The Gut-Testosterone Axis: How They're Connected

When researchers began exploring germ-free mice—animals raised without any gut bacteria—one of the findings that raised eyebrows was dramatically reduced testosterone levels compared to conventionally colonized mice.^[7] That single observation opened a field of inquiry: if the absence of gut bacteria correlates with suppressed testosterone, can deliberately supporting the gut microbiome help maintain or restore healthy androgen levels?

The relationship is bidirectional. Testosterone shapes which microbes thrive in the gut. And in turn, gut microbes influence the hormonal environment that determines how much testosterone is produced. This two-way dynamic has been described in the literature as the "microgenderome"—a term capturing the sex-specific interaction between the microbiome and reproductive hormones.^[1]

The Hypothalamic-Pituitary-Testicular (HPT) Axis

Testosterone production begins in the brain. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which signals the pituitary gland to secrete luteinizing hormone (LH). LH then travels to the testes and stimulates the Leydig cells—the primary testosterone-producing cells—to synthesize and release testosterone. Gut microbiota appear to modulate this signaling chain at multiple points, including by influencing GnRH signaling and the transcription factors controlling androgen synthesis.^[1]

Androgen Metabolism in the Gut

The gut also plays a role in how androgens are recycled and reabsorbed. Testosterone and its metabolites are excreted through bile as conjugated (glucuronidated) compounds. Certain gut bacteria—particularly those in the phylum Firmicutes—can cleave these conjugates, allowing androgens to be reabsorbed into circulation rather than excreted. A 2022 study found that fecal dihydrotestosterone (DHT) concentrations in adult mice were more than 20 times higher than serum levels, suggesting significant gut-based androgen metabolism.^[8] Disruptions to this microbial activity from dysbiosis may impair the enterohepatic circulation of testosterone.

How Gut Dysbiosis Suppresses Testosterone

Understanding how an unhealthy gut can undermine testosterone isn't just academic—it offers a roadmap for why supporting gut integrity matters for men's hormonal health. There are several interconnected pathways.

The LPS–Leydig Cell Pathway

Gram-negative bacteria in the gut produce lipopolysaccharides (LPS), which are endotoxins normally contained within the intestinal lumen. When the gut barrier is compromised—a condition associated with dysbiosis and intestinal hyperpermeability—LPS escapes into systemic circulation. Once in the bloodstream, LPS activates Toll-like receptor 4 (TLR4) on immune cells, triggering a cascade of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6.^[6]

These inflammatory cytokines directly inhibit Leydig cell function by suppressing the steroidogenic acute regulatory protein (StAR) and other enzymes essential for testosterone biosynthesis. Research has shown that even a single injection of a low dose of LPS is sufficient to meaningfully reduce Leydig cell testosterone production in animal models.^[9] In men with obesity—a condition frequently associated with gut barrier dysfunction—gut-derived bacterial endotoxins are specifically implicated in testosterone deficiency.^[1]

Chronic Systemic Inflammation

Beyond the direct LPS–Leydig cell pathway, gut dysbiosis generates chronic low-grade inflammation that broadly impairs hormonal signaling. IL-17, in particular, has emerged as a key mediator in this process. The 2014 PLOS ONE study on L. reuteri found that experimentally depleting IL-17A in aging mice recapitulated many of the positive testicular effects seen in probiotic-fed mice—including increased testicular weight, larger seminiferous tubule cross-sections, and more Leydig cells per testis. This suggests that the testosterone-supporting effects of probiotics are partly mediated by their anti-inflammatory properties.^[2]

Nutrient Absorption and Hormonal Raw Materials

Testosterone synthesis depends on several micronutrients—zinc, magnesium, vitamin D, and cholesterol among them. A gut that is inflamed, dysbiotic, or structurally compromised may fail to absorb these hormonal building blocks even when dietary intake is adequate. This is an indirect but clinically relevant connection: probiotic support for gut integrity and nutrient absorption is a supporting actor in the testosterone story, not just a background character. For a deeper look at how gut function intersects with micronutrient status, see our article on gut health and mineral deficiency signs.

Key Probiotic Strains for Male Hormonal Health

Not all probiotics are equal when it comes to testosterone research. The strains with the most compelling evidence are those also found in MicroBiome Restore—which is why we focus exclusively on strains within our formula rather than cataloging strains not available to our customers.

Lactobacillus reuteri: The Most Studied Strain for Testosterone

Lactobacillus reuteri is the probiotic strain that launched most of the modern conversation about probiotics and male hormonal health. The seminal 2014 research from MIT, published in PLOS ONE, began almost by accident. During probiotic obesity studies, researchers noticed that male mice consuming L. reuteri ATCC 6475 had unexpectedly large testes. Systematic investigation confirmed the effect was real and dose-dependent.^[2]

At every time point examined—5, 7, 9, and 12 months of age—mice receiving L. reuteri had significantly greater testicular weight than age-matched controls, and the statistical significance of this difference grew progressively with age. Histomorphometry revealed increased seminiferous tubule cross-sectional profiles, enhanced spermatogenesis, and a significantly greater number and size of Leydig cells per testis. Crucially, serum testosterone levels were measurably higher in probiotic-fed mice, and these animals were also leaner with greater muscle mass.^[2]

The researchers specifically tested whether these effects were driven by L. reuteri's anti-inflammatory properties. When they experimentally depleted IL-17A—a pro-inflammatory cytokine—in aging mice, they recapitulated the testicular benefits of probiotic feeding. This confirmed that the mechanism involves reduction of gut-derived inflammation, particularly the IL-17 pathway, rather than any direct androgenic action.^[2]

To learn more about this strain's broader health applications, visit our article on Lactobacillus reuteri benefits.

Lactobacillus rhamnosus: Testosterone, LH, FSH, and Sperm Quality

Lactobacillus rhamnosus has produced notable results in both reproductive hormone and sperm quality research. In a PLOS ONE study examining the effects of L. rhamnosus PB01 (DSM 14870) in both normal-weight and diet-induced obese mice, probiotic supplementation increased serum testosterone, LH, and FSH levels in both groups.^[4] This is significant because it suggests the hormonal effects were not simply a consequence of weight loss.

Alongside the hormonal improvements, sperm motility parameters improved significantly—including progressive motility velocity and a reduction in the percentage of immotile sperm. In obese mice, testicular weight was also significantly higher in the probiotic-supplemented group, consistent with the testosterone-preserving effects seen in the L. reuteri literature.^[4]

More recently, a 2025 study published in Andrologia examined the antioxidative properties of both L. reuteri and L. rhamnosus in a testicular ischemia/reperfusion injury model. L. rhamnosus in particular demonstrated meaningful reductions in oxidative stress markers, normalization of testosterone and gonadotropin levels, and improvements in histological testicular parameters—suggesting its benefits extend to protecting testicular tissue under physiological stress.^[10]

The broader evidence base for this strain's anti-inflammatory and metabolic effects is reviewed in our article on Lactobacillus rhamnosus benefits.

Lactobacillus acidophilus: Testicular Function and Cholesterol

Cholesterol is the direct precursor molecule to testosterone—the body literally builds testosterone from cholesterol via a series of enzymatic conversions. High circulating cholesterol can paradoxically suppress testicular function through oxidative stress and lipotoxicity. This makes L. acidophilus, a well-established cholesterol-modulating probiotic, relevant to the testosterone conversation.

A study published in Clinical and Experimental Reproductive Medicine investigated the effects of L. acidophilus supplementation in hypercholesterolemic male rats.^[5] In the high-cholesterol group without probiotic intervention, testosterone and FSH levels were significantly lower than controls, LH was suppressed, testicular tissue showed atrophy, vacuolization, and structural damage. After L. acidophilus supplementation, all these parameters—testosterone, FSH, LH, and histological integrity—moved back toward normal control levels. Cellular apoptosis markers (FAS) also decreased significantly with probiotic treatment.^[5]

Read more about this strain's documented health applications in our article on Lactobacillus acidophilus benefits.

Lactobacillus plantarum: The HPT Axis and Sexual Function

Lactobacillus plantarum has been studied in the context of male sexual and reproductive function, with research pointing to its influence on the hypothalamic-pituitary-testicular (HPT) axis. A 2021 study found that L. plantarum reversed hyperinsulinemia-induced sexual and reproductive deficits in male mice by modulating antioxidant status, lipid metabolism, and insulin signaling throughout the HPT axis.^[11] A 2024 study in Antioxidants further demonstrated that Lactiplantibacillus plantarum 1008 attenuated obesity- and age-induced declines in sperm quality, testosterone levels, and steroidogenic enzyme expression in high-fat-diet-fed mice.^[12]

This strain's broader anti-inflammatory and gut barrier-supporting properties are covered in depth in our article on Lactobacillus plantarum health benefits.

What Human Trials Have Shown

Animal studies provide mechanistic insight, but the honest picture for human testosterone and probiotics requires scrutinizing the limited RCT data that currently exists.

The Ljunggren et al. 2024 RCT: Key Findings

The most rigorous human trial to date was a 12-week, double-blind, placebo-controlled RCT published in Contemporary Clinical Trials Communications in 2024, conducted at Malmö University in Sweden.^[3] Healthy men aged 55–65 received either high-dose L. reuteri ATCC PTA 6475 (≥5 × 10⁹ CFU/capsule), low-dose (≥5 × 10⁸ CFU/capsule), or placebo. Blood samples were collected at baseline, 6 weeks, and 12 weeks.

The primary finding: neither dose of L. reuteri significantly increased testosterone levels compared to placebo. A secondary finding of genuine interest, however, was a statistically significant reduction in triglyceride levels in the high-dose group—a meaningful cardiovascular benefit in an age group at risk for metabolic dysfunction.^[3]

This trial has important limitations worth noting. The 12-week duration may be insufficient to produce measurable androgen changes in healthy older men, whose testosterone decline is gradual over years. The study population was restricted to men 55–65 with BMI between 18–30 and no known metabolic disease—precisely the group least likely to benefit from reducing dysbiosis-driven hormonal suppression. Animal studies used much longer exposure periods relative to lifespan equivalents. And the positive results seen in mice involved younger, obesogenic-diet-stressed animals—conditions closer to a gut dysbiosis state where probiotics have more to "correct."

What a 2025 Systematic Review Found

A 2025 systematic review in PeerJ, which synthesized cross-sectional, case-control, and cohort research, concluded that a significant positive correlation does exist between gut microbiome composition and testosterone levels in men—with Firmicutes bacteria (the phylum that includes Lactobacillus species) specifically implicated in testosterone production and reabsorption.^[1] Men in high-testosterone groups consistently showed greater gut microbiome diversity. The review proposed the HPG axis modulation, androgen metabolism, and intestinal homeostasis as three distinct biological pathways through which the microbiome influences androgen levels.^[1]

Probiotics, Sperm Quality, and Male Fertility

While testosterone is the central hormone in this discussion, male reproductive health involves more than androgen levels alone. Sperm quality—motility, concentration, morphology—is where probiotic research has produced some of its most consistent findings for men's health.

The L. rhamnosus animal studies described above found significant improvements in progressive motility velocity and reduced percentages of immotile sperm, alongside hormonal improvements.^[4] The 2014 L. reuteri study from MIT also reported improved sperm concentration and activity in 12-month-old mice consuming the probiotic.^[2] And the cholesterol-diet research on L. acidophilus demonstrated restoration of spermatogenesis alongside testosterone recovery.^[5]

A 2024 study in mSystems provided a complementary mechanistic piece: disruptions to gut microbiota-derived inosine metabolism impaired intestinal mucus barrier integrity, raised circulating LPS levels, and subsequently reduced testosterone production—demonstrating a direct chain of causation from gut barrier dysfunction to testicular impairment.^[9]

For men thinking about fertility as well as hormonal health, the research on probiotics is particularly relevant. Our article on probiotics for fertility and conception covers additional strain-specific evidence for reproductive support.

What to Look for in a Probiotic for Men's Hormonal Health

If you're evaluating probiotics with male hormonal and gut health in mind, the evidence points toward several criteria that separate meaningful formulas from marketing-driven ones.

Multi-Strain Diversity Across Lactobacillus and Bifidobacterium

The gut-testosterone connection operates through multiple pathways simultaneously—anti-inflammatory activity, gut barrier integrity, androgen metabolism, HPG axis modulation. A multi-strain probiotic that supports diverse microbial functions across the intestinal ecosystem is more likely to address these pathways comprehensively than any single strain. The 2025 systematic review specifically noted that men with higher testosterone showed significantly greater overall gut microbiome diversity, reinforcing the case for diversity over single-strain supplementation.^[1]

Clean Formulation: Why Fillers Undermine the Purpose

Many commercial probiotics contain inactive ingredients that can actively work against the gut health goals you're trying to achieve. Microcrystalline cellulose (MCC), magnesium stearate, and titanium dioxide are standard industry additives with documented concerns for gut function. If the goal is to reduce gut inflammation and support intestinal barrier integrity—preconditions for healthy testosterone production—introducing gut-disrupting fillers is counterproductive. Learning to read probiotic supplement labels and identify hidden additives is a skill that pays dividends.

MicroBiome Restore was formulated specifically without these additives. Our capsule material is pullulan—a fermented, plant-derived polymer that provides delayed-release without synthetic coatings. For a deeper look at why capsule material matters, see our article comparing HPMC vs. pullulan capsules for gut health.

Prebiotic Support: Feeding the Strains That Matter

Probiotic bacteria require fuel to colonize and produce their gut-health benefits—and that fuel comes from prebiotics. MicroBiome Restore includes 9 organic prebiotics, including Jerusalem artichoke (a concentrated source of inulin-type fructans), acacia fiber, maitake mushroom, fig fruit, bladderwrack, Norwegian kelp, and oarweed. Together, these prebiotic sources support the growth of the beneficial bacterial populations—particularly Lactobacillus and Bifidobacterium species—that the testosterone-relevant research has focused on. A formula that includes both probiotics and compatible prebiotics is sometimes called a synbiotic, and this combination is increasingly recognized as the most effective approach to long-term microbiome support.

Appropriate CFU Count

The animal studies that produced positive testosterone results used dose ranges roughly equivalent to 1–10 billion CFU in human terms, adjusted for body weight. Clinical probiotic doses for general gut health typically fall between 1 and 20 billion CFU daily. MicroBiome Restore delivers 15 billion CFU per serving across 26 strains—providing meaningful coverage without requiring megadoses that have no additional evidence behind them.

Frequently Asked Questions

The Gut's Role in Male Hormonal Health: What We Know and Where the Science Is Heading

The evidence connecting gut health to testosterone is genuine, growing, and mechanistically compelling—even if the human clinical picture is less definitive than the animal data would suggest. What we can say with confidence is this: a gut ecosystem characterized by dysbiosis, elevated intestinal permeability, and systemic inflammation represents a meaningful threat to optimal testosterone production. The Leydig cells that synthesize testosterone are directly vulnerable to LPS-driven inflammatory signaling from a compromised gut. The HPT axis that orchestrates androgen production is influenced by gut microbiome metabolites and signaling molecules. And men with higher gut microbiome diversity consistently show higher testosterone in population-level research.

Probiotics don't replace the other pillars of male hormonal health—resistance training, quality sleep, stress management, and appropriate nutrition matter enormously. But a filler-free, multi-strain probiotic that includes the strains most studied for their role in gut barrier integrity and anti-inflammatory function may support the upstream conditions that make testosterone production possible. That is a meaningful distinction from the overclaimed testosterone-boosting supplements that dominate the market.

Explore our complete guide to MicroBiome Restore to understand the full formulation rationale behind our 26-strain synbiotic, or visit the best probiotic strains for men over 40 article for more on how gut health supports broader male wellness.