In this study, a two-sample MR analysis was employed to investigate the impact of 196 gut microbiota species on osteoporosis. The findings revealed that five gut microbiota species act as protective factors against osteoporosis, while another five act as risk factors, with family.Christensenellaceae being notably significant. Further mediation analysis indicated that family.Christensenellaceae may increase the risk of osteoporosis through the intermediary effect of serum 3,4-dihydroxybutyrate levels.
Osteoporosis is a common disease influenced by various pathogenic factors, including gut microbiome [18], autophagy [19], abnormal iron metabolism [20], and stress [21]. Shasha Song et al. [22] systematically reviewed previous studies on osteoporosis, revealing that the gut microbiome can impact osteoporosis through multiple mechanisms: ①Probiotics could counteract the decrease in osteoblast number and bone mass induced by OVX (glucocorticoids and ovariectomy) by increasing the gene expression of Bmp-2 and Sparc through alterations in the gut microbiome composition.②Probiotics compensate for bone loss by regulating immunity.③Probiotics can prevent OVX-induced bone loss by reducing intestinal permeability and secretion of IL-17, RANKL, and TNF-α from the intestines, inhibiting osteoclastogenesis.
Family.Christensenellaceae, belonging to the Firmicutes phylum, is widely distributed in the human gut [23]. The genotype of the host is estimated to influence 30%-60% of the variation in the relative abundance of Christensenellaceae [24]. Its abundance in the human gut is associated with health [25]. Beaumont found a correlation between Christensenellaceae and a lower BMI [26]. Considering the existing research showing the correlation between Christensenellaceae and a lower BMI and considering that a lower BMI is a risk factor for osteoporosis, we speculate that Christensenellaceae may impact osteoporosis by influencing human metabolism.
3,4-dihydroxybutyrate is a common metabolic product in the human body, and its levels are elevated in the urine of succinic semialdehyde dehydrogenase deficiency patients [27]. Studies by T Minami et al. [28] found that 3,4-dihydroxybutanoic acid gamma-lactone and 2,4,5-trihydroxypentanoic acid gamma-lactone can affect the neural excitability of the lateral hypothalamic area and ventromedial hypothalamic nucleus in rats, brain areas referred to as feeding and satiety centers, respectively. N Shimizu et al. [29] found that injections of 3,4-dihydroxybutanoic acid (2.5 mumol) into the third cerebral ventricle of chronic rats suppressed food intake and single-neuron activity in the lateral hypothalamic area, suggesting that 3,4-dihydroxybutanoic acid may act as an endogenous satiety substance. These studies suggest that 3,4-dihydroxybutyrate may be associated with the feeling of satiety. We can further speculate that an elevated level of blood 3,4-dihydroxybutyrate may lead to increased satiety, thereby reducing the intake of various nutrients, including VitD and calcium, consequently increasing the risk of osteoporosis.
In summary of the above discussion and based on our MR analysis results, we consider that family.Christensenellaceae may increase the risk of osteoporosis by elevating serum 3,4-dihydroxybutyrate levels, and previous research findings seem to support this evidence chain.
To the best of our knowledge, this study is the first to use MR methods to investigate the causal relationship between gut microbiota and osteoporosis and the first to explore the mediating role of metabolites in this process. GWAS data for exposure, mediation, and outcome are derived from three different populations, aiming to avoid bias resulting from overlapping study populations. The use of MR methods allows for the effective elimination of confounding factors and reverse causation interference. We conducted sensitivity analyses using MR Egger and IVW methods, effectively addressing issues of horizontal pleiotropy and heterogeneity in the study results.
However, this study has certain limitations. The lowest taxonomic level in the exposure dataset was genus, preventing exploration of the causal association between gut microbiota and osteoporosis at the species level. To conduct sensitivity analysis and detect horizontal pleiotropy, more genetic variations need to be included as instrumental variables; therefore, SNPs used in the analysis did not reach the traditional GWAS significance threshold (P < 5×10–8). Consequently, we selected only the gut microbiota with the smallest P-values for further analysis to restrict the overgeneralization of study conclusions. The study population consists of individuals of European descent, limiting the generalizability of the findings to other ethnicities. The study utilized summary-level data, precluding subgroup analysis for more precise conclusions. Further research, using individual-level data, can explore whether the conclusions of this study can be generalized to different populations.
In conclusion, this two-sample MR-based mediation analysis revealed that family.Christensenellaceae may increase the risk of osteoporosis by elevating serum 3,4-dihydroxybutyrate levels. Further RCTs are needed to validate this conclusion.