1. Anderson KN, Anderson LE. Mosby’s Pocket Dictionary of Medicine, Nursing and Allied Health. 2nd ed. St. Louis, USA: Mosby Publishers; 1994
2. Bishop ML, Fody EP, Schoeff LE. Clinical Chemistry. 7th ed. Philadelpia, USA: Lippincott Williams and Wilkins; 2013. pp. 686-688
3. Japelt RB, Jakobsen J. Vitamin D in plants: A review of occurrence, analysis and biosynthesis. Frontiers in Plant Science. 2013;4:136
4. Henry KM, Kon SK, Thompson SY, McCallum JW, Stewart J. The vitamin D activity of pastures and hays. The British Journal of Nutrition. 1958;12:462-469
5. Wallis GC, Kennedy GH, Fishman RH. The vitamin D content of roughages. Journal of Animal Science. 1958;17:410-415