ABSTRACT

Context:  Vitamin D status is usually assessed by serum total 25-hydroxyvitamin D.  Whether free 25-hydroxyvitamin D measures better correlate with various clinical outcomes is unclear.

Objective:  To identify correlations between total 25-hydroxyvitamin D (t25-OHD), calculated and direct measures of free 25-OHD, and to identify associations of these measures with other outcomes in children, across the 6 common GC haplotypes.

Design:  Healthy urban-dwelling children underwent measurement of relevant variables.

Setting: Academic medical center

Participants: Healthy, urban-dwelling children, 6 months to 10 yrs old, predominantly of Hispanic background and representative of all common GC haplotypes.

Intervention: None

Main Outcome Measures: Total and free 25-OHD and 1,25(OH)2D, calcium, phosphate, PTH, glucose, insulin, aldosterone, and renin.

Results: Mean t25-OHD [26.3±6.7ng/ml; 65.8±16.8nmol/L] were lowest in the GC2 genotype.  Mean t1,25(OH)2D [57.6±16.5pg/ml; 143.9±41.3pmol/L], were lowest in GC1f/1f, GC1f/2, and GC2/2 groups. T25-OHD correlated strongly with circulating free 25-OHD (cf25-OHD) (r=0.89) and directly measured free 25-OHD (dmf25-OHD) (r=0.69). Cf25-OHD correlated with dmf25-OHD (r=0.69) (p<0.001 for all).  t25-OHD inversely correlated with BMI (r=-0.191; p=0.006), skin reflectometry, and systolic blood pressure. T25-OHD correlated with fasting insulin and HOMA-IR, however significance for these correlations were not evident after adjustment for BMI. PTH inversely correlated with all measures of 25-OHD, but most strongly with t25-OHD.

Conclusions:  Measure of circulating total and free 25-OHD are comparable measures of vitamin D status in heathy children. Correlations are similar with other outcome variables, however t25-OHD remains the strongest correlate of circulating PTH and other variables. These data argue against routine refinement of the t25-OHD measure using currently available assessments of free 25-OHD.