Low levels of 25-hydroxyvitamin D (less than 20 ng/mL Bjelakovic 2014) are very common in North America and have been associated with a wide variety of diseases including osteoporosis, bone fractures, malignancy, cardiovascular disease, infections, and more. Surrogate versus Clinical OutcomesĪlthough 25-hydroxyvitamin D levels are a better surrogate than a rat binding study, clinical outcomes are the most relevant. This larger dose may be seen as more convenient for patients or healthcare providers who want to provide supplementation on a weekly or monthly basis. Unlike cholecalciferol, which is typically only available as a maximum dose of 5,000 units per capsule or tablet, ergocalciferol is available as a monster 50,000 unit dose. If ergocalciferol is less potent and has a shorter duration of effect, why is it used? The most likely reason is due to dosage formulations. Healthy volunteers were given a single 50,000 unit dose of either vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol) Why is ergocalciferol prescribed? The difference in duration of effect and potency are well demonstrated in a study by Armas et al: In fact, recent literature (Trang 1998, Armas 2004, Houghton 2006) convincingly demonstrates that cholecalciferol is 1.7 to 3 times more potent and has a longer-lasting effect than ergocalciferol in increasing serum 25-hydroxyvitamin D levels, the active form of vitamin D in humans. Given that both ergocalciferol and cholecalciferol undergo metabolic changes in the human body, which differ from other animals, it should make sense that a binding study in rats may not be sufficient to show equivalence. On the basis of this animal data, most resources cite the two being equipotent and interchangeable. When the manufacturing method for ergocalciferol was created, binding studies of the vitamin D receptor in rats showed equipotency between ergocalciferol and cholecalciferol. In contrast, vitamin D2 (ergocalciferol) is not produced in the human body, but is created by exposing certain plant-derived materials to ultraviolet light. Vitamin D3 (cholecalciferol) is produced by the human body in response to sunlight and is also available through dietary sources, such as fish. Although these two have historically been considered interchangeable and equipotent, the current body of literature strongly supports the preference of Vitamin D3 (cholecalciferol) over D2 (ergocalciferol). In the United States, vitamin D supplementation is primarily available as vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).
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