People who spend more time in the sun as children subsequently have a lower risk of developing multiple sclerosis (MS), a US study shows.
Some sun can be a good thing
The University of Southern California team suggest UV rays offer protection by altering the cell immune responses or by boosting vitamin D levels.
An earlier study found women who took vitamin D supplements were 40% less likely to develop MS.
The latest research is published in the journal Neurology.
MS is among the most common neurological diseases affecting around two million people worldwide.
However, it is more common at higher latitudes, which generally have lower levels of ultraviolet radiation - the type produced by the sun.
People in these countries are exposed to less sunlight, which triggers a chemical reaction in the body leading to vitamin D production.
For the study, researchers surveyed 79 pairs of identical twins who had the same genetic risk of MS.
In each pair, one of the twins had MS.
The twins were asked to specify whether they or their twin spent more time outdoors during hot days, cold days, and summer, and which one spent more time basking in the sun, going to the beach and playing team sports as a child.
The researchers found the twin with MS spent less time in the sun as a child than the twin who did not have MS.
Depending on the activity, the twin who spent more hours outdoors had up to a 57% reduced risk of developing MS.
The study authors, Dr Talat Islam and Dr Thomas Mack, said more work into how sun exposure reduces MS risk was now needed.
They suggested: "Exposure to ultra violet rays may induce protection against MS by alternative mechanisms, either directly by altering the cellular immune response or indirectly by producing immunoactive vitamin D."
Chris Jones, chief executive of the MS Trust, said: "This work supports past work suggesting a link between sun exposure and a lower risk of MS.
"But the researchers are certainly not suggesting people go out and get skin cancer. Exposure to the sun's rays can be dangerous."