Vitamin D is also known as the sunshine vitamin. This micronutrient supports many of the body’s essential functions, including modulating the immune system. Not only does it make it easier for immune cells to fight infections, but vitamin D also helps to ease inflammation and manage diseases caused by an overactive immune system.
What Is Vitamin D?
Vitamin D is a micronutrient the body relies on to maintain and enhance many of its functions. Nearly all of the body’s cells have vitamin D receptors, indicating that the vitamin’s value throughout the body is far beyond what we know so far. The recommended daily allowance for adults is 600-800 IUs. Your body can make vitamin D from the sun, but if you live somewhere with a short sunny season or you spend most of your time inside, you’ll need to seek out other vitamin D sources. It’s not always easy to find vitamin D in food, especially for people who follow a plant-based diet since vitamin D doesn’t occur naturally in many foods—especially plants.
Common vitamin D rich foods include fish, cheese, eggs and some types of mushrooms. Some vitamin D sources are fortified, which means it’s an added nutrient. Vitamin D fortified foods include tofu, soy milk and some breakfast cereals.
There are two major types of vitamin D—vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), each playing a vital role in immunity. Vitamin D is also essential for healthy bone and teeth, as the body can’t absorb calcium without it.
Anyone can develop a vitamin D deficiency, but some people are at a higher risk—older adults, people with limited sun exposure, vegans, people with lactose intolerance and people who struggle with chronic digestive issues. Vitamin D deficiency symptoms include fatigue, muscle weakness and low mood. Over time, vitamin D deficiency can lead to weakened bones, heart disease and immune system disorders.
Vitamin D Immune System Benefits
While most people know vitamin D’s essential role in bone and teeth formation, this nutrient is less known for its immune benefits. Vitamin D encourages your body's production of immune cells and eases inflammatory responses when the immune system is threatened.
Manages Inflammation and Immune Responses
Vitamin D stimulates the production of anti-inflammatory cytokines (proteins secreted by immune cells). Cytokines act as messengers that signal cells within the innate and adaptive immune systems to activate their defense systems. In contrast, vitamin D also inhibits the production of cytokines that cause an inflammatory response. Inflammation is an important part of the body’s immune system, but it can over- or under-react. Vitamin D helps achieve and maintain balance among immune cells.
In addition to stimulating the production of cytokines, vitamin D also increases the expression of CAMP—a gene that codes for cathelicidin. Cathelicidins are antimicrobial proteins that damage the cell membranes of pathogens, destroying them and preventing them from multiplying. Like a vitamin D deficiency, a deficient number of cathelicidins leaves the body vulnerable to infection.
Regulates Production of Immune Cells
In addition to its role in managing inflammation and immune responses, vitamin D plays a regulatory role in the development of white blood cells. Vitamin D plays a key supporting role in the body’s antimicrobial response. It protects the body from harmful microorganisms by enhancing the production and function of T-cells and macrophages, which are two types of immune cells that work together to find and kill harmful microbes.
With insufficient levels of vitamin D, the body’s adaptive immune system may become less efficient at targeting specific threats.
Vitamin D Uses
Because vitamin D plays an essential role in immune system function, researchers have discovered that this nutrient may be useful for cold, flu and other respiratory illnesses, as well as the prevention and management of autoimmune diseases.
Cold and Flu
Many researchers believe that vitamin D may play a role in preventing cold and flu. In their review of existing research, a team of researchers across the U.S. and Canada identified a probable link between vitamin D and cold and flu. They note that scientists have been looking for the cause of seasonal differences in cold and flu trends for quite some time. The team proposed that the high incidence of cold and flu during winter months may be linked to the drastic drop in sun exposure, and therefore, vitamin D availability.
In one cited study, where nearly 16,000 adults were asked about their recent healthy history, those with low vitamin D levels were more likely to report having recently had a cold, flu or upper respiratory infection. In a study of nearly 340 school-aged children, researchers found that children who supplemented vitamin D had an influenza-A rate that was about 40% lower than those who didn’t.
While the research on vitamin D and its role in cold and flu prevention is promising, it’s worth noting that no amount of vitamin D can keep you completely safe from a cold or flu. If you’re at risk for developing complications, the best way to stay safe is to avoid large gatherings, keep your distance from people who are unwell, wash your hands and avoid touching your face. With the right precautions in place, vitamin D can be an excellent added-insurance against infections.
Cold and flu are not the only respiratory infections that improve with higher vitamin D intake. The connection between respiratory illness and vitamin D isn’t new—we’ve known for decades that people who are vitamin D deficient are more likely to get tuberculosis and that they usually fare worse than those who don’t have a vitamin D deficiency. In fact, before the introduction of antibiotics, sun exposure was used to treat tuberculosis.
In a study involving patients with chronic obstructive pulmonary disease (COPD), a research duo from a British university observed that patients with COPD had lower concentrations of vitamin D in their plasma than people without the disease. They hypothesized that vitamin D might improve the functioning of cells that line the airway and act as a barrier for the respiratory tract.
Notably, vitamin D is currently being assessed for its possible role in COVID-19 management. The research teams leading the clinical trials note that coronaviruses are more active in the winter. So far, COVID-19 has affected people in northern areas with cold winters more severely. They suspect that seasonal differences may be linked to the lack of vitamin D in the winter months, and they hypothesize that supplementing with vitamin D may lessen the severity of symptoms and decrease the risk of serious lung damage. Clinical trials are ongoing in the U.S., Canada and beyond(13,14).
Vitamin D, with its effects on immune and inflammatory responses, seems to be an important tool in the prevention and improvement of a range of respiratory illnesses.
Autoimmune disorders are diseases that develop when the immune system mistakes the body’s healthy cells and tissues for foreign invaders and responds by attacking them. Autoimmune disorders challenge the body in two ways. Firstly, destroying healthy cells creates undue stress on the body. Secondly, harmful invaders have a chance to sneak in while the immune system is busy defending the body against itself.
There’s no shortage of evidence linking vitamin D deficiency to autoimmune diseases(10). Low vitamin D levels have been shown to predict future development of multiple sclerosis, type 1 diabetes and rheumatoid arthritis.
In a review of current literature, a team of neurology researchers found substantial evidence that higher vitamin D levels correlate with a reduced risk of developing multiple sclerosis (MS). They found that MS patients with sufficient vitamin D levels experienced reduced disease activity, including in the brain, and had a decreased risk of relapse. MS causes the body’s immune system to attack the protective myelin sheath that coats nerve cells. The link between vitamin D and MS may be related to vitamin D’s ability to regulate how immune cells work, but more research is needed to support (or refute) this theory.
Type 1 Diabetes
A 2008 paper reviewed the connection between vitamin D and diabetes. The researcher noted that regular doses of vitamin D in childhood are linked to a reduced risk of type 1 diabetes later in life. In addition to its preventive qualities, vitamin D has been shown to improve blood sugar regulation and insulin sensitivity in people with diabetes.
In a study that compared rheumatoid arthritis patients to non-patients, researchers discovered that the rheumatoid arthritis patients had significantly lower vitamin D levels. They suggested a link not just between vitamin D and the risk of developing rheumatoid arthritis, but also between the nutrient and severity of symptoms.
While each autoimmune disease and its impact on the body’s functions is unique, it appears that vitamin D may help reduce autoimmune activity. Beyond that, a deficiency of the micronutrient may predict a higher risk for developing an autoimmune disease.
Vitamin D in Summary
Vitamin D is an essential nutrient that may be produced by the body either in response to sun exposure or by food or supplement. It’s benefits start on the surface—the skin—and extend to the body’s inner workings. It’s an irreplaceable component of many essential tasks.
Because its beneficial actions are multifold—fighting inflammation and boosting immunity, as well as aiding in calcium absorption and modulating gene expression—a vitamin D supplement is a valuable addition to a healthy lifestyle.