New insights into vitamin D regulation: is there a role for alkaline phosphatase?

Purpose The diagnosis of vitamin D deficiency is based on the determination of total plasma 25-hydroxyvitamin D (25-OHD) concentrations, but the regulation of vitamin D 25-hydroxylation is not a major consideration and very little information is available on this activity. To check what factors could interfere with the activity of vitamin D-25-hydroxylase and thus alter the 25-OHD concentrations, we looked for potential correlations between 25-OHD and results of liver function tests in healthy adults. Methods This single-centre study was retrospective and consisted of evaluating the correlations between 25-OHD and the activities of aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyl transpeptidase (GGT), alkaline phosphatase (ALP), and bone alkaline phosphatase (BALP) in 349 healthy subjects aged from 18 to 65 years. In particular, in Group 1 (n = 119), we looked for correlations between 25OHD and all liver function tests and in Group 2 (n = 230) the correlation between 25OHD and BALP. Results In Group 1, we found no correlation between 25OHD and AST (r = - 0.03; p = 0.8), ALT (r = - 0.02; p = 0.91), GGT (r = - 0.08; p = 0.68), direct bilirubin (r = - 0.02; p = 0.89), indirect bilirubin (r = - 0.24; p = 0.21), and total bilirubin (r = - 0.24; p = 0.21) but one between 25OHD and ALP (r = - 0.2; p = 0.007); in Group 2, we found a significant negative correlation between 25-OHD and BALP (r = - 0.2; p = 0.0008). Conclusions The correlations that we found suggest that ALP and BALP might be involved in the regulation of vitamin D-25-hydroxylase activity, but further studies are mandatory to confirm our assumptions.