Why Your Body Always Uses Carbohydrate during Exercise

Why Your Body Always Uses Carbohydrate during Exercise

By Sheri Colberg, Ph.D., FACSM

At rest, you’re already using about 40 percent carbohydrate to fuel your body’s energy needs under normal circumstances, but as soon as you start to do any exercise, your carbohydrate use increases. Usage depends on intensity, so harder workouts will always require greater use of blood glucose and glycogen than easier ones will, but even the easiest workouts will use some. Muscle contractions stimulate the breakdown of glycogen in your muscles, along with glucose uptake from your bloodstream. Carbohydrate is more fuel efficient, meaning that your body gets more ATP out of it for a given amount of oxygen, so for that reason and others it is your body’s number one choice of fuels.

Fatigue (defined as the inability to continue exercising at the same intensity) is often caused by depletion of glycogen stores in the muscles that you’re using, resulting in the phenomenon of “hitting the wall” that is common in longer-distance events. Reaching the point when you’re exercising at a moderate pace usually takes longer than 90 minutes, but it can take less time during intense or near-maximal activities. Your muscles use some blood glucose along with the glycogen, sometimes more depending on your insulin levels (more on this topic in the next chapter), but you’ll also start using glucose at a faster rate when glycogen stores start to get low—that’s when you really have to watch out for low blood sugars! You can deplete muscle and liver glycogen, especially if you haven’t eaten much for a while, and then you’ll really be in trouble.

If you exercise long enough, your body will use a lot of carbohydrate, so starting with adequate muscle glycogen stores to prevent both early fatigue and hypoglycemia is critical. By taking in carbohydrate during exercise, you can keep your blood sugars higher for longer and prevent fatigue. They are digested and absorbed more quickly than either protein or fat; carbohydrate usually starts to hit your bloodstream within five minutes. The amount of carbohydrate that you need to take in depends on how long and hard you’re exercising, what time of day it is, and how much insulin is in your system. You will need to monitor your blood sugars to figure out the appropriate amount (if any) for each different activity that you do. Refer to the table below for some general guidelines for increasing your carbohydrate intake for aerobic exercise.

Table: General Carbohydrate Increases for Endurance Sportsa

Notes: The recommended quantity is given in grams of rapidly absorbed carbohydrate. One fruit or one bread exchange equals 15 grams of carbohydrate. bLow-intensity activities are done at less than 50%, moderate activities at 50 to 70%, and high- intensity activities at 70 to 85% of heart rate reserve (refer to chapter 1).cFor blood sugars above this level, or when ketones are present, an additional dose of rapid-acting insulin may be required to reduce these levels during an activity, and the recommended carbohydrate intake may be higher than actually needed. dIntense (near-maximal), short-duration exercise may actually cause blood sugar levels to increase.

This column is excerpted from Diabetic Athlete’s Handbook (available November 6, 2008 from Human Kinetics), which contains essential exercise-related information and examples for type 1 and type 2 diabetic exercisers. Look for it in stores or find links to places to buy it online on www.shericolberg.com, along with additional information.