The 25(OH)D metabolite of vitamin D (comprised of 25(OH)D3 and 25(OH)D2), is the principal hydroxylated metabolite in serum and is considered a reasonable functional biomarker of vitamin D status. This is because, although subject to seasonal variation, multiple measurements in the same individual are relatively consistent over time. The most biologically active vitamin D metabolite is 1,25(OH)2D3, and may therefore be the most relevant to long-term health outcomes. Nonetheless, 1,25(OH)2D3 is not frequently used as a biomarker in epidemiologic studies as it displays diurnal variation and has a short in vivo half life.