A vitamin is defined as an organic micronutrient required in small amounts for normal metabolic function. These substances are generally acquired through diet because the body cannot synthesize them sufficiently.
No recognized essential nutrient currently holds the designation “Vitamin L.” Modern nutritional science recognizes 13 distinct essential vitamins, and L is not among them. The absence of this letter, and many others, reflects how scientific understanding evolves over time.
The Historical Classification of “Vitamin L”
The letter L was briefly assigned in the 1930s by Japanese researchers investigating factors required for successful reproduction and lactation in animal models. This early research led to the isolation of two separate factors thought to be necessary for milk production in rats. These two compounds were provisionally designated as Vitamin L1 and Vitamin L2.
Factor L1 was later identified as anthranilic acid, a compound found in bovine liver. The second factor, L2, was isolated from yeast and determined to be adenylthiomethylpentose.
Subsequent investigations determined that neither anthranilic acid nor adenylthiomethylpentose were required in the human diet. Since a substance must be indispensable for human health to be classified as a vitamin, the designation “Vitamin L” was abandoned and removed from the official nomenclature.
The Modern Alphabet of Essential Vitamins
Today, the 13 recognized vitamins are classified into two broad categories based on absorption and storage: fat-soluble and water-soluble. The fat-soluble group includes Vitamins A, D, E, and K, which are absorbed with dietary fats and stored in the liver and adipose tissue.
Vitamin A is a group of compounds, including retinol, which plays a role in vision, immune function, and cell growth. Vitamin D acts much like a hormone, regulating calcium and phosphate levels to maintain bone health. Vitamin E functions primarily as an antioxidant, protecting cell membranes from damage.
The water-soluble vitamins consist of Vitamin C and the eight B-complex vitamins. These dissolve in water and are not stored extensively, meaning they must be consumed more regularly. Vitamin C (ascorbic acid) is necessary for collagen synthesis and acts as a powerful antioxidant.
The B-complex group includes thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7), folate (B9), and cobalamin (B12). These B vitamins are involved in cellular metabolism, acting as cofactors for enzymes that convert food into energy, and are also necessary for red blood cell formation and nerve function.
Why Letters Were Removed or Skipped
The gaps in the alphabetical sequence, such as the absence of F, G, H, I, J, and L, resulted from the rigorous scientific process that followed the initial discoveries. In the early 20th century, new biological factors were often labeled with the next available letter before their chemical structure or human necessity was fully understood.
One reason for a letter’s removal was the reclassification of the substance into the B-complex. For example, Vitamin G was an early name for riboflavin (Vitamin B2). Similarly, Vitamin H was the original designation for biotin, later incorporated as Vitamin B7.
Other substances were disqualified because they were found to be non-essential to human health or were identified as mixtures of known compounds. A substance originally named Vitamin F was found to be the essential fatty acids, which are lipids rather than vitamins. Other factors, like Vitamin J, were discovered to be beneficial only to specific animal species, such as guinea pigs, but not required by humans.
The letters were retired to maintain clarity in scientific literature and public health communication. The current system avoids reintroducing these discarded letters to prevent confusion between historical findings and modern nutritional standards.