Free or Total 25OHD as Marker for Vitamin D Status?

Abstract

I n this issue of JBMR, Denburg and colleagues(1) compare several methods to measure serum vitamin D binding protein (DBP). Why should this interest the readers of JBMR? Clinicians and even laypeople are very familiar with the use of serum 25OHD concentrations as the most reliable marker of the vitamin D status. Moreover, they are familiar with the general belief that African Americans have low levels of 25OHD, reflecting a poor vitamin D status. In a remarkable study published in the New England Journal of Medicine,Powe and colleagues, however, concluded that this is owing to low serum concentration of DBP in African Americans, and after correction, the "bioavailable 25OHD levels" are similar in black and white people. This article generated a broad debate as to whether the dogma of poor vitamin D status in African Americans might be because of technical errors in using total rather than free/ bioavailable 25OHD concentrations.DBP is the major serum transport protein for all vitamin D metabolites and has both a high capacity and high affinity for all hydroxylated vitamin D metabolites. Therefore, the free (ie, unbound to proteins) concentration of 25OHD or 1,25(OH) 2 D (3)(4)(5) is below 0.1% of their total concentration. Bioavailable 25OHD is the sum of free 25OHD and 25OHD loosely bound to albumin and is equally a very small fraction of the total concentration. (2,4,(6)(7)(8) It is presently unclear whether free or bioavailable 25OHD is better reflecting the availability of 25OHD for cells or tissues. Most hormones that are ligands for nuclear receptors are bound to specific serum-binding proteins, and there are good data to support the idea that free (or bioavailable) hormones are physiologically more relevant than their total concentration. This even resulted in the generalized use of free T4 and free T3 measurements rather than total concentrations for the diagnosis of thyroid disorders.(9) This also largely applies to free testosterone concentrations, (10) whereas cortisol concentration in saliva is a good proxy for free cortisol concentrations. Therefore, the question of whether free 25OHD or free 1,25(OH) 2 D is a better marker for vitamin D status and predictor of skeletal or extra-skeletal health outcomes is relevant. Indeed, renal tubular cells have access to free 25OHD via its serosal membrane but also to 25OHD bound to DBP, filtered in the glomeruli, by using the cargo receptor megalin-cubulin expressed at the luminal site. (11) Megalin is also expressed in lung, thyroid, mammary gland, gallbladder, and thyroid, (7) whereas most other cells do not or only minimally express such uptake mechanism and, therefore, depend on non-protein-bound 25OHD as substrate for local, paracrine production and action of 1,25(OH) 2 D. A possible role of megalin in the parathyroid glands is unclear. The measurement of most free hormone concentrations has been problematic for a long time. It took more than two decades of research to (nearly) optimize the quality of measurements of free thyroid hormone concentrations, (9...

The variation in free 25-hydroxy vitamin D and vitamin D-binding protein with season and vitamin D status

“…The concentrations of DBP and its’ binding affinity for the vitamin D metabolites also vary among different populations depending on the polymorphism in the expression of the three different DBP isotypes (6, 10). Lower DBP concentrations have been observed in African-Americans compared with European-Americans when DBP was measured with an immunoassay using monoclonal DBP antibodies, but not when measured with a mass spectrometry or immunoassays using polyclonal DBP antibodies (11, 12, 13, 14, 15). …”

The Free Hormone Hypothesis: Is Free Serum 25‐Hydroxyvitamin D a Better Marker for Bone Mineral Density in Older Women?

“…Chronic vitamin D deficiency is associated with decreased calcium absorption and elevated levels of parathyroid hormone (PTH), which may lead to excessive bone resorption and reduced bone mineral density (BMD) . The main end points investigated in relation to vitamin D status are therefore serum PTH and BMD . The relatively few studies of free S‐25(OH)D in relation to BMD have used the calculated, not directly measured, free S‐25(OH)D and the results are conflicting …”

Effects of cholecalciferol supplementation on serum and urinary vitamin D metabolites and binding protein in HIV-infected youth

“…The bioavailable 25(OH)D is the sum of free 25(OH)D (unbound to proteins) and 25(OH)D loosely bound to albumin. It is the free form of 25OHD that acts as a substrate for local, paracrine production and action of 1,25(OH) 2 D on target cells and constitutes <0.1% of the total concentration [64]. …”