What Is Carbo Loading & Why Do Athletes Do This?

A big bowl of oatmeal on a table.
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Marathon runners, endurance cyclists and elite athletes all know the feeling when, during a big competition or long training session, energy levels plummet. This is due to a lack of readily available fuel to power the muscles. Hitting the wall like this can potentially turn you from beating your best time to falling way behind the pack, but you can help prevent it by implementing a dieting strategy known as carbo loading.

The Theory

When you exercise, your body uses glycogen -- a form of carbohydrate stored in the liver and muscle cells -- to provide energy. At some point, however, this glycogen runs out, and you experience a drop in performance as fatigue sets in. Topping up your glycogen stores before an event by carbo loading can prevent this, leading to a potential increase in your energy and endurance.

The Benefits

According to sports nutritionist Nancy Clark, under normal dietary circumstances an athlete has around 80 to 120 millimoles of stored glycogen per kilogram of body weight -- or around 36 to 55 millimoles per pound -- but a carb-loaded athlete can have up to 200 millimoles per kilogram, or 90 per pound. This can improve endurance by 2 to 3 percent, which could be the difference between winning and losing.

The Plan

Before you load carbohydrates, you first have to deplete your carb stores. This is to trick your body into thinking that there's a problem with your glycogen stores, so it compensates by storing more when you load, notes athletics coach Brian Mackenzie. According to the Australian Institute of Sport, you should deplete for three to four days by eating a low-carb diet, then load for three to four days. During this load you need around 7 to 12 grams of carbohydrate per kilogram of body weight each day, or 3.2 to 5.5 grams per pound.

Putting It Into Action

When carbo loading before competition, it's important that you have a trial run at least a couple of weeks beforehand. This is so you can determine exactly how many carbs you personally need to feel energetic and perform at your best. Additionally, carbo loading works far better if you choose foods that minimize gastrointestinal stress, so experiment with different food combinations before the big day. Check with your doctor before implementing a carb0 load and ask your coach for advice if it's your first time.