Evidence Level
The evidence for vitamin D in immune function is exceptionally strong. Martineau et al. (2017) published a participant-level meta-analysis in the BMJ covering 25 RCTs with 11,321 participants, demonstrating that vitamin D supplementation significantly reduced acute respiratory infections, with the greatest benefit in those with the lowest baseline levels. Aranow (2011) reviewed the immunological mechanisms, confirming that vitamin D receptors are expressed on B cells, T cells, monocytes, and dendritic cells, and that vitamin D induces cathelicidin — an antimicrobial peptide with direct bactericidal and antiviral activity. The global prevalence of vitamin D insufficiency makes this one of the most impactful and cost-effective immune interventions available.