1. Barger-Lux MJ, Heaney RP, Lanspa SJ, Healy JC, DeLuca HF (1995) An investigation of sources of variation in calcium absorption efficiency. J Clin Endocrinol Metab 80:406–411
2. Hollis BW, Wagner CL (2013) The role of the parent compound vitamin D with respect to metabolism and function: why clinical dose intervals can affect clinical outcomes. J Clin Endocrinol Metab 98:4619–4628
3. Nykjaer A, Dragun D, Walther D et al (1999) An endocytic pathway essential for renal uptake and activation of the steroid 25-(OH) vitamin D3. Cell 96:507–515
4. Haussler MR, Whitfield GK, Kaneko I, Haussler CA, Hsieh D, Hsieh J, Jurutka PW (2013) Molecular mechanisms of vitamin D action. Calcif Tissue Int 92:77–98
5. Scharla SH, Strong DD, Rosen C, Mohan S, Holick M, Baylink DJ, Linkhart TA (1993) 1,25-dihydroxyvitamin D3 increases secretion of insulin-like growth factor binding protein‑4 (IGFBP-4) by human osteoblast-like cells in vitro and elevates IGFBP‑4 serum levels in vivo. J Clin Endocrinol Metab 77:1190–1197