24,25-dihydroxyvitamin D₃and vitamin D status of community-dwelling black and white Americans

BACKGROUND: 24,25-Dihydroxyvitamin D [24,25(OH)₂D] is a metabolite of 25-hydroxyvitamin D (25D). Blacks frequently have low total 25D without manifestations of vitamin D deficiency, suggesting that total serum 25D may incorrectly reflect vitamin D status in different racial groups. The ratio of serum 24,25(OH)₂D to 25D [vitamin D metabolite ratio (VMR)] represents a new candidate biomarker for vitamin D status. METHODS: We measured 24,25(OH)₂D₃ and 25D₃ by mass spectrometry in a random community cohort of black (n = 212) and white (n = 164) Americans to evaluate VMR as a marker for vitamin D status. We measured parathyroid hormone concentrations by immunoassay to compare VMR and 25D₃ against a physiological indicator of vitamin D deficiency. RESULTS: Serum 24,25(OH)₂D₃ strongly correlated with 25D₃ in both black and white study participants (r = 0.90, P < 0.001 and r = 0.86, P < 0.001 respectively). Blacks had lower mean 25D₃ than whites [17.0 (7.8) vs 27.5 (11.3) ng/mL; 42.4 (19.5) vs 68.6 (28.2) nmol/L, P< 0.001] and lower mean 24,25(OH)₂D₃ [2.1 (1.3) vs 3.6 (2.0) ng/mL; 5.1 (3.1) vs 8.7 (4.8) nmol/L, P < 0.001]. In contrast to total 25D3 concentrations, mean VMR values were similar in blacks and whites [11.9 (4.0) vs 12.5 (3.4), P = 0.16, respectively] and were negatively correlated with parathyroid hormone concentrations in both races (r_s = -0.26, P< 0.001, and r_s = -0.25, P< 0.001, respectively). CONCLUSIONS: Our results provide further evidence that measurement of total 25D for assessment of vitamin D status in patients of African descent deserves reevaluation and suggest that alternative measures such as VMR should be considered.