According to researchers at University of Wisconsin-Madison, ultraviolet portion of sunlight plays a bigger role than vitamin D in controlling multiple sclerosis (MS).
For more than 30 years, scientists have known that multiple sclerosis (MS) is much more common in higher latitudes than in the tropics.
Because sunlight is more abundant near the equator, many researchers have wondered if the high levels of vitamin D engendered by sunlight could explain this unusual pattern of prevalence.
Vitamin D may reduce the symptoms of MS, but it is the ultraviolet portion of sunlight that has a major role in controlling MS, according to Hector DeLuca.
The ultraviolet (UV) portion of sunlight stimulates the body to produce vitamin D, and both vitamin D and UV can regulate the immune system and perhaps slow MS.
But researchers were not clear of the immune regulation result directly from the UV, indirectly from the creation of vitamin D, or both.
The study was designed to distinguish the role of vitamin D and UV light in explaining the high rate of MS away from the equator, said DeLuca, a world authority on vitamin D.
"Since the 1970s, a lot of people have believed that sunlight worked through vitamin D to reduce MS. It's true that large doses of the active form of vitamin D can block the disease in the animal model. That causes an unacceptably high level of calcium in the blood, but we know that people at the equator don't have this high blood calcium, even though they have a low incidence of MS. So it seems that something other than vitamin D could explain this geographic relationship," says DeLuca.
Using mice that are genetically susceptible to MS-like disease, the researchers triggered the disease by injecting a protein from nerve fibers.
Then the mice were exposed to moderate levels of UV radiation for a week. After they initiated disease by injecting the protein, they irradiated the mice every second or third day.
The UV exposure (equivalent to two hours of direct summer sun) did not change how many mice got the MS-like disease, but it did reduce the symptoms of MS, especially in the animals that were treated with UV every other day, said DeLuca.
The team also found that although the UV exposure did increase the level of vitamin D, that effect, by itself, could not explain the reduced MS symptoms.
In some situations, radiation does reduce immune reactions, but it's not clear what role that might play in the current study.
"We are looking to identify what compounds are produced in the skin that might play a role, but we honestly don't know what is going on. Somehow it makes the animal either tolerate what's going on, or have some reactive mechanism that blocks the autoimmune damage," said DeLuca.