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Your body is home to roughly 40 trillion bacteria, most of which reside in your gut and don’t cause any health problems.

In fact, scientists have begun to realize that some of these bacteria are essential to physical health.

What’s more, recent studies have found that these bacteria may have benefits for your brain and mental health.

This article explains how your brain is affected by gut bacteria and the role probiotics may play.

Probiotics are live microorganisms, usually bacteria. When you consume enough of them, they provide a specific health benefit (1).

Probiotics are “life-promoting” organisms — the word “probiotic” is derived from the Latin words “pro,” meaning to promote, and “biotic,” meaning life.

Importantly, for a species of bacteria to be termed “probiotic,” it must have a lot of scientific evidence behind it showing a specific health benefit.

Food and drug companies began to call some bacteria “probiotic” even when they had no scientifically proven health benefits. This led the European Food Safety Authority (EFSA) to ban the word “probiotic” on all foods in the European Union.

However, a lot of new scientific evidence shows that some bacterial species have true benefits for health.

Research suggests that probiotics may benefit those with certain medical conditions, including irritable bowel syndrome (IBS), eczema, dermatitis, high cholesterol levels, and liver disease (2, 3, 4, 5, 6).

Most probiotics belong to one of two types of bacteria —Lactobacillus and Bifidobacteria.

There are many different species and strains within these groups, and they may have different effects on the body.

Summary

Probiotics are live microorganisms that have proven health benefits.

The intestines and brain are connected physically and chemically. Changes in the gut can affect the brain.

The vagus nerve, a large nerve in the central nervous system, sends signals between the intestines and brain.

The brain and intestines also communicate through your gut microbes, which produce molecules that carry information to the brain (7).

Estimates suggest that you have roughly 30 trillion human cells and 40 trillion bacteria. This means that, by number of cells, you are more bacteria than you are human (8, 9).

The majority of these bacteria reside in your gut. This means they come into direct contact with the cells that line your intestines and everything that enters your body. That includes food, medicines, and microbes.

Many other microbes live alongside your gut bacteria, including yeasts and fungi. Collectively, these microbes are known as the gut microbiota or gut microbiome (10).

Each of these bacteria can produce different substances that can affect the brain. These include short-chain fatty acids, neurotransmitters, and amino acids (11).

Gut bacteria can also influence the brain and central nervous system by controlling inflammation and hormone production (12, 13).

Summary

Thousands of species of bacteria reside in the human body, primarily in the intestines. In general, these bacteria are good for your health and may even influence brain health.

The term “gut dysbiosis” refers to when the intestines and gut bacteria are in a diseased state. This may be due to the presence of disease-causing bacteria, which may also lead to chronic inflammation.

Researchers have identified gut dysbiosis in people with (14, 15, 16, 17):

  • obesity
  • heart disease
  • type 2 diabetes
  • other conditions

Some studies suggest that certain probiotics can restore the microbiota to a healthy state and reduce symptoms of various health conditions (18, 19, 20, 21).

Interestingly, some studies have shown that people with certain mental health conditions also have an altered microbiota. It’s unclear if this causes the conditions, or if it’s the result of diet and lifestyle factors (22, 23).

Since the gut and brain are connected, and gut bacteria produce substances that can influence the brain, probiotics may benefit the brain and mental health. Probiotics that benefit mental health have been called psychobiotics (24).

A number of recent studies have investigated this, but most have been conducted in animals. However, a few have shown interesting results in humans.

Summary

A number of diseases, including mental health conditions, are linked to having more disease-causing bacteria in the intestines. Some probiotics may help restore healthy bacteria and reduce symptoms.

Stress and anxiety are increasingly common, and depression is one of the main mental health problems worldwide (25).

A number of these disorders, especially stress and anxiety, are associated with high blood levels of cortisol, the human stress hormone (26, 27, 28).

Several studies have looked into how probiotics affect people with clinically diagnosed depression.

One study showed that taking a mixture of three Lactobacillus and Bifidobacteria strains for 8 weeks significantly reduced symptoms of depression. They also had reduced levels of inflammation (29).

A handful of other studies have examined how probiotics affect depressive symptoms in people without clinically diagnosed depression, including (30, 31, 32, 33, 34, 35):

  • symptoms of anxiety
  • depressive symptoms
  • psychological distress
  • academic stress
Summary

Certain probiotics may reduce anxiety, stress, and depressive symptoms in the general population. Yet, more studies are needed to understand their potential benefits for those with clinically diagnosed mental health conditions.

Irritable bowel syndrome (IBS) is directly related to the function of the colon, but some researchers believe it’s a psychological disorder (36, 37).

Anxiety and depression are common in people with IBS. Interestingly, people who have IBS also tend to have an altered microbiota (38, 39, 40).

Many studies have shown that certain probiotics can reduce symptoms of IBS, including pain and bloating (41, 42, 43).

In general, research suggests that probiotics are linked with digestive health.

Summary

Many people with IBS experience anxiety and depression. Probiotics appear to help reduce IBS symptoms.

In people with or without mental health conditions, some probiotics may help improve mood.

One study gave people a probiotic mix containing eight different Lactobacillus and Bifidobacteria strains every day for 4 weeks.

The researchers found that taking the supplements reduced participants’ negative thoughts associated with a sad mood (44).

Another study reported that consuming a milk drink containing a probiotic called Lactobacillus casei for 3 weeks improved mood in people who had the lowest mood before the treatment (45).

Interestingly, this study also found that people scored slightly lower on a memory test after taking the probiotic. More studies are needed to validate these results.

Summary

A few studies have shown that taking certain probiotics for a few weeks may slightly improve mood.

When someone has a traumatic brain injury, they may need to stay in an intensive care unit. Here, doctors may help them feed and breathe through tubes.

This can increase the risk of infection, and infections in people with traumatic brain injuries can lead to further complications.

A few studies have found that adding certain probiotics into the food delivered through the tube can reduce the number of infections and length of time the person spends in the intensive care unit (46, 47, 48).

Probiotics may have these effects due to their benefits for the immune system.

Summary

Giving probiotics after traumatic brain injury may reduce the rate of infections and length of time the person needs to stay in intensive care.

A handful of studies have shown that probiotics may have other interesting benefits for the brain.

One intriguing study found that taking a mix of Bifidobacteria, Streptococcus, Lactobacillus, and Lactococcus affected the brain regions that control emotion and sensation. In this study, healthy females took the mix twice daily for 4 weeks (49).

Other studies have shown that specific probiotics may reduce some symptoms of multiple sclerosis and schizophrenia, but much more research is needed (50, 51).

Summary

Some probiotics may influence brain function and symptoms of multiple sclerosis and schizophrenia. However, this research is still very new, so the results aren’t clear.

At the moment, there is not enough evidence to show that probiotics definitely benefit the brain. This means that doctors aren’t able to consider probiotics a treatment for any brain-related disorders.

If you’re looking to treat such disorders, talk to a doctor.

That said, there is good evidence that probiotics have health benefits in other areas, including heart health, digestive disorders, eczema, and dermatitis (3, 4, 5, 52).

Scientific evidence has shown a clear connection between the gut and the brain. This is an exciting area of research that’s growing rapidly.

People can usually get a healthy gut microbiota by following a healthy diet and lifestyle. A number of foods can contain beneficial bacteria, including:

  • probiotic yogurt
  • unpasteurized sauerkraut
  • kefir
  • kimchi

If necessary, taking probiotic supplements can help you increase the beneficial bacterial species in your intestines. In general, taking probiotics is safe and causes few side effects.

If you’re buying a probiotic, choose one that’s supported by scientific evidence. Lactobacillus GG (LGG) and VSL#3 have both been widely studied and shown to offer a number of health benefits.

Summary

Probiotics have been shown to benefit other aspects of health, but not enough research has been done to definitively demonstrate whether probiotics have positive effects on the brain.

Although the research is promising, it’s too soon to recommend any probiotic specifically to boost brain health.

Still, current evidence gives some food for thought about how probiotics may be used to improve brain health in the future.

If you want to try using probiotics, you can find them in drug stores and online.