Psychobiotics: Microbes and mental health

In the last few decades, the importance of the gut microbiome in health has grown clear. We know that these microbes support not just the health of your gut, but the health of your whole body, including your brain. 

Scientists have also investigated the complex conversations between gut microbes, gut, and brain, collectively termed the microbiota-gut-brain axis. In this article, we’ll focus on a growing area of research into psychobiotics.

Before diving into psychobiotics, let’s have a brief look at the microbiota-gut-brain axis. 

Interspecies conversations within your bowels

The nervous system in your gut is complex and controls many of your gut’s activities.

One example is peristalsis — the muscular contractions of your gut that push food along your intestines, which can occur without your brain or spinal cord getting involved. 

In fact, the gut’s nervous system, known as the enteric nervous system or ENS, is so large, influential, and complex that it is sometimes called the “second brain.” 

This neural highway allows your brain to communicate with your gut but it also lets your gut talk back to the brain. In fact, around 90% of the traffic along the vagus nerve runs from the gut to the brain.

This is one method of communication between gut and brain, but there are many more.

For instance, as your microbes break down fiber in your gut, they release a wide range of compounds. Some of these can interact with receptors on the lining of your gut, stimulating nerves or influencing the activity of the immune cells in your gut walls. These signals can reach your brain.

Additionally, gut bacteria can directly impact levels of neurotransmitters — the chemicals that your nerve cells use to communicate.

In fact, some species of bacteria consume or even manufacture neurotransmitters. And others can influence how much neurotransmitter, such as serotonin, is created by the cells of your intestines.

We’ve only scratched the surface, but you can already see that the gut and brain are tightly linked. If you’d like to learn more, we’ve covered it in more depth here.

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Why psychobiotics?

Scientists have identified distinct gut microbiome changes in people with depression, anxiety, schizophrenia, Parkinson’s disease, and other brain conditions.

Although we don’t know why these changes occur, it’s a sign that gut health and brain health go hand in hand.

And when you consider that mental health conditions and brain diseases are common, and that existing treatments don’t work for everyone, the idea of a simple intervention like psychobiotics is appealing.

Although this area of science is in its infancy, some studies have produced encouraging results. Below, we’ll run through some of these and explain what they found. 

Psychobiotic research: A whistle-stop tour

All of the studies we mention below were small, so they need to be replicated in larger groups of people before they're recommended to the public. But taken together, they’re certainly intriguing.

1. Probiotics and neuropsychiatric disorders in kids

Date: 2015

Participants: 75 infants

Probiotic: Lactobacillus rhamnosus GG

In this study, infants received either a probiotic or a placebo during the first 6 months of life. The scientists followed them up multiple times: At 3 weeks, 3 months, 6 months, 12 months, 18 months, 24 months, and 13 years.

The scientists found that at age 13, six children in the placebo group had developed attention deficit hyperactivity disorder (ADHD) or Asperger syndrome (AS), compared with none in the probiotic group.

It’s interesting to note that L. rhamnosus GG is also found in breastmilk.

2. Stress, anxiety, and bacteria

Date: 2019

Participants: 111 “stressed adults”

Probiotic: L. plantarum DR7

In this study, the scientists recruited adults who reported that they were generally stressed.

For 12 weeks, they took a daily probiotic or placebo. By the end of the study, and by week 8 for some, participants taking the probiotic reported feeling less stressed, but those in the placebo group did not.

Also, when the scientists analyzed their blood, those taking the probiotic had lower levels of the stress hormone cortisol and some markers of inflammation.

Participants over 30 years old also improved on cognitive and memory tests compared with the placebo group and participants under 30.

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3. Depression vs. probiotics

Date: 2021

Participants: 10 people with “moderate” depression who weren’t taking medication.

Probiotic: L. helveticus R0052 and Bifidobacterium longum R0175

In this pilot study, participants took a probiotic supplement every day for 8 weeks. By week 4, participants had improved mood and sleep, which continued until the 8-week mark.

However, the study didn’t have a control group, so it could be that simply being involved in a trial boosted their mood. And we don’t know what happened to their symptoms after 8 weeks.

4. Stressed students

Date: 2021

Participants: 80 students aged 18–24 during exam season.

Probiotic mix: Bacillus coagulans Unique IS2, L. rhamnosus UBLR58, B. lactis UBBLa70, L. plantarum UBLP40, B. breve UBBr01, B. infantis UBBI01.

Each student took either a probiotic mix or a placebo twice a day for 28 days.

At the beginning and end of the study, the scientists used questionnaires to gauge participants’ levels of stress, depression, and anxiety. They also measured levels of cortisol in the blood.

Compared with the placebo group, those taking the probiotic had significantly reduced cortisol and self-reported stress, depression, and anxiety.

We should note that this study was funded by a manufacturer of probiotics.

5. Autism spectrum disorder (ASD) and probiotics

Date: 2019

Participants: 80 children with ASD aged 7–15

Probiotic: L. plantarum PS128

In this Taiwanese study, the scientists gave children with ASD a probiotic supplement or a placebo daily for 4 weeks.

When the scientists compared overall symptoms at the start of the study to those at the end, there was little difference between the probiotic and placebo groups. 

However, when they looked at individual symptoms, some did improve for the probiotic group compared with the placebo group, such as anxiety and rule-breaking behavior.

This study was also funded by a company that manufactures psychobiotics.

6. Treatment-resistant depression

Date: 2018

Participants: 40 people with treatment-resistant depression.

Probiotic: Clostridium butyricum MIYAIRI 588

Around 60% of people with depression don’t benefit from antidepressants. If an individual has tried at least two forms of medication and not experienced relief, they are defined as having treatment-resistant depression.

In this study, participants took either a probiotic or a placebo daily for 8 weeks alongside their current depression meds.

Overall, those in the probiotic group had reduced depressive symptoms compared with the placebo. According to the authors, treatment was effective in 70% of participants, and the remission rate was 35%.

Remission means that their responses to a depression questionnaire were in the “normal” range.

We should note that this was an open-label trial, meaning that both the researchers and the participants knew whether they were receiving the probiotic or placebo. This can introduce significant bias.

Looking forward

So, the studies above are small and may be biased by industry ties or study design. And not all studies have found benefits.

This is to be expected. It’s a new science and, as our brief tour of the gut-microbiota-brain axis shows, it’s more than a little bit complicated.

It’s also important to remember that, because everyone’s gut microbiome is different, one type of bacteria may benefit some individuals but not others, depending on their starting gut microbiome.

This is really just the start of the road to psychobiotic discovery. Some of the results above are intriguing and, hopefully, scientists will eventually identify the most effective psychobiotic strains for each condition.

For now, watch this space.

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What should you do in the meantime?

At ZOE, we believe a food-first approach is always better than relying on supplements, and that includes probiotics. 

So, while the evidence on psychobiotics trickles in, your best bet is to eat more fibre-rich plants and more fermented foods, which are naturally probiotic.

While probiotic supplements generally have just one or a handful of bacterial species, each fermented food has its own uniquely complex collection of microbes. 

Also, there’s growing evidence that fermented foods can support general health, including mental health.

All forms of fermented foods are likely to be beneficial (as long as they contain live microbes and not too many other additives), but if you choose fermented plants, like kimchi or sauerkraut, you get double benefits.

This is because plant fiber is prebiotic, which means it feeds your probiotic bacteria. You can think of prebiotic fibre as the compost, and probiotic fermented foods as the seeds.

For more information on probiotic and prebiotic foods, try these next:

• ZOE Science & Nutrition podcast on fermented foods with Prof. Tim Spector.

• 8 powerful fermented foods.

• 16 delicious prebiotic foods.