What is the chemical classification of vitamin D?

November 17, 2025 •

3 min read

Over 350 years of research were required to fully understand the chemical nature of vitamin D, initially thought to be a simple nutrient. Far from a single compound, vitamin D is a group of lipid-soluble secosteroids, primarily vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Its unique chemical structure, a steroid with a broken ring, allows it to function as a prohormone, which is a pivotal aspect of its classification.

Quick Summary

Vitamin D is chemically classified as a secosteroid, a compound derived from cholesterol with a specific broken ring structure. It is more accurately described as a fat-soluble prohormone than a vitamin, as it is synthesized in the body and converted into its active hormonal form. The two major forms, D2 and D3, differ slightly in their side chains.

Key Points

Secosteroid: Vitamin D is chemically classified as a secosteroid, a steroid molecule with a broken B-ring structure, which distinguishes it from traditional steroids.
Prohormone: More accurately described as a prohormone, vitamin D can be produced endogenously and is converted into its active hormonal form by the body.
Two Main Forms: The two most important types are ergocalciferol (D2) from plant sources and cholecalciferol (D3) from skin synthesis or animal products.
Active Form (Calcitriol): The biologically active form is 1,25-dihydroxyvitamin D (calcitriol), produced via two hydroxylation steps in the liver and kidneys.
UVB Synthesis: Skin production of vitamin D begins when UVB radiation from sunlight breaks a bond in the cholesterol precursor, 7-dehydrocholesterol.
Endocrine Function: The active hormone calcitriol binds to nuclear Vitamin D Receptors (VDR) to regulate gene expression, mirroring the function of other steroid hormones.
Calcium Regulation: A primary role of vitamin D's active form is to increase intestinal absorption of calcium and phosphate for bone health and other physiological processes.

Understanding the Secosteroid Classification

Vitamin D is fundamentally a secosteroid, a class of steroid molecules where one of the characteristic four rings has been broken. Specifically, a chemical bond in the B-ring of its precursor molecule is cleaved by ultraviolet B (UVB) radiation, initiating its synthesis. This structural feature is key to its functionality and distinguishes it from traditional steroids, which have a fully intact ring system.

The Prohormone vs. Vitamin Distinction

Initially discovered during research into the cure for rickets in the 1920s, vitamin D was named and categorized alongside other vitamins. However, later research revealed it behaves more like a hormone than a vitamin because it can be synthesized endogenously by the body (with sufficient sunlight exposure) and acts on nuclear receptors to regulate gene expression in target cells. This unique dual identity explains why scientists now primarily refer to vitamin D as a prohormone.

Key Forms: Ergocalciferol (D2) and Cholecalciferol (D3)

Vitamin D exists in several forms, or vitamers, with the two most important for human health being vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Their primary structural difference lies in the side chain of the molecule. This slight variation results in differences in potency and metabolic pathways, though both ultimately produce the same active hormone.

Vitamin D3 (Cholecalciferol): Produced naturally in human skin and in animals from 7-dehydrocholesterol upon exposure to UVB sunlight. It is also found in some animal-based dietary sources, such as fatty fish and eggs.
Vitamin D2 (Ergocalciferol): Derived from the UV irradiation of ergosterol, a sterol found in plants, yeast, and fungi.

The Journey to an Active Hormone: Metabolism

For vitamin D to become biologically active, it must undergo two enzymatic hydroxylation steps. This metabolic process is a cornerstone of its hormonal function.

First Hydroxylation (in the liver): Both vitamin D2 and D3 are first converted into 25-hydroxyvitamin D (25(OH)D), also known as calcidiol, by the enzyme 25-hydroxylase. This is the major circulating form and is measured to assess a person's vitamin D status.
Second Hydroxylation (in the kidneys): Calcidiol is then converted into the biologically active hormone, 1,25-dihydroxyvitamin D (1,25(OH)2D), or calcitriol, by the enzyme 1-alpha-hydroxylase. This final product binds to the vitamin D receptor (VDR) to exert its effects throughout the body.

Comparison of Key Vitamin D Metabolites


Feature	Vitamin D3 (Cholecalciferol)	25-hydroxyvitamin D (Calcidiol)	1,25-dihydroxyvitamin D (Calcitriol)
Chemical Type	Secosteroid	Secosteroid metabolite	Secosteroid hormone
Biological Activity	Inactive (prohormone)	Relatively inactive (storage form)	Most biologically active form
Production Site	Skin (from 7-DHC) & Diet	Liver (from D3/D2)	Kidney (from calcidiol)
Circulating Half-Life	Short	Approx. 3 weeks	Approx. 4-6 hours
Primary Function	Precursor to active form	Major circulating/storage form	Regulates gene expression, calcium & phosphate absorption

Synthesis and Activation: The Sunlight Connection

The unique synthesis of vitamin D highlights its chemical origin. In the skin's living cells, 7-dehydrocholesterol, a precursor molecule derived from cholesterol, absorbs UVB radiation (290–315 nm). This absorption causes a photochemical reaction that breaks the B-ring, forming previtamin D3. A heat-sensitive process then rapidly converts the previtamin D3 into vitamin D3. This ingenious mechanism allows for the body to self-regulate vitamin D production; prolonged sun exposure does not lead to toxic levels as excess previtamin D3 and vitamin D3 are photodegraded into inactive byproducts.

Mechanism of Action: The Steroid Hormone Pathway

The active hormonal form, calcitriol, exerts its wide-ranging biological effects through a mechanism similar to other steroid hormones. Calcitriol binds to the Vitamin D Receptor (VDR), a nuclear receptor found in cells throughout the body. The activated VDR-calcitriol complex then enters the cell nucleus and influences the transcription of hundreds of genes. This genomic action is responsible for its role in regulating calcium and phosphorus homeostasis, supporting bone health, and influencing immune function.

Conclusion

In summary, the chemical classification of vitamin D extends far beyond its initial label as a vitamin. Its complex nature as a fat-soluble secosteroid, derived from a cholesterol precursor and functioning as a prohormone, underscores its central role in human health. Through a highly regulated metabolic pathway, it is converted into the potent hormone calcitriol, which mediates its effects on gene expression, calcium and phosphate absorption, and immune function. This intricate chemical journey, from sunlight-activated skin precursor to a powerful regulatory hormone, solidifies its true biochemical identity. For further reading, consult the comprehensive article on Vitamin D at the National Institutes of Health.(https://www.ncbi.nlm.nih.gov/books/NBK56061/)

Frequently Asked Questions

Vitamin D is considered a prohormone because, unlike other vitamins that must be obtained solely from the diet, it can be synthesized by the body itself. The synthesis occurs in the skin through exposure to ultraviolet B (UVB) light. Once in the body, it undergoes a metabolic process to become an active hormone (calcitriol) that regulates biological functions.

A secosteroid is a steroid molecule in which one of the four rings has been opened or 'broken'. Vitamin D is a secosteroid because its synthesis in the skin involves the breaking of the B-ring of its precursor, 7-dehydrocholesterol, by UVB light.

The primary difference lies in their side chain structure. Vitamin D2 (ergocalciferol) comes from plant and yeast sources, while vitamin D3 (cholecalciferol) is produced in the skin of animals and humans and is also found in animal-based foods. Both are metabolized to an active form, though they differ slightly in potency and how they are handled by the body.

Vitamin D is converted through two main steps. First, in the liver, it is hydroxylated into 25-hydroxyvitamin D (calcidiol). Second, in the kidneys, calcidiol is converted into the biologically active hormone, 1,25-dihydroxyvitamin D (calcitriol).

The precursor of vitamin D3 in the skin is 7-dehydrocholesterol, a compound derived from cholesterol. Exposure to UVB light converts this precursor into previtamin D3, which then thermally isomerizes to vitamin D3.

The active form of vitamin D, calcitriol, primarily regulates calcium by promoting the absorption of calcium and phosphate from the intestines. It also plays a role in mobilizing calcium from bone and stimulating its reabsorption in the kidneys, all working to maintain stable blood calcium concentrations.

Understanding that vitamin D is a secosteroid provides crucial insight into its origin and function. It explains its fat-soluble nature, its metabolic relationship to cholesterol, and why its mechanism of action—binding to a nuclear receptor—is more like a hormone than a typical dietary vitamin.