When Food Can Make You Faster

Marathon fueling strategies

January 1, 2003

Here’s the reality. At marathon pace, a typical runner burns in the range of 200 grams of carbohydrate per hour. But the average runner can absorb only 60 to 80 grams per hour when carbohydrate is consumed in the most absorbable form—a six to eight percent liquid solution. So the ACSM guidelines are based not on how much carbohydrate you would need to absorb in order to prevent depletion but on a very conservative estimate of how much you can actually cram into your stomach without suffering gastrointestinal distress (cramps, bloating, nausea). The vast majority of runners don’t even get close to this.

The situation with fluid loss is similar. Body fluid (comprising water and electrolyte minerals) is often lost at rates exceeding 1.5 liters per hour during marathon-pace running. Yet the average runner is unable to tolerate drinking at more than a third of this rate while racing. Drinking any faster—even though the body could use a lot more fluid—causes uncomfortable stomach sloshing and can eventually result in cramps, bloating and nausea as well.

The bottom line is that you simply cannot get enough fluid or carbohydrate while running a marathon. Sure, you could actually swallow as much fluid as you sweat out and as much carbohydrate as you burn, but it would only come back up the way it went down, slightly warmer.

According to Christine Rosenbloom, Ph.D., and a nutritionist at Georgia State University, however, "There are huge variations in the amount of fluid consumption individual runners can tolerate and in the amount of carbohydrate they can absorb while running." And you cannot know how much your body is able to handle—thus you can’t be certain you’re consuming the right amount of nutrition in your next marathon—unless you make the effort to discover it in your training. Also, as with every other aspect of running, practice helps. "By routinely trying to take in more nutrition in training runs and perhaps also in the occasional shorter race, you can learn to tolerate more than you could right now," says Rosenbloom. For these two reasons, "training your gut" is essential for optimal marathon performance.

How to Train Your Gut

The first thing you need to do is to start thinking about your bolus. That’s right, your bolus. It’s a funny name for the contents of your stomach. The reason it’s important is that, up to a point, the greater the volume of nutrition in your stomach, the faster it empties. And the faster your stomach empties, the faster the needed fluids and carbohydrate get into your bloodstream. So it’s a good idea to practice beginning your workouts with a larger bolus than you’re used to having.