Also...””A geographic association with northern latitudes has also been observed for multiple sclerosis and Crohn’s disease, other autoimmune diseases that may be mediated by reduced vitamin D from decreased solar exposure and the immune effects of vitamin D deficiency,” the authors wrote. “
Although there has been some link between rates of MS and northern latitudes (higher incidences in Minnesota, for instance), it certainly doesn’t explain the higher numbers in Colorado, which gets over 300 days of sunshine a year.
At the latitude of Boston there are four months of the year when you can't make any vitamin D from sunlight. Somewhere in S. Carolina is where you can year round. You pretty much have to be in the sunlight in the middle of the day to make vitamin D, during those months that it's possible to make vitamin D from sunlight. And with good skin exposure.