Depression in older adults is far more common than most people realize and far harder to treat. Standard antidepressants often fall short in this age group, with response rates hovering around 50% and a higher risk of side effects. So researchers in India set out to test something simpler: could adding a daily probiotic capsule to standard depression care actually help? Their findings, just published in the Journal of the American Geriatrics Society, offer a cautiously encouraging but nuanced answer.
The Trial: Small, Rigorous, and Focused on Seniors
The PRODG trial (PRObiotics in Depression among Geriatrics) was a randomized, double-blind, placebo-controlled pilot study conducted at two major hospitals in India: AIIMS, New Delhi, and NIMHANS, Bengaluru. Researchers enrolled 58 adults aged 60 and older, all diagnosed with moderate unipolar depression, and randomly assigned them to receive either a daily probiotic capsule or an identical-looking placebo, both alongside standard antidepressant treatment for 12 weeks. Participants were then tracked for another 12 weeks after stopping the supplement, for a total of 24 weeks of follow-up.
The probiotic capsule contained two well-studied bacterial strains, Lactobacillus helveticus and Bifidobacterium longum, delivering about 6 billion colony-forming units daily. These particular strains were chosen because earlier research has linked them to improved mood and reduced anxiety, partly through their effects on the gut-brain axis the communication network that connects digestive bacteria to brain chemistry via the nervous and immune systems and through neurotransmitter production.
What They Measured
The primary outcome was straightforward: did patients achieve at least a 50% reduction in depression severity, as measured by the Montgomery-Åsberg Depression Rating Scale (MADRS), after 12 weeks? Researchers also tracked anxiety levels (GAD-7 scale), cognitive function, quality of life across four domains (physical, psychological, environmental, social), blood levels of brain-derived neurotrophic factor (BDNF a protein tied to brain cell health and resilience), and, in a subset of patients, changes in gut bacteria composition.
The Results: Modest but Real Differences
Both groups improved substantially over the 24-week period, an important finding that underscores that standard antidepressant care works reasonably well even in older patients. But the probiotic group showed significantly lower depression and anxiety scores overall compared to the placebo group.
By 12 weeks, the probiotic group’s average MADRS score had dropped to about 11.4, compared to roughly 17.1 in the placebo group. Anxiety scores followed a similar pattern, with the probiotic group reporting notably less anxiety throughout the trial.
However, there’s an important statistical nuance here: while the overall group difference was significant, the interaction between treatment group and time was not, meaning the rate of improvement was statistically similar in both groups, even though the probiotic group consistently scored better at each checkpoint. In plain terms, both groups improved at a similar pace, but the probiotic group started with a more favorable trajectory and stayed ahead throughout.
Quality of life told a different story. All four domains physical, psychological, environmental, and social improved dramatically in both groups, with no meaningful added benefit from probiotics. So while mood and anxiety symptoms responded somewhat better with probiotics, day-to-day functioning and well-being improved equally regardless of which capsule patients received.
The Biological Evidence Behind the Numbers
Perhaps the most compelling part of this study isn’t the symptom scores it’s the biological data supporting a real mechanism. Researchers measured serum BDNF, a neurotrophin known to support neuronal growth and resilience, which tends to be reduced in depression. At 12 weeks, BDNF levels rose significantly in the probiotic group but not in the placebo group.
At the same time, fecal testing confirmed that the supplemented bacterial strains had successfully colonized participants’ guts: L. helveticus and B. longum abundances increased by roughly 8- to 10-fold in the probiotic group, with no such change in the placebo group. This combination of symptom improvement, rising BDNF, and confirmed bacterial colonization strengthens the case for a genuine gut-brain pathway rather than a coincidental finding.
The treatment also appeared safe. No serious psychiatric adverse events occurred, and the only notable side effects were mild, transient gastrointestinal symptoms (bloating, fullness, constipation) in 7 patients in the probiotic group during the first 6 weeks.
What Else Influenced the Results
The study’s statistical models also revealed some interesting side findings. Higher antidepressant doses were linked to higher depression scores likely because clinicians prescribed stronger doses to patients who started out more severely depressed, not because the medication was making things worse. Benzodiazepine use (anti-anxiety medication) was strongly associated with higher anxiety scores and poorer physical and social quality of life, again likely reflecting that these medications were prescribed to patients already struggling more. And patients with hypertension reported worse social quality of life meaningfully, highlighting how physical comorbidities intertwine with mental health in older adults.
Important Caveats
This was explicitly a pilot trial designed to test feasibility and gather preliminary data, not to provide definitive proof of efficacy. Several limitations temper how the results should be interpreted:
High dropout. More than half of participants were lost to follow-up by 24 weeks, frequently due to difficulty maintaining contact or adherence. While dropout rates were similar between groups, this level of attrition reduces confidence in the findings.
Smaller sample than planned. Researchers aimed for 80 participants but managed only 58 within their timeframe, and no formal statistical power calculation was performed; standard practice for exploratory pilot studies, but it limits how strongly conclusions can be drawn.
Inconsistent antidepressant prescribing. Because each clinician chose medications and doses based on individual judgment, this variability, while realistic, introduces an extra layer of uncontrolled confounding in the results.
Unexpected placebo-group decline. Some placebo participants worsened despite receiving standard antidepressant care, complicating direct comparisons between the two arms.
Limited mechanistic data. Gut microbiota testing was performed in only a small subset of patients, and inflammatory markers weren’t measured, leaving some mechanistic questions unanswered.
The authors themselves are candid about these constraints, noting that “the interpretation of these findings is constrained by substantial dropout and marked heterogeneity in antidepressant regimens which reduces internal validity.”
Why This Study Still Matters
Despite its limitations, PRODG fills a meaningful gap. Most prior probiotic-depression research has focused on younger or middle-aged adults; data specific to older patients with clinically diagnosed depression has been scarce. In countries like India, where geriatric depression is widely under-recognized and psychiatric care can be difficult to access, a low-cost, low-risk supplement that’s culturally familiar fermented dairy products like curd are already dietary staples could offer real practical value as a complementary approach, not a replacement for standard psychiatric treatment.
The researchers are clear that probiotics should be viewed as an adjunct, not an alternative, to established antidepressant care. Larger, better-controlled trials with standardized treatment protocols and stronger retention strategies will be needed before probiotics could be confidently recommended as part of routine depression management for older adults.
The Bottom Line
In this small but carefully designed trial, adding a two-strain probiotic supplement to standard depression treatment was associated with modestly better mood and anxiety outcomes in older adults, alongside biological signals of rising BDNF and confirmed gut colonization — consistent with a genuine gut-brain mechanism. It didn’t outperform standard care on quality of life, and the trial’s limitations mean these results should be seen as encouraging groundwork rather than a green light for widespread clinical use. For now, the most accurate takeaway is one of cautious optimism: probiotics look safe and biologically plausible as a depression adjunct in older adults, but the evidence needs to grow before they earn a routine place in geriatric mental health care.
References
Sinha P, Chatterjee P, Kathiresan P, et al. Efficacy of Adjunct PRObiotics as Compared to the Standard Care in Moderate Unipolar Depression Among Geriatric Patients: A Randomized Double-Blind Placebo-Controlled Pilot Multi-Center Trial (PRODG). Journal of the American Geriatrics Society. 2026. https://doi.org/10.1111/jgs.70530