The gut microbiome has been implicated in a variety of physiological states, but controversy over causality remains unresolved. Here, we performed bidirectional Mendelian randomization (MR) analyses on 3,432 Chinese individuals with whole genome, whole metagenome, anthropometric, and blood metabolic trait data. We identified 58 causal relationships between the gut microbiome and blood metabolites, and replicated 43 of them. Increased relative abundances of fecal Oscillibacter and Alistipes were causally linked to decreased triglyceride concentration. Conversely, blood metabolites such as glutamic acid appeared to decrease fecal Oxalobacter, and members of Proteobacteria were influenced by metabolites such as 5-methyltetrahydrofolic acid, alanine, glutamate, and selenium. Two-sample MR with data from Biobank Japan partly corroborated results with triglyceride and with uric acid, and also provided causal support for published fecal bacterial markers for cancer and cardiovascular diseases. This study illustrates the value of human genetic information to help prioritize gut microbial features for mechanistic and clinical studies.Metagenome-wide association studies (MWAS) using human stool samples, as well as animal models, especially germ-free mice, have pointed to a potential role of the gut microbiome in diseases such as cardiometabolic, autoimmune, neuropsychiatric disorders and cancer, with mechanistic investigations for diseases such as obesity, colorectal cancer and schizophrenia [1][2][3][4] . Twin-based heritability estimation and more recent metagenome-genome-wide association studies (M-GWAS) have questioned the traditional view of the gut microbiota as a purely environmental factor 5-9 , although the extent of the genetic influence remains controversial 7,10 . Yet, all these published cohorts, except for human sequences in the metagenomic data of HMP (Human Microbiome Project), utilized array data for human genetics, and most of them had 16S rRNA gene amplicon sequencing for the fecal microbiota [5][6][7][8][9] .As the gut microbiome is considered to be highly dynamic, causality has been an unresolved issue in the field. Mendelian randomization (MR) 11 offers an opportunity to distinguish between causal and non-causal effects from cross-sectional data, without animal studies or randomized controlled trials. An early study used MR to look at the gut microbiota and ischemic heart disease 12 . Recently, a study used MR to confirm that increased relative abundance of bacteria producing the fecal volatile short-chain fatty acid (SCFA) butyrate was causally linked to improved insulin response to oral glucose challenge; in contrast, another fecal SCFA, propionate, was causally related to an increased risk of T2D 13 . However, both studies used genotype data, and it was not clear to what extent the genetic factors explained the microbial feature of interest.In this study, we present a large-scale M-GWAS using whole genome and fecal microbiome, followed by bidirectional MR for the fecal microbiome and anthropometric...

Objective. Estrogen is a critical hormone that is mainly produced by the ovary in females. Estrogen deficiency leads to various syndromes and diseases, partly due to gut microbiota alterations. Previous studies have shown that estrogen deficiency affects the gut microbiota at 6–8 weeks after ovariectomy, but the immediate effect of estrogen deficiency on the gut microbiota remains poorly understood. Methods. To investigate the short time and dynamic effects of decreased estrogen levels on the gut microbiota and their potential impact on estrogen deficiency-related diseases, we performed metagenomic sequencing of 260 fecal samples from 50 ovariectomy (OVX) and 15 control C57BL/6 female mice at four time points after surgery. Results. We found that seven gut microbiota species, including E. coli, Parabacteroides unclassified, Lachnospiraceae bacterium 8_1_57FAA, Bacteroides uniformis, Veillonella unclassified, Bacteroides xylanisolvens, and Firmicutes bacterium M10_2, were abundant in OVX mice. The abundance of these species increased with time after OVX surgery. The relative abundance of the opportunistic pathogen E. coli and the Crohn's disease-related Veillonella spp. was significantly correlated with mouse weight gain in the OVX group. Butyrate production and the Entner–Doudoroff pathway were significantly enriched in the control mouse group, while the degradation of glutamic acid and aspartic acid was enriched in the OVX mouse group. As the time after OVX surgery increased, the bacterial species and metabolic pathways significantly changed and tended to suggest an inflammatory environment, indicating a subhealthy state of the gut microbiota in the OVX mouse group. Conclusions. Taken together, our results show that the dynamic gut microbiota profile alteration caused by estrogen deficiency is related to obesity and inflammation, which may lead to immune and metabolic disorders. This study provides new clues for the treatment of estrogen deficiency-related diseases.

The calculation of gut metabolic modules from gene profile

Calorie restriction (CR) has been widely recognized for its effect in reducing body weight and alleviating diabetes in humans, as well as prolonging life span in animal studies. Gut microbiome shifts contribute to part of the effects of CR, but little is known regarding their influences except on metabolism and immunity. Here we monitored gut microbiome using metagenomics and metatranscriptomics in obese individuals undergoing CR, and revealed microbial determinants that could contribute to successful weight loss. Microbiome changes are linked to changes in blood metabolome and hormones, which eventually correlate to brain functional changes as studied using functional magnetic resonance imaging (fMRI). Brain functional shifts indicate response of central neural system (CNS) to CR, and microbiome constitutes the keystone of gut-brain axis. Animal experiment further reaffirms the gut microbiome changes, metabolic and hormonal shifts of CR, while proteomic analysis of brain tissues suggest that epigenetic modifications of key proteins could explain responses of CNS to CR. Our study establishes linkage between CR, gut microbiome, metabolome/ hormones and CNS function, and demonstrates that CR has multi-facet, coordinated effects on the host, of which many could contribute to weight loss and other beneficial effects.