Quantification and kinetics of 25-hydroxyvitamin D3 by isotope dilution liquid chromatography/thermospray mass spectrometry

Abstract

A liquid chromatography/thermospray mass spectrometry method has been developed and used to measure the plasma levels and half-life of 25-hydroxyvitamin D3 in adults. The mean plasma levels of 25-hydroxyvitamin D3 averaged 40 ng ml-1 (n = 4) in summer and 30 ng ml-1 (n = 6) in winter. The method was also used to determine the half-life of serum 25-hydroxyvitamin D3 in subjects maintained on either high or low-fiber diets who had been given an intravenous infusion of (6,19,19-2H3)25-hydroxyvitamin D3 sufficient to label 5% of their estimated body pools. The half-life was determined to be 10.4 days (n = 4), which is approximately 50% of the currently accepted value of 19 days, determined using radiolabeled methods. This difference may be due to kinetic isotope effects arising as a result of the tritiated compounds being labeled at sites that undergo Cyt-P450-catalyzed oxidations.

Vitamin D and its Major Metabolites: Serum Levels after Graded Oral Dosing in Healthy Men

“…Finally, since most of this prior work did not measure the actual vitamin D 3 content of the administered supple- ments, and since many manufacturers exceed the labeled level (to allow for possible storage losses), it may be that actual intakes were underestimated by others and, correspondingly, that reported increments in serum levels were overstated. As the Food and Nutrition Board of the NAS attempts for the first time to define safe upper limits for nutrients [35], the results reported here may have some relevance for vitamin D. While it is reassuring that no hypercalcemia was detected in post-treatment testing of our vitamin D 3 -treated subjects, their treatment period was too short (8 weeks) to provide safety data needed for long-term intakes. But the slope we found of serum 25(OH)D on vitamin D intake is, we believe, relevant.…”

“…The turnover of the vitamin D pool is essentially unknown and will depend partly on its size. 25(OH)D is generally considered to have a half-life in the range of 19 days, but a methodologically better estimate is about 10 days [35]. As such, the 56-day treatment period with vitamin D 3 would have been greater than the 4+ half-times usually considered adequate in calculation of drug kinetics [36].…”

The Pharmacology of Vitamin D

“…Within 1 hour after injection of radiolabeled DBP, the radiolabel is present in a greater concentration than in plasma within the following tissues: kidney, liver, skeletal muscle, heart, lung, intestine, testis, and bone [31]. In contrast to DBP, its ligand, 25(OH)D, is cleared slowly from the body, with a halflife of about 10 days in both rabbit [31] and human [32]. The binding of 25(OH)D to DBP does not affect the turnover or the tissue uptake of DBP [31].…”

“…The discontinuation of 25(OH)D may have made sense at the time, because the objective of increasing plasma 25(OH)D concentrations can be almost as easily achieved by providing enough vitamin D 3 . Nonetheless, useful perspectives can be gained from the historic experience with 25(OH)D. Since the increase in the plateau plasma 25(OH)D concentration per mg dose is at least four times higher for 25(OH)D administration than for vitamin D 3 administration [78,82], we can conclude that only about 25% of vitamin D molecules taken orally will become 25(OH)D. One potential clinical advantage of 25(OH)D is that the half-life of its molecules is approximately 2 weeks [83], and this can offer an advantage if there is a concern about giving vitamin D to potentially hypersensitive individuals, such as those with hyperparathyroidism or granulomatous conditions. A shorter half-life with 25(OH)D treatment translates to a faster recovery from an overdose than with vitamin D. I am not aware that this potential advantage of more rapid clearance has been put to use.…”

Vitamin D and the evolution of human depigmentation

“…The paracrine system operates in diverse organs from skin to uterus (Norman, 2008). 1,25(OH)2D has a half life of 4 h (Jones, 2008) compared to that of 10-21 days for serum vitamin D (Ellis et al, 1977;Vicchio et al, 1993) and up to 2 months for adipose stores (Vieth, 2001). 1,25(OH)2D levels show a 70% dose-dependent variation for high oral doses of vitamin D (Hathcock et al, 2007) but are usually very stable.…”