Almost everyone recognises them: signals from the gut indicating that you are full. One of those satiety hormones, GLP-1, is even used nowadays to make weight-loss drugs, of which Ozempic is the best-known example. ‘This shows that there is growing interest in how the gut communicates with the brain,’ explains Aarts. ‘That communication is influenced in various ways. Our large intestine contains many bacteria that produce substances. These substances influence how many satiety hormones we produce and how strong our immune system and stress response are.’
Communication in both directions
Aarts emphasises that communication does not only flow from the gut to the brain, but also the other way round. ‘Take a feeling of nervousness: that affects your gut. The same applies to falling in love and stress. But when it comes to eating behaviour, what is particularly relevant is how signals from the gut influence the brain and what role gut bacteria play.’
According to Aarts, that influence varies from person to person. ‘It depends on lifestyle, diet and environment. People who live together often have more similar gut bacteria. Because those gut bacteria are changeable, you can influence them through diet. Fibre from vegetables, fruit and legumes is important. Gut bacteria ferment these into short-chain fatty acids: substances that stimulate the production of satiety hormones. So you can influence the communication between the gut and the brain through diet.’
Obesity and ADHD
Over the next five years, Aarts’s research will focus on people who are overweight. ‘That’s more than 50% of the Dutch population,’ she says. ‘We live in a food-rich environment that constantly tempts us. But eating behaviour isn’t driven solely by hunger. Emotion, motivation and impulsivity also play a role.’
She is investigating whether higher order cognitive processes involved in eating behaviour contribute to a loss of control. ‘For example, processes that, when we eat tasty food, prevent us from properly regulating our emotional system or reward system. I want to know: do the gut-brain axis, satiety hormones — such as GLP-1 — and short-chain fatty acids play a role in this?’
In doing so, Aarts is looking specifically at people with ADHD. ‘The prevalence of obesity is around 70% higher among people with ADHD,’ explains Aarts. ‘It is more difficult for them to maintain control. That’s because impulsivity plays a greater role for them, also when it comes to eating. They are also at greater risk of metabolic changes and type 2 diabetes, and often have lower levels of short-chain fatty acids in their stools. Greater knowledge of gut-brain communication is therefore important in order to help these people in a more targeted way.’
Due to the societal relevance of her research, Aarts recently received a prestigious Vici grant from the Dutch Research Council (NWO). ‘Research into satiety hormones and short-chain fatty acids in people with ADHD in relation to eating behaviour has not been done before,’ she notes.
Her research consists of two parts. ‘We are comparing people with obesity and ADHD who are receiving a GLP-1 medication with a placebo group. Using an MRI scanner, we then examine the effects on higher order cognitive processes involved in eating behaviour and identify the differences compared to people without ADHD. In parallel, one group is given a dietary fibre (chicory root, consisting mainly of the prebiotic fibre inulin) for three months, whilst the other group receives a placebo. I’m curious to see whether this could be a preventive approach to regulating eating behaviour, so that we don’t all have to rely on Ozempic for the rest of our lives.’
Reserves for scarcity
The question still remains: why do we keep eating when we’re no longer hungry? According to Aarts, the explanation lies partly in our evolution. ‘For more than 90% of our evolutionary history, we lived in scarcity. Overeating made sense: you didn’t know when food would be available again. That urge is still in our brains.’
According to her, our brains are still focused on calorie-rich food. ‘Fats and sugars used to be scarce; you had to work hard to get them. Now they’re available everywhere, but our brains haven’t fully adapted to that yet. Some people cope with it better than others.’
Stress also plays a role. ‘Some people eat more when they’re stressed. From an evolutionary perspective, that makes sense: stress requires energy. The gut then sends fewer signals to stop eating. The solution lies in better managing stress. People with ADHD find this more difficult. That is why I hope my research will help me better understand how those mechanisms work.’