Vitamin D and Immune Health

Vitamin D has recently made its way into the spotlight, especially following its controversial link with COVID-19. While claims remain dubious/overstated at best, vitamin D has always been a major player in modulating the body’s immune response. Despite its misnomer, vitamin D is better described as a fat-soluble prohormone, which is then converted by the liver and kidneys into a bioactive hormone.

Sources of vitamin D are widespread – it can be found naturally in select foods, fortified into others, packaged as dietary supplements, and derived from the sun’s UV (ultraviolet) rays. Though vitamin D has been long associated with maintaining bone health, it has been linked with the immune response as early as 500 BCE, where Hippocrates, the father of modern medicine, was said to have treated a tuberculosis infection with heliotherapy (sunlight exposure).  

A healthy person has two types of immune responses that work together to fight off foreign pathogens– the innate and the adaptive systems. The innate response is the body’s first line of defense. It works quickly, within 0 to 96 hours of initial infection, and in a generalized fashion to stop foreign invaders. Vitamin D is an integral ingredient in creating proteins that aid in the antimicrobial response of the innate immune systems. The vitamin also aids in the regulation of genes that code for pathogen clearing cells, like macrophages and dendritic cells.  

The adaptive immune system is the second major line of defense. Unlike the innate immune system, it takes 5-6 days to kick into gear. However, what makes the adaptive immune response so potent is its capability to specifically target the foreign invader and to develop memory cells that can quickly launch an immune response if a re-infection were to occur. Since it is such a powerful response, vitamin D plays a part in regulating the B and T cells that make up the adaptive immune system. It plays an important role in dampening these adaptive immune responses as to not accidently harm our own cells, in what is called “self-tolerance.” In fact, there has also been increasing evidence that shows a link between vitamin D deficiencies and autoimmune diseases, like lupus and inflammatory bowel disease (IBS). 

After understanding the beneficial effects vitamin D has on our health, the golden question becomes: “How can I make sure I get enough vitamin D?” It is difficult to cite a precise amount, as peoples’ health histories and needs vary so greatly. However, unless you are at risk of developing a vitamin D deficiency, 10 - 30 minutes spent in the midday sun is more than sufficient. If you intend on adding vitamin D supplements to your routine, it is important to remember that vitamin D is a fat-soluble vitamin, which means that the body absorbs the vitamin in its tissues and organs. Thus, to avoid toxicity, dosage should be limited to less than 2000 IU a day.