The aims of this study were to examine the efficacy among various vitamin D supplementation regimens on serum 25-hydroxyvitamin D (25(OH)D) concentrations and determine the minimal dose rate required to achieve sufficient serum concentrations (≥75 nmol/l) among older adults in long-term care (LTC).

A 1-year medical history was abstracted from medical records, and a one-time blood draw to measure serum 25(OH)D concentrations was obtained. Individuals were stratified into vitamin D-supplemented and non-supplemented groups. The supplemented group was further categorised into four treatment forms: single-ingredient vitamin D2or3, multivitamin, Ca with vitamin D or combination of the three, and by daily prescribed doses: 0–9·9, 10–19·9, 20–49·9, 50–99·9 and >100 μg/d.

Five LTC communities in Austin, Texas.

One hundred seventy-three older (≥65 years) adults.

Of the participants, 62% received a vitamin D supplement and 55% had insufficient (≤75 nmol/l) 25(OH)D serum concentrations. Individuals receiving single-ingredient vitamin D2or3 supplementation received the highest daily vitamin D mean dose (72·5 μg/d), while combination of forms was the most frequent treatment (44%) with the highest mean serum concentration (108 nmol/l). All supplementation doses were successful at reaching sufficient serum concentrations, except those<20 μg/d. Using a prediction model, it was observed that 0·025 μg/d of vitamin D supplementation resulted in a 0·008 nmol/l increase in serum 25(OH)D concentrations.

Based on the predictive equation, results suggest that supplementation of 37·5 μg/d of vitamin D2or3 or combination of vitamin D is most likely to achieve sufficient serum 25(OH)D concentrations in older adults in LTC.