How to Carb-Load for a Marathon (Without Overdoing It)

Most runners know they're supposed to carb-load before a marathon. Far fewer actually do it correctly. The pre-race pasta dinner has become such a ritual that it obscures what carb-loading actually is, and what it isn't.

Done right, carb-loading can improve marathon performance by 2 to 3% for events lasting more than 90 minutes. Done wrong, it leaves you bloated, lethargic, and standing at the start line feeling worse than if you'd eaten normally.

Here's what the science actually says, and how to apply it practically.

What Carb-Loading Actually Is

Your muscles store carbohydrates in the form of glycogen. That glycogen is your primary fuel source during a marathon, and a well-trained runner can hold roughly 500 grams of it. The problem is that at marathon pace, you can burn through those stores in about 90 minutes to 2 hours — well short of 26.2 miles for most runners.

Carb-loading is the process of deliberately filling those glycogen stores to capacity before race day, so you start with a full tank rather than a half-full one.

What it is not: an excuse to eat unlimited quantities of anything you want for three days straight. The goal is maximizing glycogen storage, not calorie surplus. When you increase carbohydrate intake, you should correspondingly reduce fat and protein to keep total calories roughly in check.

The Old Way vs The New Way

In the 1960s, the original carb-loading protocol involved a deliberate depletion phase — several days of near-zero carbohydrate intake combined with hard training to drain glycogen stores, followed by a high-carb rebound. The idea was that depleted muscles would absorb more glycogen on the reload.

The problem: the depletion phase made runners feel terrible, increased injury risk during the final week of taper, and the performance benefit wasn't meaningfully better than simpler approaches.

Modern research has largely discarded the depletion phase. Research published in the British Journal of Sports Medicine found that well-trained runners can effectively maximize glycogen stores simply by tapering training and increasing carbohydrate intake over 36 to 48 hours before the race — no depletion required.

How Much to Actually Eat

This is where most runners underestimate the numbers. Sports nutrition guidelines consistently recommend 10 to 12 grams of carbohydrate per kilogram of body weight per day during the carb-loading phase.

For a 70 kg runner, that's 700 to 840 grams of carbohydrates per day. To put that in concrete terms: roughly a large bowl of oatmeal at breakfast, a bagel with banana mid-morning, a big plate of white rice with lunch, a few pretzels as a snack, and a large pasta dinner. Every day for two days.

Most runners who think they're carb-loading are eating maybe 60 to 70% of what they actually need. The most common reason carb-loading doesn't work is simply not eating enough carbs.

When to Start

Begin your carb-load 48 to 60 hours before your race start time. For a Sunday morning marathon, that means Friday becomes your first high-carb day, Saturday your second, and Sunday morning you have a familiar, moderate pre-race breakfast.

Don't try to cram it all into the night before. A single pasta dinner the evening before race day does not constitute a carb-load. Your body needs time to convert carbohydrates into stored glycogen, and that process happens over 24 to 48 hours, not overnight.

What to Eat (and What to Avoid)

The goal during carb-loading is to eat high-carb, low-fiber, low-fat foods. This is a temporary shift in eating pattern, not your everyday nutrition approach.

Good carb-loading foods: white rice, white pasta, white bread, bagels, bananas, potatoes (not fried), oats, pancakes, fruit juice, and sports drinks.

Foods to limit: high-fiber vegetables, beans and lentils, whole grain bread, fatty or fried foods, large portions of meat, and anything you haven't eaten before. Fiber and fat slow digestion, reduce how much carbohydrate you can actually absorb, and increase GI risk on race day.

The Weight Gain You'll Notice

Expect the scale to go up by 1 to 4 lbs during your carb-load. This is not fat gain. For every gram of glycogen stored in your muscles, your body holds onto approximately 3 grams of water. That extra water is not a problem — it actually helps delay dehydration during the race.

If you know this in advance, it won't throw you off mentally. Don't weigh yourself during your carb-load if the number will stress you out. That weight will come off during and after the race as your body burns through its glycogen stores.

Race Morning

Your glycogen stores drop overnight as your body continues to use fuel during sleep. Add a pre-race breakfast 2 to 3 hours before the start: a familiar, carb-focused meal of 1 to 4 grams of carbohydrate per kilogram of body weight. For most runners, that's something like a bagel with jam, a banana, and a sports drink — nothing new, nothing heavy.

The Golden Rule: Test It First

Everything about your carb-load should be practiced before race day. Run a long simulation effort (20 miles or a race tune-up) after a two-day carb-load to understand how your body responds. Some runners feel great with 10 g/kg. Others feel heavy and sluggish and do better at 7 or 8 g/kg. You won't know until you test it.

Race day is not the time to experiment with food.

This is also where having the right meals already planned makes a real difference. In Fuel.fit, pre-run recipes are built around easily digestible carbohydrates with low fiber and fat — exactly what you need during the carb-loading window. No guesswork about whether a meal fits your fueling window.

Training for a marathon and want meals designed around your fueling windows? Try Fuel.fit.

Sources

1. Healthline — Carb Loading
2. Featherstone Nutrition — Carb Loading
3. PubMed — Nutrition strategies for the marathon
4. PubMed — Relationship between muscle water and glycogen recovery
5. The Running Channel — A Simple Guide to Carb-Loading
6. Baylor College of Medicine — Should you carbo load before a marathon?
7. Welsh Athletics — Carbo loading for Marathons