Introduction Increased and excess adiposity is associated with increased risk of endometrial cancer (EC) and both of these are associated with circulating metabolite profiles. However, how metabolites relate to the adiposity-EC relationship remains unclear. Methods We have brought together evidence from Mendelian randomization (MR) and observational analyses to evaluate the effect of i) adiposity traits on endometrial cancer, ii) adiposity traits on circulating metabolites and iii) adiposity-associated metabolites on EC. We have also evaluated the potential role of metabolites in the adiposity-EC relationship using multivariable MR. Observational analyses were conducted using individual level data from UK Biobank (N = 1,005 cases and 215,339 controls). MR analyses were performed using female-specific summary statistics from genome-wide association studies of body mass index (BMI; N up to 434,794), waist-to-hip ratio (WHR; N up to 381,152), 249 metabolites and ratios from targeted nuclear magnetic resonance metabolomics (N up to 140,768) and EC risk (12,906 cases and 108,979 controls). Results In observational analyses, higher BMI and WHR were associated with elevated odds of overall EC (odds ratio (OR) per standard deviation (SD) increase in BMI = 1.37; 95% confidence interval (CI) = 1.19, 1.57; OR per SD increase in WHR= 1.15; 95% CI = 1.01, 1.32). In MR analysis, higher BMI was associated with elevated odds of overall EC risk (OR per SD increase in BMI = 1.80; 95% CI = 1.56, 2.07), endometrioid cancer (OR = 1.71; 95% CI = 1.45, 2.02) and non-endometrioid cancer (OR = 2.20; 95% CI = 1.55, 3.12). There was weaker evidence for a causal relationship with WHR. BMI was associated with 165 metabolites and ratios after Bonferroni-correction in MR analyses, several of which were associated with EC and 25 of which were directionally consistent with an intermediate role in the effect of BMI on EC risk from two-step MR and observational analyses. In MVMR analyses, there was evidence suggesting that the effect of BMI on non-endometrioid EC was mediated by several lipid metabolites; for example, the univariable MR OR for non-endometrioid EC per 1 SD increase in BMI was 2.51 (95%CI = 1.47, 4.29), whereas on adjusting for free cholesterol to total lipids ratio in medium LDL, the MVMR OR for non-endometrioid EC per 1 SD increase in BMI was 1.18 (95%CI = 0.53, 2.66). Further bioinformatic analyses highlighted a mixture of other potential shared pathways (including height, adiposity traits and blood cell traits) that could influence the risk of EC. Conclusion Evidence here suggests that higher BMI causes a higher risk of overall and all histological subtypes of EC and variation in numerous circulating metabolites. Several of these metabolites showed relationships consistent with an intermediate role between BMI and non-endometrioid EC, however, further bioinformatic analyses highlighted other potential shared mechanisms that could influence the risk of EC.

DJP receives consulting fees from Johnson & Johnson and Novo Nordisk and payments for lectures, presentations, and educational events from Johnson & Johnson, Medtronic, and Novo Nordisk.

This work was supported by the Wellcome Trust through a Wellcome Trust Investigator award to NJT (202802/Z/16/Z, 2016-2023). MAL is funded by a Medical Research Council GW4 studentship (grant number: MR/R502340/1). NJT was a Wellcome Trust Investigator (202802/Z/16/Z, 2016-2023), is the PI of the Avon Longitudinal Study of Parents and Children (MRC & WT 217065/Z/19/Z), is supported by the University of Bristol NIHR Biomedical Research Centre (BRC-1215-2001), the MRC Integrative Epidemiology Unit (MC_UU_00011/1) and works within the CRUK Integrative Cancer Epidemiology Programme (C18281/A29019). VT, NJT, SW, LD and MG are supported by the Cancer Research UK (grant number: PRCPJT-May22\100028). L.J.C. was supported by N.J.T. Wellcome Investigator Award (202802/Z/16/Z) 2016-2023, and by MC_UU_00032/1 since 2023. All authors work in the MRC Integrative Epidemiology Unit at the University of Bristol, which is supported by the Medical Research Council (grant numbers: MC_UU_00011/1-7) and the University of Bristol. KHW is supported by the University of Bristol and Cancer Research UK [grant number RCCPDF\100007]. DJP has been funded by the Royal College of Surgeons of England. He receives consulting fees from Johnson & Johnson and Novo Nordisk and payments for lectures, presentations, and educational events from Johnson & Johnson, Medtronic, and Novo Nordisk. This research was funded in whole, or in part, by the Wellcome Trust [202802/Z/16/Z, 217065/Z/19/Z]. For the purpose of Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ethical approval was obtained from the Northwest Multi-centre Research Ethics Committee (11/NW/0382).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

The full summary statistics for BMI and WHR are available from the GIANT consortium (https://portals.broadinstitute.org/collaboration/giant/index.php/GIANT_consortium_data_files) and Zenodo (https://zenodo.org/record/1251813#.Yk7O25PMIUE), data files 4, 5, 7, and 8. The full summary statistics for the endometrial cancer GWAS are available from the OpenGWAS database (https://gwas.mrcieu.ac.uk/datasets/); IDs for endometrial cancer GWAS: ebi-a-GCST006464, ebi-a-GCST006465, and ebi-a-GCST006466; This can be accessed via the TwoSampleMR (https://mrcieu.github.io/TwoSampleMR/) and ieugwasr (http://gwas-api.mrcieu.ac.uk/) R packages or directly from OpenGWAS in GWAS-VCF format50. The full summary statistics for the 1H-NMR metabolites will be made available at the University of Bristol data repository. The individual level data used in this work is not publicly available and can only be obtained from UK Biobank with an approved application.