What Does Vitamin D Contain? An In-Depth Look at Its Composition

December 19, 2025 •

4 min read

While commonly referred to as a single nutrient, vitamin D is actually a group of fat-soluble secosteroids, with the two most prominent forms being D2 and D3. This unique composition is responsible for its crucial role in managing calcium and phosphate levels within the body. Knowing what does vitamin D contain is key to understanding its health benefits and identifying the best dietary and supplemental sources.

Quick Summary

This article details the chemical makeup of vitamin D, focusing on its two primary forms, ergocalciferol (D2) and cholecalciferol (D3). It explains how these fat-soluble compounds are sourced from sunlight, diet, and supplements, and how they are metabolized in the body to regulate calcium absorption for bone health and other functions.

Key Points

Secosteroid Composition: Vitamin D is a fat-soluble secosteroid, meaning it's derived from a steroid molecule with a broken ring structure.
Forms D2 and D3: The primary forms are D2 (ergocalciferol) from plants and D3 (cholecalciferol) from animal sources and sun exposure.
Activation is Necessary: Inactive D2 and D3 must be converted into the active hormone, calcitriol, by the liver and kidneys before the body can use it.
Regulates Calcium and Phosphate: Its main function is to regulate the intestinal absorption of calcium and phosphate, which is critical for bone health.
Sources from Sunlight and Diet: Sources of vitamin D include skin exposure to UVB rays, fatty fish, egg yolks, and fortified foods like milk and cereal.

Unpacking the Chemical Structure of Vitamin D

At its core, vitamin D is a group of fat-soluble compounds known as secosteroids. A secosteroid is a molecule where one of the steroid rings has been broken. In vitamin D, this happens between the 9th and 10th carbon atoms in its parent compound. The specific chemical composition differs slightly between its two most important forms for humans: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). These forms vary only in the structure of their side chains, with D2 having a double bond between carbons 22 and 23 and an additional methyl group on carbon 24 compared to D3.

Vitamin D2 (Ergocalciferol)

Ergocalciferol, or vitamin D2, is primarily derived from plant sources and fungi, like mushrooms, which produce it upon exposure to ultraviolet (UV) radiation. This form is used to fortify many foods, including milk alternatives and cereals. For a long time, D2 and D3 were considered equipotent, but recent research suggests D3 may be more effective in raising and sustaining blood vitamin D levels in humans.

Vitamin D3 (Cholecalciferol)

Cholecalciferol, or vitamin D3, is the form produced in the skin of humans and other animals when exposed to sunlight's UVB rays. It is also found in animal-based foods like fatty fish, egg yolks, and liver. Many supplements contain D3, and it is often the preferred form due to its higher potency and longer-lasting effect in the body. An animal-free version of D3, sourced from lichen, is also available for those following a vegan diet.

The Journey to an Active Hormone

Regardless of whether it starts as D2 or D3, vitamin D is biologically inactive until it is processed by the body. This activation is a two-step process involving hydroxylation in the liver and then in the kidneys. The final active hormonal form, called calcitriol (1,25-dihydroxyvitamin D), is what the body uses to regulate essential functions.

First Hydroxylation in the Liver: Upon absorption, vitamin D (D2 or D3) travels to the liver. Here, an enzyme adds a hydroxyl group, converting it to 25-hydroxyvitamin D (25(OH)D). A blood test measuring 25(OH)D levels is the standard way to assess a person's vitamin D status.
Second Hydroxylation in the Kidneys: From the liver, 25(OH)D moves to the kidneys, where another enzyme adds a second hydroxyl group. This creates calcitriol, the active hormone that binds to vitamin D receptors throughout the body.

Comparison of Vitamin D2 vs. Vitamin D3


Feature	Vitamin D2 (Ergocalciferol)	Vitamin D3 (Cholecalciferol)
Primary Source	Plants and fungi exposed to UV light	Animal sources and human skin exposed to sunlight
Availability	Often found in fortified foods, some supplements	Found naturally in fatty fish, eggs; common in supplements
Chemical Difference	Contains an extra methyl group and a double bond in the side chain	Side chain differs structurally from D2
Potency	May be less effective at raising and sustaining blood levels in humans	Generally considered more effective and longer-lasting in humans
Dietary Suitability	Sourced from plants, making it a vegan-friendly option	Primarily animal-derived, though lichen-sourced vegan options exist

Natural and Fortified Dietary Sources

While sunlight is a major source, most people also rely on dietary intake to maintain sufficient vitamin D levels. Very few foods naturally contain significant amounts of vitamin D. For this reason, many food products are fortified.

Natural sources include:

Fatty Fish: Salmon, mackerel, and tuna are excellent sources of D3.
Fish Liver Oils: Cod liver oil is a well-known, potent source.
Egg Yolks: Contain smaller amounts of D3.
Mushrooms: Provide vitamin D2 when grown under UV light.
Beef Liver: Contains modest amounts.

Fortified sources include:

Milk: Most cow's milk in the U.S. is fortified with vitamin D.
Plant-Based Milks: Soy, almond, and oat milks are often fortified.
Cereals and Oatmeal: Many breakfast cereals are enriched with vitamin D.
Orange Juice: Some brands are fortified.
Margarine: Some fat spreads and margarines contain added vitamin D.

Conclusion

To fully answer the question, "What does vitamin D contain?", one must look beyond the single name. This essential nutrient is composed of fat-soluble secosteroids, primarily ergocalciferol (D2) from plants and cholecalciferol (D3) from animals and sun exposure. These precursors are converted by the liver and kidneys into the active hormone, calcitriol, which plays a critical role in calcium regulation, bone health, and immune function. Understanding the different forms and their sources—from sunlight and fortified foods to fatty fish and supplements—is essential for maintaining adequate levels and supporting overall health.

Bone Health: Vitamin D is vital for absorbing calcium and phosphorus, which are key for building and maintaining strong bones.
Immune System Support: It plays a crucial role in modulating immune function to help fight off pathogens.
Two Main Forms: The two most common forms are ergocalciferol (D2), from plants and fungi, and cholecalciferol (D3), from animal sources and sun exposure.
Activation Process: Before it can be used by the body, inactive vitamin D is hydroxylated first in the liver and then in the kidneys to become the active hormone, calcitriol.
Best Assessment: The best way to measure a person's vitamin D status is via a blood test that measures the inactive form, 25-hydroxyvitamin D.
Sunlight vs. Diet: While sunlight exposure produces D3, dietary sources (both natural and fortified) are also important, especially during autumn and winter when sun exposure is limited.

Frequently Asked Questions

The main difference lies in their source and chemical structure. Vitamin D2 (ergocalciferol) comes from plants and fungi, while vitamin D3 (cholecalciferol) is produced in the skin when exposed to sunlight and found in animal-based foods. D3 is generally considered more effective at raising and maintaining blood levels.

Yes, vitamin D functions as both a vitamin and a prohormone. Once synthesized or consumed, it is converted into its active form, calcitriol, which functions as a steroid hormone by binding to receptors in various tissues to regulate gene expression.

While sunlight is a major source, many factors like geographical location, time of year, skin pigmentation, and use of sunscreen can limit production. Therefore, relying solely on sunlight is often not sufficient, especially during winter months, making dietary sources or supplements necessary for many people.

Few foods naturally contain high amounts of vitamin D. The best natural sources are fatty fish like salmon, mackerel, and tuna. Small amounts are also found in egg yolks, cheese, beef liver, and some mushrooms that have been exposed to UV light.

Vitamin D's most vital role is regulating the absorption of calcium and phosphorus in the intestines. Without enough vitamin D, the body cannot absorb calcium properly, which can lead to soft and brittle bones, a condition known as osteomalacia in adults or rickets in children.

Yes, excessive intake of vitamin D, almost always from high-dose supplements, can be harmful. High blood levels can lead to health problems like nausea, vomiting, weakness, kidney stones, and in extreme cases, kidney failure and heart problems.

After intake, vitamin D undergoes two hydroxylation steps to become active. First, the liver converts it to 25-hydroxyvitamin D. Then, the kidneys convert that form into the biologically active hormone, calcitriol.