Lactobacillus reuteri (L. reuteri) is a probiotic bacterium that naturally colonizes the mammalian gastrointestinal tract. It is typically associated with supporting digestive health and immune function. Recently, an unusual claim has circulated about its potential to influence reproductive biology, specifically to increase testicle size and testosterone levels. This article investigates the scientific basis of this claim, examining evidence from laboratory settings and contrasting it with established human physiology.
The Known Functions of Lactobacillus reuteri
The conventional use of L. reuteri is rooted in its ability to support the gut microbiome and interact with the host immune system. This probiotic produces various antimicrobial substances, including organic acids and a unique compound called reuterin. By generating these compounds, L. reuteri helps maintain a balanced intestinal flora, which contributes to overall digestive wellness.
The bacterium also plays a role in modulating immune responses and bolstering the intestinal barrier. Specific strains of L. reuteri have demonstrated an ability to reduce pro-inflammatory signals, suggesting anti-inflammatory properties. These established functions make it a common supplement for issues ranging from general digestive upset to infantile colic.
Investigating the Claim: Probiotics and Hormonal Pathways
The unusual claim linking L. reuteri to changes in male reproductive organs stems from animal studies conducted primarily on mice. Researchers observed that male mice consuming L. reuteri exhibited significantly larger testicles and higher serum testosterone levels compared to control mice. This effect was seen consistently across different diets and age groups, even counteracting some age-related testicular atrophy.
Microscopic analysis of the testicular tissue revealed the biological basis for this observed increase in size. The testes of the probiotic-fed mice showed an increased number and size of Leydig cells, the primary cells responsible for producing testosterone. There was also an increase in the cross-sectional area of the seminiferous tubules, suggesting enhanced overall gonadal function.
The hypothesized mechanism connecting the gut microbe to the reproductive system involves the gut-brain-endocrine axis and a reduction in systemic inflammation. Researchers proposed that the anti-inflammatory properties of L. reuteri, possibly by suppressing the pro-inflammatory cytokine interleukin-17A, reduced chronic inflammation. Reducing this inflammatory burden appeared to create an environment conducive to increased Leydig cell proliferation and testosterone synthesis.
Further research has begun to pinpoint a more direct link between the microbe and the hormonal system. Studies using human gut organoids suggest that L. reuteri can influence the secretion of various hormones from the intestinal lining. Notably, L. reuteri cultures promoted the secretion of luteinizing hormone subunit beta, a component of Luteinizing Hormone (LH). LH is the signal molecule released by the pituitary gland that stimulates testosterone production in the Leydig cells, directly linking the microbe to the hypothalamic-pituitary-gonadal (HPG) axis.
Testicle Size: Biological Reality vs. Probiotic Effect
The findings in rodent models must be viewed within the context of human biology. The size and development of human testicles are primarily determined by genetics and the complex hormonal signaling that occurs during puberty. Genetic makeup sets the blueprint for maximum potential size, while pubertal hormone surges dictate the final adult size.
There is currently no reliable human clinical evidence to support the dramatic effect seen in the mouse studies. The robust increase in Leydig cell count and testicular weight observed in mice has not been replicated or confirmed in men taking L. reuteri supplements. The difference between the controlled laboratory environment and the diverse physiology of the human population is significant.
While the preliminary findings about the HPG axis connection are intriguing, they are based on animal models and in vitro human cell cultures. The direct impact of consuming L. reuteri on the physical size of the human testis remains unproven. It is unlikely to override the established biological and genetic determinants of adult organ size. Therefore, while L. reuteri offers established benefits for gut and immune health, it should not be considered an effective means of increasing human testicle size.