Summary

Background
Randomised trials of vitamin D supplementation for cardiovascular disease and all-cause mortality have generally reported null findings. However, generalisability of results to individuals with low vitamin D status is unclear. We aimed to characterise dose-response relationships between 25-hydroxyvitamin D (25[OH]D) concentrations and risk of coronary heart disease, stroke, and all-cause mortality in observational and Mendelian randomisation frameworks.

Methods
Observational analyses were undertaken using data from 33 prospective studies comprising 500 ۹۶۲ individuals with no known history of coronary heart disease or stroke at baseline. Mendelian randomisation analyses were performed in four population-based cohort studies (UK Biobank, EPIC-CVD, and two Copenhagen population-based studies) comprising 386 ۴۰۶ middle-aged individuals of European ancestries, including 33 ۵۴۶ people who developed coronary heart disease, 18 ۱۶۶ people who had a stroke, and 27 ۸۸۵ people who died. Primary outcomes were coronary heart disease, defined as fatal ischaemic heart disease (International Classification of Diseases 10th revision code I20-I25) or non-fatal myocardial infarction (I21-I23); stroke, defined as any cerebrovascular disease (I60-I69); and all-cause mortality.

Findings
Observational analyses suggested inverse associations between incident coronary heart disease, stroke, and all-cause mortality outcomes with 25(OH)D concentration at low 25(OH)D concentrations. In population-wide genetic analyses, there were no associations of genetically predicted 25(OH)D with coronary heart disease (odds ratio [OR] per 10 nmol/L higher genetically-predicted 25(OH)D concentration 0·۹۸, ۹۵% CI 0·۹۵–۱·۰۱), stroke (1·۰۱, [۰·۹۷–۱·۰۵]), or all-cause mortality (0·۹۹, ۰·۹۵–۱·۰۲). Null findings were also observed in genetic analyses for cause-specific mortality outcomes, and in stratified genetic analyses for all outcomes at all observed levels of 25(OH)D concentrations.

Interpretation
Stratified Mendelian randomisation analyses suggest a lack of causal relationship for 25(OH)D concentrations with both cardiovascular and mortality outcomes for individuals at all levels of 25(OH)D. Our findings suggest that substantial reductions in mortality and cardiovascular morbidity due to long-term low-dose vitamin D supplementation are unlikely even if targeted at individuals with low vitamin D status.

Funding
British Heart Foundation, Medical Research Council, National Institute for Health Research, Health Data Research UK, Cancer Research UK, and International Agency for Research on Cancer.

Introduction

Vitamin D is an essential nutrient obtained from sunlight, dietary intake, and supplementation.1 Observational epidemiological studies have consistently found that low concentrations of circulating 25-hydroxyvitamin D (25[OH]D), a metabolite used as a clinical indicator of vitamin D status, are associated with an increased risk of cardiovascular disease and all-cause mortality, as well as other chronic diseases.2,  ۳ However, several large randomised trials of vitamin D supplementation have reported null results,4,  ۵,  ۶ casting doubt on the observational evidence. However, as trials have typically recruited participants irrespective of baseline 25(OH)D concentration, they have had limited power to test supplementation effects in subgroups with low 25(OH)D concentrations.7

An efficient approach for assessing the potential causal effect of vitamin D supplementation is Mendelian randomisation. Mendelian randomisation uses genetic variants specifically related to a particular exposure to compare genetically defined population subgroups with different average levels of the exposure. The independent segregation of alleles at conception means that these genetically defined subgroups should not differ systematically with respect to confounding variables, creating a natural experiment analogous to a randomised trial.8 Therefore, Mendelian randomisation analyses can provide more reliable insights into causal relationships between risk factors and disease outcomes than conventional observational analyses. Previous Mendelian randomisation analyses have reported null associations of genetically predicted 25(OH)D concentrations with coronary heart disease9,  ۱۰ and ischaemic stroke.11,  ۱۲ An inverse association has been observed between genetically predicted 25(OH)D and all-cause mortality.13 Null findings have been observed for several further outcomes, including other cardiovascular diseases and cancers.14

Results

۳۸۶ ۴۰۶ participants from the four studies were included in genetic analyses (table 1), including 33 ۵۴۶ people who had coronary heart disease, 18 ۱۶۶ people who had a stroke, and 27 ۸۸۵ people who died, and 500 ۹۶۲ participants were included in observational analyses (appendix 3 pp 11–۱۳). Mean age of participants included in the genetic analysis ranged from 54·۸ years (SD 9·۴) to 57·۵ years (12·۹), with similar numbers of men and women in each study, and the mean season-shifted 25(OH)D concentrations (corresponding to an autumn measurement) were 54·۵ nmol/L (SD 19·۶) in UK Biobank, 46·۹ nmol/L (16·۴) in EPIC-CVD, and 53·۸ nmol/L (25·۹) in the Copenhagen studies. Mean 25(OH)D estimates did not notably differ by assay type (appendix 3 p 21). The focused genetic risk score explained 4·۷% of the variance in 25(OH)D concentrations in UK Biobank study, 5·۸% in EPIC-CVD, and 1·۸% in the Copenhagen studies. This genetic risk score was not associated with a range of cardiovascular risk factors in UK Biobank, except for BMI and HDL cholesterol, although these associations were small (appendix 3 p 22). The genome-wide score was strongly associated with LDL cholesterol and triglycerides (appendix 3 p 23), and so Mendelian randomisation estimates using this score are unreliable.