It’s no secret that you need food to fuel your fitness—though what works best might vary by time and workout type. That said, when eaten before a sweat session, certain choices do more harm than good—we’re talking stomach pain, muscle cramps and lethargy.
So why does what you eat—and when you eat it—matter? “Working out is a challenge for your nervous system, and what you’re doing is giving your body something to have to respond to,” says Kelvin Gary, NASM-certified personal trainer, Precision Nutrition Coach, and owner of Body Space Fitness in New York City. When you eat before a workout, you’re making your body have to decide between an internal process (processing the food) and an external process (converting energy into work—i.e. pushing that weighted sled farther and faster). And the internal process is going to win out every time. “You can’t hit pause on digestion,” he says.
With that in mind, we rounded up the foods that might not only upset your stomach, but also hurt your performance. (As for what you should eat, this is the list to live by.) So, while many of the choices below are A-okay to eat in general, you may want to consider scratching them off the menu if your workout’s coming up.
Even if you typically eat dairy without a problem, you may not be able to handle it if you’ve had it too soon before a workout, says Tammy Lakatos Shames, RDN, CDN, CFT, author of The Nutrition Twins’ Veggie Cure. It not only takes longer to digest because of its protein content, but may also cause less-than-stellar reactions like cramps, discomfort and even diarrhea, she adds. Your best bet: Don’t have dairy two, even three hours prior to go-time.
Make no mistake: We’re all about high-fiber foods—fruits, veggies, chia seeds and flax seeds are all a part of a healthy digestive system and a good-for-you diet. That said, you may not want to load up on these choices just before hitting the trail, hopping in the ring, or getting yourself to the gym, says Lakatos Shames. Fiber gets the digestive tract moving (as does cardio exercise), she explains, meaning you might end up having to hightail it to the restroom—and interrupt your workout. Plus, since the blood in the stomach is working hard to process the fiber in these foods, this can actually cause indigestion, adds Lakatos.
Though you might feel like booking it to the gym to burn off the calories of a burger or Chinese food, you may want to hold off. Problem is, these meals are high in fat and take a lot of time to digest, and if you work out soon after, you’re making your body compete with itself for its blood supply, explains Lyssie Lakatos, RDN, CDN, CFT and author of The Nutrition Twins’ Veggie Cure. Sadly, this is also true for healthy fats like avocados, olive oil and nuts in large quantities—though fried foods and double bacon cheeseburgers are worse, she says. The less-than-ideal result? Muscle cramps.
Not only that, but filling up on fatty foods may make you feel like you’re running on empty. One 2013 study published in the online supplement of the journal SLEEP suggests that the more high-fat foods you eat, the more tired you feel. Yet another study—this one in rats—published in The Journal of the Federation of American Societies for Experimental Biology finds that a high-fat diet leads to worse exercise performance. In the 2009 study, rats that were fed the high-fat diet cut their running distance on a treadmill by 35 percent.
When it comes to green juices and how they’ll affect your performance, it all boils down to timing. If you’re sipping on the stuff a few hours before checking in at the spin studio and treating the juice as a meal, you’re not going to be getting enough carbs to fuel any sort of long, intense workout, say Lakatos and Lakatos Shames. On the other hand, drinking the juice as a light snack about an hour or so before exercise is completely fine since it doesn’t take long to digest, they add. But take note: You may end up bloated or with an upset stomach due to the fiber content—though this applies more to blended green drinks rather than juiced ones—or the sulfurous veggie content.
If you’re the kind of person who likes to immediately head to the gym to burn off an indulgence, you should know that grabbing a Snickers bar isn’t going to help you power through your workout. You’re better off waiting until later in the day. Sure, the sugars in candy will give you a quick initial boost, but you’ll end up crashing mid-exercise, says Gary. And it all boils down to how the body processes sugar. The second the sweet stuff hits your bloodstream, your blood sugar spikes, causing an immediate sugar high, explain Lakatos and Lakatos Shames. In turn, your body kicks out insulin to shuttle the sugar out of your blood and into your tissues. Once the sugar’s been expelled from your bloodstream, your energy levels crash. Plus, since exercise itself sends the sugar in your blood to the muscles, some might experience a double whammy of an energy crash.
Protein bars get a bad rap—and rightfully so, considering many of them are just glorified candy bars. But it’s hard not to reach for them when you’re short on time and looking for something to fuel your workout. Two things to look for: bars that are too high in sugar or don’t have a good proportion of carbs. If it’s high in protein (think 10 grams or more), it’ll sit in the stomach and won’t provide the fuel you need (i.e. carbs) to power through all your reps, say Lakatos and Lakatos Shames. And if it contains a mountain of sugar, you’re headed for a crash, they add. If you’re looking for a bar to nosh on before the gym, they both recommend the KIND Healthy Grain bars. With at least 18 grams of whole grains per bar, they provide the energy you need to log that extra mile.
Both the bubbles and sugar in carbonated drinks (and in juices and flavored waters for that matter) can lead to an upset stomach, says Lakatos. Plus, like candy bars, they contain sugar that can lead to an energy high, followed by a prompt—and not so pleasant—crash.