Glucose, a form of sugar, is your body's main source of energy. It's created during the process of digestion, when your digestive system breaks down the food you eat. If you don't need all that energy right away, your body has ways to store it and then use it later.
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Glucose Use and Storage
When your body converts food into glucose, that increases your blood sugar, which, in turn, causes your pancreas to make the hormone insulin. Insulin helps glucose enter the cells that need it now for energy, and it helps your body convert excess glucose into forms that can be used later on.
Your body's capacity to store excess glucose is limited, though, says Robert H. Eckel, MD, president of medicine and science at the American Diabetes Association. "It only occurs in the liver and in muscle," he says. In both those of those places, glucose is stored in a form called glycogen.
Glucose in Your Muscles
Your muscles use stored glycogen for energy when you're physically active, especially during moderate-intensity exercise and during the final phases of exercise, says Dr. Eckel. But the glycogen stores in your muscles aren't sufficient for long-term needs.
"Because there's only a limited amount, that's one reason we get tired when we exercise too much," he says. "Ultimately, when there's not enough glucose to really adequately burn in the muscle, you form lactate, and that's what makes the muscles tired."
According to the Mayo Clinic, endurance athletes — like those who run marathons or participate in triathlons — may be able to increase the energy storage in their muscles by carbohydrate loading. That involves increasing the amount of carbs you eat and drink in the days leading up to the high-endurance event. But it's typically only beneficial if you're preparing for a marathon or another athletic event that will keep you active for 90 minutes or more, Mayo notes.
Read more: What Happens to Your Blood Sugar When You Exercise?
Glucose in Your Liver
While glucose is stored in the liver as glycogen, your body doesn't typically call upon that source for physical activity.
"The glycogen content in liver is mostly there for flight and fight reactivity," says Dr. Eckel. A good example of this mechanism, he says, is in people who take insulin for diabetes. If the insulin makes their blood sugar too low, "ultimately the liver glycogen breaks down to be able to make enough glucose to circulate in the bloodstream to meet the needs of the brain," he explains.
In rare cases — the Cleveland Clinic estimates one in 20,000 to 25,000 births — children are born without the ability to store or break down glycogen. They have what's called a glycogen storage disease, which can affect the liver and muscles as well as other areas of the body.
Glucose Storage in Fat
Glucose can also be converted into a type of fat called triglycerides and stored in your fat cells, according to the Mayo Clinic. Think of this as your long-term energy reserves. "Fat storage is what keeps us alive when we don't eat for 40 or 50 days," says Dr. Eckel.
Your body typically turns to this energy source after it's tapped into your glycogen stores. "Let's say after a major fast — like you're fasting for days — you no longer have glycogen stores," says Deena Adimoolam, MD, an assistant professor of endocrinology at the Icahn School of Medicine in New York City.
"Your body starts utilizing glucose from other sources like fat. And so, what happens is, you have a breakdown of fat, a process called adipolysis, which leads to a release of glucose, which can be then used as energy," explains Dr. Adimoolam.
How long it takes before your body starts breaking down fat varies from person to person, says Dr. Adimoolam. "It depends on how much glycogen storage they have and how long they've been fasting for," she says. "Some people might utilize all of their glycogen just after an overnight fast. For others, it might take 24 hours."
- Mayo Clinic: "Diabetes Treatment: Using Insulin to Manage Blood Sugar"
- Mayo Clinic: "Carbohydrate-Loading Diet"
- Mayo Clinic: "Triglycerides: Why Do They Matter?
- Deena Adimoolam, MD, assistant professor, endocrinology, Icahn School of Medicine, New York City
- Cleveland Clinic: "Glycogen Storage Disease (GSD)"
- Robert H. Eckel, MD, president, medicine and science, American Diabetes Association