Why You Bonk at Mile 20 (Even Though You Ate)

Your body has a fuel tank. It's called glycogen, which is stored carbohydrates in your muscles and liver.

Most people can hold around 400–500 grams worth, give or take depending on your size and how well you carb-loaded.

That sounds like plenty for a marathon. It's not.

At marathon effort, not only does your body burn through carbohydrates at a rate that would drain that tank well before the finish line, but performance starts falling apart long before the tank is truly empty (research suggests the average runner will hit "the wall" around 75–90 minutes without external fuel).

Your body does burn fat alongside carbs, which helps extend the range, but fat can't keep up with the demand at marathon pace.

As your stored carbs drop and blood sugar falls, your brain starts pulling the emergency brake, which is "the bonk" you hear about — it's not just tired legs, it's your central nervous system shutting things down to protect you.

And the brutal part: you don't get a low fuel warning. You feel fine, fine, fine… and then suddenly you don't. Your legs turn to concrete, your pace craters, and no amount of willpower fixes it because there's nothing left to burn.

This is where the importance of external carbs comes into play.

Here's where it gets frustrating. You probably did eat something out there. A gel at mile 10. Maybe another at mile 15. You did what you were supposed to do. And you still hit the wall.

The problem isn't that you didn't fuel. It's when you fueled.

Most runners eat reactively. You wait until you start feeling a little flat, maybe a little hungry, and that's your cue to grab a gel. Makes sense, right? Eat when you need it.

Except your body doesn't work that way.

When you eat something mid-run, it doesn't hit your bloodstream immediately. It has to get digested, absorbed through your gut, and transported to your muscles. That process takes 15–30 minutes, sometimes longer if your stomach is already stressed from the effort.

So when you feel that first dip at mile 16 and tear open a gel, you're not solving the problem. You're placing an order that won't arrive until mile 19. By then, you're already in the hole — and you can't climb out.

This is the trap. Fueling at mile 10 feels proactive, but if you started the race with nothing and waited until mile 10 for your first calories, you spent the first 90+ minutes burning through your tank with no deposits coming in.

You weren't fueling. You were chasing.

Proactive fueling flips the script. Instead of eating when you feel like you need it, you eat on a schedule — early and often, before your body ever sends a distress signal.

Here's how it breaks down:

Start at 15 minutes. Not mile 5. Not when you "settle in." Fifteen minutes into the race, you take your first fuel. This feels ridiculous. You're fresh, your legs feel great, and eating is the last thing on your mind. Do it anyway. You're not eating for how you feel now. You're eating for mile 20.

Then every 20 minutes after that. Small amounts each time — 20 to 30 grams of carbs per dose. That's roughly one gel, a few chews, or a couple gulps of a carb-heavy drink. The goal is a steady drip of energy coming into your system, not occasional big dumps that your gut has to process all at once.

Match your intake to your burn rate. If you're burning 60 grams an hour, you need to put 60 grams an hour back in. Simple in theory. The reason most people don't do it is because they're relying on feel instead of a plan.

One more piece: sodium. Your gut needs sodium to actually absorb the carbs you're taking in. Without it, that gel sits in your stomach longer than it should, which is part of why some people feel sloshy or nauseous. We won't go deep on the science here, but just know — if your fuel doesn't have sodium in it, add some.

This whole approach feels unnatural at first. You're eating when you're not hungry. You're thinking about fuel when you'd rather just run. But that's exactly the point. By the time it feels natural to eat, you've already lost the race against the math.

You don't need fancy products to make this work.

Honey packets, maple syrup in a small flask, even dried fruit — all of it counts. A tablespoon of honey is roughly 17 grams of carbs. Add a pinch of sea salt to your water bottle and you've got the sodium piece covered. Total cost per "gel" is maybe 30 cents instead of three bucks.

The key is knowing your target. For most people running a marathon at moderate effort, somewhere between 60 and 80 grams of carbs per hour is the right range. On the lower end if you're going easy, higher if you're racing hard.

Set a timer on your watch or use mile markers as reminders. Every 20 minutes, eat something. Don't negotiate with yourself about whether you feel like it.

And please — practice this during training. Your gut needs weeks to adapt to processing fuel at effort. Race day is not the time to experiment. Your long runs are your testing ground. Figure out what sits well, what doesn't, and dial it in before you're 20 miles into something that matters.

All of this is doable on your own. But it's also a lot to keep track of mid-race when your brain is foggy and your legs are asking questions you don't want to answer.

PODIUM does the math for you. You tell it how long you're running, the intensity, and what sport — and it calculates exactly how many carbs and how much sodium you need. Then it tells you when to eat with tactical audio cues so you don't have to think about it.

No spreadsheets. No timers. Just run.

Bonking feels like your body failing you. It's not. It's your body doing exactly what it's supposed to do when the tank runs dry.

The good news is it's preventable. Not with more fitness, not with mental toughness, but with a plan you actually follow. Start early, stay consistent, don't wait for hunger to tell you what to do.

The runners who look strong at mile 24 aren't built different. They just did the math before the race started.