The Core Component: Ergosterol
To understand what vitamin D2 is made of, one must first be familiar with its primary precursor: ergosterol. Ergosterol is a type of sterol, which is an important organic molecule, structurally similar to cholesterol in animals. However, ergosterol is the main sterol found in the cell membranes of most fungi and yeast. It is responsible for maintaining the integrity, permeability, and fluidity of the cell membrane, making it a critical component for the life of these organisms.
The Unique Fungal Connection
Because ergosterol is so abundant in fungi, these organisms are the primary natural source of vitamin D2. Mushrooms, for example, naturally contain ergosterol. When wild mushrooms or commercially grown mushrooms are exposed to ultraviolet (UV) light, the ergosterol is converted into ergocalciferol, or vitamin D2. This makes UV-exposed mushrooms a rare and valuable natural plant-based source of vitamin D. This process is the key to how plants and fungi contribute to the dietary intake of this essential vitamin, especially for those following a vegan or vegetarian diet.
The Conversion Process: UV Irradiation
The creation of vitamin D2 is initiated by the absorption of UV radiation. Specifically, ultraviolet B (UVB) or ultraviolet C (UVC) light strikes the ergosterol molecule, causing a photochemical reaction that breaks a bond in its steroid structure.
- Photochemical Reaction: UV light initiates a ring-opening reaction in the ergosterol molecule, transforming it into a compound called previtamin D2.
- Thermal Isomerization: This previtamin D2 is unstable and undergoes a temperature-dependent rearrangement, or thermal isomerization, to become the more stable vitamin D2 (ergocalciferol).
- Activation in the Body: When consumed, vitamin D2 is biologically inactive. It undergoes two hydroxylation steps—one in the liver and one in the kidneys—to become the active, hormone-like form called calcitriol.
Naturally Occurring Sources
Several foods naturally contain or can be enriched with vitamin D2 through this process:
- UV-Exposed Mushrooms: Many types of mushrooms, including portobello, crimini, and shiitake, can significantly increase their vitamin D2 content when exposed to UV light.
- Yeast: Used in many supplements and foods, yeast that has been irradiated with UV light is a common source of vitamin D2.
Manufacturing for Supplements
For the large-scale production of supplements and fortified foods, vitamin D2 is created synthetically in a lab setting by irradiating purified ergosterol. This allows for a cheaper and more accessible form of vitamin D to be added to various products. Since vitamin D2 is more cost-effective to produce than vitamin D3, it is often the form used in fortification of products like cereals, plant-based milks, and orange juice.
Differentiating Vitamin D2 and D3
Understanding the composition of vitamin D2 is clearer when compared with its counterpart, vitamin D3. While both serve similar functions, their origins and effects differ in important ways.
| Feature | Vitamin D2 (Ergocalciferol) | Vitamin D3 (Cholecalciferol) |
|---|---|---|
| Origin | Plants and fungi (e.g., yeast, mushrooms) | Animals (e.g., fatty fish, egg yolks) and human skin |
| Precursor | Ergosterol | 7-Dehydrocholesterol |
| Formation | UV irradiation of ergosterol | UV exposure on skin triggers conversion of 7-dehydrocholesterol |
| Efficacy | Less effective at raising and sustaining blood vitamin D levels compared to D3 | More effective at raising and sustaining blood vitamin D levels |
| Cost | Generally less expensive to produce synthetically | More expensive to produce for supplements |
| Suitability | Plant-based, often preferred by vegans and vegetarians | Animal-derived, not suitable for strict vegans unless sourced from lichen |
Metabolism and Function
Both vitamin D2 and vitamin D3 are prohormones, meaning they are biologically inactive until metabolized by the body. The metabolic pathway is largely the same for both forms. First, the liver converts D2 into 25-hydroxyvitamin D2. This compound is then transported to the kidneys, where it is converted into the active hormone, 1,25-dihydroxyvitamin D2 (ercalcitriol). This active form is crucial for regulating the absorption of calcium and phosphorus, which are essential for bone health, immune function, and overall cellular processes.
Benefits of Ergocalciferol
Despite potential differences in potency compared to D3, vitamin D2 is a potent and valuable nutrient. It provides significant health benefits, especially for individuals who get little sun exposure or follow plant-based diets. Its role in promoting calcium absorption is fundamental for preventing bone diseases like rickets and osteoporosis. Additionally, vitamin D2 helps support immune function and reduces inflammation. For many people, particularly those in higher latitudes or with darker skin, consuming fortified foods or supplements containing vitamin D2 is an effective way to maintain adequate vitamin D levels and support overall health.
Conclusion
In summary, what vitamin D2 is made of is the plant-based sterol ergosterol. Through a natural or manufactured process of UV irradiation, ergosterol is converted into ergocalciferol. This compound serves as an important source of vitamin D, particularly for those on plant-based diets or for the fortification of foods. While often used interchangeably with vitamin D3, understanding the distinct origins of vitamin D2 from yeast and fungi helps clarify its role in nutrition and supplementation. Both forms ultimately follow the same metabolic path in the body to produce the active vitamin D hormone, essential for bone and immune health.
For more information on the different forms and functions of this vital nutrient, consult the NIH Office of Dietary Supplements fact sheet.
