Normalized Vitamin D Metabolite Concentrations are Better Correlated to Pharmacological Effects Than Measured Concentrations

Figures & data

Figure 1.  Comparison of the correlations between bone mineral density.

(A) Measured 25(OH)D, and (B) normalized 25(OH)D; each point represents one set of measurements from one volunteer; measured vitamin D was obtained by direct analysis of the blood sample; normalized vitamin D was obtained from measured vitamin D and the concentration of binding proteins; the line represents the best linear fit.

BMD: Bone mineral density.

Adapted with permission from [13].

Figure 2.  Measured concentration of total 25(OH)D and the resulting normalized concentration of 25(OH)D calculated with Equation 2.

(A) Healthy and (B) CKD volunteers; each point represents one set of measurements from one volunteer; measured vitamin D was obtained by direct analysis of the blood sample; normalized vitamin D was obtained from measured vitamin D and the concentration of binding proteins; the line represents the best linear fit.

CKD: Chronic kidney disease.

Figure 3.  Correlation of serum calcium with ln(25(OH)D) for the healthy subjects.

(A) Measured and (B) normalized metabolite concentration; each point represents one set of measurements from one volunteer; measured vitamin D was obtained by direct analysis of the blood sample; normalized vitamin D was obtained from measured vitamin D and the concentration of binding proteins; the line represents the best linear fit.

Table 1.  The binding constants of vitamin D₃ metabolites to vitamin D binding protein and albumin [32,33].

Binding constant (Ka)	DBP	Albumin
25(OH)D3	7.0 × 10^8 M^-1	6.0 × 10^5 M^-1
1,25(OH)2D3	3.7 × 10^7 M^-1	5.4 × 10^4 M^-1

Table 2.  Demographic information for the study groups.

Parameter/mean ± SD, n (%)	Healthy (n = 15)	CKD (n = 23)	p-value
Age (years)	24.3 ± 2.6	67.4 ± 7.3	<0.0001
BMI	26 ± 7	36 ± 7.7	0.0003
Weight (lbs)	166.9 ± 55.2	196.8 ± 75.4	0.2
Gender:			0.03
– Male	7 (47%)	19 (83%)
– Female	8 (53%)	4 (17%)
Race:			0.1
– White	13 (87%)	22 (96%)
– Nonwhite	2 (13%)	1 (4%)
Albumin (g/dl)	3.97 ± 0.36	3.85 ± 0.23	0.2
Calcium (mg/dl)	8.6 ± 0.30	9.4 ± 0.53	<0.0001
Blood urea nitrogen	11.2 ± 2.8	29.3 ± 9.9	<0.0001
Cholesterol	169 ± 32	161 ± 34	0.5
25(OH)D (nM)	43.8 ± 22	70.9 ± 30	0.005
DBP (µM) median [interquartile]	4.9 ± 1.5 5.0 (4.0–6.0)	6.29 ± 4.4 5.4 (3.9–7.9)	0.18 0.50
PTH (pg/ml	41 ± 16	65 ± 40	0.01
Creatinine (mg/dl)	0.7 ± 0.09	1.8 ± 0.49	<0.0001
Creatinine clearance (ml/min/1.73m^2)	121 ± 28	40 ± 14	<0.0001
Glucose (mg/dl)	82 ± 16	143 ± 54	<0.0001

CKD: Chronic kidney disease; DBP: Vitamin D binding protein; PTH: Parathyroid hormone; SD: Standard deviation.

Table 3.  Correlations between 25(OH)D and characteristics of the healthy subjects.

Parameter	Measured 25(OH)D		Normalized 25(OH)D
	r	p-value	r	p-value
Calcium	0.56	0.029^†	0.80	0.00037^†
PTH	0.46	0.084	0.56	0.028^†
BMI (normal)	0.029	0.93	0.44	0.17
BMI (overweight)	0.76	0.24	0.997	0.0029^†

Levels of 25(OH)D were log transformed before analysis.

† Indicates statistical significance.

PTH: Parathyroid hormone.

Table 4.  Correlations between 25(OH)D and characteristics of the chronic kidney disease study population.

Parameter	Measured 25(OH)D		Normalized 25(OH)D
	r	p-value	r	p-value
Calcium	0.14	0.51	0.2	0.34
Calcium adjusted for ClCr	0.14	0.52	0.49	0.017^†
PTH	0.1	0.65	0.18	0.42
PTH and calcium	0.27	0.46	0.36	0.25
PTH and calcium adjusted for ClCr	0.27	0.47	0.63	0.007^†

Levels of 25(OH)D were log transformed before analysis.

† Indicates statistical significance.

Table 5.  Correlations between 1,25(OH)₂D and characteristics of the chronic kidney disease study population.

Parameter	Measured 1,25(OH)2D		Normalized 1,25(OH)2D
	r	p-value	r	p-value
Calcium	0.047	0.83	0.3	0.17
Calcium adjusted for ClCr	0.053	0.81	0.48	0.02^†
PTH	0.056	0.80	0.0016	0.99
PTH and calcium	0.065	0.96	0.31	0.37
PTH and calcium adjusted for ClCr	0.091	0.92	0.51	0.05^†

Levels of 1,25(OH)₂D were log transformed before analysis.

† Indicates statistical significance.

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