Hank: In the bodybuilding and fitness world, everyone’s got their special diet and exercise pro-tips to try and build muscle. Like, some people swear by their pre-workout supplements. These supplements might contain things like caffeine, taurine, beta-alanine, creatine, things that end in ‘een’, chemicals that can make you more alert and get your blood pumping.

There’s a little bit of evidence that certain ingredients under certain conditions can help you build muscle. But other research suggests that supplements might just make you feel like you’re working harder and makin’ those gains. Not to mention, that pumped-up feeling might come with a weird nervous system side effect: skin-tingling.

This tingling sensation is called paresthesia, which you’ve probably experienced after trapping your arm or leg in an awkward position while you’re, like, marathoning The X-Files or after sitting on the toilet for too long and then you get up and you’re like ow-ow-ow-ow.

It’s that mildly itchy, burning, or prickly “pins and needles” feel. And it’s kind of awful. The culprit seems to be an amino acid in lots of pre-workout supplements called beta-alanine, which your body can use to make a molecule called carnosine, which hangs out in your skeletal muscles.

When you’re pushing your muscles really hard, your body is trying to convert sugars into energy as fast as possible. And when there’s not enough oxygen to go around, your cells start to make a lot of lactic acid, which can build up and cause you to “feel the burn.”

Now, researchers think that carnosine helps counteract all that acid build up in your muscles. So it kind of makes sense that beta-alanine is in some pre-workout supplements, to theoretically help you push through an intense workout and get swole.

But beta-alanine doesn’t just affect your muscles – it likes to hang out with your nerves, too. Your skin is full of nerve endings, to help you feel things like temperature, pressure, and pain when you high-five someone too hard. And researchers think that beta-alanine can bind to certain receptors on some of these nerves, stimulating them to cause that uncomfortable itch.

Scientists still haven’t figured out what specific molecular mechanisms might cause paresthesia here, but they have noticed that there seems to be a dose-dependent response. In other words, when someone ingests more beta-alanine, or gets more injected through their skin, the tingly side-effect gets worse. That might explain why some people feel skin-tingling when they take a workout supplement while others don’t – different people might be more or less tolerant of a set dose.

So don’t get too worked up if a supplement’s “Get Ripped Quick” claims don’t work for you... but they’re generally safe, as long as you don’t mind a little tingling sensation under your skin.

Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming. If you’d like to submit questions to be answered, or get some videos a few days early, you can go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!