These two organs are connected both physically and biochemically in a number of different ways.
The Vagus Nerve and the Nervous System
Neurons are cells found in your brain and central nervous system that tell your body how to behave. There are approximately 100 billion neurons in the human brain.
Interestingly, your gut contains 500 million neurons, which are connected to your brain through nerves in your nervous system (5Trusted Source).
For example, in animal studies, stress inhibits the signals sent through the vagus nerve and also causes gastrointestinal problems (8Trusted Source).
Similarly, one study in humans found that people with irritable bowel syndrome (IBS) or Crohn’s disease had reduced vagal tone, indicating a reduced function of the vagus nerve (9Trusted Source).
An interesting study in mice found that feeding them a probiotic reduced the amount of stress hormone in their blood. However, when their vagus nerve was cut, the probiotic had no effect (10Trusted Source).
This suggests that the vagus nerve is important in the gut-brain axis and its role in stress.
Your gut and brain are also connected through chemicals called neurotransmitters.
Neurotransmitters produced in the brain control feelings and emotions.
For example, the neurotransmitter serotonin contributes to feelings of happiness and also helps control your body clock (11Trusted Source).
Interestingly, many of these neurotransmitters are also produced by your gut cells and the trillions of microbes living there. A large proportion of serotonin is produced in the gut (12Trusted Source).
Your gut microbes also produce a neurotransmitter called gamma-aminobutyric acid (GABA), which helps control feelings of fear and anxiety (13Trusted Source).
Studies have shown that certain probiotics can increase the production of GABA and reduce anxiety and depression-like behaviour (14Trusted Source).
Gut Microbes (Microbiome) Make Other Chemicals That Affect the Brain
The trillions of microbes that live in your gut also make other chemicals that affect how your brain works (15Trusted Source).
They make SCFA by digesting fibre. SCFA affect brain function in a number of ways, such as reducing appetite.
One study found that consuming propionate can reduce food intake and reduce the activity in the brain related to reward from high-energy food (17Trusted Source).
Another SCFA, butyrate, and the microbes that produce it are also important for forming the barrier between the brain and the blood, which is called the blood-brain barrier (18Trusted Source).
Gut microbes also metabolize bile acids and amino acids to produce other chemicals that affect the brain (15Trusted Source).
Bile acids are chemicals made by the liver that are normally involved in absorbing dietary fats. However, they may also affect the brain.
Gut Microbes Affect Inflammation
Your gut-brain axis is also connected through the immune system.
Gut and gut microbes play an important role in your immune system and inflammation by controlling what is passed into the body and what is excreted (21Trusted Source).
If your immune system is switched on for too long, it can lead to inflammation, which is associated with a number of brain disorders like depression and Alzheimer’s disease (22Trusted Source).
Lipopolysaccharide (LPS) is an inflammatory toxin made by certain bacteria. It can cause inflammation if too much of it passes from the gut into the blood.
This can happen when the gut barrier becomes leaky, which allows bacteria and LPS to cross over into the blood.
Inflammation and high LPS in the blood have been associated with a number of brain disorders including severe depression, dementia and schizophrenia (23Trusted Source)
Your gut and brain are connected physically through millions of nerves, most importantly the vagus nerve. The gut and its microbes also control inflammation and make many different compounds that can affect brain health.
Probiotics, Prebiotics and the Gut-Brain Axis
Gut bacteria affect brain health, so changing your gut bacteria may improve your brain health.
Probiotics are live bacteria that impart health benefits if eaten. However, not all probiotics are the same.
Probiotics that affect the brain are often referred to as “psychobiotics” (24Trusted Source).
One small study of people with irritable bowel syndrome and mild-to-moderate anxiety or depression found that taking a probiotic called Bifidobacterium longum NCC3001 for six weeks significantly improved symptoms (27Trusted Source).
Prebiotics, which are typically fibres that are fermented by your gut bacteria, may also affect brain health.
One study found that taking a prebiotic called galactooligosaccharides for three weeks significantly reduced the amount of stress hormone in the body, called cortisol (28Trusted Source).
Probiotics that affect the brain are also called psychobiotics. Both probiotics and prebiotics have been shown to reduce levels of anxiety, stress and depression.
What Foods Help the Gut-Brain Axis?
Health Tip: The Foods Your Brain Craves
A few groups of foods are specifically beneficial for the gut-brain axis.
Here are some of the most important ones:
Omega-3 fats: These fats are found in oily fish and also in high quantities in the human brain. Studies in humans and animals show that omega-3s can increase good bacteria in the gut and reduce risk of brain disorders (29Trusted Source, 30Trusted Source, 31Trusted Source).
Fermented foods: Yogurt, kefir, sauerkraut and cheese all contain healthy microbes such as lactic acid bacteria. Fermented foods have been shown to alter brain activity (32Trusted Source).
High-fiber foods: Whole grains, nuts, seeds, fruits and vegetables all contain prebiotic fibers that are good for your gut bacteria. Prebiotics can reduce stress hormone in humans (33Trusted Source).
Polyphenol-rich foods: Cocoa, green tea, olive oil and coffee all contain polyphenols, which are plant chemicals that are digested by your gut bacteria. Polyphenols increase healthy gut bacteria and may improve cognition (34Trusted Source, 35Trusted Source).
Tryptophan-rich foods: Tryptophan is an amino acid that is converted into the neurotransmitter serotonin. Foods that are high in tryptophan include turkey, eggs and cheese.
A number of foods such as oily fish, fermented foods and high-fiber foods may help increase the beneficial bacteria in your gut and improve brain health.
THE BOTTOM LINE
The gut-brain axis (vagus nerve) refers to the physical and chemical connections between your gut and brain.
Millions of nerves and neurons run between your gut and brain. Neurotransmitters and other chemicals produced in your gut also affect your brain.
By altering the types of bacteria in your gut by including more probiotics and prebiotics, it may be possible to improve your brain health.
Omega-3 fatty acids, fermented foods, probiotics and other polyphenol-rich foods may improve your gut health, which may benefit the gut-brain axis.