You lace up your fastest sneakers, commit to a 6 a.m. sweat session, and never skip the cooldown stretch. But if your diet isn’t keeping pace? Hate to break it to you—your fitness results could be falling short.
Alex Larson, LD, RDN, a Minnesota-based sports dietitian who works with both beginners and elite athletes, says nearly everyone she coaches runs into the same problem: nutritional missteps that zap energy, slow recovery, and in some cases even increase injury risk.
The tricky part? These mistakes aren’t always obvious. Sometimes they’re harmless-seeming habits you don’t think twice about. Other times, they’re strategies you picked up from a workout buddy, a favorite spin instructor, or a well-meaning fitness influencer. The good news: they’re all fixable—and often, small tweaks can make a huge difference.
Here are six of the most common eating mistakes that undermine your workouts—and the simple fixes that can put you back on track.
1. Tackling Intense Morning Workouts on an Empty Stomach
Rolling out of bed and heading straight to the gym might feel efficient, but if you’re hitting weights or doing more than 45 minutes of endurance training, skipping fuel is a recipe for sluggish performance.
“Fasted training is one of the most common—and most destructive—habits I see,” says Alyssa Leib, MS, RD, a Colorado-based sports dietitian. Here’s why: muscles run on carbohydrates, and after an overnight fast, your glycogen stores are already low. Without fuel, your brain conserves energy by dialing down performance, leaving you fatigued and foggy.
Even more concerning: a 2024 study in the European Journal of Sport Science found that endurance athletes who trained fasted were 1.6 times more likely to sustain bone injuries than those who fueled up. Long-term, fasted training can also contribute to “Relative Energy Deficiency in Sport” (REDs), a condition that disrupts metabolism, bone health, and hormone balance.
Fix it: Pre-workout fuel doesn’t need to be complicated. Skip high-fiber and protein-heavy foods, and opt for quick carbs: a couple of dates, a drizzle of honey, or a small glass of juice if you’re short on time. Got 30–90 minutes before training? Try a banana with peanut butter on half an English muffin for a balanced boost.
2. Overloading on Fiber
Fiber is essential for long-term health (hello, reduced colon cancer risk), but piling on too much before a workout—say, a giant salad or a whole apple—can backfire with bloating, cramps, or bathroom emergencies.
Fix it: Save the high-fiber foods for post-workout meals. Pre-workout, stick with easy-to-digest carbs like toast, crackers, or a banana. Aim for 25 grams of fiber daily if you’re a woman, 38 grams if you’re a man—spread throughout the day, not front-loaded before class.
3. Prioritizing Protein, Forgetting Carbs
Protein is vital for muscle repair, but if you load up on it before cardio, you might actually slow yourself down. Protein digests more slowly than carbs, which means it’s not the best choice for quick fuel.
Fix it: For standard 45-minute workouts, lean on carbs for energy. For longer endurance sessions (like a two-hour run), combine carbs with a little protein—for example, a banana with peanut butter. Save the bulk of your protein for recovery meals later.
4. Demonizing Processed Foods
It’s true: in everyday life, highly processed foods don’t have the best reputation. But here’s the nuance—during workouts, some processed options are actually ideal. Think: sports drinks, dried fruit, or even pretzels. They digest quickly, are portable, and won’t upset your stomach mid-session.
Fix it: Differentiate between “performance fuel” and “daily diet.” A sports drink or gummy chews before or during a workout can be smart choices, as long as your overall diet is rich in whole foods like vegetables, whole grains, lean proteins, and healthy fats.
5. Skipping Post-Workout Refueling
Wait too long to eat after training, and you risk prolonging muscle soreness and stalling recovery. The first 60–90 minutes post-exercise is prime time for replenishing glycogen and repairing muscle tissue.
Fix it: Reach for a snack with both carbs and protein within an hour—like chocolate milk, a smoothie, or a turkey sandwich. If you’re headed straight to a meal, make sure it’s balanced with carbs, protein, veggies, and healthy fats.
6. Going Overboard on Caffeine
Yes, caffeine can improve endurance and focus, but there’s a line. Chugging multiple energy drinks or four cups of coffee before training can lead to dehydration, jitters, and electrolyte loss. Many pre-workout supplements also contain other stimulants—like taurine or guarana—with unclear long-term safety profiles.
Fix it: Stick to moderate caffeine intake—about one to two cups of coffee is plenty. And don’t forget: caffeine isn’t a substitute for proper hydration, carbs, and electrolytes.
The Bottom Line
Your workout doesn’t begin when you tie your laces or unroll your yoga mat—it starts in the kitchen. By fueling smarter, you’ll not only feel stronger in the moment but also recover faster, reduce injury risk, and get more out of every rep, mile, or sprint.
