Vitamin D plays a role in overall health, supporting bone strength, immune function, and calcium absorption. The body can produce vitamin D through sun exposure, and it can also be obtained from certain foods and supplements. This nutrient exists in different forms, primarily Vitamin D2 and Vitamin D3. Doctors sometimes prescribe one form over the other, leading to questions about their distinct roles and applications.
Understanding Vitamin D Forms
Vitamin D2, known as ergocalciferol, is primarily derived from plant sources, such as fungi and yeast, particularly when exposed to ultraviolet (UV) light. Many fortified foods, like some dairy products and cereals, also contain Vitamin D2. Vitamin D3, or cholecalciferol, is the form produced in human skin upon exposure to sunlight. It is also found in animal-based foods, including fatty fish, fish liver oils, and egg yolks, and can be derived from lanolin. Both D2 and D3 are fat-soluble vitamins, meaning they are absorbed with dietary fats and can be stored in the body.
How the Body Processes Vitamin D
Once ingested or produced, both Vitamin D2 and D3 undergo a two-step activation process in the body. The first step occurs in the liver, where both forms are converted into 25-hydroxyvitamin D, also called calcidiol. This calcidiol is the main circulating form of vitamin D, and its levels in the blood are used to assess a person’s vitamin D status.
The second activation step primarily takes place in the kidneys, where 25-hydroxyvitamin D is further converted into the biologically active form, 1,25-dihydroxyvitamin D, or calcitriol. This active form is responsible for regulating calcium and phosphate levels, which are important for bone health. While both D2 and D3 follow this metabolic pathway, studies indicate that D3 is generally more effective at raising and maintaining circulating levels of 25-hydroxyvitamin D compared to D2.
Reasons for Prescribing Vitamin D2
Despite Vitamin D3 often being considered more potent, doctors may prescribe Vitamin D2 in specific situations. Historically, D2 was one of the first widely available prescription forms of vitamin D, leading to its established use in clinical practice. It is frequently available in high-dose prescription formulations, such as 50,000 International Units (IU), which are used to treat severe vitamin D deficiency or conditions involving malabsorption.
Vitamin D2 is also suitable for patients following vegan or strict vegetarian diets, as it is plant-derived. Many D3 supplements typically come from animal sources. In some healthcare systems, prescription D2 might be more cost-effective or more readily covered by insurance plans, influencing prescribing patterns. D2 may also be preferred for certain rare conditions or specific malabsorption issues requiring high doses.
Why Vitamin D3 is Commonly Preferred
Vitamin D3 is generally favored for supplementation because it is the form naturally produced in human skin upon sun exposure. This makes it physiologically similar to the vitamin D the body generates itself.
This greater potency means D3 can maintain higher vitamin D levels for a longer duration. D3 is also considered to have better bioavailability and conversion efficiency within the body. D3 is the most common form found in over-the-counter vitamin D supplements and fortified foods, making it widely accessible for general use.