Purpose: Results from recent clinical trials suggest that vitamin D efficacy against cancer may be influenced by body mass index. As suppression of parathyroid hormone (PTH) is one indicator of vitamin D efficacy, we examined to what extent doses of vitamin D3 supplementation suppress PTH levels in individuals with and without obesity.
Methods: A total of 328 healthy African Americans were randomized into the following four groups and treated for 3 months: placebo, 1,000, 2,000, or 4,000 IU/day of vitamin D3 supplementation.
Results: Among the participants, 250 individuals with PTH measurements were included in the analysis. Obese individuals (n = 141) experienced a steep reduction of 3-month PTH from placebo to 1,000 IU/day of vitamin D3 supplementation, but no further reduction at 2,000 or 4,000 IU/day. For non-obese individuals (n = 109), the reduction of 3-month PTH was approximately linear for increasing vitamin D3 doses. At supplementation of 2,000 to 4,000 IU/day, 3-month 25(OH)vitamin D levels were high in both non-obese and obese individuals, but the 3-month PTH levels remained about 10 pg/mL higher in individuals with obesity.
Conclusion: Our findings suggest that excess adiposity confers resistance to vitamin D efficacy in suppressing PTH levels, even when given at high doses.
Keywords: Cancer; Circulating 25(OH)D; Effect modification; Obesity; Parathyroid hormone; Vitamin D3 supplementation.