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What Happens to Muscles When Glucose Is Not Available?

By Jill Corleone, RDN, LD

Glucose is to your muscles what gas is to your car -- it's the fuel that makes it go. While your body has a backup system when it comes to muscle fuel, if you're in the middle of a workout or sporting event and your muscles run out of glucose, your body may not be able to respond quick enough. This can lead to weakness and poor performance. If you think your muscles are running low on glucose often, consult a dietitian to help you design a diet that keeps them fueled.

Glucose for Muscle Energy

When you eat food that contains carbohydrates, such as bread or fruit, your body breaks it down into glucose. This is absorbed by your intestines, sent to your bloodstream and delivered to not only your muscles but also your nervous system. The glucose that is not used immediately for energy is stored as glycogen in your muscles and liver; it is used in between meals or during periods of exercise. Your body is able to store up to 2,000 calories worth of glycogen, according to Iowa State University Extension and Outreach.

Low Muscle Glucose

Running out of glucose is often referred to as "hitting the wall," and it is most often associated with distance runners. It's the point where you suddenly feel very tired and lack any energy to go farther. In addition to affecting your exercise performance, the low blood sugar may also impair your mental abilities because glucose is also the primary fuel for your brain. Not getting enough carbohydrates in your diet may increase your risk running out of glucose.

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The Body's Backup System

When you've run out of glucose for energy, your body may turn to fat for fuel. In fact, fat serves as a primary source of fuel for low-intensity and endurance exercise, says the Iowa State University Extension and Outreach. That doesn't mean you shouldn't eat carbohydrates, however. Getting enough carbs in your diet when you exercise spares the use of protein for fuel, which is important when you're trying to maintain or improve muscle strength.

Fueling to Maximize Stores

If you're trying to prevent your muscles from running out glucose, you need to eat enough carbohydrates. How much you need depends on your exercise routine, but it ranges from 45 percent to 65 percent of calories. To maximize glycogen stores, Iowa State University Extension and Outreach suggests you aim for 60 percent of calories from carbs. Try to include mostly healthy carbs to up the nutrition in your diet. In addition to fruit, that means whole grains, beans, vegetables and low-fat dairy or plant-based alternatives.

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