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In the largest study yet 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.
Vitamin C has been used for the prevention of colds for decades, but little scientific evidence supports its effectiveness. In contrast, evidence has accumulated that vitamin D plays a key role in the immune system.
The wintertime deficiency of vitamin D, which the body produces in response to sunlight, has been implicated in the seasonal increase in colds and flu, and previous small studies have suggested an association between low blood levels of vitamin D and a higher risk of respiratory infections.