The variation in free 25-hydroxy vitamin D and vitamin D-binding protein with season and vitamin D status.
OBJECTIVE
Serum 25-hydroxy vitamin D [25(OH)D] varies greatly with season at northern latitudes. The purpose of this study was to determine if the seasonal variations in serum total 25(OH)D are followed by a concomitant variation in free 25(OH)D or if the variation is damped by alterations in the binding capacity of DBP.


METHODS
Serum was collected from 540 healthy blood donors (60% men; mean age 41 ± 13 years) during 12 months and analyzed for total 25(OH)D, directly measured free 25(OH)D, vitamin D-binding protein (DBP) and albumin. Calculated free 25(OH)D was estimated.


RESULTS
The UV-B radiation during the sampling month was positively correlated with the serum levels of total 25(OH)D (r= 0.355,P< 0.001), directly measured free (r= 0.336,P< 0.001) and calculated free 25(OH)D (r= 0.275,P< 0.001), but not with DBP and albumin. The percentage of free 25(OH)D was higher during the winter months than that during the summer months (0.020 ± 0.005% vs 0.019 ± 0.004%;P= 0.007) and higher in participants with a serum 25(OH)D below 25 nmol/L than that in participants with a serum 25(OH)D above 75 nmol/L (0.031 ± 0.007% vs 0.017 ± 0.003%;P< 0.001). iPTH was correlated with directly measured free 25(OH)D (r= -0.226;P< 0.001), but only weakly with calculated free 25(OH)D (r= -0.095;P= 0.027).


CONCLUSIONS
Directly measured free serum 25(OH)D was highly correlated with total serum 25(OH)D and followed the same seasonal variation, whereas the serum concentrations of DBP and albumin were stable. The fluctuation in free 25(OH)D was only marginally damped with an increase in the percentage of free 25(OH)D during the winter months and in participants with vitamin D deficiency.
Department of Clinical Chemistry and Transfusion MedicineSahlgrenska Academy at the University of Gothenburg, Gothenburg, Sweden.
Göran
G
Oleröd
Department of Clinical ChemistrySahlgrenska University Hospital, Gothenburg, Sweden.
Lillemor Mattsson
LM
Hultén
Department of Clinical Chemistry and Transfusion MedicineSahlgrenska Academy at the University of Gothenburg, Gothenburg, Sweden.
Ola
O
Hammarsten
Department of Rheumatology and Inflammation ResearchSahlgrenska Academy at the University of Gothenburg, Gothenburg, Sweden Eva.Klingberg@vgregion.se.
Eva
E
Klingberg
25-hydroxy vitamin D; free 25-hydroxy vitamin D; parathyroid hormone; vitamin D deficiency; vitamin D-binding protein