Quantifying minor vitamin D metabolites using liquid chromatography-tandem mass spectrometry and determining their contribution to vitamin D nutritional status

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Dowling, Kirsten
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University College Cork
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Adequate vitamin D status is necessary throughout life in terms of maintaining bone strength, calcium and phosphorus balance, as well as prevention of rickets in children and osteomalacia in adults. The accepted biomarker used for the assessment of vitamin D status is circulating total 25-hydroxyvitamin D (25(OH)D) concentration, which is comprised of the sum of 25-hydroxyvitamin D3 (25(OH)D3) and 25-hydroxvitamin D2 (25(OH)D2) The use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the analysis of total 25(OH)D has also enabled the detection of low concentration, minor vitamin D metabolites in serum. The aims of this work were to examine the impact of three such minor vitamin D metabolites, 25(OH)D2, 24,25-dihydroxyvitamin D (24,25(OH)2D), and 3 epimer of 25(OH)D3 (3-epi-25(OH)D3) on the assessment of vitamin D nutritional status and also to provide further insight and refinement of our understanding of the vitamin D metabolic pathway. This required the development and validation of LC-MS/MS methods for measuring these three metabolites in multiple matrices (serum/plasma, meat, milk). Serum 25(OH)D2, not only contributes to the total 25(OH)D estimate, but also serves as a marker for vitamin D2 intake. Its presence in sera from a diverse collection of European populations, as part of this work, suggests that vitamin D2 is present in the food chain. Preliminary analysis of Irish milk and beef, as prioritized candidate food sources, for 25(OH)D2 and vitamin D2 were unable to pinpoint the primary dietary contributors to vitamin D2 intake. In the present work, an improved method for LC-MS/MS analysis was developed which produced accurate results for both total 25(OH)D and the metabolite 24,25(OH)2D. Application of this method allowed us to demonstrate that the presence of 24,25(OH)2D in serum led to an overestimated measurement of serum total 25(OH)D when employing an exemplar immunoassay technique. 24,25(OH)2D is the first metabolite in the vitamin D catabolic cascade, and thus considered a marker of vitamin D catabolism. The new LC-MS/MS method for 24,25(OH)2D was applied as a means to determine differential catabolic rates between 25(OH)D2 and 25(OH)D3 in multiple species. This analysis also highlighted novel inter-species differences in the ratio of 24,25(OH)2D to 25(OH)D. The availability of a large sample set of age-stratified population studies across Europe revealed that serum 3-epi-25(OH)D3 was primarily dependent on age, with concentrations highest at birth and decreasing steadily up until adolescence. While some 3-epi-25(OH)D3 was detected in pork and beef, as potential exogenous sources of the epimer, the majority of circulating 3-epi-25(OH)D3 is produced endogenously via an enzyme distinct from 25-hydroxlase. Overall, this work highlights the potential of using LC-MS/MS to analyse minor vitamin D metabolites which can help add new information to the research area of vitamin D metabolism and status, and potentially better inform clinical interpretations of vitamin D status.
25-hydroxyvitamin D , 24,25-dihydroxyvitamin D , 3-epi-25-hydroxyvitamin D , Vitamin D status , Liquid chromatography-mass spectrometry , Vitamin D2 , Vitamin D in food , Vitamin D metabolism
Dowling, K. 2019. Quantifying minor vitamin D metabolites using liquid chromatography-tandem mass spectrometry and determining their contribution to vitamin D nutritional status. PhD Thesis, University College Cork.