When you think of vitamin D, you probably think about enjoying being outside in the sun because it makes sense since our bodies create vitamin D when our skin is exposed to sunlight. It’s well known for helping the body absorb calcium and build strong, healthy bones. But did you know that it has another important function? Vitamin D also plays a key role in fighting disease. It’s one of the building blocks of a healthy, strong immune system. Studies show that vitamin D is particularly adept at helping our bodies fight off respiratory viruses, such as seasonal flu and even COVID-19. Where exactly do humans get vitamin D? The best (and most well-known) source of vitamin D is the sun. When our skin is exposed to the sun’s ultraviolet B (UVB) rays, a chain of reactions occurs within the skin’s cells that cause the body to manufacture vitamin D. It’s one of only two vitamins that our bodies create (vitamin K is the other). While the sun is not the only way to get vitamin D, it is certainly the most convenient. Vitamin D can also be found in foods, but relatively few foods contain vitamin D naturally. Fatty fish like tuna, salmon , and mackerel are good sources of vitamin D, and some foods like cheese, egg yolks , and beef liver contain smaller amounts. But natural food sources of vitamin D are limited. That’s one reason many food manufacturers fortify their products with this essential vitamin. Milk, orange juice , and certain cereals often contain added vitamin D. Supplements are another source of vitamin D and they may be particularly important for those who are unable to get enough sunlight to produce adequate amounts for optimal immune function. What are the different types of vitamin D? Not all supplements are the same, and that is especially true when it comes to vitamin D. There are two commonly available forms of vitamin D supplements – vitamin D2 and vitamin D3. The main difference is that D2 comes mainly from plants and fortified foods , while D3 is found in animal-sourced foods. D2 is produced when plants are exposed to sunlight whereas D3 is the natural form our bodies create from sunlight. When it comes to supplements, it is typically cheaper to produce the D2 form and this is commonly found in multivitamin supplements, however , D3 is thought to more effective at raising vitamin D levels in your blood. Why are so many of us vitamin D deficient? It is estimated that a significant portion of the world’s population is vitamin D deficient. Sunscreen is thought to be a contributing factor. While sunscreen helps protect our skin from sun damage and ultraviolet B (UVB) rays, it also blocks our ability to create vitamin D, which requires some UVB exposure Of course, there are other reasons that humans don’t get enough sunlight exposure. Winter is an obvious one, but air pollution and high altitudes can also keep us from getting the sunshine our bodies need. Additionally, people with darker skin types require longer sun exposure than people with lighter skin to create vitamin D. How does vitamin D affect the immune system? Several studies have concluded that vitamin D plays a vital role in maintaining a healthy immune system. In fact, low levels of vitamin D have been linked to increased risk for several immune-related diseases including psoriasis, type 1 diabetes, multiple sclerosis, rheumatoid arthritis, tuberculosis, sepsis, respiratory infection , and COVID-19. What role does vitamin D play exactly? Vitamin D reduces concentrations of cell secretions that produce inflammation and injure the lining of the lungs, which can lead to pneumonia. At the same time, vitamin D increases concentrations of anti-inflammatory secretions. It also lowers a virus’s ability to replicate and even helps to form certain proteins that protect the lungs and heart. Some preliminary studies show possible improvement in COVID-19 patients when given high doses of vitamin D. But what is known for sure, though, is that getting adequate amounts of vitamin D is important to many of the body’s functions and a healthy immune system is How much vitamin D do you need? For most people, 10-30 minutes of the midday sun on thin-skinned areas, a few times a week (without sunscreen) is probably sufficient. People with darker skin may need more exposure. But depending on your lifestyle and where you live, it may be difficult to get enough sunlight, especially in the winter months. That’s when supplementation may be beneficial. While experts disagree on the exact daily vitamin D requirement, most recommend between 1,500 – 5,000 IUs when only minimal sunlight exposure is possible. If deficient, higher levels may be required for a short period of time (up to 8 weeks) before switching to a maintenance dose. It’s important to consult with your doctor before taking supplements, including vitamin D.