Vitamin D (also known as calciferol) is a fat-soluble vitamin that can be obtained from sunlight exposure, food, and supplements [1]. Vitamin D helps with calcium absorption, maintaining bone health, and regulating various physiological functions, such as antioxidant function, anti-inflammatory response, supporting immune function, and preserving muscle function, among others [2].
Especially in terms of immune function, recent studies suggest that vitamin D can assist in regulating innate and adaptive immunity by reducing viral replication, slowing inflammation, and modulating immune cell activities[3]. Even before the COVID-19 pandemic, vitamin D was being evaluated for its potential to help prevent viral infections, such as influenza, hepatitis C virus, human immunodeficiency virus (HIV), and others. Thus, it is hypothesized that vitamin D might help prevent COVID-19 infection and potentially aid in treatment [4].
How does vitamin D prevent COVID-19?
Vitamin D has several abilities to reduce the risk of infection and death. Three potential ways of preventing infection include physical barriers, natural immunity, and adaptive immunity [5].
- Physical barriers: Vitamin D helps maintain the adhesion of mucosal cells and intercellular adhesion, preventing bacterial invasion.
- Natural immunity: Vitamin D metabolites can induce innate antibacterial mechanisms, including inducing immune cell phagocytosis of bacteria and activating oxygen antibacterial capabilities, preventing excessive production of cell inflammatory factors, and enhancing macrophage protective capabilities.
- Adaptive immunity: Vitamin D is a modulator of adaptive immunity, mainly inhibiting the production of cell inflammatory factors IL-2 and interferon gamma (IFNγ), suppressing the response of helper T type 1 cells (Th1), and promoting the action of T helper type 2 (Th2) cells, regulating the inhibition of the inflammatory process.
Analytical study results have found that vitamin D supplementation is helpful in preventing acute respiratory infections[6]. Another study found that vitamin D has the effects of accelerating healing and suppressing inflammation in lung tissues[7]. Other studies have indicated that vitamin D supplementation can reduce the risk of infection by influenza A virus, rotavirus, and dengue virus[5]. Taken together, a deficiency in vitamin D may increase the risk of viral infections, while adequate intake of vitamin D can help prevent such infections, making it plausible for the prevention of COVID-19[8]. However, more research evidence is needed to support this theory.
Is taking vitamin D useful for those who have already contracted COVID-19?
Research suggests that vitamin D has regulatory functions on both the innate and adaptive immune systems in the human body. It helps reduce viral replication, slow down inflammatory responses, and regulate immune cell activity[9]. Clinical studies have found that COVID-19 patients with lower levels of vitamin D in their bodies have higher mortality rates, showing a negative correlation between the two[10]. Another study found that COVID-19 confirmed patients who were given high-dose vitamin D supplementation had shorter durations of COVID-19 symptoms, especially cough and loss of taste[11].
In addition, an analytical study found that children infected with COVID-19 who had lower levels of vitamin D in their bodies were at increased risk of developing severe symptoms. It was also observed that children and adolescents with vitamin D deficiency had a higher risk of contracting COVID-19. Furthermore, the study noted that vitamin D supplementation could improve the severity of COVID-19 symptoms[12]. However, more clinical trials are needed to obtain stronger evidence.
Can vitamin D prevent or improve long COVID?
In general, long COVID refers to symptoms that persist for more than 4 weeks or even months in COVID-19 confirmed patients, with symptoms that may disappear and reappear, including fatigue, reduced physical fitness (noticeable during exercise), difficulty breathing, or shortness of breath, cough, neurological symptoms (difficulty concentrating), loss of smell or taste, sleep problems, dizziness when standing, and more[13].
In addition to enhancing immunity, vitamin D also plays a role in cell growth, neural muscle function, and regulating inflammatory responses. Studies suggest that vitamin D deficiency can lead to fatigue and decreased exercise tolerance. Therefore, researchers hypothesize that vitamin D supplementation may be helpful for patients with long COVID experiencing fatigue and reduced physical fitness[7].
A clinical study evaluated the vitamin D levels in the bodies of COVID-19 recoveries and found that 35.6% of the participants had vitamin D deficiency, and these individuals performed poorly in a six-minute walk test (exercise tolerance test). Researchers suggest that vitamin D deficiency is common in COVID-19 recoveries, and low vitamin D levels are associated with reduced physical function, especially in the elderly population[14]. More clinical studies are needed to confirm the role of vitamin D in helping with long COVID.
What are the other benefits and effects of vitamin D?
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Assisting calcium absorption
Vitamin D can help with calcium absorption in the small intestine. In the absence of vitamin D, the absorption rate of calcium by the small intestine is only about 10-15%. With the presence of vitamin D, the absorption rate of calcium increases to around 30-40%. It also helps regulate the balance of blood calcium in the body[15].
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Supporting immune function
Research indicates that vitamin D can enhance immune function by regulating physical barriers, innate immunity, and adaptive immunity. It promotes immune function and suppresses inflammatory responses[5]. For example, vitamin D can help prevent upper respiratory infections and delay the inflammatory state[7].
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Maintaining bone and dental health
Vitamin D can help regulate blood calcium balance. When blood calcium levels are sufficient, it provides adequate calcium for bone mineralization. When blood calcium levels decrease, it regulates the release of calcium from bones, thus maintaining the balance of blood calcium in the body[16]. Therefore, the FDA recommends the simultaneous intake of vitamin D and calcium, which can help reduce the risk of osteoporosis in older adults[17].
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Helping regulate blood sugar
Research suggests that vitamin D deficiency increases the risk of type 2 diabetes. Vitamin D deficiency can lead to reduced synthesis and secretion of insulin, potentially causing type 2 diabetes. Additionally, vitamin D can improve insulin sensitivity, aiding better blood sugar control[18].
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Anticancer effects
Recent research has found that vitamin D can help prevent cancer by inhibiting the growth of cancerous cells. Vitamin D deficiency has been linked to an increased incidence of cancer cell proliferation[19].
Currently, many studies suggest that vitamin D may be helpful in preventing COVID-19 infection, improving COVID-19 symptoms, and potentially aiding long COVID. However, more scientific research is needed to support these claims. The U.S. FDA, among others, still maintains a conservative perspective on this matter. Nevertheless, many studies on the role of vitamin D in relation to COVID-19 are ongoing, and more supportive evidence is expected in the future.
