and scientists. “The database with faecal samples and microbiomes that we are creating in this project will be of benefit for years to come. We are making all the data freely available so that other researchers can use it.” What advice does Belzer have for European policymakers? “I hope to see a much bigger role for diet in combating diseases, for example by referring people with certain diseases to dieticians rather than just having them treated by doctors. The crucial role of nutrition in our health is becoming increasingly clear. And just offering optional dietary advice is often not enough.” She also emphasises that it is one thing to know which choices you should be making, but the choices you actually make also depend on the financial situation and the range of food that is available. “Which products are in the supermarket, what adverts are aimed at children and how expensive are fresh fruit and vegetables? Policy can have a big influence on these factors.”

European research context

Eat2beNICE addresses the following European policy challenges: Public health: how can we prevent diseases and combat obesity? Wageningen University & Research groups involved: Laboratory of Microbiology European and other countries involved: Brazil, Estonia, Germany, Hungary, the Netherlands, Norway, Spain, Sweden and the United States Duration: 2017 – 2023

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