People who have spent more time in the sun and those with higher vitamin D levels may be less likely to develop multiple sclerosis (MS), according to an Australian study.

Previous studies have shown that people living close to the equator are less likely to get multiple sclerosis than those at higher latitudes, a difference that may be explained by more sun exposure and higher vitamin D levels.

According to a report in "Neurology," Robyn Lucas of The Australian National University and colleagues studied 216 adults who have just started having the first symptoms of MS between 2003 and 2006.

They also found a comparison group of nearly 400 people from the same regions of Australia, who matched the subjects in age and gender, but had no signs or symptoms of MS.

Participants in both groups were asked how much time they had spent in the sun and where they had lived at different points in their lives, with skin damage from the sun and the level of vitamin D in their blood also checked.

On average, people with the first signs of MS had been exposed to a smaller "UV dose" -- based on how much time they had spent in the sun and how close to the equator they had lived -- over the course of their lives.

People with early MS were also less than half as likely to have high levels of skin damage caused by sun exposure, with vitamin D levels 5 to 10 per cent lower than those without MS.

"Our study is the first to be able to look at both sun exposure and vitamin D status right at the very first symptoms that might precede development of MS," Lucas told Reuters Health in an e-mail.

The latest study does not prove that being exposed to very little sunlight or having low vitamin D levels causes MS, and while the authors tried to show that both sun exposure and vitamin D levels influence risk of MS on their own, some experts were doubtful that this was possible.

"They may have independent roles, but the reality is it's extremely difficult to sort them out," said Alberto Ascherio, who studies the link between vitamin D and MS at the Harvard School of Public Health and was not involved in the current study.

He noted that the authors didn't know the participants' blood levels of vitamin D over the course of their lives, and that it's possible that measuring somebody's sun exposure over the years is really just another way of measuring how much vitamin D they had at those times.

Since sun exposure is associated with a higher risk of skin cancer, more time outside is not always better. Nor do the results mean that everybody should load up on vitamin D, health experts said.

The main message of the study, Lucas said, is that "small amounts of sun exposure... occurring frequently, are probably optimal both for maintaining vitamin D levels and for other health effects." – Reuters