Serum free 1,25‐dihydroxyvitamin d and the free 1,25‐dihydroxyvitamin d index during a longitudinal study of human pregnancy and lactation

SCOTT G. WILSON, ROBERT W. RETALLACK, JACQUELINE C. KENT, GRAEME K. WORTH, DONALD H. GUTTERIDGE

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Abstract

The changes in three different indices of 1,25‐dihydroxyvitamin D (1,25(OH) 2 D) biological activity were studied longitudinally in 35 women during late pregnancy and lactation and in 26 control women. Measurements were made of maternal serum total 1,25(OH) 2 D and free 1,25(OH) 2 D concentration (by centrifugal ultrafiltration) and the free 1,25(OH) 2 D index (the molar ratio of total 1,25(OH) 2 D and vitamin D binding protein (DBP)). During late pregnancy total 1,25(OH) 2 D concentrations were significantly elevated when compared to controls, as were free 1,25(OH) 2 D and DBP concentrations and the free 1,25(OH) 2 D index. Serum total 1,25(OH) 2 D, free 1,25(OH) 2 D and DBP concentrations all fell dramatically during the first 2 weeks of lactation with total 1,25(OH) 2 D and free 1,25(OH) 2 D concentrations falling to levels below those of controls. During the course of lactation both total I,25(OH) 2 D and free 1,25(OH) 2 D levels rose significantly although they were not different from controls at 18 weeks of lactation. In contrast, the free 1,25(OH) 2 D index fell during the first 2 weeks of lactation, but remained at this level, significantly lower than controls. Neither urinary calcium excretion nor dietary calcium intake correlated with total or free 1,25(OH) 2 D, DBP, or the free 1,25(OH) 2 D index. The disagreement in the results of free 1,25(OH) 2 D concentration and free 1,25(OH) 2 D index demonstrates that these two approaches to measuring biologically active 1,25(OH) 2 D are not equivalent. In attempting to account for the increased calcium requirements of human reproduction we conclude that in pregnancy any of the 1,25(OH) 2 D measurements may be appropriate. In lactation, however, either 1,25(OH) 2 D is not a major factor or 1,25(OH) 2 D biological activity is inadequately represented by any of the currently available methods.

Original languageEnglish
Pages (from-to)613-622
Number of pages10
JournalClinical Endocrinology
Volume32
Issue number5
DOIs
Publication statusPublished - 1 Jan 1990
Externally publishedYes

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