The Gut-Brain Connection: How it Works and The Role of Nutrition
How Are the Gut and Brain Connected?
The gut-brain axis is a term for the communication network that connects your gut and brain (1Trusted Source, 2Trusted Source, 3Trusted Source). 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 (4Trusted Source).
Interestingly, your gut contains 500 million neurons, which are connected to your brain through nerves in your nervous system (5Trusted Source).
The vagus nerve is one of the biggest nerves connecting your gut and brain. It sends signals in both directions (6Trusted Source, 7Trusted 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.
Neurotransmitters
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 in laboratory mice have shown that certain probiotics can increase the production of GABA and reduce anxiety and depression-like behavior (14Trusted Source).
Gut Microbes 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).
Your gut microbes produce lots of short-chain fatty acids (SCFA) such as butyrate, propionate and acetate (16Trusted Source).
They make SCFA by digesting fiber. 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.
Two studies in mice found that stress and social disorders reduce the production of bile acids by gut bacteria and alter the genes involved in their production (19Trusted Source, 20Trusted Source).
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)
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