Why is vitamin B2 called Vitamin G? Unraveling the Naming History

December 29, 2025 •

3 min read

In the early 20th century, scientists identified a water-soluble compound vital for growth, which they initially labeled 'Vitamin G'. This nutrient was later reclassified, explaining why vitamin B2 was called Vitamin G during a period of evolving vitamin nomenclature. The change reflects a pivotal moment in nutritional science as researchers began to distinguish individual components of the B-complex.

Quick Summary

Before modern classification, vitamin B2, also known as riboflavin, was historically referred to as Vitamin G. This reflects an early stage in nutritional science when scientists grouped similar water-soluble compounds, later reclassifying them based on chemical properties.

Key Points

Historical Naming: Before being officially named Riboflavin (B2), the compound was temporarily called Vitamin G, reflecting an early, less precise phase of vitamin classification.
Complex Unveiled: The name change happened after scientists realized the initial 'water-soluble B' was actually a complex of different compounds, with Vitamin G being the heat-stable, growth-promoting factor.
Chemical Definition: The name Riboflavin was adopted once the chemical structure was determined, combining 'ribose' (a sugar part) and 'flavin' (the yellow pigment).
Essential Coenzymes: Riboflavin is crucial for forming the coenzymes FAD and FMN, which are essential for energy metabolism, cell growth, and antioxidant functions.
Dietary Importance: As a water-soluble nutrient, riboflavin is not stored in the body and must be consumed regularly through foods like dairy, eggs, and fortified grains.
Scientific Evolution: The shift from 'Vitamin G' to 'Riboflavin (B2)' reflects the maturation of nutritional science from initial observation to detailed biochemical understanding.

The Early Days of Vitamin Discovery

The history of why vitamin B2 was called Vitamin G is linked to the early 20th-century discovery of vitamins. Casimir Funk coined the term 'vitamine' in 1912, a combination of 'vital' and 'amine', though not all vitamins contain an amine group. Early classifications, like Elmer McCollum's 'fat-soluble A' and 'water-soluble B', were based on solubility, which proved to be too simple as research progressed.

Unraveling the Vitamin B Complex

Scientists soon discovered that 'water-soluble B' was a complex of several substances. Experiments showed that a heated extract of 'water-soluble B' still promoted growth but didn't prevent beriberi, indicating at least two factors: a heat-labile anti-beriberi factor (B1) and a heat-stable growth factor.

The Discovery of Riboflavin

In 1933, a yellow-green fluorescent pigment from milk whey was isolated and identified as the heat-stable growth factor. Initially called 'lactoflavin' and later found in egg whites ('ovoflavin'), this compound was sometimes referred to as Vitamin G, particularly in the United States, due to its growth-promoting properties. This followed an alphabetical naming convention for newly discovered vitamins.

From 'Growth Factor G' to Riboflavin (B2)

The alphabetical naming system became confusing as more vitamins were discovered. In the 1930s, the chemical structure of the yellow pigment was identified, containing a ribose sugar and a flavin molecule. This led to the descriptive name riboflavin. It was then classified into the B-complex and designated B2, being the second factor identified. The name Vitamin G gradually became obsolete.

Comparing Early 'Vitamin G' and Modern Riboflavin (B2)


Feature	Early 'Vitamin G'	Modern Riboflavin (B2)
Naming Basis	Alphabetical designation for a growth-promoting factor identified within the 'water-soluble B' complex.	Chemically descriptive name (ribose + flavin) and numerical assignment (B2) within the B-complex.
Scientific Understanding	Broadly understood as a heat-stable, water-soluble factor necessary for growth.	Precisely defined as a precursor to the coenzymes FAD and FMN, essential for metabolic functions.
Primary Function Emphasis	Focused primarily on its role in animal growth.	Recognized for a wide range of functions, including energy production, cell growth, and antioxidant protection.
Nomenclature Status	Obsolete and primarily of historical interest.	Standard and universally accepted in modern nutritional and medical fields.

Key Functions of Riboflavin (Vitamin B2)

Riboflavin is a crucial water-soluble nutrient required for various bodily functions, needing regular dietary intake as it's not significantly stored. It's a precursor to the coenzymes FMN and FAD, vital for metabolism.

Key roles include:

Energy Production: FAD and FMN are essential for converting nutrients into energy.
Antioxidant Support: Riboflavin helps protect cells from damage.
Nutrient Metabolism: It aids the metabolism of other B vitamins.
Red Blood Cell Formation: Important for producing red blood cells and oxygen transport.
Vision Health: Supports healthy vision.
Nervous System and Liver: Necessary for their proper function.

The Evolution of Vitamin Nomenclature

The change from Vitamin G to B2 was part of a larger shift towards a systematic vitamin naming system based on chemical properties. This led to the numbered B vitamins, replacing the inconsistent alphabetical system and explaining gaps in the sequence.

The End of an Era: Conclusion

Vitamin B2 is not currently called Vitamin G; the latter name is a historical term from early nutritional science when the B-complex was being understood. The transition from the ambiguous Vitamin G to the chemically defined Riboflavin (B2) reflects scientific progress. Although Vitamin G is a historical term, riboflavin remains a vital nutrient for health, energy, and metabolism. This history underscores the importance of systematic research in modern nutrition. For more information on riboflavin, consult resources like the National Institutes of Health.

Frequently Asked Questions

Yes, they refer to the same substance, riboflavin. Vitamin G is the older, obsolete name that was used by some researchers in the United States during the early 20th century before the vitamin was officially reclassified as B2.

The official and modern name for the substance formerly known as Vitamin G is riboflavin, which is also designated as Vitamin B2.

The name was changed as part of a more systematic classification effort. Researchers discovered that the original 'water-soluble B' was a complex of several different vitamins. As they were identified and their chemical structures determined, they were assigned numbers (B1, B2, etc.).

Early nutritional research showed that a single 'water-soluble B' factor was needed for health. Further work demonstrated that this factor could be separated into heat-labile and heat-stable components, leading to the realization that it was a complex of multiple vitamins.

Rich sources of riboflavin include dairy products (milk, cheese), eggs, lean meats, organ meats, fortified cereals, green vegetables like spinach and broccoli, and almonds.

Riboflavin is essential for energy production, cell growth, and metabolism. It acts as a precursor for the coenzymes FAD and FMN, which help the body convert food into energy and protect against free radical damage.

The gaps exist because some substances were initially thought to be vitamins and given a letter or number, but were later found to be non-essential or were reclassified. This is why the sequence jumps from B3 to B5, for example.