What Is Vitamin D?
Vitamin D is a fat-soluble secosteroid responsible for increasing intestinal absorption of calcium, phosphate, and magnesium. In humans, the most important compounds in this group are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Unlike most other vitamins, vitamin D is only conditionally essential in humans, as the body can produce it in the skin upon exposure to sunlight. However, dietary intake is necessary for many people to meet their needs due to factors like limited sun exposure, geography, and skin pigmentation.
The Many Names of Vitamin D
Beyond its common name, vitamin D is known by several other important terms. Understanding these different names is key to recognizing the vitamin in different contexts, from scientific literature to supplement labels.
- Calciferol: This is the scientific or generic name for vitamin D, referring to the entire group of vitamin D compounds.
- Ergocalciferol (Vitamin D2): This form is derived from plant sources and is produced by the ultraviolet irradiation of ergosterol found in fungi and yeast.
- Cholecalciferol (Vitamin D3): This is the form produced in the skin of animals, including humans, when exposed to UVB rays from sunlight. It is also found in animal-based foods and supplements.
- Calcitriol: The biologically active form of vitamin D. Both D2 and D3 are converted in the liver and kidneys into calcitriol, which then interacts with vitamin D receptors throughout the body.
- The Sunshine Vitamin: A common nickname for vitamin D, referencing the body's ability to produce it when exposed to sunlight.
Synthesis and Activation of Vitamin D
Regardless of whether it comes from sunlight, food, or supplements, vitamin D is biologically inactive until it is processed by the body. The journey from inactive precursor to active hormone is a two-step process:
- First Hydroxylation (in the liver): The liver converts both D2 and D3 into 25-hydroxyvitamin D, also known as calcidiol. This is the major circulating form of vitamin D, and its blood level is measured to assess a person's vitamin D status.
- Second Hydroxylation (in the kidneys): The kidneys convert calcidiol into the final, biologically active form, calcitriol. Calcitriol is the potent steroid hormone that performs the critical functions of vitamin D.
Sources of Vitamin D in a Nutrition Diet
While sunlight is a major source for most, obtaining vitamin D through your diet is also important, especially during months with less sun. Here are key sources to include in your nutrition diet:
- Oily Fish: Excellent natural sources include salmon, mackerel, herring, and sardines.
- Fortified Foods: Many products have vitamin D added to them. Common examples include milk (dairy and plant-based), some cereals, and orange juice.
- Mushrooms: Some mushrooms contain vitamin D2, especially those exposed to ultraviolet (UV) light.
- Egg Yolks: A moderate source of vitamin D3.
- Beef Liver: Contains a small amount of vitamin D.
Comparison of Vitamin D2 vs. Vitamin D3
When choosing supplements or understanding food sources, it is useful to know the difference between the two primary forms of the vitamin.
| Feature | Vitamin D2 (Ergocalciferol) | Vitamin D3 (Cholecalciferol) |
|---|---|---|
| Source | Plants, fungi (e.g., mushrooms), and fortified foods. | Sunlight exposure, animal-based foods (e.g., fatty fish), and supplements. |
| Potency | Less effective at raising and maintaining serum 25(OH)D levels compared to D3. | More effective at raising and maintaining serum 25(OH)D levels compared to D2. |
| Absorption | Well-absorbed in the small intestine. | Well-absorbed in the small intestine. |
| Availability | Available in fortified foods and supplements, including prescription options. | Widely available over-the-counter and in many fortified products. |
| Metabolism | Follows the same two-step hydroxylation process as D3 to become active calcitriol. | Follows the same two-step hydroxylation process as D2 to become active calcitriol. |
The Consequences of Vitamin D Deficiency
If a person does not have adequate intake of vitamin D, either from sunlight or diet, they are at risk for deficiency. This is especially true for those with limited sun exposure, darker skin tones, or conditions that affect fat absorption. The consequences of a deficiency can be significant, particularly for bone health.
- Rickets: In children, a severe deficiency can cause rickets, a condition leading to bone deformities and skeletal weakness.
- Osteomalacia: In adults, a deficiency can cause osteomalacia, resulting in soft and weakened bones and persistent muscle aches and pains.
- Osteoporosis: Insufficient vitamin D contributes to poor calcium absorption, which can lead to osteoporosis, a condition characterized by brittle bones and an increased risk of fractures, especially in older adults.
- Impaired Immune Function: Research suggests a link between vitamin D deficiency and an increased susceptibility to infections and autoimmune diseases.
Conclusion
Vitamin D is a crucial nutrient for a healthy body, known by its scientific name, calciferol. Whether obtained from the sun as cholecalciferol (D3) or from plants as ergocalciferol (D2), its proper metabolism is vital for calcium regulation, bone health, and immune support. Including a variety of dietary sources like oily fish, fortified foods, and supplements—when necessary—is essential for maintaining adequate levels, preventing deficiency-related bone disorders, and supporting overall health and well-being.
For more in-depth information on nutrition and vitamins, consult reputable health resources like The Nutrition Source from the Harvard T.H. Chan School of Public Health.
