Vitamin D plays a crucial role in regulating the immune system, prompting significant scientific exploration into its effects. Researchers are investigating two key areas: whether vitamin D deficiency contributes to autoimmune diseases like multiple sclerosis and type 1 diabetes, and if vitamin D supplements can enhance the body’s defenses against infectious diseases such as tuberculosis and seasonal flu.
Vitamin D and Multiple Sclerosis
The incidence of multiple sclerosis (MS) is rising globally, yet the exact causes remain unclear. However, research suggests a combination of genetic predisposition and environmental factors, notably insufficient vitamin D and UVB exposure, increases MS risk. [51] The idea of vitamin D’s involvement in MS emerged over four decades ago, sparked by observations that MS rates were significantly higher in regions far from the equator with less sunlight, and lower in areas with diets rich in fish. [52]
A prospective dietary study indicated that women with a daily vitamin D intake exceeding 400 IU had a 40% reduced risk of MS. [53] Another study in the U.S. among young adults found that white men and women with the highest serum vitamin D levels experienced a 62% lower MS risk compared to those with the lowest levels. [54] This effect wasn’t observed in Black participants, potentially due to a smaller sample size and prevalent low vitamin D levels in this group, making it harder to detect a correlation. Similar findings emerged from a Swedish prospective study of young adults, showing a 61% lower MS risk with higher serum vitamin D. [55] Additionally, a Finnish study of young women linked low serum vitamin D levels to a 43% increased risk of MS. [56]
For individuals already diagnosed with MS, higher vitamin D levels have been associated with decreased disease activity and slower progression in prospective studies. [57, 58] Although clinical trials are ongoing to assess vitamin D as an MS treatment, prevention trials are scarce, likely because MS is relatively rare and would require extensive, long-term studies. Current evidence strongly suggests that low vitamin D may be a causal factor in MS, potentially accounting for about 40% of cases preventable through vitamin D sufficiency. [59] This conclusion is bolstered by recent evidence indicating that genetically predisposed low vitamin D levels correlate with a higher risk of multiple sclerosis.
Vitamin D and Type 1 Diabetes
Type 1 diabetes (T1D) also exhibits geographic variation; for instance, a child in Finland is significantly more likely to develop T1D than one in Venezuela. [60] While genetics play a major role, some research suggests lower T1D rates in sunnier climates. Early indications of vitamin D’s role in T1D come from a 30-year Finnish study tracking over 10,000 children from birth. This study found that infants receiving regular vitamin D supplements had nearly a 90% lower risk of developing type 1 diabetes compared to those not supplemented. [61]
However, subsequent studies examining dietary vitamin D and supplementation trials in children at high T1D risk have yielded inconsistent results. [62] Approximately 40% of T1D cases develop in adulthood. A U.S. prospective study among young adults revealed that white individuals with the highest serum vitamin D levels had a 44% lower risk of adult-onset T1D compared to those with the lowest levels. [63] Randomized controlled trials focusing on vitamin D and adult-onset T1D are lacking, and their feasibility is uncertain. Further research is necessary to clarify vitamin D’s role in type 1 diabetes.
Vitamin D, Flu, and the Common Cold
The seasonal nature of flu outbreaks, peaking in winter and subsiding in summer, led to the hypothesis of a sunlight-related “seasonal stimulus” triggering influenza. [64] Over two decades later, scientists proposed vitamin D as this seasonal stimulus. [65] Evidence supporting this includes:
- Winter months correlate with the lowest vitamin D levels. [65]
- The active form of vitamin D moderates inflammatory responses from certain white blood cells while enhancing immune cell production of microbe-fighting proteins. [65]
- Children with vitamin D deficiency rickets are more susceptible to respiratory infections, whereas sun exposure seems to reduce these infections in children. [65]
- Adults with low vitamin D levels are more likely to report recent coughs, colds, or upper respiratory infections. [66]
A randomized controlled trial in Japanese schoolchildren investigated daily vitamin D supplements for preventing seasonal flu. [67] Nearly 340 children were monitored for four months during peak flu season. Half received 1,200 IU of vitamin D daily, and half received a placebo. Type A influenza rates were about 40% lower in the vitamin D group, though no significant difference was found for type B influenza.
While randomized controlled trials on vitamin D for preventing other acute respiratory infections show mixed outcomes, a large meta-analysis of individual participant data indicated that daily or weekly vitamin D supplementation reduces the risk of acute respiratory infections. [68] This effect was particularly pronounced in individuals with severe vitamin D deficiency.
These meta-analysis findings raise the possibility that low vitamin D levels might also increase the risk or severity of novel coronavirus 2019 (COVID-19) infection. Although direct evidence is still emerging due to the novelty of COVID-19, maintaining adequate vitamin D levels is advisable for this and other health reasons. For those with risk factors for vitamin D deficiency, such as darker skin or limited sun exposure, supplementing with 1000 to 2000 IU per day is reasonable. This dosage is commonly found in multivitamins and is readily available.
More research is needed to definitively confirm vitamin D’s protective effects against flu and other acute respiratory infections. Even with potential benefits, vitamin D supplementation should not replace flu vaccinations and preventive measures like social distancing and hand washing to minimize COVID-19 risk.
Vitamin D and Tuberculosis
Before antibiotics, sunlight and sun lamps were standard tuberculosis (TB) treatments. [69] Recent research supports a link between vitamin D and TB risk. Combined analysis of case-control studies suggests that individuals diagnosed with tuberculosis have lower vitamin D levels compared to healthy individuals with similar demographics. [70] However, these studies don’t establish causality or whether vitamin D supplementation prevents TB. Genetic variations in vitamin D receptors might also influence TB risk. [71] Further research is warranted to explore this connection.
Vitamin D and Other Autoimmune Conditions
The Vitamin D and Omega 3 trial (VITAL), a large randomized, double-blind, placebo-controlled study involving over 25,000 adults aged 50 and older, assessed the impact of vitamin D supplements (2,000 IU/day) and omega-3 fatty acids (1,000 mg/day) over five years. The study found that vitamin D supplementation, alone or with omega-3s, reduced the incidence of autoimmune diseases by about 22% compared to placebo. [80] Observed autoimmune conditions included rheumatoid arthritis, psoriasis, polymyalgia rheumatica, and autoimmune thyroid diseases like Hashimoto’s thyroiditis and Graves’ disease. The supplement doses used are widely accessible and generally well-tolerated. The researchers recommend further trials to evaluate the effectiveness of these supplements in younger populations and those at higher risk of autoimmune diseases.
