Circulating 25-hydroxyvitamin D in man

Measurement by bioassay of the antiricketic sterol content of crude biologic extracts is tedious, insensitive and incapable of estimating the separate contributions made by vitamin D and its metabolites, which vary in their biologic potency. A specific binding assay for 25-hydroxyvitamin D (25-OHD) has permitted analyses of this hepatic metabolite of vitamin D in human serum. Over physiologic ranges of serum calcium, inorganic phosphorus and parathyroid hormone concentrations, no correlation of these measurements could be made with serum 25-OHD. In 11 of 13 normal subjects 25-OHD levels were higher in the summer on a regimen of vitamin D ingestion similar to their intake in the winter. Vitamin D administration, by oral or parenteral routes, indicated a regulation of 25-OHD production that apparently was proportional to initial serum 25-OHD concentration and was overpowered by the introduction of massive amounts of substrate. During long-term constant intake of large amounts of vitamin D, serum 25-OHD concentration correlated well with the dose. Ingestion of 25-OHD results in prompt (4 to 8 hours) peaks of serum 25-OHD concentration and might offer a quicker and more facile alternative in the clinical use of antiricketic sterols.