Bio-available Vitamin D is reduced by half during pregnancy – Jan 2017

Clinical Utility of Measurement of Vitamin D-Binding Protein and Calculation of Bioavailable Vitamin D in Assessment of Vitamin D Status.

Ann Lab Med. 2017 Jan;37(1):34-38. doi: 10.3343/alm.2017.37.1.34.
Kim HJ1, Ji M2, Song J3, Moon HW4, Hur M4, Yun YM5.

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BACKGROUND:
The associations of vitamin D deficiency with various clinical conditions highlighted the importance of vitamin D testing. Currently, clinicians measure only the total 25-hydroxyvitamin D [25(OH)D] concentration, regardless of its bioavailability. We aimed to determine the effect of vitamin D-binding protein (VDBP) on 25(OH)D bioavailability.
METHODS:
Serum samples were collected from 60 healthy controls, 50 pregnant women, and 50 patients in intensive care units (ICUs). Total 25(OH)D was quantified by liquid chromatography with tandem mass spectrometry, and VDBP levels were determined by using an ELISA kit (R&D Systems, USA). The bioavailable 25(OH)D levels were calculated by using total 25(OH)D, VDBP, and albumin concentrations.
RESULTS:
In comparison with healthy controls, the total 25(OH)D concentration was significantly lower in ICU patients (median, 11.65 vs 18.25 ng/mL; P<0.00001), but no significant difference was noted between pregnant women (18.25 ng/mL) and healthy controls. The VDBP level was significantly lower in ICU patients (95.58 vs 167.18 μg/mL, P=0.0002) and higher in pregnant women (225.01 vs 167.18 μg/mL, P=0.008) compared with healthy controls. Nonetheless, the calculated bioavailable 25(OH)D levels of ICU patients and pregnant women were significantly lower than those of healthy controls (1.97 and 1.93 ng/mL vs 2.56 ng/mL; P=0.0073 and 0.0027).
CONCLUSIONS:
A single marker of the total 25(OH)D level is not sufficient to accurately evaluate vitamin D status, especially in pregnant women. In cases where VDBP concentrations may be altered, VDBP measurements and bioavailable 25(OH)D calculations may help to determine vitamin D status accurately.

PMID: 27834063 DOI: 10.3343/alm.2017.37.1.34

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Fig. 2. Comparison of total 25-hydroxyvitamin D [25(OH)D], vitamin D-binding protein (VDBP), and calculated bioavailable 25(OH)D by gestational stage.

• (A) Total 25(OH)D levels were not significantly different between the 1st and 2nd and 3rd trimesters (median, interquartile range: 1st trimester: 18.20, 15.07-23.87 ng/mL; 2nd and 3rd trimesters: 18.30, 13.75-25.70 ng/mL).
• (B) VDBP levels during the 2nd and 3rd trimesters were significantly higher than those during the 1st trimester (1st trimester: 102.20, 84.75-259.90 µg/mL, 2nd and 3rd trimesters: 273.65, 163.75-453.98 µg/mL).
• (C) The calculated bioavailable 25(OH)D level in the 2nd and 3rd trimesters was significantly lower than that in the 1st trimester (1st trimester: 4.19, 2.09-5.67 ng/mL; 2nd and 3rd trimesters, 1.73, 0.90-2.43 ng/mL).
• P values were calculated by the Mann-Whitney test.
• Two dashed lines denote vitamin D deficiency and severe vitamin D deficiency.
• The arrowheads and dots represent the outside value (>1.5× interquartile ranges) and far-out value (>3× interquartile ranges), respectively.
• The horizontal lines represent the maximum and minimum values, except for the outside value and far-out value.

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Fig. 1. Comparison of total 25-hydroxyvitamin D [25(OH)D], vitamin D binding protein (VDBP), and calculated bioavailable 25(OH)D in the three study groups.

• (A) The total 25(OH)D level in intensive care unit (ICU) patients (median, interquartile range: 11.65, 7.86-14.87 ng/mL) was significantly lower than that in healthy controls (18.25, 13.48-23.78 ng/mL) or in pregnant women (18.25, 13.98-25.24 ng/mL).
• (B) The VDBP level in
  pregnant women (225.01, 130.24-422.92 μg/mL) was significantly higher, and the
  VDBP level in ICU patients (95.58, 61.15-167.34 μg/mL) was significantly lower
  than that in healthy controls (167.18, 105.99-257.70 μg/mL). (
• C) The calculated bioavailable 25(OH)D levels of
  ICU patients (1.97, 1.48-3.15 ng/mL) and
  pregnant women (1.93, 1.03-3.41 ng/mL) were significantly lower than those in
  healthy controls (2.56, 1.95-4.22 ng/mL).
• P values were calculated by the Mann-Whitney test.
• Two dashed lines denote vitamin D deficiency and severe vitamin D deficiency.
• The arrowheads and dots represent the outside value (>1.5? interquartile ranges) and far-out value (>3? interquartile ranges), respectively.
• The horizontal lines represent maximum and minimum values, except for the outside value and far-out value.

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