The immune system is a complex, yet efficient, barrier against harmful pathogens trying to enter the human body. The immune system’s first line of defense is made up of external barriers that protect the body, primarily the skin. The second line of defense is much more specialized, and is triggered by the entrance of foreign cells into the body. One important component of the second line of defense is innate, or nonspecific, immunity. The innate immune system attacks and destroys these pathogens that break past the first line of defense.
The respiratory system relies on the immune system to protect the airway, as the oral and nasal cavities are both easily accessible spots for pathogens to enter. The trachea is lined with a mucous layer called airway surface liquid (ASL), which is meant to protect the respiratory tract from infection. A major component of ASL are antimicrobial peptides, which are part of the innate immune response against foreign cells and pathogens.
Vitamin D regulates the production of antimicrobial peptides, leading researchers to theorize that vitamin D status may play a role in respiratory tract infections. There has been an abundance of research that has linked low vitamin D status to increased occurrence of respiratory infections. Additionally, vitamin D supplementation has been shown to decrease the frequency of upper respiratory tract infections. Yet, there has been no research to date that has evaluated the role of vitamin D supplementation on ASL in healthy individuals. Therefore, researchers decided to explore the effect of vitamin D supplementation on lung immunity in a single-centre, community-based, randomized, placebo-controlled double-blind trial.