The Big Idea: Can What You Eat Change Your Mind?

Probotic drinks, brain-boosting superfoods, gut-healthy snack bars: every day we are bombarded with information about what we should and should not eat. Some foods switch abruptly between these categories depending on who you ask: If a gut-healthy snack bar is ultra-processed, does that negate its benefits? You’d be forgiven for thinking that every food choice you make has immediate, direct consequences for your health. And, increasingly, your brain. Does Broccoli Make You Brighter? Can a pickle really be a pick-me-up?

There is a scientific truth in this magic. By now you may have heard that people with poorer mental health, such as those who suffer from depression or anxiety, have a different balance of bacteria in their gut – a decrease in the richness and diversity of their microbiome. This ties in nicely with elegant (if somewhat crude) laboratory experiments in which transplant feces of people with depression in rats leads to ‘depression-like’ behavior. How is this possible? This is because your intestines send signals to your brain via the brain vagus nerveyour immune system and other resources, passing on information about how things are going there. And so, the theory goes, a less diverse microbiome could be the cause of a depressed or anxious brain. Maybe a gut-healthy snack bar would really help?

Maybe. But the bacterial world we live in is not so tidy. Your microbiome, unlike the microbiomes of rats living in a germ-free laboratory, is messy. It is influenced by many factors other than the snack you just ate, including your appetite genesyour history of taking certain medicationsand even yours social interactions – every day, all your life. Disentangling cause and effect is difficult. Someone with poor mental health may have a less diverse microbiome simply because they do Also more likely you ate a less varied diet, interacted less with other peopleor taken antidepressants. Probiotics can promote gut microbiome diversity, but whether this is enough to impact the mental health of most people is unclear.

And in any case, the brain is far from just a passive receiver. Signals from the intestines are filtered and changed by the brain itself: dampened or amplified by neural processes. The influence your feelings have on higher processes depends on the state of your brain, its priorities and its predictions. Ultimately, it’s the brain that matters.

And what matters most to the brain is survival. Signals from the body are incorporated into your brain’s broader representation of the world – the world around you and the world within you – so that your brain can monitor and adapt your behavior to help you survive. In the case of food, two pieces of information are absolutely essential for survival: Does something you eat contain energy that your body can use? And is there anything in it that could make you sick?

When you take a sip of a sugary drink, your brain processes the sensory information – the taste, bubbles on the tongue, and so on. But it doesn’t stop there. Once you’ve swallowed, your digestive system tells the brain’s deep reward structures (again via the vagus nerve) that what you’ve eaten contains calories. You also register that it is moisturizing. These signals reinforce your behavior: they tell your brain to drink that drink again in the future.

This is an age-old form of reward-based learning. It keeps us alive. It helps us find foods that satisfy us and drinks that quench our thirst. It also happens unconsciously: When you’re in the store thinking about which food is better for your brain, your brain also subtly and secretly influences your decision based on its past experiences with rewards.

Your life experience with food and drink differs from mine, so the accumulated information about reward is somewhat idiosyncratic. Not only that, but different brains may also be more or less sensitive to food-related signals from the body. This means that people adjust their energy intake (through food) or expenditure (exercise) to varying degrees. Most of the genes often associated with obesity work in the brain, and part of the appetite-reducing effect of drugs like Ozempic may be due to altered dopamine signaling.

So much for the things you like, what about the things you hate? As with rewards, we differ in our sensitivity to punishing, aversive signals from the body, such as nausea. And just as our brain learns from internal signals what it needs to keep us alive – things that contain calories, fat, hydration – it also learns what we need to avoid to protect ourselves from disease.

That’s why, once something has made you sick, your brain learns strongly and quickly to stay away from it. One Sunday I went out for a delicious brunch with smoked salmon and scrambled eggs. I then got a horrible case of food poisoning. Nausea and vomiting are a kind of internal punishment: a clear message from the intestines to the brain that we have ingested something bad.

It seems likely that once we’ve had a disgusting experience with something, it’s much harder to “unlearn” it than other types of experiences that tend to lose their power over time. In other words, disgust can be more persistent than other emotions. To this day, I’m put off by the thought of eating scrambled eggs and smoked salmon together, even though it’s relatively unlikely that it would make me sick again.

Disgust can be a blessing if it helps you avoid something potentially toxic. But it can also be a burden, underlying phobias, food avoidance and some forms of post-traumatic stress. Sometimes your brain needs aversive signals from the gut to keep you healthy, but sometimes their influence can soften somewhat. Recently, neuroscientists in Aarhus, Denmark discovered that stomach-brain coupling existed strengthened in people with increased symptoms of stress, anxiety and depression.

Can what you eat change your mind? In a sense, yes. The most special property of the gut-brain axis is that it is plastic. In the same way that your brain is constantly taking in new information about the world around you, strengthening or changing connections through neuroplasticity, it also adapts to signals from within. Dodgy leftovers, or a specially formulated drink that boosts your microbiome in some way, can indeed have a measurable or observable effect. However, the precise way in which food affects your mood and well-being depends largely on your specific brain: its past experiences with reward and punishment, its genetic and environmental characteristics. There is no single answer, so beware of strong claims. What we know for sure is that no matter what is on the menu, your brain really listens to its feelings.

Camilla Nord is a neuroscientist and author of The Balanced Brain (Penguin)

Read more

Intestine by Giulia Enders (writer, £12.99)

Dark matter: the new science of the microbiome by James Kinross (Penguin, £20)

The Inflamed Mind: A Radical New Approach to Depression by Edward Bullmore (Octopus, £9.99)