Vitamin D3 (cholecalciferol) — evidence review

• Prevent or treat **rickets / osteomalacia** and correct **vitamin D deficiency** (raise 25(OH)D).
• Reduce **osteoporotic fractures** (hip, nonvertebral, any fracture) and raise BMD.
• Reduce **falls**.
• Reduce **all-cause mortality**.
• Prevent **acute respiratory infections**.
• Prevent or treat **COVID-19**.
• Prevent or treat **depression** / improve mood.
• Improve **athletic performance** (strength, power, jump).
• Raise **testosterone**.
• Prevent **cancer** (incidence) and reduce **cancer death**.

Vitamin D3 is a real vitamin. In frank deficiency it prevents and treats rickets and osteomalacia; that is **strong**. In very old, institutionalized, calcium-and-D-deficient women, **800 IU D3 plus calcium** cut hip fractures (Chapuy; Cochrane institutional absolute effect ~9 fewer hip fractures / 1,000 / year). That is **moderate** and **not** the 2,000–5,000 IU, K2-optional protocol used in replete adults. For the claimed 2,000 IU/day stack — the exact VITAL dose — large, long, blinded RCTs show **no** reduction in invasive cancer, major CVD, total/hip/nonvertebral fracture, depression, or all-cause death, and no COVID/ARI benefit in CORONAVIT. Monthly-equivalent trials (ViDA, D-Health) are likewise null on CVD, cancer incidence, falls, fractures, and death; D-Health’s cancer-mortality point estimate was higher, not lower. **~5,000 IU/day has no comparable hard-outcome RCT** and sits above the NAM UL. Athletic and testosterone RCTs at a few thousand IU/day are null. K2 with D changes carboxylation labs and has not changed 3-year BMD or 2-year valve/coronary calcification.