Carb loading for a marathon: how much, when, and how many gels

Carb loading means eating much more carbohydrate than usual for the last day or two before a long race, while training tapers, so you start with your muscle and liver glycogen stores as full as they can be. For the marathon it's a standard recommendation in sports nutrition reviews. It's also easy to get wrong: too little, too late, too much fiber, or a single giant dinner.

This guide covers how much to eat, for how long, a worked example in pounds and kilograms, and how much carbohydrate to take during the race, including how many gels that means.

A health note first. Carb loading is a big, fast change in diet. If you have diabetes or prediabetes, a GI condition such as IBS or celiac disease, kidney disease, a history of disordered eating, or you're pregnant, don't carb load without a plan from your doctor or a registered dietitian. Everything here is general information, not individual advice.

Who benefits from carb loading

Glycogen is the main fuel for running at marathon effort, and a marathon is long enough to run it low. Louise Burke's review of marathon nutrition says carb loading can improve marathon performance by letting you run at your optimal pace for longer before fatigue sets in.

Her review table on competition strategies places carb loading with prolonged events, usually longer than 90 minutes, that would otherwise deplete muscle glycogen, naming the marathon as an example. For shorter events, a Gatorade Sports Science Institute review notes that such aggressive intakes may not be necessary. For a half marathon, see what to eat before a half marathon instead.

What you don't need: the old "depletion" phase of several low-carb days before loading. Burke's 2007 review says there's no evidence it provides any benefit.

How much carbohydrate to eat

The reviews we checked agree closely:

• 10 to 12 g of carbohydrate per kilogram of body weight per day, for the 36 to 48 hours before the race, plus a taper (Burke 2007, written for well-trained marathoners, and Burke 2015; the GSSI review gives the same range).
• The International Society of Sports Nutrition's ultra-marathon position stand recommends about 10 g/kg/day in the 48 hours before an ultra. For the food you want waiting on the course, see our ultra drop bag checklist.

In pounds, 10 to 12 g/kg is about 4.5 to 5.4 g of carbohydrate per pound per day.

These are much bigger numbers than most people expect. A typical day of eating doesn't come close, which is why "pasta dinner the night before" on its own isn't carb loading.

Worked example: a 150 lb runner

1. Convert to kilograms. 150 lb ÷ 2.2046 = 68.0 kg.
2. Daily target. 68.0 kg × 10 g = 680 g; 68.0 kg × 12 g = 816 g. Aim for 680 to 816 g of carbohydrate per day.
3. Duration. For a Sunday morning marathon, starting Friday morning gives about 48 hours; starting Friday evening gives about 36.
4. Race breakfast. Burke's table adds 1 to 4 g/kg in the 1 to 4 hours before the start: 68 to 272 g for this runner. Our suggestion for a middle ground to test in training: about 2 g/kg (136 g) about 3 hours out.

What 680 grams looks like

Per-100 g carbohydrate values from USDA FoodData Central: cooked white rice 28.2 g, cooked pasta 30.9 g, white bread 49.4 g, plain bagel 52.4 g, banana 22.8 g, honey 82.4 g. Using those numbers, one loading day for our 150 lb runner could look like this (our example, not a prescribed menu):

That's a lot of food and still about 120 g short of 680 g. Drinks are an easy way to close the gap without more bulk: juice, sports drinks, chocolate milk, sodas. Read the labels and add them up. Small side portions of protein and vegetables are fine; keep them small.

How to carb load without wrecking your stomach

• Go low-fiber. Swap whole grains, beans and big salads for white rice, white bread, pasta, potatoes without skin and low-fiber fruit for these two days. Cleveland Clinic's marathon advice recommends simple, low-fiber carbs in race week and limiting high-fiber, high-fat, spicy and dairy-heavy foods the night before.
• Go low-fat. Fat fills you up without adding carbohydrate. Plain starches beat pizza, Alfredo sauce and fries.
• Eat little and often. Five or six carb-heavy meals and snacks are easier than three huge ones.
• Drink. Carbohydrate is stored with water, which Cleveland Clinic gives as a reason to hydrate well. It's also why a small gain on the scale is nothing to worry about.
• Taper. Loading works alongside reduced training, not on top of hard sessions.
• Rehearse. Try a loading day before a long run or a tune-up race during your 16-week marathon plan, so race week holds no surprises.
• Nothing new. Cleveland Clinic is blunt: race week isn't the time to experiment.

Carbs per hour while running

Loading fills the tank; it doesn't last 26.2 miles on its own. Burke's 2015 table recommends:

Other sources fill in the details:

• Asker Jeukendrup's 2014 review explains that a single type of carbohydrate (such as glucose alone) can be used at up to about 60 g per hour; going higher requires multiple transportable carbohydrates (such as glucose plus fructose), which the gut absorbs by different routes. The GSSI review likewise says glucose-fructose mixtures are preferable at 60 to 90 g per hour.
• Jeukendrup recommends practicing race fueling in training, and notes that the gut adapts: a study he cites found that 28 days of a high-carbohydrate diet increased how much of the carbohydrate athletes took in during exercise they actually burned. He also says runners working at lower absolute intensities burn less carbohydrate and should adjust intake downward.
• The ISSN nutrient timing position stand recommends about 30 to 60 g of carbohydrate per hour, taken every 10 to 15 minutes, particularly in sessions longer than about 70 minutes. On the race course, aid stations set the rhythm; Cleveland Clinic suggests taking carbs every 25 to 30 minutes.

Our suggestion for a first marathon: start in the 30 to 60 g per hour range and build up in training. If you aim for the higher rates, work up to them over many long runs, as Jeukendrup's gut-training point suggests; don't try 90 g per hour for the first time on race day.

How many gels for a marathon

The math is simple:

gels = finish time in hours × grams per hour ÷ grams per gel

Gels differ, so use the carbohydrate number on your gel's label. The table uses 25 g per gel as an example, and counts everything as gels; any sports drink you take reduces the count.

Example: a runner aiming for a sub 4 hour marathon at 60 g per hour needs about 240 g of carbohydrate. If they get 60 g from sports drink at aid stations, that leaves 180 g, or about 7 gels at 25 g each. Don't know your finish time yet? Check the marathon pace chart.

Carry one or two more than you plan to use, in case you drop one. And find out which drink and gels the race offers on the course: if you've trained with them, you can carry less.

Common carb loading mistakes

• Loading only at dinner the night before. No realistic single meal delivers 10 g/kg.
• Loading with fat and fiber. Pizza, burgers, big salads and bean chili are not carb loading.
• Panicking about the scale. A small gain from stored water is expected.
• Doing hard training during the load. The taper is part of the protocol.
• Racing at 90 g per hour without practice. Build tolerance over weeks.
• Ignoring a medical condition. Diabetes and GI conditions need a personal plan from a professional.

For the meal-by-meal version of race week, see what to eat before a marathon.