Estimation of the dietary requirement for vitamin D in free-living adults >=64 y of age.

Dose-ranging interventional study in apparently healthy, free-living white men and women aged 64 years or older recruited from Cork and Coleraine. Participants were not using vitamin D supplements, and baseline sun exposure and dietary intake were assessed.

Daily oral vitamin D3 capsules were given for 22 weeks at 5, 10, or 15 µg/day, with a placebo comparator arm. The trial was conducted over winter in free-living older adults at northern latitudes.

Daily vitamin D3 increased late-winter serum 25(OH)D in a dose-related manner, and the findings support modest supplementation for many healthy older adults at these latitudes. End-of-study median 25(OH)D was 41.6 nmol/L with placebo, 53.2 with 5 µg/day, 69.5 with 10 µg/day, and 73.8 with 15 µg/day. The treatment-by-time interaction was significant (P = 0.004), and the postintervention comparison across groups was highly significant (P < 0.0001). The estimated slope relating total vitamin D intake to serum 25(OH)D was 1.97 nmol/L per µg/day, with no adverse events reported.

The study was short, winter-only, and limited to healthy white adults aged 64 years or older living at 51-55°N, which narrows generalizability. Active-arm sample sizes were modest, and sex differences in the intake-response relationship were noted but not fully explained. Arm-specific data for some characteristics, such as ethnicity and BMI, were incomplete or reported only at the cohort level.