Your Gut Microbiome and Testosterone: The Connection Most Men Don't Know About

Men focused on testosterone optimisation spend a lot of time thinking about sleep, training, body composition, and supplements. Almost none of them are thinking about their gut microbiome.

This is a significant oversight. The emerging research on the gut-hormone axis is making it increasingly clear that microbiome composition has direct and indirect effects on testosterone, oestrogen metabolism, inflammation, and overall hormonal health in men. If you want a proper at-home read of your own microbiome, my GI Cognition gut health test review covers what the report actually gives you.

The cognitive side of this is worth a closer look too — see my piece on gut health and mental performance in men.

The Oestrobolome

The starting point is the oestrobolome - the collection of gut bacteria capable of metabolising oestrogens. This is primarily a female hormone health topic in the popular literature, but it's equally relevant to men.

Here's why: oestrogen is metabolised in the liver through glucuronidation - a process that conjugates it to glucuronic acid and makes it water-soluble for excretion. In the gut, bacteria with the enzyme beta-glucuronidase can deconjugate oestrogen - breaking it free from glucuronic acid and allowing it to be reabsorbed into circulation rather than excreted.

If your gut microbiome has excessive beta-glucuronidase activity (associated with dysbiosis - imbalanced microbiome), more oestrogen gets recirculated. For men, this means higher circulating oestradiol - which suppresses testosterone via the hypothalamic feedback loop, promotes fat storage (which drives further aromatisation of testosterone to oestrogen), and contributes to symptoms like low libido, fatigue, and mood disruption.

Supporting a diverse, healthy microbiome with appropriate microbial balance reduces beta-glucuronidase activity and improves oestrogen clearance. This indirectly supports a more favourable testosterone:oestrogen ratio.

Inflammation: The Core Link

The most established pathway between gut health and testosterone is inflammation. Gut dysbiosis - an imbalanced microbiome - is one of the most common drivers of chronic systemic inflammation, mediated through increased intestinal permeability (the so-called "leaky gut" phenomenon) and elevated lipopolysaccharide (LPS) in circulation.

LPS is a component of gram-negative bacterial cell walls. When gut barrier integrity is compromised, LPS enters the bloodstream and triggers a systemic inflammatory response via Toll-like receptor 4 (TLR4) activation. This produces elevated TNF-α, IL-6, and hsCRP.

These inflammatory cytokines directly impair testosterone production:

• IL-1β and TNF-α have been shown to directly inhibit Leydig cell steroidogenesis (testosterone synthesis) in animal and cell models
• hsCRP elevation correlates with reduced testosterone in epidemiological studies
• Inflammation suppresses GnRH and LH production at the hypothalamic-pituitary level

Reducing gut inflammation through microbiome support therefore has a plausible and well-mechanised pathway to improved hormonal health.

The SHBG-Gut Connection

The liver produces SHBG, and liver function is significantly influenced by gut health. Dysbiosis and gut-derived inflammation promote liver inflammation, which alters SHBG production.

Research has found that conditions strongly associated with dysbiosis - obesity, metabolic syndrome, non-alcoholic fatty liver disease - are also associated with altered SHBG levels. Improving gut health as part of a broader metabolic intervention reduces this driver of SHBG disruption.

Direct Evidence From Probiotic Research

Animal studies have been compelling. A 2014 study (PLOS ONE) found that mice fed Lactobacillus reuteri had significantly higher testosterone levels, larger testes, and more favourable body composition compared to controls. The effect was partly attributed to improved gut integrity reducing systemic inflammation.

Human evidence is emerging. A 2022 randomised trial found that probiotic supplementation in men with metabolic syndrome significantly reduced inflammatory markers and improved testosterone levels over 12 weeks. A 2020 systematic review of probiotics and male fertility found consistent improvements in sperm parameters and reductions in oxidative stress markers across multiple trials.

The specific strains matter. Lactobacillus reuteri has the most direct animal evidence for testosterone effects. More broadly, strains with documented anti-inflammatory effects and bile salt hydrolase activity are most relevant for the oestrogen recycling and inflammation pathways.

How to Support Your Gut Microbiome

Dietary diversity: The single most powerful predictor of microbiome diversity is dietary diversity - specifically variety of plant foods. Research (the American Gut Project, 2018) found that consuming 30+ different plant foods per week was associated with significantly more diverse microbiomes than consuming fewer than 10. Each plant food type carries different fibre types that feed different bacterial species.

Fermented foods: Regular consumption of fermented foods (yoghurt, kefir, kimchi, sauerkraut, kombucha) increases microbiome diversity and reduces inflammatory markers. A 2021 Stanford RCT comparing a high-fermented-food diet versus high-fibre diet found that fermented foods significantly outperformed fibre for increasing microbiome diversity and reducing 19 inflammatory proteins, including IL-6.

Prebiotic fibre: Specific fibres - inulin, FOS (fructooligosaccharides), beta-glucan, resistant starch - preferentially feed beneficial bacteria. Sources include chicory, garlic, onion, leek, oats, green bananas, and cooked-then-cooled potatoes.

Probiotics: For targeted support, clinically validated probiotic strains provide reliable, dose-controlled delivery of specific bacteria. BioGaia is one of the most extensively researched probiotic brands globally, with specific strains validated in multiple human trials. Their Lactobacillus reuteri DSM 17938 and ATCC PTA 6475 strains are among the most studied for systemic effects including bone health, immune function, and the testosterone-relevant pathways discussed above.

Reduce microbiome disruptors: Alcohol, chronic NSAID use, proton pump inhibitors, and artificial sweeteners all negatively impact microbiome diversity and integrity. These aren't always avoidable, but awareness matters.

Testing Your Microbiome

If you want to understand your specific microbiome composition, at-home stool microbiome tests (from services like Biomesight or Thryve) provide a detailed profile of bacterial species and diversity. These aren't diagnostic but give directional information about dysbiosis patterns.

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This article is for educational purposes. Probiotic supplementation is generally safe for healthy adults but should be discussed with a GP if you are immunocompromised or have significant gastrointestinal conditions.