If you've ever trained for an athletic event or researched what to eat after a hard workout, you've likely heard of glycogen: A type of energy — stored in the muscles and liver — that fuels daily activity. In general, glycogen should be replenished after exercise. If you're following a low-carb diet, however, that may not be so easy.
That's because glycogen is made when the body breaks down carbohydrates into glucose, according to the Centers for Disease Control and Prevention (CDC). Without carbs, you lack an external source of glucose, which can result in depleted glycogen stores.
Glycogen is important not only for muscle development, but also for fueling the brain, red blood cells and internal organs, according to the National Center for Biotechnology Information (NCBI). So what happens when you don't give the body what it needs? That depends on the specific diet you're following and the metabolic state your body is in. Here's what you should know about glycogen stores and low-carb or ketogenic diets.
Refueling on a Very Low-Carb Diet
The current Dietary Guidelines for Americans recommend getting 45 to 65 percent of calories from carbs, which is generally enough to keep the glycogen stores in your muscles and liver full — especially if you're consuming some carbohydrates during and after long workouts. In a 2,000-calorie diet, for example, this is between 225 and 325 grams of carbs daily.
Read more: Healthy Low-Carb Eating Plan
But some low-carb diets recommend scaling your carbohydrate consumption back to 50 or fewer grams per day. Examples of this type of plan include the first phase of the Atkins 20 diet or some versions of the ketogenic diet.
The issue: This doesn't provide enough carbs to fully restore liver or muscle glycogen, says David Bridges, PhD, assistant professor of nutritional sciences at the University of Michigan School of Public Health. And actually, the body doesn't need to produce much glycogen at this point, because it shifts into ketosis — a state in which the body runs off a different fuel source consisting of fatty acids and ketones. (Ketones are compounds your body naturally produces when too little external glucose is available, according to NCBI.)
"When you're in ketosis, the liver takes the fat and very quickly turns it into ketone bodies for the heart and the brain and the muscles to use for fuel," says Bridges. At the same time, he adds, the body also produce a small amount of glucose through a process called gluconeogenesis, using amino acids instead of carbohydrates.
Humans can certainly function in ketosis, although the science isn't exactly clear as to whether that functioning is more or less efficient in this state. In a July 2014 study published in Nutrients, researchers found that during high-intensity efforts, exercise may be impaired by a ketogenic diet — most likely due to diminished glycogen stores.
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For shorter or easier workouts, the glycogen produced through gluconeogenesis is generally enough to keep athletes feeling fast and strong, according to Bridges. But for longer or harder sessions — like endurance bike rides or marathon runs — it's not adequate. "Scientists are learning that a ketogenic diet is either bad for your athletic performance or at the very least, not good," he says. "With this particular diet, the body just can't keep up with the demand on energy for really long workouts."
If you do choose to follow a ketogenic diet while working out regularly, it's important to eat enough fat and protein to make up for your low carbohydrate intake, says Amy Goss, PhD, a registered dietitian and assistant professor of nutrition sciences at the University of Alabama at Birmingham. Otherwise, you'd take in too few calories and send your body into a starvation state, which could cause you to feel lethargic and weak.
If you're committed to a very-low carb diet, be sure to consume — along with lots of leafy greens and proteins — ample amounts of healthy fats, including nuts and seeds, olive oil and avocado, says Goss. You aren't restoring glycogen, but you're providing fat calories to use for fuel.
Concentrate on Quality Carbs Post-Workout
There are also more moderate low-carb diets, which advise limiting carbs to between 50 and 150 grams a day. A plan like this may make more sense for serious athletes, says Bridges, since they'll still take in enough carbs to restore glycogen in their liver and muscles.
It's smart to refuel with carbohydrates after a long or hard workout, even if you're sticking to a moderate- or low-carb diet. But that doesn't mean you should go overboard.
"I'm a big proponent of training the way you're going to compete, and that goes for what you eat, as well," says Bridges. "If you're an athlete and you're limiting your carbs in everyday life, you don't have to suddenly eat a ton of bread or pasta after exercise or right before a race."
Instead, plan your meals and snacks so that you can use part of your carb allowance after every sweat session, or schedule your workouts so you can eat a full meal within an hour afterward. Include some protein, as well, to help with muscle repair, recommends the Academy of Nutrition and Dietetics. Examples of appropriate post-workout snacks include a whole-wheat turkey wrap, a glass of chocolate milk or plain yogurt with fresh berries.
To make room for these carbohydrates in your diet, Bridges suggests cutting out added sugars and empty calories, like soda and refined grains. "That's basically a low-carb diet in the broadest sense, and it's a pretty healthy way to do it," he says. "Especially for athletes who need the energy that high-quality carbohydrates can provide."
- Nutrients: "The Effects of a Ketogenic Diet on Exercise Metabolism and Physical Performance in Off-Road Cyclists"
- Academy of Nutrition and Dietetics: "Timing Your Pre- and Post-Workout Nutrition"
- Health.gov: "Dietary Guidelines for Americans"
- Atkins.com: "Atkins 20, Phase 1: Induction"
- University of Michigan School of Public Health: "Dave Bridges, PhD"
- The University of Alabama at Birmingham Nutrition Obesity Research Center: "Amy Miskimon Goss, PhD"
- Centers for Disease Control and Prevention: "Manage Blood Sugar"
- National Center for Biotechnology Information: "Biochemistry. 5th edition"