Vitamin D’s Winter Immune Benefits

November 14, 2020 2 Comments

Vitamin D’s Winter Immune Benefits


Clinical trials show that vitamin D decreases rates and severity of viral respiratory tract infections. More than 70% of Americans have either deficient or insufficient vitamin D blood levels. Vitamin D has shown promise against winter illness because it plays a critical role in supporting the immune system. Low vitamin D levels have been associated with higher rates of many chronic diseases. This includes an increased risk for acute communicable diseases, including viral infections in vitamin D deficient people. A meta-analysis of randomized, controlled clinical trials showed a protective effect against acute respiratory infections with vitamin D supplementation. More than 40% of Americans have been found to have insufficient blood levels of vitamin D (defined as levels between 20-30 ng/mL). An additional nearly 30% of Americans have lower vitamin D levels (below 20 ng/mL) that qualify as deficiency. This factor may be especially important among adults aged 60 and over. Oral intake of vitamin D to ensure healthy levels may help protect against winter-season conditions.

Impact on Immune Function

For the body to produce its own vitamin D, we need direct skin exposure to sunlight. But we spend most of our time indoors or covered up by clothes and sunscreen. And spending more time in the sun raises the risk of skin cancer and accelerated skin aging. The other way to get vitamin D is through diet, but most foods contain only modest amounts. As a result, a majority of people are getting too little of this crucial vitamin. Having low levels of vitamin D is associated with a greater risk for many health problems, from cognitive decline to heart disease.

Vitamin D supports immune health by helping:

  • Optimize immune function that protects us from infectious disease.
  • Control overly aggressive inflammatory immune responses, which can inflict systemic damage.

When excessive levels of immune-system proteins called cytokines provoke attacks on healthy tissues, the result is called a “cytokine storm.” This is a dangerous reaction that can lead to acute respiratory distress syndrome (ARDS), an often-fatal complication in which fluid collects in the lungs.


Vitamin D’s Immune Benefits

  • Vitamin D supports the immune system’s response to illnesses of all kinds.
  • More than 40% of Americans have insufficient blood levels of vitamin D.
  • Past studies show that low levels of vitamin D are associated with increased rates and severity of viral infections.
  • Clinical trials have shown that vitamin D has a protective effect against respiratory tract infections.

Vitamin D and Viral Illness

Viral respiratory tract infections, such as the flu, are more common during winter. One of the reasons for this may be seasonal variations in our vitamin D levels. During winter, we get less sun, leading to lower vitamin D production. That puts us at increased risk for viral infection. Research shows that infections are more common and more severe in those with vitamin D deficiency. Low vitamin D is also a risk factor for more severe lung disease, including acute respiratory distress syndrome (ARDS). Research suggests that those with insufficient vitamin D are at increased risk of a cytokine storm.

This hyperproduction of inflammatory factors leads to worsening disease severity and increased risk of death. Low vitamin D levels may be associated with the dangerous inflammation that occurs in ARDS.14,15

Vitamin D’s Protective Actions

Vitamin D contributes to many functions that help shield the body from infections and lessen their severity.

Maintaining adequate levels of vitamin D:

  • Interferes with the ability of viruses to replicate and produce more viruses,
  • Helps support and repair healthy cellular linings in the body, including in the airways of the lungs,
  • Increases production of proteins that shield against bacteria and viruses, enhancing the ability of cells to protect themselves from infection,
  • Improves the ability of immune cells to mount an effective attack against specific viruses, and
  • Helps prevent the immune system from going overboard and producing excessive pro-inflammatory compounds in the lungs.

Oral Vitamin D Reduces Risk

Many studies have evaluated whether daily oral intake of vitamin D can reduce rates of viral respiratory illness. Meta-analyses of clinical trials have shown that vitamin D has a protective effect against respiratory tract infections. The impact of vitamin D treatment is greatest in those who, to begin with, have low levels of vitamin D.


Vitamin D supports the immune system in many different ways, helping to shield the respiratory tract from viral illness. A large majority of adults have vitamin D levels below the optimal level. Trials have shown that oral vitamin D intake modestly decreases rates of viral respiratory tract infections.


There are no universal guidelines for frequency of vitamin D testing. However, given the high prevalence of vitamin D deficiency and the strong association of low vitamin D levels with several health issues, annual testing and supplementation to achieve adequate blood levels is highly recommended. Annual blood tests can let people know whether they are taking the correct dosage to ensure optimal blood levels of vitamin D.

If you do not already maintain an optimal blood level of 25-hydroxyvitamin D of 50 to 80 ng/mL, then take between 5,000 to 8,000 IU of vitamin D daily with meals.


Scientifically reviewed by: Julia Dosik, MPH, on October 2020. Written By Julie Myers.



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December 09, 2020



December 09, 2020


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