Optimum dose of vitamin D for disease prevention in older people: BEST-D trial of vitamin D in primary care

Double-blind, randomized, placebo-controlled, parallel-group trial; 305 community-dwelling adults aged 65 years or older in Banbury, Oxfordshire, UK; ambulatory and not currently taking more than 400 IU/day vitamin D3.

Vitamin D3 (cholecalciferol): 4000 IU daily, 2000 IU daily, or placebo daily for 12 months; taken as soft gel capsules.

4000 IU/day vitamin D3 for 12 months increased mean 25(OH)D from about 50 to 137 nmol/L; 88% achieved >90 nmol/L at 12 months vs 1% for placebo. 2000 IU/day increased to about 102 nmol/L with 70% achieving >90 nmol/L. Parathyroid hormone decreased with both doses; greater reduction with 4000 IU (22% vs 14%). Albumin-corrected calcium rose slightly but remained within normal range. No significant effects on cardiovascular risk factors or physical function; no adverse vitamin D–related events. Conclusion: When adherence is around 70%, 4000 IU daily may be required to reach 25(OH)D levels linked with the lowest disease risk; these findings support testing this higher dose in large trials for fracture prevention, though fracture outcomes were not assessed in this study.

Almost all participants were Caucasian; recruitment from a single general practice; 1-year duration; did not assess fracture outcomes; generalizability to diverse populations uncertain.