Bidirectional relationships between the gut microbiome and sexual traits

The human gut microbiota is known to be shaped by a variety of environmental factors (diet, drugs, geography and sanitation) and host intrinsic factors (age and sexual development). The differences in gut microbiota between sexes are minimal before adulthood and late adulthood, and marked during adulthood. For instance, consistent higher abundances of Akkermansia and Ruminococcus have been observed in adult women compared to men and most studies have found higher abundances of Prevotella and Fusobacterium (linked to a diet rich in animal proteins) in adult men compared to women. The gut microbiota taxonomy and functionality present in women is more similar to men once reached the menopause. In fact, specific taxa have been associated with the levels of different sexual hormones and their precursors in blood. The gut microbiota composition and circulating testosterone levels are also tightly linked to the extent that microbial signatures can predict its levels in blood. Not only sexual hormones seem to influence the gut microbiome, but also the latter participates in the metabolism of these hormones, with some bacteria being able to metabolize gonadal steroid hormones (one example is 3β-hydroxysteroid dehydrogenase, a testosterone degrading enzyme). In summary, the relationships between the gut microbiome and sexual traits are bidirectional. In addition, other phenotypes and cultural gender-related factors could drive sex-related differences. It is important to note that other members of the microbiome (Archeae, viruses and fungi) have been largely unexplored in relation to this sexual dimorphism. More research is needed on this topic.