Vitamin D and asthma

Several studies have associated low levels of 25-hydroxyvitamin D (25[OH]D) with higher risk of asthma.1,2 However, these studies could have residual confounding or reverse causation, making it difficult to draw conclusions on whether 25(OH)D is causally associated with asthma. Instrumental variable analysis was developed for assessing causality using genetic variants in epidemiologic research known as Mendelian randomization (MR). The rationale of MR is to find genetic variants associated with the exposure (eg, 25[OH]D) but not with other risk factors (eg, obesity) that affect the outcome (eg, asthma).

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