As you know, there are a lot of things that can mess with your workout. You could not get enough sleep the night before, you could exercise on an empty stomach, or you could simply do the moves wrong, to name a few. Well, here is the latest fitness faux pas to add to your list: taking a vitamin D2 supplement. According to a new study from Appalachian State University, published in the journal Nutrients, doing so could damage your muscles bigtime.

For the study, researchers divided 28 NASCAR pit crew athletes into two groups. Half of the group took a vitamin D2 supplement for six weeks, and the other half took a placebo pill. After six weeks, the scientists had both groups do 90mins of intense exercise, and then took their blood samples (they'd also taken samples at the start of the study, before the participants took the supplements at all). The results? After the exercise session, participants who'd taken the vitamin D2 supplement had higher muscle damage than those who didn't take the pill. And even though the participants in the study were all male, lead study author David Neiman, PhD, says that the results can apply to females as well.