Discovery and Characterization of Vitamin D: A Milestone in Nutritional Science

The early 20th century marked a pivotal period in the understanding of vitamins, particularly the discovery and characterization of vitamin D. The groundwork for this discovery was laid by Sir Edward Mellanby in 1919/20 when he conducted groundbreaking experiments to investigate the cause of rickets, a bone disease that was prevalent at the time. Mellanby’s work involved raising dogs in an environment devoid of sunlight or ultraviolet light, thereby mimicking conditions that could lead to rickets. He meticulously devised a diet that allowed him to determine that rickets was indeed caused by a deficiency of a trace component in the diet. This crucial finding was a significant step forward in understanding the role of nutrition in preventing disease.

In 1921, Mellanby published his findings, stating, "The action of fats in rickets is due to a vitamin or accessory food factor which they contain, probably identical with the fat-soluble vitamin." This statement was revolutionary, as it suggested that rickets could be prevented by a dietary component found in certain fats, particularly cod liver oil. Mellanby’s identification of cod liver oil as an excellent antirachitic agent was a milestone in nutritional science, as it provided a practical means of preventing rickets through dietary supplementation.

Mellanby’s work laid the foundation for further research into the fat-soluble vitamins. Shortly after Mellanby’s discovery, E.V. McCollum and his associates conducted experiments that distinguished between two fat-soluble vitamins, vitamin A and vitamin D. By bubbling oxygen through a preparation of the "fat-soluble vitamin," they observed that vitamin A was inactivated, whereas vitamin D retained its activity. This distinction was critical in understanding the specific roles of these vitamins in human health.

The identification of the role of sunlight in vitamin D production came soon after. In 1923, Goldblatt and Soames made a breakthrough discovery when they identified that 7-dehydrocholesterol, a precursor of vitamin D found in the skin, could be converted into an active form of vitamin D when exposed to sunlight or ultraviolet light. This finding provided a biological explanation for the protective effect of sunlight against rickets, further cementing the connection between sunlight exposure and vitamin D synthesis.

Hess and Weinstock later confirmed this connection through an innovative experiment. They excised small portions of skin from rats, irradiated the skin with ultraviolet light, and then fed it to groups of rachitic rats. The irradiated skin provided absolute protection against rickets, while the unirradiated skin did not, conclusively demonstrating that irradiated skin could produce sufficient quantities of vitamin D to prevent rickets. This experiment also suggested that vitamin D might not be an essential dietary trace constituent, as it could be synthesized in the body through exposure to light.

In parallel studies, Steenbock and Black discovered that food irradiated with ultraviolet light also acquired antirachitic properties. This finding underscored the importance of vitamin D in preventing rickets and further solidified its classification as a vitamin. The rapid advances in nutritional science during this period, particularly the identification of water-soluble and fat-soluble vitamins, played a crucial role in establishing vitamin D as a key nutrient necessary for bone health and the prevention of rickets.
Discovery and Characterization of Vitamin D: A Milestone in Nutritional Science