New Insight into the Role of Vitamin D in the Stroke Risk: A Meta-Analysis of Stratified Data by 25(OH)D Levels.

Mendelian Randomization (MR) studies have emerged as a powerful tool for investigating causal relationships between modifiable risk factors and clinical outcomes, using genetic variants as instrumental variables. In the context of vitamin D research, MR is a promising approach to elucidate the effects of vitamin D on various health outcomes, including adverse cardiovascular events. However, the validity of MR analyses relies heavily on the strength of the genetic associations found. "Weak instrument bias", arising from instruments with low explanatory power for the exposure of interest, can lead to biased estimates and compromise causal inference. We have, herein, briefly reviewed the challenges posed by weak instrument bias in a large MR study on vitamin D [25(OH)D] and stroke, exploring implications for the study's validity and reliability of findings. We have then added an original meta-analysis stratified by 25(OH)D levels. By using aggregated data from a recent MR study, an original meta-analysis stratified by population mean levels of 25(OH)D has indicated that interventions based on vitamin D supplementations in population mean levels ranging from 50 to 70 nmol/L are likely to translate into a 13% reduction of stroke risk (pooled odds ratio=0.873, 95% CI: 0.764-0.997, p-value=0.04). MR studies are a valuable approach for discerning causal relationships between exposures, such as vitamin D, and health outcomes. However, the effectiveness of MR analyses depends on the robustness of the genetic instruments employed. By recognizing and addressing weak instrument bias in MR studies of vitamin D, researchers can enhance the credibility and utility of causal inference in understanding the health effects of this essential nutrient. A metaanalysis stratified by population mean levels of 25(OH)D has revealed the potential benefits of targeted interventions with vitamin D supplementations for stroke.