Taking probiotics can help reduce negative mood, a Leiden University research study by Dr Katerina Johnson and Dr Laura Steenbergen has found. Their research also identifies traits of individuals who were more likely to benefit from probiotics.
There is growing interest in the potential of probiotics to bolster not only gut health, but also mental health.
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Katerina explained: “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.”
While studies in animals have previously found promising effects of probiotics on the brain and behaviour, human studies have yielded inconsistent results.
Capturing changes
So Johnson and Steenbergen used a combination of methods to capture how probiotics might influence the ability to regulate our emotions and affect our mood. These included psychological questionnaires, daily mood reports and computer tasks testing how people process emotions. The study was conducted in young, healthy adults using a multispecies probiotic (including Lactobacillus and Bifidobacterium), taken daily for a month.
This is the first study to implement daily mood monitoring to assess the effects of probiotics – and it revealed a clear signal that probiotics can improve negative mood compared to placebo.
Laura commented: “It is striking that by simply asking participants how they were feeling each day, we could detect the beneficial effects of probiotics on mood, but in contrast the standard psychological questionnaires that are common practice in the field were not sensitive enough to pick up these changes.”
Dissecting emotions
In recent decades, research in the psychological sciences has moved away from assessing subjective experience (i.e. how one is feeling) and favoured more objective measurements like questionnaires in an attempt to evaluate emotional states. “But by trying to dissect someone’s emotions into discrete categories e.g. stress, anger, anxiety and their depressive tendencies, we may be more likely to miss the bigger picture,” she added.
On a similar timescale as antidepressants, the researchers found it took about two weeks for probiotics to improve negative mood. But unlike antidepressants which tend to reduce both negative and positive mood, known as emotional blunting, their results showed that the probiotics only reduced negative mood, which could represent a possible benefit (though they should not be considered a substitute for antidepressants).
As Katerina explained: “Once we observed this drop in low mood, we were then keen to explore whether we could predict who would best respond to probiotics based on their psychological traits. We found that various traits, most notably people who tended to be more risk averse, were the ones whose mood benefited most from taking probiotics.”
Emotional cues
Their results also included some evidence that probiotics may affect the way the brain processes emotional cues as those given probiotics were slightly more accurate at recognising facial expressions.
The researchers note that there still remain unknowns surrounding precisely how they work and their long-term effects. Laura added: “Perhaps in the future probiotics could be used in a targeted way as an early intervention to reduce the chances of low mood progressing to mental health conditions such as depression, though more research would be needed to confirm such a use.”
Beyond the immediate field, Johnson and Steenbergen also hope their findings will spur others in mental health research to incorporate simple daily measures of how positive or negative their participants are feeling. As the authors conclude in their paper “In an attempt to delineate the complexity of the human brain and emotion… we cannot lose sight of asking the obvious. Sometimes the most simple questions can reveal the most meaningful answers.”
The study is published in Nature Portfolio’s npj Mental Health Research: https://doi.org/10.1038/s44184-025-00123-z
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