Stress and anxiety can be reduced over time in one key way, study suggests

Probiotics are good bacteria found naturally in fermented foods like sauerkraut, cheese and kimchi, as well as in drinks such as kombucha and supplements in the form of tablets and pills, according to the Cleveland Clinic.

"Probiotic supplements add to your existing supply of friendly microbes. They help fight off the less friendly types and boost your immunity against infections," according to the clinic's website.

Now, in a new study published in the journal npj Mental Health Research, researchers sought to understand how probiotics could also improve mental health.

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"The gut–brain connection provides various routes through which bacteria in the gut can influence how we feel and behave, including via the vagus nerve, immune system and hormones," said Katerina Johnson, a research associate at the University of Oxford and a co-author of the study.

Lindsay Allen, a registered dietitian nutritionist and owner of Back in Balance Nutrition in Florida, said, "Part of having a healthy gut means having plentiful and diverse microorganisms living in there."

"These good bacteria cells actually outnumber the amount of human cells we have in our body," she said.

"So, it absolutely makes sense that taking probiotics can make us feel good."

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For the study, one group of young healthy adults took daily probiotics over the course of a month, while a second group took a placebo. 

Then, each participant submitted a daily mood report.

"It's particularly notable that by tracking how people felt on a daily basis, we could detect the beneficial effects of probiotics on mood," the researchers told Fox News Digital.

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The standard psychological questionnaires that are commonly used in the field aren't sensitive enough, they said. 

This study is the first of its kind to implement daily mood monitoring to assess the psychological effects of probiotics.

Using daily mood reports, psychological questionnaires and computer tasks testing how people process emotions, the researchers discovered a possible link between regular probiotic use and improved mood.

"It is striking that by simply asking participants how they were feeling each day, we could detect the beneficial effects of probiotics on mood," said Laura Steenberg, an assistant professor at Leiden University and a study co-author.

The daily mood reports showed that after about two weeks of daily use, probiotics can reduce negative feelings such as anxiety, stress and fatigue, compared to a placebo.

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Antidepressants work on a similar timescale, taking about two weeks to kick in.

"The comparable timescales are an interesting aspect of our findings. It may be a sign that both antidepressants and probiotics can affect our mood through shared pathways," the researchers said.

It has previously been proposed that probiotics — like antidepressants — may reduce both positive and negative emotions, according to the report.

But the results of this study indicate that positive feelings remained unchanged when people take daily probiotics.

"If indeed it is the case that probiotics specifically reduce negative mood without simultaneously reducing positive mood," the study said, "this could provide a notable benefit."

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"However, since most probiotic studies investigate changes in depression, stress and anxiety, little is known about how probiotics impact positive effects."

Despite the possible implications of the study, researchers stress that probiotics are no substitute for antidepressants and other psychiatric medication.

"My philosophy is always 'test, don't guess,' to find out exactly what the body needs," Allen said.

If a person doesn't want to take a test to decide on a supplement, Allen said, some foods to focus on for a diverse microbiome are berries, apples, spinach and kale. These are called polyphenols.

"This means they feed the bacteria and help them grow," Allen said.

Probiotic foods are different, she said, and have live organisms. These include foods such as coconut milk kefir, kefir, pickles and Greek and regular yogurt.

Also, since "probiotics can affect your health positively and negatively under different circumstances, talking with your doctor before you add probiotics to your diet can help reduce the chances of experiencing side effects, especially if you live with underlying health conditions," according to Healthline.