Carb Loading Still Works. You're Just Doing the 1990s Version.

Yes, carb loading still works, but only for races longer than about 90 minutes, and the modern version looks nothing like the old one. Skip the depletion week and the giant pasta party. Eat about 10 to 12 g of carbohydrate per kg of body weight per day for the 36 to 48 hours before your race, while you taper, and keep fibre and fat low. That fills your muscles without wrecking your gut.

Key takeaways

• Carb loading helps in events lasting more than about 90 minutes, such as marathons, long half marathons and ultras. It's unnecessary for a 5K or 10K.
• The old "deplete then load" method isn't needed. A short taper plus 1 to 2 days of high carbohydrate intake does the job.
• Target 10 to 12 g/kg/day for 36 to 48 hours. For a 60 kg (132 lb) runner, that's 600 to 720 g of carbohydrate a day.
• Most runners under-eat. A London Marathon study found those who ate more than 7 g/kg the day before ran faster.
• Expect to gain 1 to 2 kg (2 to 4 lb). That's stored glycogen and water, which is exactly what you want.

The old-school version (and why it faded)

Carb loading began with Scandinavian research in the 1960s. In a classic 1967 study, Bergström, Hermansen, Hultman and Saltin showed that muscle glycogen levels, and endurance time to exhaustion, depended heavily on diet. Their protocol became famous: a hard, glycogen-draining workout, about three days of very low carbohydrate eating while still training, then three days of very high carbohydrate eating. Versions of that ritual were still being copied by runners for decades afterwards.

It worked, but it was miserable. Runners felt flat, irritable and fragile during the low-carb phase, and doing hard training while depleted raised the risk of a bad session or an illness just before the big day.

In 1981, a study by Sherman, Costill and colleagues found that a gradual taper combined with a high-carbohydrate diet in the final three days produced glycogen levels similar to the brutal depletion method. No crash, no misery. That "modified" approach became the standard most coaches still recommend.

Then research went further. In 2002, Bussau and colleagues at the University of Western Australia had trained athletes rest and eat about 10 g of high-glycaemic carbohydrate per kg of body weight. Muscle glycogen nearly doubled after just one day and didn't rise further after two more days. So if you're still doing a week-long, depletion-first ritual, you're running a protocol that science has moved past.

Does carb loading actually make you faster?

For the right race, yes. A 1997 review in Sports Medicine by Hawley and colleagues concluded that carbohydrate loading can improve performance over a set distance by roughly 2 to 3% in events longer than 90 minutes. Over a 4-hour marathon, 2% is close to five minutes.

Real-world data point the same way. In a 2011 study in the International Journal of Sports Medicine, Atkinson and colleagues tracked 257 runners preparing for the 2009 London Marathon. After accounting for sex, body size and training, those who ate more than 7 g/kg of carbohydrate the day before the race ran faster overall and held their pace better late in the race than those who ate less. It was an observational study, so it can't prove cause and effect, but it lines up with the lab research.

The mechanism is simple. Your muscles can store a limited amount of glycogen. At marathon pace you burn through a large share of it, and when it runs low your pace falls apart. That's the famous "wall." Starting with full tanks pushes that wall further down the road.

Who should carb load (and who shouldn't)

Do it for

• Marathons and ultramarathons.
• Half marathons if you expect to take longer than about 90 minutes and want to race them hard.
• Long trail races, triathlons and any continuous effort of 90 minutes or more.

Skip it for

• 5K and 10K races. Your normal glycogen stores are enough. Just eat normal, carb-rich meals in the day before. The 2016 ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position statement suggests 7 to 12 g/kg in the 24 hours before shorter events as general fuelling.
• Easy long runs in training. Loading is a race-day tool, not a weekly habit.

A note for women

Early research in the 1990s, including work by Mark Tarnopolsky's group at McMaster University, suggested women stored less extra glycogen than men when they simply raised the percentage of carbohydrate in their diet. Follow-up studies indicated that much of the gap came from not eating enough total energy. The practical message: women can carb load effectively, but you must actually eat more food, not just swap protein for pasta.

The modern carb-loading plan

Step 1: Taper first

Loading only works if you stop draining the tank. Reduce your training volume in the final week, and keep the last 2 days to rest or very easy running of 20 to 30 minutes.

Step 2: Load for 36 to 48 hours

For a Sunday race, start loading on Friday morning. Aim for 10 to 12 g/kg/day. Some recreational runners find 8 to 10 g/kg more manageable; that's still far better than simply eating your normal diet.

Step 3: Choose low-fibre, low-fat carbohydrate

Eating 600 g or more of carbohydrate in a day is hard if it all comes from wholegrains and vegetables. Your gut will rebel. Instead:

• White rice, white pasta, white bread, bagels, peeled potatoes, rice noodles, idli, plain dosa.
• Fruit juice, sports drinks, honey, jam, maple syrup, sweets, sorbet. Liquid and simple carbohydrate count, and they're easy on the stomach.
• Bananas, ripe mango, tinned peaches.

Step 4: Make room by trimming fat and protein

Don't just add carbohydrate on top of your usual meals. Keep protein moderate and reduce fat, such as butter, oil, cheese, nuts and fried food, so total calories rise without leaving you bloated.

Step 5: Spread it out

Eat every 3 to 4 hours instead of stuffing it into one or two meals. Make your biggest carbohydrate meal lunch the day before, then have a normal, familiar dinner. See the night-before-race dinner guide for what that last meal should look like.

A sample loading day (60 kg / 132 lb runner, about 620 g)

• Breakfast: 2 white bagels with 2 tablespoons of jam and 500 ml (17 oz) of orange juice. About 180 g.
• Mid-morning: 2 bananas and 500 ml of sports drink. About 85 g.
• Lunch: 3 cups of cooked white rice with a little chicken or tofu and soy sauce. About 135 g.
• Afternoon: 4 rice cakes with 2 tablespoons of honey. About 60 g.
• Dinner: 3 cups of cooked pasta with simple tomato sauce and 2 slices of white bread. About 165 g.

Scale every portion by your body weight. A 75 kg (165 lb) runner needs about 25% more. Then on race morning, eat a familiar breakfast of 1 to 4 g/kg of carbohydrate, 1 to 4 hours before the start, as the ACSM statement recommends.

Five carb-loading mistakes that still happen

• The one-meal load. A huge pasta dinner the night before can't fill your muscles on its own, and it's the meal most likely to upset your gut.
• Loading on wholegrains and salads. Great for daily health, terrible in volume 24 hours before a race.
• Starting too early. A full week of extra eating while you taper mostly adds unneeded calories. Two days is enough.
• Panicking at the scale. Each gram of stored glycogen holds roughly 3 g of water. A 1 to 2 kg gain is a sign the loading worked.
• Never rehearsing. Practise your loading menu before one or two of your longest training runs so race weekend holds no surprises.

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FAQ

How many days before a marathon should I start carb loading?

Start 36 to 48 hours before the race, so Friday morning for a Sunday marathon. Research shows trained athletes can top up muscle glycogen in about 24 to 48 hours if they're resting and eating enough carbohydrate. A full week of loading isn't necessary.

How many grams of carbs should I eat when carb loading?

Aim for 10 to 12 g of carbohydrate per kg of body weight per day. For a 60 kg (132 lb) runner, that's 600 to 720 g, and for a 75 kg (165 lb) runner, about 750 to 900 g. If that feels impossible, 8 to 10 g/kg is still a big improvement over most people's normal diet.

Should I carb load for a half marathon?

Only if you expect to run longer than about 90 minutes and plan to race hard. Faster runners finishing in under 90 minutes usually do well with a normal, carb-rich day before. Either way, a familiar, carb-focused dinner and breakfast are smart.

Why do I gain weight when carb loading?

Your muscles store each gram of glycogen with about 3 g of water, so a 1 to 2 kg (2 to 4 lb) gain is normal. It isn't fat. That extra glycogen and water is fuel and fluid you'll use during the race.

Is carb loading bad for your stomach?

It can be if you load with high-fibre or fatty foods, or cram everything into one meal. Choose white rice, bread, pasta, potatoes, juice and sports drinks, spread across the day, and reduce fat to make room. Rehearse the plan before a long training run to see how your gut responds.