An estimated 2.5 million people in the world have MS but it is much more common in women than men - at a ratio of 2 men to 3 women, according to the Multiple Sclerosis International Federation.
The cause of the disease is unknown and without cure. It is thought to be a combination of environmental factors that trigger a reaction in the bodies of people with a genetic predisposition. The combination causes the immune system to malfunction and to attack its own myelin (the protective layer around nerves), altering the way messages, or nerve impulses, are conducted to and from the brain. This in turn disrupts functions of the body.
Researchers do know that the number of cases of MS increases the farther you get from the equator. This led to the hypothesis that sunlight exposure and high levels of vitamin D may reduce the risk of MS, said study author Kassandra Munger from the Harvard School of Public Health in Boston. Food is a source of vitamin D but the body also makes vitamin D through exposure to sunlight.
"This is the first prospective study to look at this question," she said. "The results need to be confirmed with additional research, but it's exciting to think that something as simple as taking a multivitamin could reduce your risk of developing MS."
The researchers examined data from two large studies involving women, the 20-year Nurses' Health Study and the 10-year Nurses' Health Study II. The women's diets and use of multivitamin supplements were assessed as the studies began and then again every four years. Women with MS symptoms starting before beginning the study were not included in the results. There were 187,563 women in the study. Of those, 173 women developed MS during the study period.
The women were divided into groups based on vitamin D use. Those with the highest intake of vitamin D from supplements (400 IU or more per day) were 40 per cent less likely to develop MS than those who used no supplements, reports the team in today's issue of Neurology (62: 60-65).
The risk of developing MS was lower both for those high in intake of vitamin D from supplements only and for those high in intake from both supplements and food. However, those whose intake of vitamin D was from food only did not have any lesser risk of developing MS.
The researchers also analysed the data while adjusting for smoking and latitude at birth, but the results did not change.
Since supplemental vitamin D intake was mainly from multivitamins, it is difficult to isolate the effects of vitamin D from the potential effects of other vitamins found in multivitamins, noted Munger.
"However, none of these vitamins was itself significantly associated with risk of MS after adjusting for total vitamin D intake or vitamin D from supplements," she added.
Earlier research on mice with experimental autoimmune encephalomyelitis (EAE), an autoimmune disease in animals that is used as a model of MS, found that vitamin D supplements can prevent or favourably affect the course of the disease. Other studies have shown that people with MS tend to have insufficient levels of vitamin D, and that periods of low vitamin D occur before times of high disease activity, and periods of high vitamin D precede times of low disease activity.
Munger said that future prospective studies should measure the levels of vitamin D in the blood prior to the onset of MS.
Yesterday we reported that the vitamin could be implicated in risk of rheumatoid arthritis, another autoimmune disease.