You’ve probably heard a lot (either from me or other sources) about the benefits of vitamin D: decreased heart disease risk, better blood sugar control, decreased risk of autoimmune disease, less chronic pain and depression…. just for starters.
However, anything can have its downside, and researchers at the American Heart Association found that excess vitamin D levels can lead to greater risk of an abnormal heart rhythm called atrial fibrillation. This is a serious condition that might precipitate heart muscle damage or clots inside the arteries and veins.
The probable cause for this is that with more vitamin D, the body can absorb more calcium. While calcium is good for bones, too much calcium can affect nerves and muscles — including the heart.
How much is too much according to this study? Greater than 100 ng/ml of serum 25-hydroxyvitamin D.
This is why I recommend that nearly every patient get blood levels tested for 25-hydroxyvitamin D — not only to check for deficiency, but also to monitor for excess. I recommend an optimal range of 40-100 ng/ml, but based on this research, it seems prudent to bring the top end of that range down a bit to be on the safe side.
At this time of year, at any latitude north of Virginia, the sun’s rays are not strong enough to produce any vitamin D in human skin. This a good time to get your levels checked, to be sure you’re set for the winter. And if you’re taking a supplement greater than 2000 IU per day, get your levels checked every 3-4 months to monitor for excess.
By the way, this article claims that 25-hydroxyvitamin D levels of 41-80 ng/ml are “far above what most doctors suggest.” This is an inaccurate statement, since every standard reference laboratory in the country puts the top end of the range between 80 and 100 ng/ml.