| Summary: | Circulating 25-hydroxyvitamin D (25(OH)D) is regarded as the most reliable biomarker of vitamin D status. However, limited data exist concerning the suitability of 25(OH)D as an indicator of body vitamin D stores and the ability of adipose tissue to mobilize vitamin D. In the first study, in which male mice received different vitamin D<sub>3</sub> doses for three weeks, we found strong linear response relationships between vitamin D<sub>3</sub> intake and levels of vitamin D<sub>3</sub> in the plasma (<i>p</i> < 0.001), liver (<i>p</i> < 0.001) and adipose tissues (<i>p</i> < 0.001), and strong positive correlations between plasma and tissue stores of vitamin D<sub>3</sub> (<i>p</i> < 0.001). Plasma levels of 25(OH)D<sub>3</sub> and 24,25-dihydroxyvitamin D<sub>3</sub> (24,25(OH)<sub>2</sub>D<sub>3</sub>) showed weak or no correlations with tissue vitamin D<sub>3</sub> stores. Data from a second study demonstrate a strong and rapid response of plasma 25(OH)D<sub>3</sub> in vitamin D<sub>3</sub>-treated mice with a low vitamin D status. Additionally, mice fed a vitamin D-free diet showed a strong and rapid decline in vitamin D<sub>3</sub> in the liver, whereas the decline in different adipose tissues was distinctly lower than that in the liver. To conclude, tissue stores of vitamin D<sub>3</sub> were best reflected by plasma vitamin D<sub>3</sub>. In contrast to the liver, adipose tissues responded less sensitively to an absence of vitamin D intake.
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