Higher sun exposure during childhood and early adolescence may reduce the risk of developing multiple sclerosis, research suggests.
Sun exposure may not be entirely bad
Multiple sclerosis is more common at higher latitudes, which generally have lower levels of ultraviolet radiation - the type produced by the sun.
Researchers in Tasmania surveyed 136 MS patients, and 272 people who did not have the condition.
All the participants were asked about how much they had exposed their skin to the sun in the past, and what measures they had taken to protect themselves.
The researchers found people whose skin had been exposed to higher amounts of sun during the ages of six to 15 showed greater signs of skin damage - but a lower risk of MS.
But the key seemed to be exposure to sun during the winter, rather than the summer.
Writing in the British Medical Journal, the researchers suggest MS may be linked to insufficient exposure to ultraviolet radiation, or the vitamin D which this exposure generates.
Early exposure crucial
But they found no link between MS and sun exposure in the decade before the onset of symptoms.
This, they suggest, indicates exposure to sunlight may only have an effect on the chances of developing the disease if it takes place at a time when the immune system is still developing.
MS it thought to be an autoimmune disease caused by specialised T helper 1 cells attacking myelin - the insulatory material which sheathes the nerves.
Researcher Dr Terry Dwyer said vitamin D had been show to reduce the activity of these rogue cells.
He said most people were probably exposed to enough sun during the summer to generate the protective effect.
But it was possible there could be a problem during the winter when levels of sun exposure were so low.
He told BBC News Online: "Only one hour of winter sun exposure during weekends and holidays was sufficient to confer most of the risk reduction our study observed."
Skin cancer concern
Many health experts are concerned that unprotected exposure to the sun, particularly during the summer, is leading to rising levels of skin cancer.
But there is some scientific evidence to suggest rickets is returning in some UK Asian communities, possibly because strict dress codes prevent children from being exposed to sufficient sun to generate bone-strengthening vitamin D.
Mike O'Donovan, chief executive of the MS Society, said: "This is an interesting study.
"Other researchers are already looking at whether vitamin D could be helpful in the treatment of MS."
The MS Trust issued a statement, which said: "This case study usefully adds to the growing body of knowledge about the relationship between sunlight, vitamin D and multiple sclerosis."