Why Runners Bonk — And Why the Fix Isn't Just 'Eat More Gels'

You trained for months. You had a fueling plan. You took your gels on schedule. And at mile 20, the lights went out anyway.

If you've ever hit the wall during a marathon or long race, you've probably been told the same thing: you didn't eat enough. Take more gels. Start fueling earlier. But for most runners who bonk, the problem isn't that simple — and the fix definitely isn't just cramming in more sugar.

Bonking is one of the most feared and least understood experiences in endurance sport. Understanding why it actually happens — and why it's different for every runner — is the first step toward making sure it doesn't happen again.

What bonking actually is

When you run, your muscles burn a mix of fat and carbohydrate. The harder you go, the more you rely on carbohydrate, which your body stores as glycogen in your muscles and liver. You can store roughly 1,500 to 2,000 calories of glycogen, depending on your body size, fitness, and how well you loaded before the race.

A marathon burns somewhere between 2,500 and 3,500 calories. The maths doesn't work without topping up along the way.

When glycogen runs critically low, your body can't sustain the pace. Your legs feel like concrete. Your brain fogs. Your pace drops off a cliff. That's the bonk — and it's a metabolic emergency, not a mental weakness.

Why "eat more gels" misses the point

Here's where most advice goes wrong. Bonking is treated as a simple input-output problem: you burned X calories, you ate Y calories, and if Y was less than X, you bonked. Eat more next time.

But runners who bonk often report that they were following a fueling plan. They were taking gels. They still hit the wall. That's because at least four other factors are in play.

1. Your gut has a carb ceiling

Your intestines can only absorb carbohydrate at a limited rate. For most runners, that ceiling is somewhere between 60 and 90 grams of carbs per hour — and that's only if you're using a mix of glucose and fructose sources, and only if you've trained your gut to handle it.

If you take in more than your gut can process, the excess sits in your stomach. It doesn't become fuel. It becomes nausea, bloating, and cramping — which slows you down further and makes the bonk worse.

The generic advice to "aim for 60–90g per hour" ignores the fact that your personal ceiling might be 45g. Or it might be 100g. You don't know until you test it, and most runners never do.

2. Pacing burns fuel faster than you think

Start 15 seconds per mile faster than your goal pace, and you shift your fuel mix toward carbohydrate and away from fat. That sounds small, but over 26.2 miles, it means you burn through your glycogen stores significantly faster.

A runner who goes out at their target pace and a runner who goes out 20 seconds too fast can hit the wall at completely different points — even if their fueling plans are identical. The first ten miles set the fuel budget for the last ten.

This is why GPS pace data and heart rate from your watch matter for fueling, not just for pacing. If your heart rate drifts above zone 2 early in the race, you're burning carbs at a rate your fueling plan may not be designed to replace.

3. Dehydration compounds the problem

When you're dehydrated, blood flow to your gut decreases. That means less efficient absorption of whatever fuel you're taking in. It also means your heart rate rises for the same pace, which shifts you further into carbohydrate burning.

Dehydration doesn't cause the bonk directly, but it makes every other factor worse. A runner who loses more than 3–4% of body weight through sweat is fighting on two fronts: they're burning fuel faster and absorbing it slower.

4. Heat amplifies everything

In warm conditions, your body diverts blood to the skin for cooling, leaving less for your working muscles and digestive system. Sweat rates increase, dehydration accelerates, and your gut becomes less tolerant of concentrated fuel.

A fueling plan that worked perfectly in 12°C training conditions can fail completely on a 22°C race day. This is one of the most common causes of bonking in fall marathons, where athletes trained through cooler months and race on an unexpectedly warm day.

Why generic fueling plans fail

Most online fueling calculators ask for your body weight and target pace, then spit out a grams-per-hour number. That number assumes:

• Average sweat rates
• Average gut absorption capacity
• Average glycogen stores
• Average fat oxidation rates
• Perfect pacing execution

You are not average. Nobody is. Your sweat rate depends on your genetics, fitness, acclimatisation, and the weather on race day. Your gut's carb ceiling depends on how much you've trained it. Your glycogen stores depend on your body composition, your carb loading, and how well you slept the night before.

A plan built on averages gets average results — and for a lot of runners, average means bonking at mile 20.

How to actually prevent bonking

Preventing the bonk requires attacking all four factors, not just fuel quantity.

Test your personal carb ceiling. During long runs, gradually increase your carb intake per hour and note when your stomach starts protesting. That's your current ceiling. Over weeks, you can train it higher — but you need to know where you're starting.

Respect your pace plan. If your watch shows your heart rate running higher than expected in the first half, back off. Saving 10 seconds per mile early can save you 10 minutes in the final 10K.

Match your hydration to your actual sweat rate. Weigh yourself before and after a hard training run. Every kilogram lost is roughly a litre of sweat you didn't replace. Your hydration plan should be based on this number, not on how thirsty you feel.

Adjust for conditions. If race day is warmer than your training, reduce your pace target, increase fluid intake, and consider cutting your carb concentration to help absorption.

Use your wearable data. Your watch already tracks heart rate, pace, and estimated calorie burn during every training run. That data, accumulated over weeks and months, paints a picture of how your body uses fuel at different intensities and conditions. The problem is that most watches show you the data without telling you what to do with it.

The gap between data and action

Your Garmin or Apple Watch records everything you need to build a personalised fueling plan. Heart rate zones tell you your fuel mix. GPS pace shows your intensity. Estimated sweat loss gives you a hydration baseline. Training load over time reveals how your fitness and fuel efficiency are changing.

But no watch turns that data into a race-day plan. Garmin's 2026 nutrition tracking lets you log meals and count calories — useful for daily nutrition, but it doesn't calculate how many grams of carbs you need at mile 15 based on your pace, the forecast temperature, and your sweat rate from last week's long run.

That's the problem Sweatr solves. It pulls your data from Apple Watch, Garmin, and Strava, calculates your personal sweat rate and carb needs, and builds a fueling plan that adapts to your actual training — not population averages. When race day conditions change, your plan changes with them.

If you've ever bonked and thought "I did everything right," you probably did everything the generic advice told you to do. The issue wasn't effort. It was that the plan wasn't built for your body.

Sweatr calculates your personal fueling plan from your wearable data — try it free and stop guessing before your next race.