The known health benefits of probiotics are numerous. They are most commonly used to help alleviate digestive disorders, but that’s just the beginning. The beneficial bacteria have also been positively linked to lower cholesterol, the prevention of allergies, liver disease, yeast infections and cancer. They have even been shown to protect against oxidative stress caused by exercise and to help reduce upper respiratory infections in athletes. (See 2 More Reasons You Should Be Taking Probiotics.)
The good news about these good “bugs” just keeps piling up. But can probiotics do more than just a body good? Can they also help to reduce stress and improve your mood? According to recent research, yes they can.
Animal studies have shown that manipulations of bacteria found in the stomach and intestines can modify neural function and affect mood and behavior. 
In another study, researchers administered a daily a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) to rats for two weeks, finding that it significantly reduced anxiety-like behavior. 
The same researchers then moved on to healthy human subjects, having them take the probiotic formulation every day for 30 days, and assessing them before and after the trial period with the following tests:
- Hopkins Symptom Checklist (HSCL-90)
- Hospital Anxiety and Depression Scale (HADS)
- Perceived Stress Scale (PSS)
- Coping Checklist (CCL)
- 24-Hour Urinary Free Cortisol (UFC)
The results showed that the probiotic regimen alleviated psychological distress in the subjects. Specifically, their HADS score improved, and in the HSCL-90, the probiotic supplementation decreased the scores for “somatization,” “depression” and “anger-hostility.”
These results suggested that the probiotic formulation “is beneficial in reducing anxiety and stress response, as well as improving mood in moderately stressed human subjects.”
In a follow-up study, researchers decided to break down these results further and focus on those subjects who scored the lowest in the 24-hour urinary free cortisol (UFC) test before taking the probiotics. Cortisol is a hormone your body releases in response to stress, and while the name may seem a bit misleading, this test measures the amount of cortisol found in your urine. Therefore, those with the lowest levels would have been the least stressed individuals at the beginning of the study.
Their criteria left them with a 25-person subpopulation, 10 of which were taking the probiotic and 15 of which were taking a placebo.
In addition to the improvements seen in the larger study population that we mentioned above, this less-stressed subgroup saw some extra benefits from the probiotic supplementation. Specifically, the Perceived Stress Scale scores improved, as well as three more sub-scores of the HSCL-90 test: “obsessive compulsive,” “anxiety” and “paranoid-ideation.”
Researchers concluded: “Exposure to chronic stress could disrupt the balance of intestinal microbiota and induce various diseases. Therefore, whatever the stress level, daily intake of probiotics could prevent such an imbalance, and the improvement in symptoms among subjects with low to mild stress levels suggests the value of prophylactic intake of probiotics in terms of digestive comfort and general well-being.”
How Do Probiotics Improve Your Mood?
The fact that probiotics may help to improve your mood and reduce your stress levels is great news, but you may be wondering how exactly these little buggers do it. Well, unfortunately, we don’t have a good answer for you.
The researchers admit that despite the growing interest in the role of probiotics in alleviating stress, anxiety and depression, the how is still quite unclear to the medical community. There are, however, some hypotheses the study authors cite, which we will cover briefly.
The first, and seemingly the one they most subscribe to, is related to gut-brain signaling. This is the concept that the gut and the brain are closely connected, and that this interaction plays an important part in gastrointestinal function and certain emotions. So, if gut microbiota play a role in central nervous system functions, probiotics, which are known to aid with digestive functions, may have a beneficial effect on mood and psychological distress.
That’s one theory. Others include the anti-inflammatory properties of these good bacteria, as well as the idea that probiotics may modulate the activity of brain structures involved in the processing of emotions related to anxiety, mood and aggression.
It may also be that probiotics produce neurotransmitters that act directly or indirectly on specific targets in the central nervous system. Or it could be that they regulate glycemic control since research shows that intestinal microbiota contribute to glucose tolerance, and glucose intolerance has been linked to the risk of depression.
Finally, it could be that probiotics cause a reduction of substance P in your stomach. Substance P is a neurotransmitter associated with pain and inflammation, and linked to anxious, depressive and aggressive behaviors.
What it boils down to is that we just don’t know.
Start Taking a Probiotic Today
But just because we don’t understand how probiotics positively affect our mood doesn’t change the fact that research has shown that they do in fact work.
So if you’re looking to improve your mood naturally, in addition to aiding your digestive system (among other health benefits), consider trying a probiotic dietary supplement. Look for products that contain live cultures of the three most common strains of gut-friendly bacteria: Lactobacillus acidophilus, Bifidobacterium longum and Bifidobacterium bifidum. The supplement facts box on the bottle of any reputable probiotic product should identify the bacteria strains.
Additionally, you can seek out good probiotic food sources such as yogurt with live or active bacteria cultures, sauerkraut, miso soup, gouda cheese and buttermilk.
 Messaoudi, M, et al. Beneficial psychological effects of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in healthy human volunteers. Gut Microbes July/August 2011; 2:4, 256-261.
 Sudo N, Chida Y, Aiba Y, Sonoda J, Oyama N, Yu XN, et al. Postnatal microbial colonization programs the hypothalamic-pitutary-adrenal system for stress response in mice. J Physiol 2004; 558:263-75.
 Sudo N. Stress and gut microbiota: does postnatal microbial colonization programs the hypothalamicpituitary- adrenal system for stress response? Int Congr Ser 2006; 1,287:350-4.
 Messaoudi M, et al. Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects. Br J Nutr 2011; 105:755-64.