The Calorie Myth That Makes Marathoners Undereat on Race Day

You finish your marathon, glance at your Garmin, and see it: 3,247 calories burned.

Then you think back to what you actually consumed during the race. Three gels, some sports drink at aid stations, maybe a few swigs of water. Generously, 600 calories.

That leaves a 2,600-calorie "deficit." And the question every marathoner asks at some point:

Did I eat enough? Should I have eaten more? Is that why I bonked at mile 22?

The answer isn't what you think. That calorie number on your watch is real, but it has almost nothing to do with how much you should eat during a race. And confusing the two is the single most common fueling mistake in endurance sport.

The problem with thinking in calories

When your Garmin or Apple Watch reports calories burned, it's estimating total energy expenditure — everything your body used to keep you moving for 3, 4, or 5 hours. That includes the glycogen stored in your muscles and liver (about 2,000 calories' worth for most trained runners), the fat your body oxidises during exercise (a virtually unlimited supply), and the carbohydrates you eat and absorb along the way.

Your body doesn't need you to replace all 3,000+ calories in real time. It already has most of that energy on board before you cross the start line.

The calorie number on your watch is an accounting figure. It tells you the total cost of the work. It does not tell you how much fuel to carry.

What your body actually needs mid-race

Here's where it gets simple. During a marathon, the limiting fuel source isn't total energy — it's carbohydrate availability. Your muscles and brain need a steady supply of glucose to keep working. Stored glycogen handles the first 60–90 minutes, but after that it starts to run low. That's when you bonk.

The fix isn't replacing 3,000 calories. It's maintaining a steady flow of carbohydrates into your bloodstream so your glycogen stores last the full distance.

Current sports nutrition research — including a comprehensive 2026 review in The Journal of Nutrition — recommends targeting 60 to 90 grams of carbohydrates per hour for events lasting longer than two hours. Not 60 to 90 grams of food. Not 60 to 90 grams of "energy." Specifically carbohydrates.

In calorie terms, that's only 240 to 360 calories per hour from carbs. Over a 4-hour marathon, that's 960 to 1,440 total calories consumed — nowhere near 3,000. And that's by design. Your body handles the rest with what it already has stored.

Why the calorie framing causes runners to undereat

Here's the paradox: thinking about calories makes runners eat less, not more.

When you frame race-day fueling as "replace the calories you burn," the numbers are overwhelming. You can't eat 3,000 calories during a marathon. Your stomach can't process that volume. So many runners do the mental math, decide it's impossible, and default to eating barely anything — hoping that their pre-race carb loading and maybe a gel or two will get them through.

The data confirms this. Research on recreational marathoners found that the average in-race carbohydrate intake was just 21.7 grams per hour. That's about one gel every 90 minutes. It's less than a third of the minimum recommended intake.

These runners aren't failing to fuel because they don't care. They're failing because the calorie framing makes the task seem impossible, so they give up before they start.

The simple reframe that changes everything

Stop thinking about calories. Start thinking about carbs per hour.

When you shift to this framing, the numbers suddenly make sense:

• 60 g/hr = about 2 standard gels per hour, or one gel plus 500 ml of sports drink
• 75 g/hr = a comfortable middle ground for most trained runners
• 90 g/hr = the upper end for athletes who have trained their gut (more on that below)

These are real, achievable numbers. Two gels per hour with some sports drink. Or one gel, a few chews, and a drink at every other aid station. It's not glamorous. But it works — and it's manageable for every runner who practises it in training.

Your carb target is personal (and your watch already holds the clues)

The 60–90 g/hr range is a starting point, not a prescription. Your actual target depends on several things your wearable already tracks:

Your finishing time matters. A 3-hour marathoner needs to sustain a higher carb oxidation rate than a 5-hour marathoner covering the same distance at a lower intensity. But the 5-hour runner is out there longer, meaning more total carbs consumed even if the hourly rate is lower. A personalised calculation accounts for this.

Your body weight matters. A 55 kg runner and an 85 kg runner burning carbs at different absolute rates need different intake targets. Body weight is part of the equation.

Your training data matters. Heart rate drift, pace consistency, and how your body responds to carbohydrate intake on long runs all provide data points for dialling in your number. That's information your Apple Watch or Garmin captures every session.

Your gut tolerance matters. The 2026 Journal of Nutrition review found that athletes who progressively trained their gut tolerated 90 g/hr or more of multiple transportable carbohydrates (glucose + fructose blends). But most recreational runners haven't done this work. Starting at 60 g/hr and building up over 4–8 weeks of long runs is the evidence-based approach.

What about fat? Protein? "Real food"?

During a marathon, fat oxidation handles a meaningful portion of your energy needs — roughly 30–50% at typical marathon intensities, more if you're well-trained. That fat comes from your body's own stores, not from food you eat mid-race.

Protein is irrelevant during a race. Your body doesn't use dietary protein for fuel in any meaningful way over 2–5 hours. Save the recovery shake for after.

As for "real food" — dates, banana pieces, rice balls — these work fine if your stomach tolerates them. What matters isn't the food format; it's whether it delivers the right amount of carbohydrate at the right time. A gel with 25 g of carbs and a date with 25 g of carbs are doing the same job for your muscles. Pick what you can tolerate, and make sure the total adds up.

How to build your in-race carb plan

1. Pick your hourly target. If you've never fuelled deliberately on long runs, start at 60 g/hr. If you've been practising, work toward 75 g/hr. Only push toward 90 g/hr if you've done structured gut training.

2. Map it to products you'll carry. Know the carb content of every gel, chew, and drink you plan to use. A standard gel is 20–25 g of carbs. A 500 ml bottle of sports drink is typically 30–40 g. Do the maths for your target finish time.

3. Set timing reminders. Don't rely on "feeling hungry" — by the time you feel it, you're already behind. Set alerts on your watch every 20–30 minutes as fuel checkpoints.

4. Test in training. Practise your exact race-day fueling on at least three long runs before taper. Use the same products, the same timing, the same volume.

5. Ignore the calorie number. After the race, your watch will show some impressive calorie total. Smile, note it, and do not use it to plan next time.

Let your data do the maths

Everything above requires a few calculations — your body weight, your target finish time, your training intensity, your sweat rate, and the carb content of your chosen products. Sweatr pulls your training data from Apple Watch, Garmin, and Strava and calculates your personal carb-per-hour target automatically. It builds a race-day fueling plan you can follow gel by gel, mapped to real products and timed to your pace.

No calorie counting. No mental arithmetic at mile 16. Just a plan that matches your body and your race.

The bottom line

Your watch's calorie estimate isn't wrong. It's just answering the wrong question. It tells you how much energy the work cost. It doesn't tell you how much fuel to carry.

The right question is: How many grams of carbohydrate per hour do I need to keep my brain and muscles fuelled from start to finish?

Answer that question — and practise the answer on your long runs — and the bonk at mile 22 stops being inevitable. It becomes preventable.