People who have the most vitamin D in their blood are the least likely to suffer from colds and the flu, researchers said.

A team from the University of Colorado Denver School of Medicine and Massachusetts General Hospital interviewed almost 19,000 people and took blood samples to measure their vitamin D levels.

Those with the lowest levels reported having significantly more recent colds or cases of the flu, according to a news release about the work.

While many people think that vitamin C fights colds, the researchers said there is little scientific evidence it works.

Vitamin D levels tend to drop in the winter, because sunlight helps the body produce it. Winter is also a common time for respiratory infections such as a cold or the flu. But the study found that the results were true regardless of season.

Researchers said the knowledge that vitamin D fights the cold and flu could be especially important for those with other breathing problems such as asthma or chronic obstructive pulmonary disease, known as COPD.

They noted that the results were not based on a true, randomized experiment with controls, so more study is needed.