Vitamin D-binding protein (DBP) is a multifunctional protein that is the subject of increasing research interest regarding its role in free vitamin D and DBP, as well as its possible benefits for non-skeletal health. Recent studies have revealed an association between serum DBP levels and vitamin D status in several physiological and pathophysiological conditions, and this association supports the free vitamin D hypothesis. However, the etiology behind changes in DBP levels and the relationships between DBP, parathyroid hormone (PTH), and vitamin D levels remain unclear. It is important to test the free vitamin D hypothesis again as most research uses total vitamin D as an indicator for vitamin nutrition status. The serum intact PTH (iPTH) level begins to increase when the total serum 25(OH)D level falls below ∼30 ng/ml. Thus, PTH may offer a measurable biomarker of vitamin deficiency or insufficiency.

A central controversy in the field of vitamin D clinical research is how to define hypovitaminosis D. There are many questions that remain unanswered, including whether total or free vitamin D acts as the better marker of vitamin D status, what constitutes a level that is sufficient to promote the health benefits of vitamin D, and whether free or total 25(OH)D is more strongly correlated with serum PTH levels.

The aim of this Special Issue is to invite original research articles as well as review articles that seek to define the relationship between DBP and vitamin D levels in different physiological and disease conditions in both human and animal models. We are interested in articles that explore DBP and parathyroid disorders and their relation to the levels of total and free vitamin D metabolites. We are also interested in manuscripts that investigate DBP, 25(OH)D, PHT, and their association with the risks of various diseases including cancer, osteoporosis, and diabetes mellitus.

Potential topics include but are not limited to the following:

• DBP, total, and free vitamin D metabolite levels in different physiological conditions
• DBP and vitamin D levels in different ethnic and age populations
• Hormone regulation of DBP and vitamin D metabolites
• Genetic variation of DBP and 25(OH)D levels
• Parathyroid disorders and vitamin D levels
• Methods of measuring of DBP and vitamin D metabolites
• Calculated and measured free 25(OH) levels
• The role of PTH levels in defining vitamin nutritional status
• Possible roles of DBP, vitamin D, and PTH in inflammation
• Possible benefits of DBP, vitamin D, and PTH for non-skeletal health
• Association of DBP, vitamin D, and PTH with various diseases