The Role of Vitamin D in the Immune System

A large body of scientific evidence shows that vitamin D impacts the immune system and helps support systemic health. Vitamin D functions as an immune booster and vitamin D deficiency is linked to immune disorders and increased risk of infections. Vitamin D is not just a nutrient; it’s a hormone with receptors in nearly every cell and tissue of the body. Research has correlated vitamin D levels with over 53 different disease states including Alzheimer’s, cancer, MS, osteoporosis, diabetes, depression, and COVID-19. It is estimated that about 3% of the human genome is regulated by the vitamin D endocrine system and over 1000 genes in the human body are impacted.[1, 2] 

Many diseases including influenza show winter seasonality. Some researchers hypothesize that this seasonality is due to decreased sun exposure during the winter months.[3,4] In northern climates, between November and March, vitamin D cannot be produced from sun exposure. During these months the angle of the sun prevents the majority of UVB light from getting through the atmosphere, and therefore from reaching our skin to stimulate vitamin D production. As a result, our vitamin D levels noticeably decrease during the winter. 

How vitamin D boosts the immune system

Vitamin D is involved in immunity through several mechanisms and plays a role in regulating both the innate and adaptive immune response in the body. Vitamin D impacts immune health by interacting with vitamin D receptors on innate immune cells throughout the body. The innate immune is the body’s non- specific, first line of defense against foreign pathogens. 

Vitamin D builds immunity and helps fight off infection by regulating the activity of immune cells which elicit antiviral responses. Vitamin D binds to receptors on immune cells such as neutrophiles, macrophages and natural killer cells. Vitamin D stimulates these cells to produce peptides with antiviral properties that defend the body against foreign pathogens.[5] Cathelicidins and defensins are peptides that have anti-microbial and immune modulating capabilities and act as anti-viral agents as part of the innate immune system.[6] Multiple studies have shown that vitamin D metabolites play a role in enhancing production of the human form of cathelicidins and defensins, thus enhancing innate immune response.[7] Studies have shown that vitamin D plays a role in both the activation of these cells for helping the body fight off infections, and also in suppressing immune response in cases of over inflammation.

Vitamin D acts as an immune system modulator by suppressing inflammation associated with both the innate and adaptive immune response.[8] In response to foreign pathogens, the immune system releases small proteins called cytokines, which trigger an immune response to help fight off the virus. Sometimes this response to infection goes into overdrive and excessive amounts of cytokines are released. This is known as a cytokine storm and can cause hyperinflammation. This inflammation can damage organs throughout the body, leading to pulmonary infiltrates, lung injury, acute respiratory distress syndrome, disseminated intravascular coagulant, renal failure, and eventually death.[9]

Vitamin D immune system benefits

Many studies have looked at vitamin D’s role in immunity against respiratory infections. A systematic review of 25 randomized controlled trials concluded that vitamin D supplementation significantly reduced the risk of acute respiratory tract infection.[10] Another study looking at the impact of vitamin D on respiratory function found that every 10 nM/L increase in vitamin D blood level reduced the risk of respiratory infection by 7%.[11]

Research has also found that vitamin D builds immunity against flu and colds. Vitamin D deficiency is most prevalent in the winter when cold and flu season peaks. Clinical trials have demonstrated that increasing vitamin D level has been shown to decrease incidents of influenza.[12] 

Research shows that low levels of vitamin D increases risk of autoimmune conditions, including multiple sclerosis, rheumatoid arthritis, diabetes mellitus, and inflammatory bowel disease.[13] Individuals affected by these conditions often have low levels of vitamin D. Studies have suggested that vitamin D deficiency may contribute to both the development and severity of these conditions. 

Early correlational research strongly suggests a link between vitamin D level and the risk and severity of coronavirus outcomes. In one study, researchers found that patients over the age of 40 were 51.5 percent less likely to die from COVID-19 if they had a sufficient level of vitamin D, which they define as being at least 30 ng/mL.[14] Another study of 185 patients diagnosed with COVID-19 found that vitamin D deficient patients had a higher hospitalization rate and required more intensive oxygen therapy and invasive mechanical ventilation. In these patients, vitamin D deficiency was associated with a 6-fold higher hazard of severe course of disease and about a 15-fold higher risk of death.[15] To read more about the link between vitamin D and COVID-19, check out our page The Impact of Vitamin D on COVID-19.

Vitamin D and your immune system

More research is needed to determine the exact vitamin D dosage required to boost immune system function. 

The Institute of Medicine’s (IOM) recommended dietary allowance for vitamin D is 600 International Units (IU) per day. This recommendation is based on what is needed to prevent rickets and osteoporosis, not what is needed to support the immune system. Many vitamin D researchers disagree with this recommendation and say that the IOM’s recommended allowance isn’t enough to prevent deficiency or support bone health. Generally, 4,000 IU or less per day is considered safe, as long as your blood values are being monitored. It is important to avoid excessive doses of oral vitamin D. If vitamin D intake is too high, it can cause elevated blood calcium levels and potentially dangerous consequences.

Obtaining vitamin D from light allows your body to self-regulate vitamin D production and make as much as it needs to stay healthy, without any risk of overdose or toxicity. Unlike with oral supplements, you cannot overdose on vitamin D3 produced by your skin. If you have enough vitamin D, your body will simply produce less.