Race Fueling
From Bonk to PR: How Smart Nutrition Changes Your Training
That wall you hit at mile 20 might not be your training — it might be your fueling. Learn how proper nutrition can transform your race results.
Quick answer
Avoid bonking by carb loading 2–3 days before, starting fuel within 30 minutes of racing, and maintaining 60–90 g carbs/hour — proper nutrition can unlock the PR your training deserves.
You've trained for months. Your fitness is there. But somewhere around mile 18, 20, or 22, your legs turn to jelly and your pace crashes.
You've been hit by the bonk — and it's devastating.
Here's the thing: the bonk isn't always about your training. It's often about your nutrition.
What Is the Bonk, Really?
The bonk — or "hitting the wall" — happens when your muscle glycogen stores run dry. Your body has two fuel sources: glycogen (carbs stored in muscle) and fat.
At marathon pace, you're burning mostly glycogen. Your muscles can store about 400–500 grams of glycogen, which translates to roughly 1,600–2,000 calories.
At race pace, you burn through that in 90–120 minutes. Once it's gone, your body can't maintain intensity, and you crash.
Why Your Training Doesn't Matter If Your Nutrition Fails
Fitness is the engine. Fuel is the gasoline.
You can have the most finely-tuned engine in the world, but if you run out of gas at mile 20, you're not finishing strong.
Proper nutrition during training and racing keeps your glycogen tanks from emptying so your fitness can actually shine through.
The 4 Pillars of Bonk-Proof Racing
- Carb loading before the race to start with full glycogen stores
- Starting fuel early (within 30 minutes) rather than waiting for fatigue
- Maintaining steady carb intake during the race (60–90 g/hour for most)
- Practicing your race fueling in training so your gut knows what to expect
The Morning-of Checklist
- Eat 3–4 hours before start: 1–4 g carbs/kg body weight
- Include a small amount of protein for blood sugar stability
- Avoid high fiber and fat to protect your gut
- Sip 400–600 ml fluids with electrolytes
- Top off with a small carb snack 30 minutes before the gun
The During-Race Formula
For races over 90 minutes, you need to replace carbs as you burn them.
- Start within 30 minutes of the start — don't wait for fatigue
- Aim for 60–90 g carbs per hour (one gel every 25–30 minutes)
- Pair gels with electrolyte drinks for hydration
- Practice in training so your gut tolerates the load
What Happens If You Don't Fuel
| Time Without Fuel | What Happens |
|---|---|
| 60 minutes | Liver glycogen starts dropping, blood sugar dips |
| 90 minutes | Muscle glycogen depleted, fatigue sets in |
| 2+ hours | Bonk: severe fatigue, mental fog, pace collapse |
How to Practice Bonk-Proofing in Training
Your race fueling strategy should be rehearsed in at least three long training sessions before race day.
- Use the same gels, drinks, and timing you plan for race day
- Practice in similar conditions (heat, humidity, terrain)
- Start fueling at the same time you will on race day
- Note any gut distress and adjust products or timing
The One-Sentence Bonk-Proof Rule
If you strip everything above down to a single rule, it is this: start full, and stay ahead. Start full means arriving with topped-off glycogen from a proper carb load and a race-morning breakfast. Stay ahead means taking your first carbs within 30 minutes — before you feel any dip — and holding 60-90g per hour rather than waiting for the warning signs, because once you actually feel the bonk you are 20-30 minutes behind and cannot fully recover at race pace. Almost every wall at mile 20 is a violation of one half of that sentence.
Calculating your carb-loading targets by race distance and building the gel-and-hydration schedule is the math a tool like MAVR does for you. But the start-full-stay-ahead rule is the whole strategy in miniature, and you can execute it with a watch alarm and a pocket of gels.
Bonk-proof your next race with AI-powered fueling.
Frequently Asked Questions (FAQ)
How many carbs do I need to avoid bonking?
Start with full glycogen stores via carb loading, then consume 60–90 g carbs per hour during the race. Your gut can adapt to higher rates with training.
What if I get GI distress from fueling?
Train your gut by practicing race fueling in training. Start with lower rates (30–40 g/hour) and increase gradually over 4–6 weeks.
Can I bonk in a half marathon?
Possibly if you're underfueled going in or if the pace is very aggressive. Most athletes can finish a half without additional carbs, but fast times may require fueling.
Does fat adaptation help avoid bonking?
Fat adaptation improves metabolic flexibility but doesn't eliminate the need for carbs at race intensity. You still need glycogen for high-intensity effort.
How does MAVR prevent bonking?
MAVR calculates your carb needs, builds race-day fueling schedules, and helps you practice your strategy in training so you arrive at the start line prepared.
Written by
MAVR Sports Nutrition Team
The MAVR sports-nutrition team translates peer-reviewed endurance research (Jeukendrup, Burke, the ISSN) into daily fueling for runners and triathletes.