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I've been trying to figure out what makes the vitamin B group really big with 8 vitamins. Chemically they have different structures. Solubility: not only vitamins from this group are water soluble vitamins (also vitamin C). Function: not only this group of vitamins are cofactors for metabolic processes.

So what's the reason for this grouping, or why we can't include other vitamins in this group? Also, what does the letter B stands for?

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The common scientific belief around 1900 was that the only dietary components needed to support life were protein, fats, carbohydrates, and salts.[1] In 1912 Hopkins reported on experiments showing that "accessory factors" seemed to be needed.[1] In 1916, some of the factors began to be characterized in the paper "The Relation of the Unidentified Dietary Factors, the Fat-Soluble A, and Water-Soluble B, of the Diet to the Growth Promoting Properties of Milk."[2] The title itself designates two factors as A and B. Lack of factor A seemed to be associated with various problems particularly with the eyes, and factor B with a condition similar to beriberi. Before long, another factor was found that was associated with scurvy. In 1920, the literature had a confusion of terminology in use, and Drummond, using a variant of Funk's word, proposed:

that the substances be spoken of as Vitamin A, B, C, etc. This simplified scheme should be quite sufficient until such time as the factors are isolated, and their true nature identified.[3]

It was only later that the factors became well characterized. Vitamin B came to be known to be a complex mixture, and its components took decades to be characterized.(Wikipedia) As components of Vitamin B came to be known, subscripts were assigned. Some of the subscripts were assigned to compounds that later turned out to not to be fundamental and essential, and so are no longer used. Vitamin B12 took a particularly long time to characterize.(Wikipedia)

The vitamins are now well understood, and each has a specific name unrelated to the vitamin naming scheme, yet the vitamin terminology continues to be used because it is well known and useful to people at many different levels of technical knowledge. The significance of the name of the B group of vitamins is that "B" follows "A" and precedes "C" in the alphabet, and the history of discovery is still embedded in the names we use.

[1] Hopkins FG. Feeding experiments illustrating the importance of accessory factors in normal dietaries. J Physiol. 1912 Jul 15;44(5-6):425-60. doi: 10.1113/jphysiol.1912.sp001524. PMID: 16993143; PMCID: PMC1512834.

[2] EV McCollum, N Simmonds, W Pitz. The Relation of the Unidentified Dietary Factors, the Fat-Soluble A, and Water-Soluble B, of the Diet to the Growth Promoting Properties of Milk - Journal of Biological Chemistry, 1916

[3] Drummond JC. The Nomenclature of the so-called Accessory Food Factors (Vitamins). Biochem J. 1920 Oct;14(5):660. doi: 10.1042/bj0140660. PMID: 16742922; PMCID: PMC1258930.

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According to PeaceHealth:

The vitamin B-complex refers to all of the known essential water-soluble vitamins except for vitamin C.

Vitamin B was once thought to be a single nutrient. Researchers later discovered these extracts contained several vitamins.

Each member of the B-complex has a unique structure and performs unique functions in the human body. Vitamins B1, B2, B3, and biotin participate in different aspects of energy production, vitamin B6 is essential for amino acid metabolism, and vitamin B12 and folic acid facilitate steps required for cell division.

So, vitamins B have different functions, but most of them appear to be involved in the metabolism, especially of carbohydrates (Medical Libre Texts).

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B doesn't stand for anything. Originally, the vitamins were name alphabetically which can be traced to Cornelia Kennedy's 1916 Master's Thesis. What separates B-vitamins (and all vitamins) is there function. The function of B-vitamins is in metabolism and certain metabolic cycles, such as the Citric Acid Cycle.

Citations: http://vc.bridgew.edu/cgi/viewcontent.cgi?article=1029&context=mahpls_fac https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4772032/

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    $\begingroup$ Please cite a reference in support of this assertion about B vitamins being classified as such because of an involvement in metabolism. It may be true, but I find it strange as the requirement for a vitamin must have been established before its function was known. $\endgroup$ – David Jan 12 at 20:23

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