Carb loading before a half marathon: how and when to do it

Carb loading before a half marathon, short answer: worth doing if you'll be running for longer than about 90 minutes, which is most half marathoners. It means eating a lot more carbohydrate than usual for the last day or two, while training very little. It does not mean one enormous pasta dinner the night before. That's just dinner with ambition.

The rest is the how: how much, when to start, what to eat, what it does and doesn't do, and how it changes for a 5K, a 10K or a full marathon. If you want the training side handled, a coach that fits training around real life builds the taper that makes a load work. The rice is on you.

One note up front: this is general practice, not a personal prescription. If you have diabetes, a GI condition, a history of disordered eating or anything else clinical, talk to a doctor or a registered dietitian before you try a carb load.

What carb loading actually is

Your muscles store carbohydrate as glycogen, and long, hard running burns through it. Carb loading is the deliberate version of topping those stores up before a race.

The joint position stand from the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine lays it out plainly (Thomas, Erdman and Burke, 2016):

• ›In the absence of severe muscle damage, glycogen stores can be normalized with 24 hours of reduced training and adequate fuel intake.
• ›Events longer than 90 minutes may benefit from higher glycogen stores, which is where carb loading comes in.
• ›The protocol evolved from glycogen studies in the 1960s, and in trained athletes it can be achieved by extending a carbohydrate-rich diet and tapering training over 48 hours.

Two things to notice. First, there's no week-long depletion phase in that description, so you can skip the old-school "starve the carbs, then binge" routine. Second, the load and the taper are a package. Eating 10 g/kg while still doing hard sessions isn't loading. It's just eating a lot.

If you're following a half marathon training schedule, the taper in race week already gives you the "reduced training" half of the deal. The food is the other half. And if you're new to fueling in general, what to eat before long run days is the place to start, because every rule there carries into race week.

Carb loading before half marathon: do you need it?

It depends on how long you'll be running, not on the distance.

The position stand splits it at 90 minutes (Thomas, Erdman and Burke, 2016):

The median half marathon finish in RunRepeat's race data is 2:14:59. So the typical half marathoner is out there well past the 90-minute line, and a load fits the guideline.

• ›Finishing in under about 90 minutes? A formal load isn't what the guideline is for. Eat your normal carb-heavy diet in the days before and keep training light.
• ›Finishing in 90 minutes or more? A load over the last 36 to 48 hours is worth trying, ideally in training first.

Either way, race morning is its own meal, and what to eat before half marathon start lines covers the breakfast, timing and nerves.

How much carbohydrate is a carb load?

The guideline is 10 to 12 g of carbohydrate per kilogram of body weight per day (Thomas, Erdman and Burke, 2016). Here's the multiplication:

That is a lot of food, and it's easy to fall well short without noticing. If you want to hit the number, read nutrition labels and add it up for a day. Guessing from memory is how a "load" turns into a large lunch.

You don't have to hit the top of the range to benefit from eating more carbohydrate than usual. But if you're going to bother, it helps to know what the target actually looks like.

When to carb load: a simple timeline

For a Sunday morning half marathon:

That's roughly 36 to 48 hours of loading, which matches the guideline. Race morning then adds the pre-event meal: 1 to 4 g of carbohydrate per kilogram, 1 to 4 hours before exercise longer than 60 minutes (Thomas, Erdman and Burke, 2016).

The final dinner is just one meal of the load, not the whole thing. It should look like a guide to what to eat the night before a 10K run: carb-forward, familiar, not too late. The work happened in the 36 hours before it.

What to eat when carb loading

The position stand notes that athletes may choose carbohydrate-rich sources that are low in fiber and residue and easy to consume, to make sure fuel targets are met and to support gut comfort (Thomas, Erdman and Burke, 2016). In normal language: this is the one time a week where white bread beats the seeded loaf.

Common carb-loading foods:

• ›White rice, white pasta, noodles
• ›Bagels, white bread, wraps
• ›Potatoes without the skin
• ›Plain breakfast cereal, oats if they sit well with you
• ›Juice, sports drink, milk if dairy agrees with you
• ›Honey, jam, syrup, sweets
• ›Pretzels, crackers, rice cakes

And how to actually get it in:

• ›Spread it out. Three meals plus snacks beats one giant dinner. Your stomach has a capacity limit, and the night before a race is a bad time to find it.
• ›Drink some of it. Juice and sports drink add carbohydrate without adding a lot of bulk.
• ›Turn down fat and fiber, not protein entirely. Swap some of the usual vegetables, beans and whole grains for lower-fiber options for these two days. Keep a normal amount of protein.
• ›Stick to foods you know. Two days before a race is not the time to discover a new cuisine.

Expect to feel full. That's the point. If you feel genuinely unwell, ease off; a partial load beats a race spent looking for toilets.

Practice it before race week

A carb load is a lot of food your stomach isn't used to. Try it before a long run in your build: two days of higher carbohydrate, then your race breakfast, then the run. You'll learn which foods work, how full you feel and whether anything needs changing.

Your long runs are also where you practice fueling during the race. For runs past an hour, the position stand targets 30 to 60 g of carbohydrate per hour for efforts of 1 to 2.5 hours, and up to 90 g per hour beyond about 2.5 to 3 hours (Thomas, Erdman and Burke, 2016). A carb load fills the tank before the start. It doesn't replace fueling on the way.

Carb loading for a full marathon, a 10K or a 5K

Marathon. Every marathoner is out there well beyond 90 minutes, so the load applies. The numbers are the same; the stakes are higher, and fueling during the race matters more. Energy gels for marathon training covers that side: how many carbs per hour and how to practice them.

10K. The median 10K finish in RunRepeat's data is 1:02:08, a long way inside 90 minutes, so for most runners the guideline points to normal daily fuel, not a load. Eat a normal, carb-forward dinner the night before and a light breakfast.

5K. No. A 5K needs a familiar snack and a decent warm-up. What to eat before running a 5K is about four sentences long for good reason.

Hyrox and other mixed events. The 90-minute line is about how long you'll be working, not what kind of event it is. Estimate your finish time and apply the same rule.

Lifting during race week

If you lift, the carb load is not your biggest race-week risk. Your last heavy lower-body session is.

Keep heavy lower-body lifting a week or more away from the race, and keep the last few days easy or off. If you want to lift in race week, keep it light and upper-body, early in the week. The "reduced training" part of the position stand's description is doing real work here. Loading carbs while doing squats on Friday gets you a full stomach and tired legs.

After the race

Once your stomach settles, eat something with carbs and protein, and drink. For the full distance, what to eat after a marathon is mostly a bigger version of the same idea, spread over a few days.

Across a whole build, the bigger lever is your everyday eating, not two days of rice. Healthy eating for marathon training is mostly about eating enough on your biggest training days. And for shorter races, what to eat after 5K races is normal food, reasonably soon.

Common mistakes

• ›One giant pasta dinner, then calling it a load. It's a nice dinner. It isn't 36 to 48 hours of high carbohydrate.
• ›Loading while still training hard. The taper is half the method.
• ›Loading on high-fiber food. Lentils, bran and big salads are excellent most weeks. Less excellent at mile nine.
• ›Loading for a race that's over in 40 minutes. The guideline doesn't apply. Eat dinner.
• ›Trying it for the first time in race week. Rehearse on a long run.
• ›Eating more of everything. More carbohydrate, not more fat, not more of the burger.

How exrsz handles race week

exrsz builds a periodized plan from your goal, your schedule, your level and your equipment, with running and lifting balanced in one plan, so the taper is already there and your last heavy leg day isn't the Friday before the race. It adapts each week to how you trained and reshuffles missed sessions. The bagels are your responsibility.

FAQ

Should I carb load before a half marathon?

If you'll be running longer than about 90 minutes, a load fits the guideline. The ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position stand gives 10 to 12 g of carbohydrate per kg per day for 36 to 48 hours before events longer than 90 minutes (Thomas, Erdman and Burke, 2016).

When should I start carb loading before a half marathon?

About 36 to 48 hours before the start, alongside a taper. For a Sunday morning race, that usually means starting on Friday and finishing with a familiar, carb-forward dinner on Saturday.

How many carbs should I eat when carb loading?

The guideline is 10 to 12 g of carbohydrate per kilogram of body weight per day (Thomas, Erdman and Burke, 2016). For a 70 kg runner, that's 700 to 840 g a day, which is far more than most people expect.

Is a big pasta dinner the night before enough?

Not for a full load. The guideline describes 36 to 48 hours of high carbohydrate intake with reduced training (Thomas, Erdman and Burke, 2016). The night-before dinner is just the last meal of it.

Do I need to carb load for a 10K?

Usually not, unless you'll be running longer than 90 minutes. Events under 90 minutes fall under normal daily fuel needs of 7 to 12 g/kg per day in the position stand (Thomas, Erdman and Burke, 2016), so a normal, carb-forward dinner the night before is enough.