MS is a chronic neurological disorder that affects the central nervous system, and while the exact causes of the disease are not known, researchers believe that environmental, viral, and genetic factors may each have an effect on developing the condition. Onset typically happens between the ages of 20 and 50.

The connection between vitamin D and MS has been suggested for at least three decades. Vitamin D is crucial for a healthy immune system, and a lack of the vitamin has also been linked to cancer, high blood pressure, and other medical problems. The most significant source of vitamin D is sunlight–exposure to the sun spurs the human skin into producing the vitamin. Not surprisingly, MS is more prevalent in regions where sunlight is sparse, a fact that first led scientists to investigate the correlation between the disease and the vitamin. Vitamin D is also found in smaller amounts in foods such as oily fish, egg yolks, and liver. Much of the milk sold today is fortified with vitamin D, a policy that was started to help eliminate rickets.

The study was conducted through the examination of the medical records and blood samples of 7 million U.S. military personnel. The conclusions were based on the cases of 257 people who had developed the disease from 1992 through 2004. Researchers also note that their results only apply to whites; data related to blacks and Latinos were insufficient to draw a correlation. Dr. Alberto Ascherio of the Harvard School of Public Health, one of the study’s authors, called for a large, controlled, randomized trial in order to collect more detailed information about the connection.