We know the purpose of the immune system. It defends your body from invading pathogens, promotes protective immunity, and maintains tolerance to your own healthy cells.
We also know that vitamin D is crucial to our bone health primarily by making calcium more bioavailable for your body to use. Scientists are discovering that vitamin D also has a critical role in modulating your innate and adaptive immune responses. A deficiency in vitamin D is linked to a higher susceptibility to infection and a higher risk of autoimmunity. Thus, the benefits of vitamin D may extend beyond the benefits of bone and calcium homeostasis and provide benefits to people with autoimmune diseases.
Vitamin D and bone health
The most well-known benefit of vitamin D is that it promotes calcium homeostasis and bone health. It enhances the absorption of calcium in the small intestine and stimulates calcium reabsorption of bone. In addition, it promotes the mineralization of the collagen matrix in bones.
For humans, vitamin D is primarily synthesized in the skin from exposure to the sun, although that process can be influenced by season, latitude, skin pigmentation, or use of sunblock. However, vitamin D must go through a second process in the liver to form vitamin D3. Then it acts in the intestine to stimulate calcium reabsorption and promote osteoblast differentiation and matrix calcification.
Vitamin D is vital to many tissues in our bodies including cells in the bone marrow, brain, colon, breast and malignant cells, and immune cells. Thus, vitamin D has many functions other than promoting bone health. It impacts the endocrine system, and it may act in a paracrine or autocrine manner.
In addition, scientists now know that vitamin D has an impact on cell proliferation and differentiation as well as immunologic effects resulting in an ability to maintain tolerance and to promote protective immunity.
We know this because antigen-presenting cells (macrophages and dendritic cells), T cells and B cells have the mechanisms to synthesize and respond to vitamin D. Thus, it may act in a paracrine or autocrine manner in our immune system. Moreover, local levels of 1,25 D may differ from systemic, circulating levels as local regulation of the enzymes synthesizing and inactivating vitamin D are different from the controls originating in the kidney.
Vitamin D and protective immunity
Prior to antibiotics, doctors inadvertently used vitamin D to treat infections like tuberculosis. Tuberculosis patients were sent to sanitariums where they were treated with exposure to sunlight. Doctors believed that exposure to sunlight could kill the tuberculosis bacterium. In addition, treatment with cod liver oil was common, and the liquid is rich in vitamin D.
Many studies have shown that deficiencies in vitamin D are associated with a higher risk of infection. One study with nearly 19,000 participants between 1988 and 1994 showed that individuals with lower vitamin D levels were more likely to self-report a recent upper respiratory tract infection than those with sufficient levels, even after adjusting for variables including season, age, gender, body mass, and race.
Vitamin D levels vary throughout the year. Although rates of seasonal infections varied and were lowest in the summer and highest in the winter, the association of lower serum vitamin D levels and infection was steady throughout each season.
Another study of 800 military recruits in Finland grouped the men by serum vitamin D levels. Those recruits with lower vitamin D levels lost significantly more days from active-duty personnel due to upper respiratory infections than recruits with higher vitamin D levels.
Many more studies show an association between higher levels of vitamin D and lower rates of influenza and some bacterial infections. All of them also report the converse: lower vitamin D levels are linked to higher rates of infections.
One recent double-blind placebo study used an objective outcome, a nasopharyngeal swab culture rather than self-reporting. A therapeutic dose of vitamin D showed that vitamin D administration resulted in a statistically significant (42%) decrease in the incidence of influenza.
The benefits of vitamin D on protective immunity are due in part to its effects on the innate immune system. It is known that macrophages recognize lipopolysaccharide LPS, a surrogate for bacterial infection, through toll-like receptors (TLR). Engagement of TLRs leads to a cascade of events that produce peptides with potent bactericidal activity. These peptides disrupt the membranes of harmful bacteria leading to a potent anti-bacterial property.
Vitamin D and autoimmune disease
Evidence is growing that shows a link between vitamin D deficiency and autoimmune diseases like multiple sclerosis (MS), rheumatoid arthritis (RA), diabetes mellitus (DM), inflammatory bowel disease, and systemic lupus erythematosus (SLE).
In addition, studies are increasingly showing a link between low serum vitamin D and the risk of developing an autoimmune disease in the future. There is also data linking decreased in-utero exposure to vitamin D and islet cell autoimmunity. Lower in-utero exposure of vitamin D during pregnancy may be associated with a higher risk of the child developing an autoimmune disease.
A lack of vitamin D has also been shown to add to the progression of an existing autoimmune disease. In one study, 161 patients with an early undifferentiated connective tissue disease were followed for over 2 years. Most of the participants experienced no progression and remained in an undifferentiated state. However, 21% of the participants eventually developed a defined rheumatologic diagnosis including RA, SLE, Mixed Connective Tissue Disease, and Sjogren’s Disease. While the baseline characteristics of the 2 groups were similar, those in the 21% had a vitamin D level that was significantly lower than the other group.
In addition, vitamin D levels have been studied extensively in lupus patients. Vitamin D levels are typically lower in patients than in disease or normal controls. Deficiency of vitamin D is extremely common, often with more than 50% of lupus patients with deficient levels and severe deficiency is common. Similar correlations between low levels of vitamin D and disease activity and severity have been observed in other autoimmune diseases such as MS and RA.
Vitamin D and immunologic function
Vitamin D has a significant effect on immune system cells. It inhibits B cell proliferation and blocks B cell differentiation and immunoglobulin secretion. In addition, vitamin D suppresses T cell proliferation, and it affects T cell maturation by skewing it away from the inflammatory Th17 phenotype while promoting the induction of T regulatory cells. The result is lower production of inflammatory cytokines and higher production of anti-inflammatory cytokines.
The importance of vitamin D to a healthy immune system cannot be overstated. A deficiency in vitamin D is clearly linked to the development and progression of an autoimmune disease.
Many immune cells in people with autoimmune diseases seem to respond to the immunomodulatory effects of vitamin D. These include B cells, T cells, monocytes, and DCs.
Final thoughts
Vitamin D has a crucial role that goes beyond calcium absorption and bone health. It also plays a key role in the modulation of the innate and adaptive immune responses. Vitamin D deficiency is prevalent in autoimmune diseases. Immune system cells respond to vitamin D and can synthesize it, which suggests that vitamin D supplements for those with autoimmune disease and a deficiency in the vitamin may provide significant benefits.