What is Vitamin D? Vitamin D is a nutrient that is vital for human health. Unlike other Vitamins, however, naturally we get very little of Vitamin D from our food, but instead from our skin when it has the sun shining on it.
In the deeper levels of our skin, our cells use ultraviolet light (UV-B) to manufacture the Vitamin D precursor molecule. This precursor then travels first to our liver, and then to our kidney, to get activated in separate steps to the form of Vitamin D that promotes our health. This active form then travels through our blood, getting delivered to different cell types that need it [1]. For example, cells that make bone need Vitamin D so they can absorb Calcium, and without it, people suffer from soft bones in the disease caused Rickets.
However, recent research has found that Vitamin D is important for much more than bone health, and is especially important for a healthy immune system [2,3]. During the winter, because our skin is exposed to much less sunlight, our Vitamin D levels drop. This is thought to be the reason that winter is cold and flu season [4,5]. Many medical experts consider that there is an epidemic of Vitamin D deficiency, especially among people with dark skin [6,7].
Although we don’t generally get much Vitamin D through food, it is possible to increase our Vitamin D levels through supplementation. Some fatty fish have Vitamin D, and long ago, it was discovered that cod liver oil could help people recover from illness; more recently it was found that Vitamin D was the active ingredient in cod liver oil [8]. Nowadays, oral Vitamin D supplementation is more frequently done by fortification of milk and infant formula, and by capsules or softgels, at either nonprescription or prescription strength.
REFERENCES
Bikle, D. et al. 2017. Vitamin D: Production, Metabolism, and Mechanisms of Action. Endotext [Internet]. South Dartmouth (MA): MDText.com.
Aronow, C. 2011. Vitamin D and the immune system. J. Investig. Med. 59:881.
Bouillon, R. et al. 2019. Skeletal and Extraskeletal Actions of Vitamin D: Current Evidence and Outstanding Questions. Endocrinol. Rev. 40:1109.
Cannell, J. et al. 2006. Epidemic influenza and vitamin D. Epidemiol. Infect. 134:1129.
Juzeniene, A. et al. 2010. The seasonality of pandemic and non-pandemic influenzas: the roles of solar radiation and vitamin D. Int. J. Infecious. Diseases. 14:e1099.
Holick, M. 2007. Vitamin D Deficiency. N. Engl. J. Med. 357:266.
Harris, S. 2006. Vitamin D and African Americans. J. Nutr. 136:1126.
Cannell, J. 2008. Cod Liver Oil, Vitamin A Toxicity, Frequent Respiratory Infections, and the Vitamin D Deficiency Epidemic. Ann. Otol. Rhinol. Laryngol. 117:864.