Within the last year I’ve blogged on three studies that presented results based on bioavailable vitamin D levels, the Powe and Chun studies and the Ponda RCT. Bioavailable or free vitamin D refers to circulating vitamin D compounds that aren’t attached to vitamin D binding protein (DBP), a blood protein related to albumin but more than 100 times less abundant.
About 85% of vitamin D compounds in the blood attach to DBP. Consequently, this vitamin D is available only to cells that can absorb DBP. Many types of cells don’t have the appropriate receptors, which are called megalin and cubilin and are found on many of the cells involved in the classic bone health understanding of vitamin D. The remaining vitamin D compounds attach to albumin and tiny amounts circulate unattached to anything. The albumin-bound and totally free vitamin D compounds are considered to be bioavailable.
This leads to the hypothesis that free 25(OH)D may be more important for many of the non-classical actions of vitamin D than total 25(OH)D, which in turn has led to research on the association between bioavailable vitamin D and various markers of health.