Transgender individuals, an underrepresented minority, experience medically induced hormonal changes during their transition, which might impact sexual dimorphism with regards to disease susceptibility [28]. We here describe a significant impact of sex hormones on the composition and metabolic capacity of fecal microbiota in a prospective longitudinal study in a cohort involving both trans women and trans men. Knowledge regarding the microbiome's potential mediating or mitigating role in this context remains scarce. Nonetheless, such a role is highly plausible, given its pivotal involvement in various health conditions exhibiting sex or gender-related disparities [4, 11, 29]. Previously, there was some evidence that sex might exert an influence on the diversity, composition, and function of the gut bacterial microbiota, although the findings have been inconclusive. Early investigations, involving a relatively small sample size and constrained by limited technological capabilities, yielded minimal to no discernible gender-related distinctions. For instance, a 2005 study involving 91 individuals of northern European descent (from France, Denmark, Germany, the Netherlands, and the UK) did not reveal any significant variations in gut microbiota between the sexes, as determined by principal component analysis [30]. In a 2008 Chinese study employing group-specific denaturing gradient gel electrophoresis (DGGE) profiling of Bacteroides species, a greater prevalence of Bacteroides thetaiotaomicron was observed in males [31]. Recent extensive population-wide studies have similarly not demonstrated significant sex-specific variations in the diversity, intricacy, or composition of gut microbiota [13, 32]. Nevertheless, these initial studies were methodologically limited, mainly due to the absence of shotgun metagenomic sequencing in most of them. In a substantial cohort study encompassing two independently well-characterized groups, namely the Belgian Flemish Gut Flora Project (consisting of 1,106 individuals) and the Dutch LifeLines-DEEP study (comprising 1,135 individuals), sex exhibited a relatively minor effect size among the 69 factors that were found to be significantly associated with the overall variation in the microbiome community [33]. In a large Dutch research study employing metagenomic shotgun sequencing and after accounting for multiple variables, the only discernible association with sex was related to Akkermansia muciniphila, where women were observed to have a higher prevalence of this particular species [34]. Nevertheless, the outcomes of each study pertaining to variations in microbial taxa between the sexes display inconsistencies [34–37]. This lack of consistency is to be expected, given the myriad potential confounding variables and the multitude of factors related to sex and gender identity that can impact the microbiome, extending beyond the influence of sex steroids [33]. It is worth noting that many of these studies did not factor in menopausal status, which signifies a pivotal juncture in a woman's life cycle marked by profound hormonal changes. Therefore, precious evidence is furnished by a study encompassing both pre and post-menopausal women, as well as men [12]. This study demonstrated that the gut microbiota of post-menopausal women exhibited greater similarity to that of men rather than pre-menopausal women [12]. Comprehensive analyses of metagenome functionality unveiled no significant disparities between post-menopausal women and men [12]. These findings underscore the impact of sex-specific hormonal status in shaping sex-specific patterns within the gut microbiome. Nonetheless, due to the cross-sectional design and the absence of a longitudinal within-subject approach with a high degree of covariate stability, this study cannot definitively assert that the observed changes are exclusively linked to sex steroid status.
Our results robustly demonstrate that GAHT induces specific transitional changes in bacterial species that align with the desired gender features. This notably pertains to the species Parabacteroides goldsteinii, Coprococcus sp. ART55/1, Coprococcus eutactus, and Escherichia coli. Coprococcus species inhabit the human colon and are strictly anaerobic bacteria [38]. C. eutactus plays a significant role in butyrate production. This microorganism is recognized for its immunomodulatory properties, associated with favorable metabolic effects, and is acknowledged for its neuroprotective potential [39]. Notably, C. eutactus has consistently been found to be reduced in individuals with Parkinson's disease, a neurodegenerative disorder that tends to affect men more frequently [39]. Certain E. coli strains are recognized as commensal microorganisms, coexisting with the host without causing harm. These strains might contribute to the preservation of a well-balanced gut microbiome [40]. The presence of pathogenic E. coli strains in the gut has been linked to various illnesses. For instance, E. coli strains that produce the genotoxin colibactin have been associated with colorectal cancer, a condition that exhibits a higher prevalence in men [41].
More pronounced alterations in the gut metagenome became evident in response to GAHT when we analyzed metagenomic functionality. This analysis revealed significant variations in both the overall composition and a multitude of individual metabolic pathways. Remarkably, among the pathways exhibiting a stronger association with the male phenotype (both before and after transition), several functions related to lipid metabolism were identified. These included processes such as palmitate biosynthesis, unsaturated fatty acids biosynthesis (predominantly by E. coli), and Fatty acid & beta oxidation IV. While considerable focus has been directed toward investigating the short-chain fatty acid metabolism of the gut microbiota, substantially less research has been undertaken to elucidate the importance of its anabolic and catabolic metabolism of medium- and long-chain fatty acids [42]. The majority of research examining the interplay between dietary lipids and the microbiome is derived from rodent studies. However, these investigations are inherently constrained by the fundamental disparity in nutritional requirements between humans and mice [42]. Nonetheless, the existing results provide significant insights into the implications of microbiota-lipid interactions on body composition, insulin sensitivity, inflammation, and cardiovascular disease risk. Given that men typically exhibit higher proportions of lean muscle mass and a greater susceptibility to visceral obesity and cardiovascular disease, the current findings may offer a significant clue regarding the potential contribution of the gut microbiota to these gender-related health disparities [43]. These discoveries may also improve the provision of post-transition care for transgender individuals by elucidating how the process of transitioning could potentially alter future medical risks and health outcomes.
Our study has limitations that warrant consideration. First, the sample size was relatively small and heterogenous with regard to delivery of GAHT (e.g., oral vs. transdermal estradiol), which restricted our ability to comprehensively explore potential covariates. Second, since our study did not include a control group receiving placebo instead of GAHT, we are unable to definitively prove that the observed changes in gut microbiota within our cohort occurred exclusively due to alterations in the sex hormone phenotype. However, due to ethical considerations, implementing such a study design will remain untenable in the future as well. Future research should also aim to expand the functional analysis by incorporating both bacterial and host metabolomes.