Understanding Nutrition: Why is vitamin D called vitamin D?

October 11, 2025 •

4 min read

Did you know that the "D" in vitamin D simply stands for the fourth letter of the alphabet? In the early 20th century, scientists began identifying essential nutrients and, as a matter of convenience, named them alphabetically as they were discovered. This is the simple yet historically significant reason why is vitamin D called vitamin D, a name tied to the fight against the bone disease, rickets.

Quick Summary

Named alphabetically as the fourth identified nutrient in the early 20th century, vitamin D's name reflects a historical era of scientific discovery. It was designated as the anti-rachitic factor after being separated from other vitamins like A and B.

Key Points

Alphabetical Designation: The 'D' stands for the fourth vitamin discovered and named alphabetically by scientists in the early 20th century.
Pioneering Research: Elmer McCollum's work in 1922 identified vitamin D as the anti-rachitic factor, distinct from vitamin A, by experimenting with cod liver oil.
Misnomer: Technically, vitamin D is a steroid hormone because the body can produce it endogenously with sufficient sun exposure, unlike a true vitamin which must be obtained from diet.
Forms of Vitamin D: The two major forms are D2 (ergocalciferol), derived from plants, and D3 (cholecalciferol), synthesized in skin or found in animal products.
Health Impact: This fat-soluble nutrient is crucial for calcium absorption, bone health, and preventing diseases like rickets and osteoporosis.

The Origins of Vitamin Names

The naming of vitamins, including vitamin D, is a testament to the early days of nutritional science. In 1912, Polish biochemist Casimir Funk coined the term "vitamine" (vital amine) to describe essential organic compounds, believing they all contained nitrogen. While not all vitamins turned out to be amines, the name stuck, with the 'e' later dropped to form the word we use today: "vitamin".

During this time, American researcher Elmer McCollum was conducting groundbreaking experiments. He and his team discovered that certain fats contained an unidentified substance essential for growth in rats. This became known as the fat-soluble factor "A". Shortly after, the water-soluble factor "B" was identified. Vitamins were subsequently named alphabetically based on the order of their discovery.

Elmer McCollum and the Anti-Rachitic Factor

By 1922, McCollum and his team had shifted their focus to a devastating and widespread childhood disease: rickets. The disease, characterized by soft, weak bones, was prevalent in the early 20th century. Scientists had observed that cod liver oil could prevent or cure rickets, but they initially believed it was the vitamin A in the oil that was responsible.

McCollum set out to prove this wrong. He conducted an experiment where he destroyed the vitamin A content of cod liver oil by heating and aerating it. He found that the oil, despite lacking vitamin A, still retained its anti-rachitic properties. This proved that the factor preventing rickets was a separate, distinct substance. Following the alphabetical naming convention, McCollum named this new compound “vitamin D” as it was the fourth vitamin identified.

The Misnomer: Vitamin D as a Hormone

Interestingly, the term "vitamin D" is now considered a bit of a misnomer in a technical sense. A true vitamin is a nutrient that the body cannot synthesize itself and must obtain from the diet. However, vitamin D can be produced endogenously by the human body. When skin is exposed to ultraviolet B (UVB) radiation from sunlight, it triggers the synthesis of vitamin D3 (cholecalciferol) from a precursor molecule. Because of this, it functions more like a steroid hormone that is regulated by the body.

Vitamin D2 vs. D3

While the term vitamin D refers to the general nutrient, there are two primary forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). They differ in their origins and structure, but both are essential for health.


Feature	Vitamin D2 (Ergocalciferol)	Vitamin D3 (Cholecalciferol)
Primary Source	Plants and fungi, such as yeast (often irradiated)	Animal sources (fatty fish, eggs) and human skin exposed to sunlight
Precursor	Ergosterol, a plant sterol	7-dehydrocholesterol, found in the skin
Chemical Structure	Contains an additional double bond and a methyl group in its side chain	Single bond in the side chain
Use in Supplements	Commonly used in fortified foods and supplements	Often derived from sheep's lanolin for supplements

The Role of Vitamin D in Health

Vitamin D plays a critical role in human health beyond preventing rickets. Its primary function is promoting calcium absorption in the gut and maintaining adequate serum calcium and phosphate concentrations. This is vital for bone growth, remodeling by osteoblasts and osteoclasts, and preventing conditions like osteoporosis in adults. A severe deficiency of vitamin D can lead to thin, brittle, or misshapen bones.

Key Sources of Vitamin D

To maintain sufficient levels, people primarily rely on three main sources:

Sunlight: The body’s primary and most efficient method of producing vitamin D3 is through sun exposure. The amount produced depends on factors like time of day, skin type, season, and latitude.
Foods: Few foods naturally contain vitamin D. The best dietary sources are fatty fish like salmon, tuna, and mackerel. Fortified foods, such as milk, cereals, and orange juice, are also significant sources.
Supplements: Vitamin D supplements are widely available in both D2 and D3 forms and are recommended for those with insufficient sun exposure or dietary intake.

Conclusion: The Story Behind the Letter

While the name “vitamin D” is a simple relic of an early alphabetical naming system, the story behind it reveals a fascinating period in medical and nutritional history. It highlights the pioneering work of researchers like Elmer McCollum who, in their quest to eradicate diseases like rickets, uncovered essential truths about human nutrition. Today, our understanding has evolved, recognizing vitamin D's dual nature as both a dietary nutrient and a crucial, hormone-like regulator of calcium metabolism. However, the name itself serves as a historical marker for a vital discovery that fundamentally changed public health.

Frequently Asked Questions

The 'D' in vitamin D signifies that it was the fourth vitamin identified and named alphabetically during the early 20th century.

American biochemist Elmer McCollum is credited with discovering vitamin D in 1922. He isolated the substance responsible for preventing rickets, which he initially found in cod liver oil.

Vitamin D is sometimes referred to as a steroid hormone because the human body can synthesize it endogenously when the skin is exposed to sunlight. This differs from true vitamins, which the body cannot produce on its own.

While letters F, G, H, and others were initially used for substances believed to be vitamins, they were later reclassified as either non-essential or part of the B vitamin complex. For example, vitamin G was reclassified as B2 (riboflavin).

Vitamin D2 (ergocalciferol) is primarily found in plants and fungi, while vitamin D3 (cholecalciferol) is produced in the skin upon sun exposure and is also found in animal products. Both forms are used by the body, though their potency can vary slightly.

The best natural source of vitamin D is direct sunlight on the skin. Dietary sources include fatty fish (salmon, mackerel), fortified milk, and certain cereals. Many people also take supplements.

Yes, vitamin D is crucial for bone health. It promotes the absorption of calcium and phosphate in the gut, which is essential for bone mineralization. A deficiency can lead to rickets in children and osteomalacia in adults.