All-source basal vitamin D inputs are greater than previously thought and cutaneous inputs are smaller.
Level 1 - systematic review of randomized trials
Meta-analysis/pooled individual participant data analysis of 8 supplementation trials
PubMed 23514768 · doi:10.3945/jn.112.168641
What was done
Researchers reanalyzed individual serum 25-hydroxyvitamin D [25(OH)D] concentration data from 8 cholecalciferol supplementation studies (total n = 3000). Linear and curvilinear models were used to extrapolate dose-response curves to zero concentration values to estimate basal input, and seasonal oscillation in serum 25(OH)D was measured across 4 studies to quantify peak cutaneous synthesis.
What was found
Calculated total basal input (food plus solar) ranged from 778 IU/d in end-stage renal disease patients to 2667 IU/d in healthy Caucasian adults. Obese individuals had lower baseline 25(OH)D concentrations and smaller responses to supplementation; African Americans also had lower baseline concentrations and lower calculated basal inputs. Seasonal oscillation ranged from 5.20 to 11.4 nmol/L, translating to mean peak summer cutaneous synthesis of 209 to 651 IU/d. Cutaneous synthesis accounted for only 10% to 25% of unsupplemented input at summer peak.
Why it matters
This analysis suggests that basal, unsupplemented vitamin D input is an order of magnitude higher than expected from recognized food sources, while cutaneous synthesis contributes a smaller fraction than previously assumed.
Limits
Values are derived from mathematical extrapolations of dose-response curves rather than direct measurements of dietary intake or sun exposure. The specific sources accounting for the remaining basal input (such as preformed 25(OH)D in foods) were hypothesized rather than directly quantified.
Cited by
- supports Vitamin D supplementation has a non-linear dose-response where the first 1,000 IU increases serum 25(OH)D by 4.8–5 ng/mL, while at higher doses (15,000–30,000 IU) each 1,000 IU increases levels by approximately a tenth of that rate.