Decreased bioavailability of vitamin D in obesity.
Level 3 - non-randomized controlled study
Non-randomized matched comparative physiological study
PubMed 10966885 · doi:10.1093/ajcn/72.3.690
What was done
Healthy, white obese adults (BMI ≥ 30 kg/m²) and matched lean controls (BMI ≤ 25 kg/m²) were evaluated under two experimental conditions: whole-body ultraviolet radiation or an oral dose of 50,000 IU (1.25 mg) vitamin D₂. Researchers measured basal 25-hydroxyvitamin D, parathyroid hormone, blood vitamin D₃ 24 hours post-irradiation, skin 7-dehydrocholesterol content, in vitro previtamin D₃ photoproduction, and peak serum vitamin D₂ concentrations after oral administration.
What was found
Obese individuals had significantly lower baseline 25-hydroxyvitamin D and higher parathyroid hormone levels than matched controls. The 24-hour incremental rise in serum vitamin D₃ after whole-body irradiation was 57% lower in obese participants. Skin 7-dehydrocholesterol levels and in vitro conversion to previtamin D₃ did not differ significantly between groups. BMI was inversely correlated with serum vitamin D₃ after irradiation (r = -0.55, P = 0.003) and with peak serum vitamin D₂ after oral intake (r = -0.56, P = 0.007).
Why it matters
This study shows that obesity-related vitamin D deficiency stems from decreased systemic bioavailability and adipose sequestration rather than impaired cutaneous synthesis. This provides a mechanistic basis for why individuals with obesity often require higher vitamin D doses.
Limits
The abstract does not disclose the sample size. The cohort was restricted entirely to white participants, limiting direct generalizability to other skin phototypes. Direct measurement of adipose tissue vitamin D storage was not reported in the abstract.
Cited by
- supports Vitamin D bioavailability is reduced in individuals with obesity compared to non-obese individuals.