We tend to think of the digestive tract and the brain as separate systems doing separate jobs for the body; but for decades, scientists have been studying how the brain in your head is closely connected to the “second brain” in your digestive tract.
What has brought new energy to this field in the past 10 years is the discovery that the trillions of “friendly” microbes residing in the gut—bacteria, but also potentially fungi and even viruses—can affect what happens in the brain.
The latest microbiota-gut-brain research supports the idea that gut health really matters to brain health. And, more importantly, it opens up new possibilities for controlling brain health through nutrition and a healthy lifestyle.
The gut-brain dialogue
For the most part, the brain is “sealed off” from the rest of the body by the blood-brain barrier. But, in fact, the gut and the brain have an ongoing dialogue.
The main two-way channel of direct gut-brain communication is the vagus nerve, a superhighway that runs between the central and enteric nervous systems. Yet it’s becoming clear that the micro-organisms residing in the gut also contribute to the messages that reach the brain.
Meghan Hockey, accredited practising dietitian and nutrition researcher in the Food & Mood Centre at Deakin University, Australia, says, “The gut and the brain are constantly talking to one another through microbial metabolites and immune, neuronal, and metabolic pathways.”
Microbes are known to affect messages to the brain in at least three ways:
They can directly stimulate the vagus nerve.
They can produce small molecules that escape the gut and circulate through the body to affect the brain.
They can cause changes in the immune cells of the gut, which has a cascade of effects through the immune system that eventually affects the brain.
Different gut bugs, different brain-related conditions
Scientists are starting to uncover the gut correlates of brain-related conditions. For example, individuals with major depressive disorder tend to have a different set of gut microbes than non-depressed individuals.
Different patterns in gut microbial communities have also been found in people with anxiety, schizophrenia, multiple sclerosis, Parkinson’s disease, Alzheimer’s disease, and, neurodevelopmentally, even in autism spectrum disorder.
While this doesn’t mean the gut microbes caused these conditions, it does mean that scientists can start looking at whether intervening at the distant site of the gut can affect how these conditions play out—or perhaps whether it’s possible to prevent the condition in the first place in susceptible individuals.
The contributions of gut microbes are under investigation, too, in several conditions that are widely understood to be confined to the digestive tract: inflammatory bowel disease and irritable bowel syndrome.
These conditions have long been associated with depression and anxiety, and recent gut bug-related discoveries indicate the gut disruptions may be triggering the brain problems, not the other way around.
Using diet to shape gut microbes
Hockey says now that we know gut bacteria are intricately involved in gut-brain communication, it bolsters the idea that nutritional changes can have an impact on brain health.
“Diet is one of the key factors in shaping the gut microbiota and can have profound impacts on microbial diversity, abundance, and the function of different bacteria,” she says.
In Hockey’s view, this dovetails perfectly with an emerging scientific field called nutritional psychiatry, which looks at how diet relates to brain health and how nutritional interventions can play a role in treatment regimens for mental illness.
“Many things relating to our mental health are beyond our control,” she says. “But we have direct influence over what we eat, and this can, in turn, help shape our gut microbiota and, potentially, our mental health.”
Eat more plant-based foods
While there’s no diet or supplement that alone can alleviate mental illness, Hockey emphasizes that plant foods feed the gut microbes in multiple ways that support brain health.
When advising clients, she says, “As a first step, I recommend increasing the intake and variety of plant foods such as vegetables, fruits, beans, lentils, nuts, seeds, and whole grain cereals. These foods contain a variety of fibres and polyphenols that can promote the growth of beneficial bacteria within the gut.”
Include probiotics and prebiotics
Probiotics are a potential way to achieve positive changes in the gut microbiota, and some have shown promise for helping depression, although most available probiotic strains have not been studied for their specific effects on the brain or mental health. Ditto for prebiotics, which are substances that act as “food” for beneficial gut microbes.
Remember that variety is key
Rather than obsessing over a single dietary component, however, Hockey advises looking at the big picture. “Overall diet quality and patterns, rather than individual foods, matter most to mental health,” she says. “We don’t eat individual nutrients and foods in isolation; we eat meals and snacks which contain a variety of foods and nutrients that interact with one another.”
Healthy lifestyle, good bugs
Nutrition is one of the main ways to influence the composition of our gut bacteria, but other lifestyle factors can contribute, too. Exercise, for example, has a modest effect on the gut microbial community, and so do household pets (which increase gut microbial diversity) and getting enough sleep.
Bringing a multifaceted approach to shaping your gut microbes—which respond to these lifestyle tweaks day in and day out—is the best way to build a solid foundation for brain and mental health.
The brain’s trillions of friends
In science and medicine, the digestive tract and the brain have long been treated separately. But now we know the brain is intricately connected with the gut, and in particular with the trillions of microbes that reside there. By paying attention to gut microbes and modifying them through positive lifestyle changes, including nutrition, we can make sure our brains have strong support from the bottom up.
Connecting the gut-brain dots
Talk with your health care practitioner about these supplements to support both gut and brain health.
Type of supplement
modulating the immune system or generally increasing levels of potentially beneficial bacteria in the gut
increasing brain energy metabolism in certain conditions
protecting against neurodegeneration in older adults
mitigating cognitive impairments in some mental health conditions
Fuel your brain with food
Meghan Hockey, accredited practising dietitian and nutrition researcher in the Food & Mood Centre at Deakin University, Australia, offers the following nutritional tips for better gut-brain health:
Eat more plant foods—aim for 30 per week.
Change up your fruit and vegetable routine to get more diversity: if you eat a banana each day, try swapping it for another type of fruit.
Eat animal proteins, such as red meat, in moderate amounts only.
Save sweets and ultra-processed foods for special occasions.
Snack on a handful of unsalted nuts each day.
Neurotransmitters in the gut
Many of the same neurochemicals your brain uses to calibrate your mood and cognition are, in fact, also produced by gut bacteria. These include the following:
GABA (gamma aminobutyric acid)
Although these molecules are the same as the ones in the brain, not all of the gut-produced versions reach the brain, and scientists are still puzzling through how they function in the rest of the body.