Background. Different dosing protocols have been used for vitamin D supplementation, but there has been a lack of comparative data among them.

Objective. To determine whether the same cumulative dose of vitamin D3 produces different effects if it is given daily, weekly or monthly.

Design. Women, age 81 ± 8 yr (±SD, n=48), who had undergone surgery to repair hip fracture were randomized to vitamin D3-supplementation protocols at 1,500 IU daily, or 10,500 IU once weekly, or 45,000 IU once every 28 days. The primary outcome measure was the serum 25-hydroxyvitamin D [25(OH)D] concentration attained.

Results. Initially, serum 25(OH)D concentrations for daily, weekly and monthly groups were, respectively, 15.13±6.9, 15.7±10.1, and 16.2±10.1 ng/mL. By Day 7, these had risen significantly in all the groups (p<0.001). On the first day after the monthly dose, both serum 25(OH)D and serum 1,25(OH)D had increased significantly (p<0.012 each) whereas these did not change significantly on the day after daily or weekly doses. After two months, serum 25(OH)D with daily, weekly and monthly dosing were respectively, 33.2±8.5, 29.2±8.9, 37.1±10.3 ng/mL (there were no significant differences among these values).

Conclusion. Supplementation with vitamin D can be achieved equally well with daily, weekly or monthly dosing frequencies. Therefore, the choice of dose frequency can be based on whichever approach will optimize an individual's adherence with long-term vitamin D supplementation.