How much is too much? Or not enough?

As avid runners, most of us know that we should ingest carbohydrates when we complete training runs longer than 90 minutes or compete in longer races, from the 15K to an ultramarathon. But how much is too much or not enough?

While debate exists over how carbohydrates affect the body, the scientific theory I tend to side with these days is the central governor theory, or what happens to the central nervous system when we eat carbohydrates. When we run for longer than two hours at race pace, our bodies must use both stored fat and carbohydrates, or glycogen, for fuel. Unfortunately, glycogen reserves last for only 60 to 90 minutes; after that, the body turns to stored fat. To fend off fatigue, the nervous system needs glucose to keep stimulating and firing the muscles, otherwise performance suffers; on the extreme end, we bonk and see stars. Our bodies can be trained to burn fat while running at fast paces, but at some point, carbohydrates are needed to give the nervous system a jolt.

Now that it’s understood why it’s necessary to take in carbohydrates while running, it’s back to the main question: How many? The No. 1 factor in determining how many carbohydrates an endurance athlete should eat while running is body mass—that’s right, step on the scale and the question will be answered. To put it simply, the heavier the runner, the more glucose he needs to fuel his muscles and vice versa. Don’t try to eat the same amount on the run as your smaller or larger training partner.

RELATED: Is Your Diet Holding You Back?

Over the years, with trial and error and plenty of research on carbohydrate intake, I have found the following formulas work for almost all endurance athletes:

Body weight in kilograms
(BW in pounds/2.2) X 0.7 = grams of carbohydrates per hour

Body weight in kilograms
(BW in pounds/2.2) X 1.0 = grams of carbohydrates per hour

These two formulas will calculate a high end and a low end to yield a range for appropriate hourly carbohydrate intake while training or racing. The range can be adjusted based on individual differences and effort level.

For efforts lasting 90 minutes to three hours, where a majority of the effort is at lactate threshold (85 to 90 percent of maximum effort) or 10K to 15K race pace, I recommend using the lower end of the range. For longer efforts lasting three hours or more, where a majority of the effort is below lactate threshold, use the upper end of the range. As the effort level goes up, most runners find it harder to ingest carbohydrates at the upper end of the range due to decreased blood flow to the digestive system. The key point is to experiment with the range and find out what amount works best for you, your effort level and the duration of the race or training run.

RELATED: Training And Racing With Whole Foods

It’s time to put the results from above to use: Find out how many grams of carbohydrates are in your favorite running foods—whether they are sport foods, such as gels or sports drinks, or good old-fashioned whole foods. On training runs, practice taking the recommended amount of carbohydrates in regular 20- to 30-minute intervals so that the nervous system receives a steady flow of glucose throughout the hour. The first carbohydrates can be taken right before the run or 20 to 30 minutes into the run. The most important aspect is that carbohydrate intake should be steady and consistent. Don’t wait until you feel hungry or see stars before carbing up because by then it’s too late. As the old saying goes, “Eat early and eat often!”


About The Author:

Based in Boulder, Colo., Scott Jurek is a seven-time winner of the Western States 100-mile trail run and the author of Eat and Run: My Journey To Ultramarathon Greatness.