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 [1]
● Your gut contains 500 million neurons, which are connected to your brain through nerves in your nervous system [2]● The vagus nerve is one of the biggest nerves connecting your gut and brain. It sends signals in both directions [3]For example, in animal studies, stress inhibits the signals sent through the vagus nerve and also causes gastrointestinal problems [4]
● 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 [5]● Interestingly, Vagus Nerve can "taste" sugar in your gut! [6]

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 [7].
● 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 [8].
● Your gut microbes also produce a neurotransmitter called gamma-aminobutyric acid (GABA), which helps control feelings of fear and anxiety [9].
● Studies in laboratory mice have shown that certain probiotics can increase the production of GABA and reduce anxiety and depression-like behavior

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 [10].
● Your gut microbes produce lots of short-chain fatty acids (SCFA) such as butyrate, propionate, and acetate [11].
● They make SCFA by digesting fiber. SCFA affects 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 rewards from high-energy food [12].
● 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 [13].
● Gut microbes also metabolize bile acids and amino acids to produce other chemicals that affect the brain [14].
● 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 [15], [16].

What Foods Help the Gut-Brain Axis?

Here are some of the most important groups for the Gut-Brain Axis:
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 the risk of brain disorders [17], [18], [19].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 [20].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 hormones in humans [21].Polyphenol-rich foods: Cocoa, green tea, olive oil, and coffee all contain polyphenols, which are plant chemicals that are digested by gut bacteria. Polyphenols increase healthy gut bacteria and may improve cognition [22], [23].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.