Running on Empty: When Your Body Runs Out of Glycogen

You've probably seen pre-marathon gatherings of athletes chowing down on pasta. But there's more that goes into creating the glycogen needed to fuel a long workout — or any workout. Here's what happens when you run out of this essential energy source and how you can keep that from happening.

Think of your body as your own personal sugar refinery. When you eat a meal containing carbohydrates, your digestive system breaks down those sugars and starches into glucose, a simple sugar that serves as an immediate energy source. Excess glucose — whatever isn't needed right away to power the body — is stored as glycogen in a process called glycogenesis.

Glycogen is stored mostly in your muscles for use during activity and in your liver to fuel your brain and other organs. When glycogen is needed, your body reconverts it into glucose. That process is called glycogenolysis.

The important thing to remember is that both processes take time. That's why you shouldn't exercise immediately after eating — your body is focused on breaking down that food. "Two hours after a meal is the optimal time," says Richard Peng, MBA, CDE, a clinical exercise physiologist and diabetes educator with HealthCare Partners Medical Group in Los Angeles.

However, you also shouldn't wait too long or exercise on a completely empty stomach, like in the morning before breakfast. "If you're weak from a lack of food, you won't benefit as much as you could from exercise — your muscles won't work as well as they could," Peng says.

Read more: Is Cardio Good to Do on an Empty Stomach

It's not just endurance athletes who should worry about running out of glycogen, Peng says. How this affects you, though, will depend on your fitness level. If you're new to exercise, you can run out within 5 or 10 minutes. Here's why: "It takes training for your muscles to increase their glycogen stores, so someone just starting out likely has very little," he says. "When you train regularly, you'll have more stamina."

Read more: What Causes Muscular Fatigue?

"If your body runs out of stored glycogen, it will try to break down fat cells for the energy you need," Peng adds. This process is called gluconeogenesis, or the formation of glucose from new sources. The problem is that this takes more time than converting glycogen. You may run out of steam in the middle of your workout and feel fatigued, according to a February 2018 report in the journal Nutrients. "If you chug along and eventually get a second wind, it's likely that you've processed stored fat, but in the interim you won't be working out efficiently," Peng says.

There's also a more serious concern. Between the time your glycogen is depleted and gluconeogenesis kicks in, you might experience symptoms of low blood sugar (hypoglycemia). Those symptoms can include extreme fatigue, dizziness, a near complete loss of energy and the inability to focus and react to directions. It's not uncommon to collapse from the extreme fatigue. You could even experience hallucinations under these conditions.

Read more: How to Train the Week of a 10K

If you're planning on running a 10K or even a 5K and you've never done it before, it's important to train gradually. "Increase your workouts by 10 percent a week so that your body adapts [and] stores and uses glycogen most efficiently," Peng says. A great way to do this, he adds, is "weight training to strengthen muscles rather than to build bulk, so you want sets of 15 reps using low weight."

During very long workouts, you will need sugar. "Energy gels or blocks are the best ways to replenish, but you need to take them before you think you'll need them," he says. For instance, if training has shown that you lose steam at mile six, start to replenish at mile four. Remember, your body needs time to absorb the glucose for energy—there will be a gap in glucose availability if you wait until you feel yourself petering out.

Refueling after exercise is important because it prepares your body for the next workout, according to the American Council on Exercise. The report in Nutrients suggests that a combination of protein and carbohydrate might be as effective as carbs alone, especially for people who want to limit carbs for other reasons.

For the endurance athlete, the guidelines for daily nutrient needs—carbs, protein, fat and supplements—should be personalized, taking into account your body weight, the type of exercise you're doing, the length of time you'll be working out and the weather conditions if your exercise is outdoors, according to a June 2019 report in Nutrients.