We often know that vitamin C increases the body’s resistance, while vitamin D is famous for its classic role in maintaining bone mineral density. Studies increasingly demonstrate the important role of vitamin D in strengthening the body’s immune system, especially against infectious diseases.
1. The relationship between vitamin D and infection
The relationship between vitamin D deficiency and susceptibility to infection has been of interest for more than a century. With the initial observation that malnourished children with rickets are more likely to have respiratory infections. The isolation of vitamin D3 from cod liver oil, used to treat tuberculosis in the 1930s, led to its widespread use in the treatment and prevention of tuberculosis, until the introduction of anti-infective chemotherapy in the 1950s. More recently, epidemiological studies have demonstrated a strong association between seasonal variations in vitamin D levels and the incidence of various infectious diseases, including septic shock, respiratory infections, and influenza.
2. Vitamin D deficiency increases the risk of infectious diseases
Recent studies have described high rates of vitamin D deficiency worldwide and vitamin D deficiency is reaching epidemic proportions in the global population. As our knowledge of the extraskeletal functions of vitamin D continues to grow, the benefits of maintaining vitamin D sufficiency become increasingly clear. Studies have shown an association between vitamin D supplementation and reduced susceptibility to infection across a variety of infectious processes. The strongest evidence (in the form of rigorous clinical trials) is for adjunctive vitamin D therapy for tuberculosis, influenza, and viral upper respiratory infections. Several studies have demonstrated that adequate vitamin D status can reduce the incidence of all-cause infections in the populations analyzed, and even improve mortality.
3. The Role of Vitamin D in Preventing Infections
Pioneering work by Rook and Crowle in the 1980s demonstrated that vitamin D increases the bactericidal activity of the human body against Mycobacterium tuberculosis, the causative agent of tuberculosis. This discovery led to a new era of interest in the role of vitamin D in pathogen identification and immune responses to bacterial pathogens.
Adequate vitamin D levels can maintain protection and effectively control pathogens such as fungal infections.
It is well known that it is the hyperresponsiveness of the host immune system, rather than the viral pathogen, that causes the clinical severity and mortality associated with viral diseases such as influenza. Vitamin D modulates cytokine profiles in animal models of autoimmune diseases by limiting the overproduction of proinflammatory cytokines, such as TNF and interleukin-12, and thus leading to inhibition of inflammation. In addition, the antimicrobial peptides cathelicidin and defensin, which are partially regulated by vitamin D, also play a key role in the immune defense of the respiratory system through direct neutralization of viral pathogens and enhancement of phagocytic events that kill viruses. Collectively, these studies support the hypothesis that optimal human vitamin D status may contribute to important immunoregulatory functions in the setting of respiratory viral infections by modulating the hyperresponsiveness of cytokines, while also allowing for improved clearance of infectious agents.
The immunomodulatory role of vitamin D in diseases caused by fungi, protozoa or parasites is still being studied. Adequate vitamin D levels in the body are also important for maintaining effective protection and control of various extracellular pathogens, especially parasites, protozoans and some fungal infections.
In summary, the role of vitamin D in regulating the body’s immune response to infectious agents is being elucidated through trials. Scientists also found that vitamin D supplementation is effective not only for the skeleton but also essential for the body.
