Researchers at the Karolinska Institute, in a large case-control study of over 1000 people with MS, have examined the risk of developing MS in relation to sun exposure habits and vitamin D levels.
Confirming the huge literature now on the increased risk of MS with low levels of exposure to sunlight, they showed roughly a doubling of the risk (2.2 times) for those with the lowest levels of reported sun exposure compared with those with the most sun exposure.
Interestingly, they also looked at relationship with vitamin D levels, and while they found an increased risk of Multiple Sclerosis with lower D levels, the effect was not as strong (1.4 times) as the effect for sunlight.
This implies that there are other protective factors derived from sun exposure than just vitamin D. As argued on p130 of the book Overcoming Multiple Sclerosis, there are probably several ways in which vitamin D works to prevent MS than just through vitamin D.
This reinforces the need for people with MS to ensure that they and their relatives get regular low-dose sun exposure when possible rather than just rely on vitamin D supplementation.