The relationship between the microorganisms residing in the human gut and the body’s overall health has become a major area of scientific interest. Among the billions of bacteria that make up the gut microbiome, Lactobacillus reuteri has drawn attention for its potential effects far beyond the digestive tract. This probiotic is being investigated for its influence on systemic endocrine function, particularly its ability to affect testosterone levels. The purpose of this article is to examine the current scientific evidence regarding this claim, distinguishing between laboratory findings and the limitations of human data.
Understanding Lactobacillus Reuteri
Lactobacillus reuteri, recently reclassified as Limosilactobacillus reuteri, is a Gram-positive bacterium naturally found in the gastrointestinal tract, mouth, and breast milk. It is recognized as a probiotic, meaning it confers a health benefit to the host. This bacterium promotes digestive health and modulates the gut environment. One established mechanism involves the production of reuterin, an antimicrobial substance that helps inhibit the growth of harmful pathogens. Supplementation with specific strains of L. reuteri is supported for managing common gastrointestinal issues, including reducing symptoms of infantile colic and assisting in the management of Helicobacter pylori infections.
Reviewing the Scientific Evidence
The claim that L. reuteri can increase testosterone levels stems primarily from compelling findings in rodent models rather than conclusive human clinical trials. A widely cited 2014 study on male mice demonstrated that routine consumption of the L. reuteri strain ATCC PTA 6475 led to significant changes in reproductive biology.
Mice given the probiotic displayed notably larger testicles and showed elevated serum testosterone levels compared to their age-matched control counterparts. Further microscopic analysis of the testicular tissue revealed that the probiotic-fed mice had increased Leydig cell numbers, which are the primary cells responsible for testosterone production. The study also found that the probiotic seemed to reverse markers of reproductive aging, suggesting a maintenance of youthful gonadal function.
Evidence in humans for this specific hormonal effect remains limited and less direct. While the animal data is robust, large-scale, randomized controlled trials confirming a direct testosterone increase in healthy human males are currently lacking. The promising results from animal models provide a strong hypothesis, but they do not yet translate into a guaranteed physiological outcome.
Biological Pathways Linking Gut Health and Hormone Production
The proposed connection between a gut microbe and systemic hormone levels is explained by the intricate network known as the gut-brain axis, which includes endocrine signaling pathways. The gut microbiome communicates with the brain and the rest of the body through neural, endocrine, and immune mechanisms. This communication involves the Hypothalamic-Pituitary-Gonadal (HPG) axis, which centrally regulates testosterone production.
One of the most plausible mechanisms involves L. reuteri’s anti-inflammatory properties. Chronic, low-grade inflammation is known to suppress the HPG axis and inhibit testosterone synthesis. Rodent studies highlighted that the beneficial effects of L. reuteri were linked to the deactivation of the pro-inflammatory signaling protein interleukin-17A (IL-17A). By reducing this systemic inflammation, the probiotic may remove a major inhibitory factor, allowing the body’s natural hormone production to function optimally.
Role of Metabolites
The gut microbiota produces metabolites such as short-chain fatty acids (SCFAs), which act as signaling molecules that can influence the endocrine system. These SCFAs, including butyrate and propionate, are produced when gut bacteria ferment dietary fiber and can affect the activity of various hormone-producing glands and pathways. While the precise SCFA-mediated link to testosterone is complex, the overall modulation of the gut environment by L. reuteri is thought to create a more favorable environment for hormone balance.
Practical Considerations for Supplementation
Individuals interested in trialing L. reuteri should focus on selecting a product with a verified strain, as effects are highly strain-specific. The strain ATCC PTA 6475 is the one specifically associated with the testosterone and reproductive findings in animal models. Other strains, like DSM 17938, are primarily studied for their established benefits in digestive health and infantile colic.
Dosages in human studies typically range widely, often from 100 million to 50 billion Colony Forming Units (CFUs) daily. Since there is no standardized clinical dosage for testosterone enhancement, users often rely on the doses used for general gut health. L. reuteri is generally considered safe and well-tolerated.
It is important to maintain realistic expectations regarding supplementation. While L. reuteri may support overall health and alleviate factors that inhibit hormone production, it is not a direct hormonal replacement or a guaranteed testosterone booster. The best approach involves combining the supplement with a healthy lifestyle, including proper diet and exercise, to support the body’s natural endocrine function.