Ask a smart personal trainer how to give yourself a pre-workout boost to push harder during HIIT, and many will have a common answer: coffee. But while caffeine has long been known for its ergogenic (a fancy science term for performance-enhancing) effects, a new study suggests that depending on your genes, it might actually slow down your speed in spin class.

Here’s the gist: scientists have known for a while that individuals metabolize caffeine differently—fast, moderately, or slowly—depending of which variation of the CYP1A2 gene they have. In fact, it’s one of the results delivered in most of the trendy at-home genetic tests that have been popping up lately. But how those variations of the gene might affect how caffeine impacts an individual’s sweat session was unknown. In other words, so you find out your body processes caffeine at a sprint, what would that do for your sprint?

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To find out, researchers compared the workout performance of 100 young male athletes who pedaled on stationary bikes after consuming both caffeine and a placebo.  Participants who were fast caffeine metabolizers (as measured by a genetic test) pedaled seven perfect faster when they had caffeine and moderate metabolizers performed almost exactly the same whether they had caffeine or a placebo.

The biggest surprise? Slow caffeine metabolizers pedaled 14 percent slower when they had caffeine compared to the placebo. It didn’t just not give them a boost; it slowed them down.

According to the New York Times, the researchers theorized that it’s probably because caffeine can cause blood vessels to narrow. So while fast metabolizers got a quick gush of energy from the caffeine, which was then quickly cleared out of their bodies, “caffeine lingered in the slow metabolizers, narrowing their blood vessels and reducing the flow of blood and oxygen to tiring muscles.”

So, should you get a genetic test to find out whether coffee might be your workout friend or foe? It’s important to remember that this was a very small study that looked at one very specific demographic (young, male, athletes) and at one specific type of exercise (indoor cycling). But if you always drink coffee before hitting the treadmill and tend to feel like you’re dragging your feet, why not try going without and see if you feel any different?

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