Mavr Sports Nutrition Team
Updated October 21, 2026. 9 min read.
You trained for months. Your long runs felt strong through 10 miles. Then, somewhere between mile 12 and 18, your legs turned to cement, your pace dropped, and your brain started bargaining for a walking break.
That wall isn't a fitness problem. It's a fueling problem. And it's fixable.
Every runner who bonks is making at least one of these three mistakes:
1. Underfueling
What's HappeningNot enough total carbs before and during the run
How CommonVery common — affects 70%+ of recreational runners
2. Wrong carb sources
What's HappeningUsing single-transport carbs that top out at about 60g/hour
How CommonCommon: most gels are glucose-only
3. Bad timing
What's HappeningStarting fueling too late or fueling inconsistently
How CommonVery common — most runners wait until they feel hungry
| Root Cause | What's Happening | How Common |
|---|---|---|
| 1. Underfueling | Not enough total carbs before and during the run | Very common — affects 70%+ of recreational runners |
| 2. Wrong carb sources | Using single-transport carbs that top out at about 60g/hour | Common: most gels are glucose-only |
| 3. Bad timing | Starting fueling too late or fueling inconsistently | Very common — most runners wait until they feel hungry |
Your body stores about 300–500g of glycogen in your muscles and liver. At moderate running pace, you burn through roughly 1g of carbs per minute, sometimes more. That gives you about 90–120 minutes of fuel before the tank runs dry.
Here is roughly how it plays out if you take in nothing during the run:
60 minutes
Liver glycogen starts dropping and blood sugar dips
90 minutes
Muscle glycogen runs low and fatigue sets in
2+ hours
The bonk: severe fatigue, mental fog, pace collapse
| Time Without Fuel | What Happens |
|---|---|
| 60 minutes | Liver glycogen starts dropping and blood sugar dips |
| 90 minutes | Muscle glycogen runs low and fatigue sets in |
| 2+ hours | The bonk: severe fatigue, mental fog, pace collapse |
If you start your long run with glycogen that's only 70% full (because you ate low-carb the day before, or skipped breakfast), you'll hit the wall even sooner — maybe 60–70 minutes in.
The fix: eat 7–10g of carbs per kg of body weight the day before a long run, and 1–2g/kg 2–3 hours before starting. For a 70kg runner, that's roughly 490–700g of carbs the day before and 70–140g at breakfast.
Not all carbs are absorbed the same way. Your gut uses specific transport proteins to move carbs from your intestine into your bloodstream. Each transport protein has a speed limit.
Glucose
TransportSGLT1 transporter
Max Absorption~60g/hour (about 1g/min)
ExamplesMost gels, maltodextrin, dextrose
Fructose
TransportGLUT5 transporter
Max Absorption~30g/hour more on top of glucose
ExamplesFruit, some gels, honey
Glucose + Fructose
TransportBoth transporters
Max Absorptionup to about 90g/hour combined
ExamplesMaurten, SiS Beta Fuel, Precision Fuel PF 30
| Carb Type | Transport | Max Absorption | Examples |
|---|---|---|---|
| Glucose | SGLT1 transporter | ~60g/hour (about 1g/min) | Most gels, maltodextrin, dextrose |
| Fructose | GLUT5 transporter | ~30g/hour more on top of glucose | Fruit, some gels, honey |
| Glucose + Fructose | Both transporters | up to about 90g/hour combined | Maurten, SiS Beta Fuel, Precision Fuel PF 30 |
If you're already taking about 60g of glucose-only carbs per hour and want more, you've hit your gut's limit for glucose. Switching to a dual-source fuel (glucose + fructose) lets you absorb up to about 90g per hour.
The most common timing mistake: waiting until you feel hungry or tired to start fueling. By the time you notice your energy dropping, your blood glucose is already falling and glycogen is depleted. Playing catch-up is almost impossible.
You need to start fueling before you feel like you need it. That means your first gel or fuel source should come 20–30 minutes into your run — not 45–60 minutes in.
Here's a practical fueling plan for long runs that eliminates all three root causes at once:
2–3 hours before
Full pre-run meal (carbs + some protein, low fat/fiber)
Carbs1–2g/kg
15 min before
Quick top-up: gel or banana
Carbs20–25g
20–30 min in
First gel or fuel source (do not wait)
Carbs20–25g
Every 20–30 min after
Continue fueling consistently
Carbs20–30g per dose
Target per hour
Total from all sources combined
Carbs30–60g/hr (60–90g with dual-source)
Within 30 min after
Recovery: carbs + protein
Carbs1–1.2g/kg carbs + 20–30g protein
| Time | Action | Carbs |
|---|---|---|
| 2–3 hours before | Full pre-run meal (carbs + some protein, low fat/fiber) | 1–2g/kg |
| 15 min before | Quick top-up: gel or banana | 20–25g |
| 20–30 min in | First gel or fuel source (do not wait) | 20–25g |
| Every 20–30 min after | Continue fueling consistently | 20–30g per dose |
| Target per hour | Total from all sources combined | 30–60g/hr (60–90g with dual-source) |
| Within 30 min after | Recovery: carbs + protein | 1–1.2g/kg carbs + 20–30g protein |
The key insight: fuel early and consistently. Think of it like watering a plant — small amounts at regular intervals work far better than one big pour when it's already wilting.
The same wall hits in races, usually around mile 18 to 22 of a marathon. Race day comes down to one rule: start full, and stay ahead. Start full means arriving with topped-off glycogen. Stay ahead means taking carbs before you feel any dip, because once the bonk arrives you can't fully recover at race pace.
Sarah, 34, kept crashing around mile 15 of her long runs during marathon training. She ate a light breakfast, carried two gels, and took them when she felt tired — usually around 50 and 80 minutes. She was getting maybe 40g of carbs total during a 2.5-hour run.
After switching to the protocol above — a bigger breakfast (80g carbs), a pre-run gel, and fueling every 25 minutes with dual-source carbs — she got to 20 miles without slowing down. Total in-run carbs: about 120g over 2.5 hours. Her body could finally absorb and use the fuel.
The story above turns on one habit nearly everyone skips: actually calculating in-run carbs as a total for the session, not a vague "I'll grab a gel when I feel low." Work it backward from your target rate — say 60g per hour for a 2.5-hour run is 150g total — then divide it into specific feeds (a gel every 25-30 minutes, plus carbs from your drink) so the plan is a timeline, not a guess. For higher rates, use dual-source carbs (glucose plus fructose) so your gut can actually absorb 60-90g an hour, and build toward that number gradually rather than attempting it cold.
Running that calculation and turning it into a minute-by-minute timeline is precisely what Mavr does for you. But the math itself is simple arithmetic, and doing it once for your next long run — total carbs, divided into timed feeds — is what separates bonking from finishing strong, app or no app.
Because your fueling strategy is the same every time. If you consistently bonk around mile 14–16, that's roughly 90–120 minutes of running — exactly when glycogen runs out for most runners. The fix is to start fueling earlier (minute 20–30) and consume more carbs per hour.
For most runners, 30–60g per hour prevents bonking. If you're using dual-source fuels (glucose + fructose), you can absorb up to 60–90g per hour. Start with 30g/hour and increase over a few weeks as your gut adapts.
Partially. Fitter runners burn fat more efficiently, which extends the time before glycogen depletion. But even elite runners bonk without proper fueling — glycogen depletion is a mathematical certainty. No amount of fitness replaces eating enough carbs.
Bonking feels like someone flipped a switch. Your pace suddenly drops, your legs feel heavy and weak, your brain gets foggy, you might feel shaky or irritable, and even walking feels hard. It's distinct from normal fatigue because it comes on suddenly rather than gradually.
Practice fueling during long runs. Start with 30g carbs/hour for 2–3 weeks, then increase to 45g/hour, then 60g/hour. Your gut adapts over time — the transport proteins that absorb carbs actually increase with practice. Give it 4–6 weeks to see significant improvement.
It's possible if you start underfueled or run a very aggressive pace. Most runners can finish a half marathon without extra carbs if they start with full glycogen, but a fast time often goes better with some fuel on the way.
It improves metabolic flexibility, so you burn fat a little more efficiently. But it doesn't remove the need for carbs at race intensity. Hard efforts still run on glycogen, so you still need to fuel.
General guidance for healthy adults, not medical advice. Talk to a doctor or registered dietitian if you have a health condition, take medicines or have special dietary needs.
Stop bonking on long runs. Mavr builds a fueling timeline matched to your pace, distance, and gut tolerance.
Ask Kai, the coach in the app