Down‐to‐Earth Studies of the Gut Microbiome in Bone Health and Disease

The gut microbiome is considered a key modulator of human health, with growing evidence for effects across multiple body systems and where skeletal health outcomes are no exception. Hundreds of publications have emerged in the last years, extending from fundamental (in vitro and in vivo animal) studies to interventional trials (mostly focusing on probiotics and prebiotics), supporting a putative relationship between variation in the gut microbiome and bone metabolism. Nevertheless, in what way the gut microbiome triggers bone disease remains to be clearly elucidated. Therefore, the bench-to-bedside trajectory of gut microbiome research could benefit from the contribution of large epidemiological (observational) studies that can aid the understanding of the underlying biological mechanisms and, in theory, orient the design of interventional studies to successfully improve bone health. In this issue, Orwoll and colleagues describe a comprehensive fecal microbiome study embedded in the well-known MrOS (US) cohort study of community-dwelling older men (n = 831, age range 78 to 98 years). The authors report four genera (Anaerofilum, Methanomassiliicoccus, Ruminiclostridium 9, and Tyzzerella) weakly associated with one or more of the rich set of skeletal parameters assessed in the participants (eg, dual-energy X-ray absorptiometry [DXA] or high-resolution peripheral quantitative computed tomography [HR-pQCT]). Lacking independent replication or functional follow-up of these associations, we praise the authors for humbly recognizing the hypothesisgenerating nature of their findings. Orwoll and colleagues drill down on the implications of the low statistical power in their study and advocate for larger well-powered studies (with sample size in the tens of thousands) invested in generating robust findings to improve the understanding of the gut microbiome–bone axis. They also highlight the crucial need to correctly evaluate the level of evidence and intrinsic limitations of their study. To place in perspective the complexity of interpreting gut microbiome findings across the different taxonomic levels (setting aside power limitations), let us consider the “negative” association between the genus Lactobacillus spp. and bone health outcomes, reported by Orwoll and colleagues. To begin, the genus Lactobacillus comprises more than 170 species and many more subspecies and strains. In the last years, clinical trials of probiotics have reported positive effects for different types of Lactobacillus spp. on bone health outcomes, both in humans (ie, Lactobacillus rheuteri and the combination of Lactobacillus paracasei DSM 13434, Lactobacillus plantarum DSM 15312, and Lactobacillus plantarum DSM 15313) and in animal models (ie, Lactobacillus acidophilus and Lactobacillus rhamnosus). As stated by the authors, this body of evidence is in contrast with their current negative findings. It is still plausible that other Lactobacillus spp. are detrimental to bone health or even that the activity of the same microorganisms is modulated differently by lifestyle of the host (ie, diet, exercise) or by interaction with other species within the same niche in the gut, contributing to the conflicting results. When conducting human microbiome research, adherence to traditionalepidemiologicalprinciples iswarranted.Yet, researchers should alsobecomeacquaintedwith specificmicrobiome research pitfalls, such as shortcomings intrinsic to sequence technologies and the limitations ofmodeling complex ecosystems (as illustrated by the multiplicity of methods used in the field). Previously, JBMR has provided author guidelines for the publication of genomic andother formsof -omicsstudies thatareapplicable tomicrobiome research. Among them, replication of the associations in independentsettingsand/or functionalvalidation insupportofmechanistic principles are sine qua non elements to be considered. However, microbiome screenings confined to the assessment of taxonomic composition and diversity are simplistic, underestimating the full characterization of an ecosystem, which is a notable drawback for guiding functional evaluations. In addition, replication efforts remain challenging given the large interindividual variation of the gut microbiome and its dynamic response to environmental factors. We highlight here some additional fundamental considerations when drawing up human microbiome studies in epidemiological settings.