You Ask, I Answer: Carb Loading

With the New York City marathon happening tomorrow, I thought this would be a good time to ask you this.

What is "carb loading"? I know runners do it before a race, but how does it work, and why do they do it?

-- Alice Hanover
New York, NY

Carb loading is about optimizing glycogen stores in muscle tissue (glycogen is the biochemistry way of saying "stored energy.")

Depleting these stores and then providing the body with an extreme amount of carbohydrates makes an enzyme known as glycogen synthase store the incoming carbohydrate (which is converted to glucose and then, ultimately, glycogen) very effectively.

Think of this as equivalent to a master suitcase packer who can fit in one suitcase what most people would need three for!

Why do this? Well, the higher the glycogen stores, the longer athletes can last in extended aerobic exercises (that is why long-distance runners -- as opposed to bodybuilders -- practice this.)

Carb loading can double the amount of glycogen stored in muscle tissue, so it can potentially provide marathon participants with significant advantage, provided it is done correctly.

There are two ways to do this (both methods take place over the course of seven days prior to the athletic event.)

The traditional way (developed in the 1960s) had athletes sharply decreasing their carbohydrate intake to Stage 1 Atkins levels (no more than 30 grams of carbohydrates -- what you find in a slice of bread -- a day) while vigorously training for three days.

FYI: Putting a long-distance runner on a low-carb regimen is pretty much the most cruel thing you can do.

The next three days, carbohydrate intake would skyrocket to approximately 80% of calories while physical activity continually decreased.

On the seventh day (the day before the race), carbohydrate intake would remain extremely high and physical activity was not to be performed.

Newer methods are less extreme.

For the first three days, athletes consume roughly 60 percent of their calories from carbohydrates. Once that stage is complete, they take in approximately 80% of their calories from carbohydrates for the next three days.

In this method, exercise is on a constant decline (from very intense in Day 1 to absolutely none in Day 6.)

Since carb loading asks for high amounts of carbohydrates, this is one of the few times you will hear Registered Dietitians recommend low-fiber foods.

This serves two purposes -- it allows athletes to fill up less quickly and also prevents stomach complications (80 percent of calories from high-fiber foods could get rather uncomfortable.)