Evidence Level
Strong
Vitamin D has one of the largest evidence bases of any supplement. The landmark VITAL trial (2019, NEJM, n=25,871) found that while vitamin D3 at 2,000 IU/day did not significantly reduce overall cancer incidence, it reduced cancer mortality by 25% in follow-up analysis. A 2017 BMJ individual participant data meta-analysis of 25 RCTs demonstrated that vitamin D supplementation reduced acute respiratory infections, with the greatest benefit in those with baseline 25(OH)D levels below 25 nmol/L. Bone health benefits are well-established by decades of research.