Vitamin D turnover studies

ING Staff: Kerry Jones, Inez Schoenmakers, Tony Fulford, Ann Prentice

Vitamin D is obtained through cutaneous synthesis or from the diet, and is hydroxylated to 25OHD or catabolised in the liver. The endocrine functions of vitamin D are controlled by further hydroxylation in the kidneys to produce the major active form 1,25(OH)2D. The role of vitamin D in skeletal health is well-established, however optimum levels, particularly in a population with low calcium intake, remain uncertain. Recent evidence also suggests a broader role for vitamin D in conditions unrelated to bone such as cancer, pregnancy related disorders and infectious disease.

As with other nutrients, there is a need to determine the physiological requirements of vitamin D and find a measurement or biomarker to provide information about the adequacy of vitamin D status. Plasma 25OHD is a frequently used biomarker but is a function of both the supply of vitamin D from diet and cutaneous synthesis and usage of 25OHD.

We have proposed the measurement of 25OHD half-life as an additional biomarker of vitamin D status to provide information on vitamin D usage and requirements as determined by differences in environment, physiology, disease or genetics in different populations. The method involves giving an oral dose of 25OHD and collecting small blood samples at intervals over the following 2-3 weeks.

Following the success of our pilot studies in The Gambia and UK, the vitamin D turnover method now provides a tool to investigate vitamin D metabolism. It may help to define optimum 25OHD concentrations in plasma and contribute to the formulation of recommendations for treatment of deficiencies and in the identification of populations with different vitamin D requirements.

Gambian studies in which we are currently using this method and/or studies being planned include those in children, pregnant and lactating women and older people.


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