Estimation of the dietary requirement for vitamin D in healthy adults.

Interventional human study in apparently healthy white men and women aged 20-40 years recruited from Cork, Ireland, and Coleraine, Northern Ireland. Active-arm sizes were 48 for 5 g/day, 57 for 10 g/day, and 53 for 15 g/day vitamin D3, with treatment started in October-November 2006 and followed for 22 weeks.

Oral vitamin D3 capsules were given daily for 22 weeks in three dose arms: 5 g/day, 10 g/day, or 15 g/day. The study was conducted during winter after assessment of summer sun exposure and diet.

Higher oral vitamin D3 doses were associated with higher end-of-winter 25(OH)D concentrations, but concentrations still declined from baseline in all three dose groups. Median 25(OH)D fell from 60.0 to 49.7 nmol/L with 5 g/day, from 72.2 to 60.0 nmol/L with 10 g/day, and from 75.9 to 69.0 nmol/L with 15 g/day. PTH increased from 46.9 to 52.0 ng/mL, 43.1 to 50.5 ng/mL, and 38.4 to 43.0 ng/mL, respectively. No adverse events were reported. Overall, the findings support a dose-response relationship and indicate that winter vitamin D needs vary with baseline status and prior sun exposure.

No placebo or control arm is described in the extracted active-arm data, so interpretation is based on dose-response and pre-post changes rather than a direct untreated comparison. Outcomes are reported for modest-sized groups over a single 22-week winter period in healthy white adults aged 20-40 years from northern latitudes, limiting generalizability. The study population had varying summer sun exposure, which may have influenced winter status despite adjustment.