You slip on the fastest sneaks, consistently drag your butt to 6 a.m. spin class, and never skip post-exercise stretching. But if your eating habits aren’t on point? Then sorry, but you’re leaving fitness results on the table.
Alex Larson, LD, RDN, a Minnesota-based registered dietitian nutritionist, works exclusively with endurance athletes, and nearly everyone she’s coached—from beginners to expert-level competitors—“has had gaps in their nutrition that affected performance, energy, or recovery,” she tells SELF. Even more concerning, certain fueling mishaps can increase your chances of injury and even harm your overall health.
These mistakes aren’t always glaring. Sometimes they’re small, innocuous-seeming habits you may not even be aware of. Other times, they’re actions you intentionally take because someone—maybe your workout buddy, fave spin instructor, or a GymTok influencer—erroneously extolled their virtues. In either case, take heart: These missteps are completely fixable! In fact, making a few small tweaks to your fueling habits “can make a big difference,” Larson says.
Read on for some of the most common nutrition mistakes sports dietitians see that wreak havoc on people’s workouts—plus what to do instead.
1. You don’t eat before tough morning sessions.
There are plenty of reasons why people roll out of bed and work out on an empty stomach, including feeling strapped for time, being worried about GI discomfort, or (falsely) believing it’ll improve their performance. And while fasted exercise may be okay for short or low intensity morning workouts—think: light 20 to 30 minute resistance work, yoga, or walks—if you’re doing heavy lifting sessions and endurance exercise lasting 45 minutes or more, you really should prioritize fueling first, Kelly Jones, MS, RD, CSSD, a board-certified specialist in sports dietetics and owner of Student Athlete Nutrition, tells SELF.
Unfortunately, a lot of people have missed this memo. In fact, jumping into a workout on empty is the most popular—and problematic—fueling mistake that Colorado-based sports dietitian Alyssa Lieb, MS, RD, sees people make.
So why’s it so bad? For one, when you exercise, your muscles need fuel to perform their best, and carbs are the best source of that fuel. The thing is, your body can only store a certain amount before you need to replace them by eating more. And if you haven’t eaten in many hours (you know, like…while you’re sleeping), it doesn’t take long to deplete that stash.
When that happens, your brain recognizes that you’re low on energy, and to conserve it, it reduces your ability for high-intensity movements, Jones says. The result? You feel sluggish, fatigued, and your performance tanks, Larson explains.
More concerning, though, is that fasted workouts are linked with an increased risk of bone injuries. One 2024 study, published in the European Journal of Sport Science, found that endurance athletes who practiced fasted training were 1.61 times as likely to have a bone injury than athletes who ate beforehand. What’s more, if you don’t fuel up before hitting the gym, you’re missing out on an important window to get calories in. Research shows that people who do fasted workouts end up eating fewer calories over the course of the day. Taking in less cals than your body needs can increase the risk of issues like relative energy deficiency in sport (REDs, where your body doesn’t have enough energy to function properly), Lieb says. In short, fasted workouts can not only sabotage today’s gym sesh—they can also set you up for some more concerning longer-term health issues.
