I can see why people would be a little concerned about these findings, but it's too early to know whether they are significant or not. Firstly, the research hasn't yet been peer reviewed. When the authors try to publish this, the peer review process will appraise the methodology and significance of the results. Then we will know more. Secondly, it is hard to imagine the numbers in the group who had high vitamin D levels (>250nmol/L) were very large, even though the overall study numbers were large. After all, in the middle of this vitamin D epidemic, how many Americans will have abnormally high levels? A small number clearly. If the numbers in that group are small, it may reduce the significance of the finding. And who would the people in this group be? Typically, those with illnesses trying to improve their condition by taking supplements (it would be very hard to get to that level without supplements). So the intercurrent illnesses suffered by this group (for instance cardiovascular disease) might be confounders that could explain the higher incidence of atrial fibrillation. Thirdly, as one of you has suggested, what mechanism could explain such a finding? Although we know vitamin D plays a role in cardiovascular disease, generally it is protective. So, all in all, we should await the detail in the published paper (assuming it gets through peer review) before coming to any conclusion. In the meantime, we have always suggested here that it is sensible if you have MS to aim for a level that is high normal, over 150nmol/L, but preferably not above the upper limit of the normal range (75-225nmol/L).