Wednesday, March 28, 2007

A Visit From Our Resident Science Geek

Carbohydrate during exercise

Your muscles and brain are fueled by carbohydrates (glucose). At lower exercise intensities, your body is able to convert stored fat into fuel for your muscles. As the intensity increases carbohydrate becomes the primary fuel for the muscles. An average well nourished person's, body stores carbohydrate (as glycogen) in the muscle (about 325g) and liver (90-110g). This is about 2000 calories - enough to fuel a 20 mile run. These glycogen stores can be spared by taking in carbohydrate during exercise.

During exercise try to consume 30-60g of carbohydrates per hour. Most people cannot metabolize more than 60g of carbohydrate per hour so increasing intake will not increase your performance. In fact, over consumption of carbohydrates (and food in general - especially fats) may result in an upset stomach due to slower gastric emptying (think "sugar belly"). The more intense the exercise the higher the probability of getting stomach issues.

The concentration of carbohydrates also plays a role in gastric emptying. A 6-8% carbohydrate solution (most sports drinks are formulated in this range) is most quickly absorbed. If you are using gels or "real" food for your carbohydrate source be sure to ingest enough water to help speed absorption.

Theoretically, during a race you could fuel yourself on just a sports drink (I have worked with Iron man triathletes who use only sports drinks for their hydration and nutrition needs during competition). If you consume about 1L per hour of a sports drink to hydrate, you are also getting about 60g of carbohydrate. For longer races, you will most likely not want to do this. You will feel hunger and want to eat. Try eating during less intense sections of the course to minimize the risk of stomach upset.

Things you might try (during training) to test your nutrition strategy:

1. If you are consuming more than the 60g per hour and have stomach issues, try cutting back and see if the issue goes away.

2. Even if you are not having stomach issues and are consuming higher amounts of food/carbohydrates, try cutting back and see if it affects your performance. You may find you need to carry less food during a race.

3. Focus on the concentration of carbohydrate in your stomach. Use a sports drink for your energy source, or know the carbohydrate content of your food and drink enough water to achieve a 6-8% solution in your stomach.

4. You can "train" your digestive system to handle exercise with food in the stomach. Practice training after eating. Also you'll learn which foods you handle more readily than others.

Next post I will focus on carbohydrates and recovery.

Happy Training,


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