How to carb load to maximize your glycogen stores

What a carb load does

More glycogen at the start, more power in the last hour

Carb loading means eating more carbohydrate than usual for a few days so your muscles store more glycogen than they normally hold.

In a 1995 study, 8 cyclists each completed 2 rides of 3 hours, one after raising their carbohydrate intake by about 72% and one on their normal diet.

Muscle biopsies showed the athletes who carb loaded started with 47% more muscle glycogen, and the riders covered more distance and held more power over the final 60 minutes.

That means the extra glycogen shows up in the final hour of your race, when your pace would otherwise start to fade.

The same pattern turns up in runners. A study of 257 runners at the 2009 London Marathon compared what they ate the day before with how they ran.

Runners who ate more than 7 grams of carbohydrate per kilogram of body weight (3.2 g per lb) ran faster, and slowed down less over the back half, than the runners who ate less than 7 grams.

Do you need one?

Your finish time decides

You store enough glycogen to cover about 90 minutes of running.

If your race takes less than that, you finish with glycogen to spare, so loading adds nothing. Eat normally, keep carbohydrate up the day before, and get race morning right.

Between 90 minutes and 2 hours, one elevated day is enough. Raise carbohydrate to 6 to 8 g per kilogram of body weight (2.7 to 3.6 g per lb) for the final 24 hours and skip the multi-day version.

Past 2 hours, do a full load. This is where the studies measured the biggest gains.

Total calories have to go up

The load needs extra calories

Older advice said to keep your calories the same and swap fat for carbohydrate. That method has been tested, and it does not raise muscle glycogen.

In a 2001 study, women who raised carbohydrate as a share of their calories did not raise their muscle glycogen at all. When their total calories rose by 34% alongside the carbohydrate, muscle glycogen rose by about 17%.

That means you have to eat more total calories, and the extra should be carbohydrate. For a 150 lb (68 kg) runner that is roughly 700 to 1,000 calories a day above a normal race-week day.

Cut fat back a little, because it fills you up and you need room for 600 g of carbohydrate. Keep protein at about 1.5 g per kilogram of body weight (0.7 g per lb).

Skip the depletion run

Rest plus the right dose fills your muscles in 24 hours

The old protocol had you do a hard run and cut carbohydrate a few days out, on the theory that emptying your muscles first makes them store more later.

Researchers tested this directly in 2002. 8 endurance-trained men did no depleting exercise at all, stayed inactive, and ate 10 g of carbohydrate per kilogram of body weight (4.5 g per lb) each day. Muscle glycogen went from 95 to 180 mmol per kg of wet muscle in 24 hours, close to double, and 2 further days of loading added nothing.

That means you do not need the depletion run. It adds fatigue in race week and no extra glycogen.

Race morning

Race morning is where most runners get it wrong

Most carb-loading guides stop at the day before the race and say nothing about breakfast.

Your muscles keep their glycogen overnight, but your liver uses some of its glycogen while you sleep. Breakfast refills it.

Starting 3 hours before the gun, take 1 g of carbohydrate per kilogram of body weight (0.45 g per lb) for every hour between your meal and the start. For a 150 lb (68 kg) runner eating 3 hours out, that is about 205 g of carbohydrate at breakfast.

A bagel with jam, a banana, a large glass of juice and a sports drink gets you close. Nothing on that plate should be new.

Expect the scale to go up

The extra weight is glycogen and water

Your body stores about 3 g of water with every gram of glycogen, so a full load adds 2 to 4 lb (1 to 2 kg). That weight means the load worked. You lose it again over the first few miles as you burn the glycogen off.

Go low on fiber for the final 24 to 48 hours, make lunch the day before your biggest meal, and use liquid carbohydrate (juice, sports drink) when you cannot eat any more solid food.