Endurance athletes love numbers. Paces, heart rate, VO2 max—we track it all. But when it comes to fueling, too many runners get stuck on one number: calories. The problem? Calories alone don’t tell you whether your fuel is actually doing its job.
Carbohydrates are your body’s preferred fuel source for running. They’re like gasoline in a race car—without enough, you’ll sputter out long before the finish line. But I see it all the time: runners choosing low-carb energy gels or sports drinks because they assume “calories = energy,” without considering what kind of energy they’re getting. And then they wonder why they’re slowing down.
Let’s talk about why this happens and how fixing your fueling strategy could completely change your performance.
The Fuel Lag Effect: Today’s Mistakes Show Up Tomorrow
Most runners think fueling is about this workout or this race, but your body doesn’t reset overnight. If you underfuel a long run, your muscles start the next day’s session already depleted. You might not notice it right away, but over time, that deficit adds up.
The result? Cumulative fatigue, slower recovery, and workouts that feel way harder than they should. If you’ve ever felt like you’re grinding through training even though you’re hitting the right paces and recovery days, look at your fueling. What you eat today determines how strong you feel tomorrow.
Underfueling Makes You Feel Slower—Not Just Perform Slower
Have you ever had a run where everything felt like a struggle, even though your splits looked fine? That’s not just fatigue—it’s your brain slamming on the brakes.
When your carb intake is too low, your brain perceives running as harder, even when your body could keep going. This is your central nervous system protecting you from running out of fuel. If you’re constantly underfueling, running will feel like an uphill battle, even on fresh legs.
This is where so many runners go wrong. They think they’re “out of shape” when really, their brain is just responding to an energy crisis. Imagine putting the wrong type of fuel in a race car and then blaming the engine for not running smoothly. It’s not the engine—it’s the fuel.
Your Gut Needs Training, Too
Here’s something no one talks about: if you don’t train with carbs, your body forgets how to use them efficiently.
Some athletes try to “save” carbs in training, then dump 90 grams per hour into their system on race day, expecting everything to go smoothly. But just like your legs need training, your gut does, too. If you don’t practice fueling, your stomach won’t know what to do with it. Instead of giving you energy, it’ll sit there like a brick, causing bloating, nausea, or GI distress.
This is why runners who underfuel in training often struggle on race day, even if they try to do things “right” when it counts. Your body has to be taught how to absorb and use carbs efficiently. The good news? You can train this just like any other part of running.
What You Can Do Now
Start by looking at your current fueling. Are you choosing gels or drinks based on calories instead of carbs? Are you only taking in 10-15g of carbs per hour when your body could use 30-90g? Have you actually trained your gut to handle more fuel?
Small changes make a massive difference. More carbs mean more energy, stronger workouts, and a body that adapts to training instead of breaking down from it.
If fueling has been an afterthought, it’s time to change that. Let’s build a strategy that actually supports your running. Work with me, and let’s get this dialed in so you can run stronger, recover faster, and stop wondering why your training isn’t clicking.
Your next PR might not come from running harder—it might just come from fueling smarter.