Did you know that there
are roughly 40 trillion bacteria living in and on you?
Most of these bacteria reside in your gut and don’t cause any
In fact, scientists have begun to realize that these bacteria
are essential for your physical health.
Now compelling new research has found these bacteria may also
be beneficial for your brain and mental health.
This article explains how your brain is affected by gut
bacteria and the role probiotics may play.
What Are Probiotics?
The word “probiotic” is derived from the Latin words “pro,”
meaning to promote, and “biotic,” meaning life.
Importantly, in order for a certain species of bacteria to be
termed “probiotic,” it must have a lot of scientific evidence
behind it showing a specific health benefit.
Unfortunately, the word probiotic has become overused by food
and pharmaceutical companies who were calling some bacteria
probiotics even if they had no scientifically proven health
This led the European Food Safety Authority (EFSA) to ban the
word “probiotic” on all foods in the European Union.
However, there is a lot of new scientific evidence showing that
some bacterial species have true benefits for health.
Most probiotics belong to one of two types of bacteria:
Lactobacillus or Bifidobacteria. There are
many different species and strains within these groups, and
they may have different effects on the body.
Bottom Line: Probiotics are live
microorganisms that have a proven health benefit for the
How Are Your Intestines and Brain Connected?
The intestines and brain are connected physically and
The physical connection between the intestines and brain is
through the central nervous system, which controls all of the
activities of the body.
The vagus nerve is a large nerve that sends signals between the
intestines and brain (7).
The brain is also connected to the intestines through your gut
microbes. Molecules that they produce can act as signals that
the brain can detect (7).
In the past, scientists have estimated that a person has
approximately 100 trillion bacterial cells in their body and
only 10 trillion human cells, meaning your own cells are
outnumbered by 10 to 1 (8).
However, recent estimates suggest that you have roughly 30
trillion human cells and 40 trillion bacteria. This is still
pretty impressive and means that, by number of cells, you are
more bacteria than human (9).
The majority of these bacteria are in your gut, so they’re in
direct contact with the cells that line your intestines and
with everything that enters your body. That includes food,
medicines and microbes.
Alongside your gut bacteria, there are many other microbes,
such as yeasts and fungi. Collectively, these microbes are
known as the gut microbiota or gut microbiome (10).
Each of these bacteria can produce different compounds, such as
short-chain fatty acids, neurotransmitters and amino acids.
Many of these substances have effects on the brain (11).
Bottom Line: There are thousands of
different species of bacteria in the human body, mostly in
the intestines. In general, these bacteria are good for your
health and may even influence brain health.
An Altered Microbiota May Contribute to a Number of Diseases
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
Interestingly, some studies have shown that people with certain
mental disorders also have an altered microbiota. However, it
is unclear if this is a cause of such diseases or the result of
an altered diet and lifestyle (22, 23, 24, 25).
Since the gut and brain are connected, and gut bacteria produce
substances that can influence the brain, probiotics may be able
to benefit the brain and mental health.
A number of recent studies have investigated this, but most
have been in animals. However, a few have shown interesting
results in humans.
Bottom Line: A number of diseases, including
mental disorders, are associated with higher disease-causing
bacteria in the intestines. Some probiotics may be able to
restore healthy bacteria to reduce symptoms.
Taking Probiotic Supplements May Reduce Symptoms of Stress,
Anxiety and Depression
Only one study has examined how probiotics affect patients with
clinically diagnosed depression.
In the study, consuming a mixture of three
Lactobacillus and Bifidobacteria strains for
eight weeks significantly reduced depressive symptoms. The
probiotic also reduced inflammation in the patients (30).
A handful of other studies have examined how probiotics affect
depressive symptoms in healthy subjects. In healthy people,
certain probiotics may reduce:
- Symptoms of anxiety (31,
- Depressive symptoms (34)
- Psychological distress (35)
- Academic stress (36)
Bottom Line: Certain probiotics may reduce
anxiety, stress and depressive symptoms in healthy people.
More research is needed about potential benefits for people
with clinically diagnosed psychological disorders.
Taking Probiotic Supplements May Reduce Symptoms of Irritable
Bottom Line: IBS is commonly associated with
elevated levels of anxiety and depression. Probiotics appear
to help reduce IBS symptoms.
Certain Probiotics May Enhance Mood
In healthy people without a psychological disorder, some
probiotics may help improve mood.
One study treated participants daily for four weeks with a
probiotic mix containing eight different Lactobacillus
and Bifidobacteria strains.
The researchers found that taking the supplements reduced
participants’ negative thoughts associated with a sad mood
Another study showed that consuming a milk drink containing a
probiotic called Lactobacillus casei for three weeks
improved mood in people who had the lowest mood before the
Interestingly, this study also found that people scored
slightly lower on a memory test after taking the probiotic.
Further research is needed to validate these results.
Bottom Line: A few studies have shown that
taking certain probiotics for a few weeks may slightly
Certain Probiotics May Have Benefits After Traumatic Brain
Traumatic brain injury usually involves admission to an
intensive care unit. During this time, food is usually
administered enterally, meaning through a tube.
In some cases, breathing also has to be assisted with a tube,
which can increase the risk of infection. Infections in people
with traumatic brain injuries can lead to further
A few studies have found that adding certain probiotics into
enteral nutrition can reduce the number of infections in the
patients with traumatic brain injury and the length of time
they spend in the intensive care unit (47, 48, 49).
The effects of the probiotics on these outcomes may be due to
their benefits for the immune system.
Bottom Line: Administering probiotics after
traumatic brain injury may reduce patients’ rate of
infections and length of stay in intensive care.
Other Beneficial Effects of Probiotics on the Brain
A handful of other studies have shown that probiotics may have
interesting benefits for the brain.
One intriguing study looked at images of women’s brains after
they consumed a mix of Bifidobacteria,
Streptococcus, Lactobacillus and
Consuming the probiotic affected regions of the brain that
control emotion and sensation (50).
Bottom Line: Some probiotics may influence
the function of the brain and symptoms of multiple sclerosis
and schizophrenia. However, this research is still very new,
so the results aren’t clear.
Should You Be Taking a Probiotic for Your Brain?
At the moment, there is not enough evidence to definitively say
that probiotics benefit the brain. Therefore, they cannot yet
be considered a treatment for any brain-related disorders.
If you’re looking to treat such disorders, consult a doctor.
Thus far, scientific evidence does show a clear connection
between the gut and the brain. It’s an exciting area of
research that is expanding rapidly.
If necessary, taking probiotic supplements can help you
increase the beneficial bacterial species in your intestines.
In general, consuming probiotics is safe and causes few side
If you’re buying a probiotic, choose one that has scientific
evidence behind it. Lactobacillus GG (LGG) and VSL#3
have both been widely studied and shown a number of health
Bottom Line: 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.
Take Home Message
Although the research is promising, it is 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 enhance brain health in the future.