A recent study conducted by researchers at Harvard-affiliated hospitals and the University of Colorado found that people with low levels of vitamin D are 40 percent more likely to report respiratory infections such as colds and flu.

The study analyzed data from the Third National Health and Nutrition Examination Survey, in which 19,000 participants were tested for their blood levels of 25-hydroxyvitamin D—a precursor of the vitamin that is considered the best measure of its concentration. Study participants with the lowest vitamin D blood levels were found to be much more likely to report having a recent respiratory infection. This result was even more pronounced in those with a history of asthma or chronic obstructive pulmonary disease.

Jonathan M. Mansbach—one of the co-authors of the study and an assistant professor of pediatrics at Harvard-affiliated Children’s Hospital in Boston—said that although vitamin D is commonly associated only with the development and maintenance of strong bones, it should be seen as a compound that human immune systems need in order to function properly.

Mansbach said that this study is by no means conclusive and said that further research will be necessary to confirm the finding.

“We have found a strong association between vitamin D deficiency and respiratory infection,” he said. “However, association does not imply causation. vitamin D does not cure all colds and flu.”

Vitamin D is produced in the skin when it is exposed to the sun, particularly UVB rays. It can also be acquired through certain foods such as fortified milk and dietary supplements.

Lona Sandon, national spokesperson of the American Dietetic Association, says that she believes this study is just one piece of the puzzle in determining the role of vitamin D. “So far we’ve only had observational data,” said Sandon, who is also an assistant professor of clinical nutrition at the University of Texas Southwestern Medical Center. “I also would love to see more clinical trials done for this study to test the effectiveness of vitamin D to boost immunity and fight respiratory infection.”

Sandon, a registered dietician, said she has always recommended that her patients spend at least 15 minutes a day in the sun and consume foods rich in vitamin D.