Gut Instincts: The Bacteria Behind Your Behavior

Image Citation: [13]

Written by Alicia Kim ‘28

Edited by Kevin Pham ‘28

Most people wouldn’t expect their behavior to be shaped by the bacteria living in their gut. But as strange as it sounds, growing evidence suggests that probiotics—those live microorganisms often found in supplements and fermented foods—may play a subtle yet meaningful role in how we think, feel, and respond to the world around us. It turns out, your “gut feeling” might be more scientific than you think.

The gut-brain axis is the communication superhighway linking your central nervous system (CNS) and your enteric nervous system (ENS), often dubbed as the “second brain” for its autonomy and complexity. These two systems stay in constant contact via multiple channels—neuronal (especially through the vagus nerve), endocrine (hormonal), immune, and metabolic signaling [1]. The trillions of bacteria in the gut microbiome contribute to this crosstalk by producing a range of neuroactive compounds, including gamma-aminobutyric acid (GABA), serotonin, dopamine, and short-chain fatty acids like butyrate, which can affect mood, stress response, and even cognition [2,3].

Disruptions to this axis—such as an imbalance in gut bacteria, known as dysbiosis—have been linked to a number of neurological and psychiatric conditions, including anxiety, depression, autism spectrum disorder, and Parkinson’s disease [4]. Researchers are now exploring whether modifying the gut microbiome through probiotics can support mental well-being and cognitive health.

Probiotics are live microorganisms that offer health benefits when taken in the right amounts—and some of them might also benefit the brain. These are sometimes referred to as “psychobiotics” because of their potential effects on mood, stress, and cognitive function [5]. Animal studies provide some of the strongest early evidence. In one study, mice that were given Lactobacillus rhamnosus showed less anxiety and depression-like behavior, and changes in GABA receptor expression in the brain. But when researchers cut the vagus nerve—the main highway between gut and brain—the effects disappeared, showing just how critical that link is [6].

Human studies have begun to echo these findings. A randomized controlled trial found that a daily probiotic mix of Lactobacillus helveticus and Bifidobacterium longum reduced stress and improved mood in healthy adults after 30 days [7]. Another study in people with irritable bowel syndrome found that Bifidobacterium longum not only reduced depression symptoms but also changed brain activity in emotional processing regions [8]. Interestingly, many of these effects don’t seem to come from big changes in the gut microbiome itself, but rather from chemical signals sent by the microbes already present [9].

There’s also some evidence that probiotics could support memory and attention. In one trial, older adults who consumed fermented milk containing Lactobacillus helveticus performed better on memory tests under fatigue and reported feeling less anxious [10]. These effects may be due in part to probiotics lowering levels of systemic inflammation through the gut microbiota, which is known to affect both mood and cognition [11].

Even in high-stress situations, probiotics might help. A study involving medical students found that drinking a fermented milk drink with Lactobacillus casei during exam season led to fewer gastrointestinal issues and less anxiety compared to a placebo group [12]. While this doesn't mean probiotics are a cure-all, it does suggest they could help buffer against stress in challenging times.

Despite the promising results, it’s still early days for psychobiotics. Not all probiotics affect the brain, and those that do may not work for everyone. Strain specificity, dosage, duration, and individual differences all play a role in determining who benefits from these supplements and how. More large-scale, well-controlled studies are needed to understand the full potential and limitations of probiotics in mental health treatment. Still, the idea that tiny organisms in your gut might be shaping your behavior, mood, and thoughts is one of the most exciting frontiers in neuroscience and microbiology. As we learn more, we may begin to think of probiotics not just as digestive aids—but as subtle mood shifters and brain boosters too.

References

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6. Bravo JA, Forsythe P, Chew MV, Escaravage E, Savignac HM, Dinan TG, et al. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proc Natl Acad Sci USA. 2011;108(38):16050–5.

7. Messaoudi M, Lalonde R, Violle N, Javelot H, Desor D, Nejdi A, 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(5):755–64.

8. Pinto-Sanchez MI, Hall GB, Ghajar K, Nardelli A, Bolino C, Lau JT, et al. Probiotic Bifidobacterium longum NCC3001 reduces depression scores and alters brain activity in patients with irritable bowel syndrome. Gastroenterology. 2017;153(2):448–59.e8.

9. Kristensen NB, Bryrup T, Allin KH, Nielsen T, Hansen TH, Pedersen O. Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials. Genome Med. 2016;8(1):52.

10. Chung YC, Jin HM, Cui Y, Kim DS, Jung JM, Park JI, et al. Fermented milk of Lactobacillus helveticus IDCC3801 improves cognitive functioning during cognitive fatigue in healthy older adults: a randomized, double-blind, placebo-controlled study. J Funct Foods. 2014;10:465–74.

11. Dantzer R, O’Connor JC, Freund GG, Johnson RW, Kelley KW. From inflammation to sickness and depression: when the immune system subjugates the brain. Nat Rev Neurosci. 2008;9(1):46–56.

12. Kato-Kataoka A, Nishida K, Takada M, Suda K, Kawai M, Shimizu K, et al. Fermented milk containing Lactobacillus casei strain Shirota preserves the diversity of the gut microbiota and relieves abdominal dysfunction in healthy medical students exposed to academic stress. Appl Environ Microbiol. 2016;82(12):3649–58.

13. Harvard Health Publishing. Should you take probiotics? Harvard Health. February 2, 2022. Available from: https://www.health.harvard.edu/staying-healthy/should-you-take-probiotics