Vitamin D may be an important way to arm the immune system against disorders like the common cold, report investigators from the University of Colorado Denver (UC Denver) School of Medicine, Massachusetts General Hospital (MGH) and Children's Hospital Boston.

In the largest and most nationally representative study of the association between vitamin D and respiratory infections, people with the lowest blood vitamin D levels reported having significantly more recent colds or cases of the flu. The risks were even higher for those with chronic respiratory disorders, such as asthma and emphysema. The report appears in the February 23 Archives of Internal Medicine.

"The findings of our study support an important role for vitamin D in prevention of common respiratory infections, such as colds and the flu," says Adit Ginde, MD, MPH, UC Denver Division of Emergency Medicine and lead author of the study. "Individuals with common lung diseases, such as asthma or emphysema, may be particularly susceptible to respiratory infections from vitamin D deficiency."

While vitamin C has been used for the prevention of colds and other respiratory disorders for decades, little scientific evidence supports its effectiveness. In contrast, in recent years evidence has accumulated that vitamin D – most commonly associated with the development and maintenance of strong bones – may also play a key role in the immune system. Circumstantial evidence has implicated the wintertime deficiency of vitamin D, which the body produces in response to sunlight, in the seasonal increase in colds and flu; and small studies have suggested an association between low blood levels of vitamin D and a higher risk of respiratory infections.