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

Randomized, placebo-controlled primary-care trial in community-dwelling adults aged 65 years and older in Banbury, Oxfordshire, United Kingdom. For the 4000 IU/day vitamin D3 group, 102 participants were randomized and 97 had complete data; for the 2000 IU/day vitamin D3 group, 102 were randomized and 98 had complete data.

Daily vitamin D3 (cholecalciferol) was given as soft gel capsules for 12 months at 4000 IU/day (100 bcg) or 2000 IU/day (50 bcg). Matching placebo capsules were used in the trial, and the supplement supply was provided by Tishcon Corporation.

Daily vitamin D3 effectively increased vitamin D status, and the 4000 IU/day dose was more effective than 2000 IU/day for reaching sufficiency. Plasma 25(OH)D rose from 49 to 137 nmol/L with 4000 IU/day and from 55 to 102 nmol/L with 2000 IU/day by 12 months; 88% versus 70% respectively had 25(OH)D >90 nmol/L. Both active doses reduced PTH compared with placebo, but there were no significant differences in cardiovascular risk factors or physical function over 1 year. The regimen was well tolerated, with no clinical hypercalcaemia or kidney stones reported.

The trial was conducted at a single primary-care site in a relatively specific older UK population, which limits generalizability. Clinical outcomes such as fractures were not assessed, so the findings mainly support biochemical efficacy rather than disease prevention. Follow-up was 12 months, which may be insufficient to detect longer-term safety or clinical benefits.