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While vitamin C is arguably the best-known nutrient for immune defense, vitamin D has a lot to do with immunity, too. This fat-soluble essential vitamin supports a multitude of cellular functions that support the body’s immune response1—including protecting against pathogens and scavenging free radicals.
What are autoimmune diseases?
Autoimmune disease triggers the immune system to target its own body, rather than the infection itself. It also makes people with autoimmune diseases more susceptible to infections, viruses, and illnesses.
“Autoimmune disease is a condition in which the body’s immune system malfunctions and begins attacking healthy tissues,” functional medicine specialist and clinical pharmacist Jennifer Bourgeois, Pharm.D., IHP, FAIS tells mindbodygreen. “The immune system produces a specific antibody that targets a particular tissue or enzyme naturally found within the body, and tissue damage occurs as a direct effect of the inflammatory process.”
How does vitamin D status impact autoimmune disease risk?
Healthy vitamin D levels help decrease risk of autoimmune disease because vitamin D plays a role in regulating inflammation and helps recognize if cells are “good” or “bad”—i.e., which ones are a threat to the body, and which ones aren’t.
“Vitamin D pathways impact autoimmune disease pathologies through [vitamin D’s] role as an immunomodulator,” family medicine physician Laura Purdy, M.D. explains. “Vitamin D has been shown to regulate the immune system by promoting the differentiation of regulatory T cells3, which help prevent autoimmunity by suppressing the activity of other immune cells that attack the body’s own tissues.”
She adds: “Additionally, vitamin D can reduce the production of pro-inflammatory cytokines4, which are molecules that promote inflammation and contribute to the development and progression of autoimmune diseases.”
Due to the relationship between vitamin D status and autoimmunity, it’s no surprise that VDD is prevalent amongst autoimmune disease populations—including rheumatoid arthritis (RA), lupus, multiple sclerosis (MS), and diabetes (types 1 and 2).
Rheumatoid arthritis
“Rheumatoid arthritis is caused by inflammation that leads to tissue damage in the joints,” emergency medicine physician and medical editor at GoodRx, Katie Golden, M.D., explains. “Research suggests that vitamin D deficiency may contribute to the development of rheumatoid arthritis5, as well as correlate with disease severity.”
According to a 2016 review from Clinical and Experimental Rheumatology, approximately 55% of RA patients6 are deficient in vitamin D. Additionally, research shows an inverse relationship between RA and vitamin D, in that activity increases when D status decreases.
“Other studies have suggested that increasing vitamin D intake may help alleviate symptoms,” Golden shares. “This makes sense, given vitamin D plays an important role in bone health.” She adds that most of the existing studies researching the connection between RA and vitamin D are small, so more research is needed.
Lupus
“We know that vitamin D plays an important role in bone health, heart health, and immune system function,” Golden explains. “This makes vitamin D deficiency an important topic for people with lupus, an autoimmune disease that can affect both joints and blood vessels. We know that vitamin D deficiency is common in people with lupus, but it’s unclear whether this is a cause or a result of the condition.”
Multiple sclerosis
In a 2017 Neurology and Therapy review, lower vitamin D levels were found to be strongly associated with the development of new T2 lesions7—i.e., damage to the outer layer of neurons caused by MS activity.
Additionally, the review found that each 10 ng/ml increase of 25(OH)D serum levels lowered both the risk of new T2 lesions in MS patients (by 15%) and the risk of enhancing existing lesions (by 32%).
“Research has suggested that vitamin D deficiency increases someone’s risk for developing multiple sclerosis,” Golden explains. On the other hand, achieving optimal vitamin D status has also been shown to reduce the likelihood of developing MS by 62%8.
“[Vitamin D deficiency] may also correlate with disease severity in those living with the condition, but it’s unclear if increasing vitamin D intake can help MS symptoms,” Golden says.
Diabetes
In a 2022 Chinese cohort study published in Diabetes, approximately 49% of type 2 diabetes patients had VDD, while a 2022 systematic review from The Review of Diabetic Studies found that vitamin levels were up to 50% lower10 in children with type 1 diabetes.
How biological sex impacts vitamin D status & autoimmune risk
“[Biological] sex can affect the relationship between autoimmune disease risk and vitamin D, as studies have shown that women are more likely to develop autoimmune diseases than men, and also tend to have lower levels of vitamin D,” Purdy explains. “This suggests that there may be a link between sex hormones and vitamin D metabolism, which could influence the development and progression of autoimmune diseases.”
Interestingly, 85% of women with polycystic ovary syndrome (PCOS) have vitamin D deficiency as well. Additionally, maintaining vitamin D sufficiency can help reduce breast cancer risk by 50%. Given all these factors, it’s especially important for women to sustain truly optimal vitamin D levels (which may be higher than you think).
How to achieve and maintain vitamin D sufficiency
While food rich in vitamin D (e.g., egg yolk, fatty fish, liver, and grass-fed butter, according to Bourgeois) and sun exposure (it’s called the “sunshine vitamin,” after all) do increase vitamin D levels, neither are reliable sources of vitamin D for most people—especially in the long term.
For that reason, the best way to reach and sustain healthy vitamin D status is to take a high-quality vitamin D supplement that delivers 5,000 IU of vitamin D3 (like the ones in this comprehensive roundup).
The takeaway
Failing to maintain adequate vitamin D levels can increase autoimmune disease risk. Reaching and sustaining a healthy vitamin D status is key to promoting not only immune function, but whole-body health as well.
