In addition to variable concentrations VDBP also exists as different polymorphic forms. The affinity of the different VDBP forms towards 25OH Vitamin D may vary although this is still under debate. A polymorphic form with a high affinity for 25OH Vitamin D will decrease the fraction of free 25OH Vitamin D available. Conversely, a low affinity VDBP form will result in higher levels of free 25OH Vitamin D. The major form of VDBP present in individuals depends on the ethnicity. E.g. among blacks the predominant form is Gc1F, while among whites, the predominant form is Gc1S. If the difference in affinity really exists, the measurement of free 25OH Vitamin D may again be a better marker of the Vitamin D action than the measurement of total 25OH Vitamin D.
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