Probiotic vs. placebo and metformin: probiotic dietary intervention in polycystic ovary syndrome - A randomized controlled trial.

The short version

This phase IV, double-blind randomized controlled trial with 180 participants (ProPCO-RCT) explored whether a multi-strain probiotic supplement can improve hormone and metabolic markers in women with Polycystic Ovary Syndrome (PCOS), a condition affecting up to one in five women that combines irregular periods, excess androgen (male hormone) activity, and polycystic ovaries. The trial's premise builds on prior findings that women with PCOS have a less diverse gut microbiome, which past research has linked to higher testosterone and greater insulin resistance. The study also examines equol, a compound gut bacteria make from soy isoflavones that has anti-androgenic properties, framing the gut microbiome as a possible lever on PCOS hormone levels. Participants (ages 18-45, meeting standard Rotterdam PCOS diagnostic criteria) were split into three groups of 60: a seven-strain probiotic sachet, a matching placebo, or open-label metformin (a standard PCOS drug), each taken daily for six months. The probiotic-versus-placebo comparison was blinded and allocation-concealed; the metformin group was not blinded. Measurements included blood hormone panels, an oral glucose tolerance test, a hirsutism (excess hair growth) score, body measurements, gut permeability and stool-microbiome testing (via genetic sequencing), and symptom and mood questionnaires, taken at the start and repeated at six months. As a protocol paper, this publication describes the trial's design rather than its findings — the primary comparison (change in free testosterone, probiotic versus placebo) had not yet been unblinded or analyzed at the time of publication. The one concrete number reported comes from the research group's earlier, smaller pilot study: 10 women taking the same probiotic for three months saw their free testosterone drop by a median of 0.3 pg/ml (range 1.7 down to 2.5 up), a result the team used to calculate that 60 participants per group would give the trial 80% power to detect a larger, expected 0.6 pg/ml reduction. The full trial adds metformin as a benchmark and follows participants for twice as long as most prior probiotic-PCOS studies the authors cite, several of which reported androgen and metabolic improvements with about 60 participants total. Because the results are not yet in, the paper's value lies in its design: registered with clinical trial authorities (ClinicalTrials.gov NCT04593459), ethics-approved, and built to isolate a probiotic's effect from both an inert placebo and an established drug. The authors describe it as the first trial to test a probiotic against both comparators in PCOS. The protocol also flags a limitation the authors state plainly: recruitment began before the COVID-19 pandemic, and the trial's long six-month intervention window may make some participants unable to fully follow the schedule, which is why the statistical plan pre-specifies analyzing everyone as originally assigned. Readers looking for evidence of probiotic effectiveness in PCOS will need the trial's later results publication, not this one.

Key findings

• This paper is a published trial protocol (registered as ClinicalTrials.gov NCT04593459), not a report of study results; it describes how a randomized controlled trial of probiotics for Polycystic Ovary Syndrome (PCOS) was designed, before any outcome data were analyzed.
• The planned trial randomizes 180 women with PCOS into three groups of 60: a seven-strain probiotic supplement, a matching placebo, or open-label metformin (500 mg twice daily, a standard PCOS medication), each taken for six months.
• The main planned measurement (called the primary endpoint) is the change in free testosterone, a hormone, comparing the probiotic group to the placebo group; the metformin comparison is labeled exploratory rather than the trial's main question.
• The sample size of 60 per group was calculated using data from an earlier small pilot study of 10 women, in which three months of the same probiotic lowered free testosterone by a median of 0.3 pg/ml.
• Thirty of the 180 participants will also undergo gastroscopy and sigmoidoscopy (endoscopic procedures) to collect gut tissue samples, since the trial's broader premise is that gut bacteria composition may influence PCOS hormone levels.

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