In the last three decades, the importance of vitamin D for many functions in our bodies has become increasingly recognized. In 2006, vitamin D was shown to be involved with the activation of or innate immune system for fighting infections. Supporting this discovery, lower levels of vitamin D have recently been associated with increased risk of respiratory infection.
Young Finnish men are required to complete at least 6 months of military service. 800 of these men serving on a military base in Finland were enrolled in a study to determine whether low blood levels of vitamin D are associated with acute respiratory tract infections.
The men with low blood levels of vitamin D missed almost twice the amount of duty due to respiratory infections as men with sufficient levels of vitamin D. The association was statistically significant. The authors corrected for the effect of smoking. Smokers had lower vitamin D levels and a 20 percent higher rate of absence from repiratory tract infections than non-smokers.
Note: Other important nutrients for immune function are vitamin E, vitamin A, vitamin C, vitamin B6, selenium and zinc. Iron is also required for healthy immune functions. However with the exception of women of childbearing years, some children and endurance athletes, most people have sufficient levels of iron without supplementation.
Laaksi and others. "An assocation of serum vitamin D concentration < 40 nmol/L with acute respiratory tract infection in young Finnish men." Am J Clin Nutr. 2007;86:714-7.