Bioavailable vitamin D is more tightly linked to mineral metabolism than total vitamin D in incident hemodialysis patients

Abstract

Prior studies showed conflicting results regarding the association between 25-hydroxyvitamin D (25(OH)D) levels and mineral metabolism in end-stage renal disease. In order to determine whether the bioavailable vitamin D (that fraction not bound to vitamin D binding protein) associates more strongly with measures of mineral metabolism than total levels, we identified 94 patients with previously measured 25(OH)D and 1,25-dihydroxyvitamin D (1,25(OH)2D) from a cohort of incident hemodialysis patients. Vitamin D binding protein was measured from stored serum samples. Bioavailable 25(OH)D and 1,25(OH)2D were determined using previously validated formulae. Associations with demographic factors and measures of mineral metabolism were examined. When compared with whites, black patients had lower levels of total, but not bioavailable, 25(OH)D. Bioavailable, but not total, 25(OH)D and 1,25(OH)2D were each significantly correlated with serum calcium. In univariate and multivariate regression analysis, only bioavailable 25(OH)D was significantly associated with parathyroid hormone levels. Hence, bioavailable vitamin D levels are better correlated with measures of mineral metabolism than total levels in patients on hemodialysis.

Assessment of vitamin D status – a changing landscape

“…These concepts are not new since they were described in 1989 [149]. However, they have been recently put under the spotlight in different publications dealing with the free hormone hypothesis [150], the difference in DBP concentrations in Whites and Blacks [106] (even if this has recently been challenged [151] and the better association of free or bioavailable vitamin D with mineral parameters, compared to total 25-OHD in postmenopausal women [152] or hemodialysed patients [153].…”

Nutritional Vitamin D Supplementation in Dialysis

“…Similarly, DIVINE's geographic restriction to the Northeast United States may limit generalizability to other regions. Lastly, this analysis did not incorporate measures of vitamin D binding protein or "bioavailable" vitamin D, which has been gaining increasing attention as a potentially more accurate measure of vitamin D status; debate is ongoing regarding optimal methods to determine bioavailable vitamin D status (37)(38)(39)(40)(41).…”

Non-classical effects of vitamin D

“…Метаболиты витамина D могут также связываться с альбумином, так как, несмотря на меньшее сродство к лиганду в сравнении с DBP, кон-центрация альбумина в сыворотке значительно (~10 раз) превышает концентрацию DBP. Некоторые исследова-тели выделяют связанный с альбумином и свободный витамин D под общим термином "биодоступный" и отме-чают его большую ассоциацию с минеральным обменом по сравнению с общим уровнем витамина D как у здоро-вых пациентов, так и у пациентов на гемодиализе [10].…”