Also known as: cholecalciferol, vitamin d, d3
Latest evidence update: 2026-11-01
Strongest in Sample size (95). Held back by Recency (40).
Solid mix of RCTs with some methodological gaps.
Confirmed across many independent studies with significant findings.
Tens of thousands of participants pooled across studies.
Mixed direction across studies.
Evidence base skews older; field may have moved on.
Recommended: 2000-5000 IU daily
Vitamin D3 is a form of Vitamin D. The studies above are specifically about Vitamin D3; broader vitamin d research differs in dose and bioavailability and lives on the parent page.
See all Vitamin Dresearch →ProtocolEngine provides general health information based on published research. This is not medical advice. Consult a healthcare professional before starting any supplement or health protocol.
May help reduce disease activity and progression in rheumatoid arthritis, with higher vitamin D levels associated with lower disease activity
Associated with reduced risk of multiple sclerosis and fewer disease relapses, with higher vitamin D levels linked to fewer brain lesions
May improve outcomes in inflammatory bowel disease by reducing inflammation markers and disease activity scores
Vitamin D deficiency is common in people with autoimmune thyroid disease and other autoimmune conditions, though supplementation effectiveness requires further study
Higher vitamin D levels are associated with lower COVID-19 mortality risk in hospitalized patients, though benefits in prevention remain unclear