Glycogen is energy stored within your muscles and liver that fuels daily activity, from running on the treadmill to doing the dishes after dinner. This stored energy also plays an important role in fueling your brain, red blood cells and internal organs.
Your body breaks down carbohydrates into glucose, storing it as glycogen. Without carbs, you lack an external source of glucose, resulting in depleted glycogen stores. Some low-carb diets that contain 100 to 150 grams per day can still provide enough carbs to restore glycogen adequately for the average person. When you're on a very restrictive low-carb diet of 50 grams or fewer daily, your body switches to a different fuel source and you don't need to worry about replenishing glycogen stores.
Follow a Moderate Low-Carb Diet
Glycogen stored in the muscles is the body's first choice for fuel for hard physical efforts. You use it, along with stored fat, to power workouts, a walk to the bus or household chores. Liver glycogen is usually used by the brain, kidneys and red blood cells for fuel. Sometimes your body uses liver glycogen to fuel activity too.
A standard American diet contains 45 to 65 percent of calories from carbs and keeps the glycogen stores in your muscles and liver full. In a 2,000-calorie diet, for example, this is between 225 and 325 grams of carbs daily. In a moderately restrictive low-carb diet consisting of 50 to 150 grams of carbs per day, you'll still take in enough carbs to keep your liver glycogen full and to restore some glycogen in your muscles.
Mostly, however, your body will adapt to using a greater amount of fat for fuel. If you eat lots of carbohydrates, your body relies on that glucose and the resulting glycogen for energy. But, as you reduce carb intake, your body adjusts to using a greater percentage of fat to fuel workouts and other physical activity. The carbs you do consume will be converted to liver glycogen to fuel the brain and other organ cells, and some may go to your muscles for repair after an especially intense workout.
Ketogenic Diets and Glycogen
A very restrictive low-carb diet of 50 or fewer grams daily does not provide enough carbs to restore liver or muscle glycogen. But, you don't need to, because your body shifts into ketosis, where it 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.
If you combine a very low-carb diet with low fat intake, you usually take in too few calories and send your body into a starvation state. Women need at least 1,200 calories per day and men need 1,800 to prevent metabolic slowdown. You'll feel lethargic due to too few calories, depleted glycogen stores and muscle loss. You're basically left without an adequate fuel source.
With such a diet, your body isn't getting enough calories to fuel you -- it has no glycogen or fat for energy. You can't restore glycogen stores, or any energy system, without eating more carbs or fats. If you're committed to a very-low carb diet, be sure to consume -- along with your leafy greens and proteins -- ample amounts of quality fats, such as coconut and olive oils, avocado, full-fat dairy and fatty cuts of meat. You aren't restoring glycogen, but you're providing fat calories to use for fuel.
Athletes and Glycogen Replenishment
Athletes accustomed to standard carb intakes of 200 grams or more daily may find adaptation to a ketogenic diet challenging at first. They're accustomed to using glycogen for fuel during a workout. Marathon runners or triathletes often have to refuel regularly during training and events since muscles and the liver hold only about two hour's worth of glycogen.
After several weeks, endurance athletes who train for long periods at a low to moderate intensity may benefit from a ketogenic diet by losing weight, improving recovery and enhancing fat metabolism. But it takes a while to adapt to using more fat for fuel and using fewer carbohydrate replacements as you exercise. Exactly how long depends on how the individual body reacts.
Research published in Nutrients in 2014 showed that, during high-intensity efforts, exercise may be impaired by a ketogenic diet. This is largely due to diminished glycogen stores.
Concentrate Carbs Post-Workout
If you are an athlete and want the weight-loss benefits and blood sugar-stabilizing benefits of eating fewer carbs, but are not going to the extremely low levels of a ketogenic diet, shoot for some post-workout carb intake. Plan to snack about 20 minutes after you finish your session so you can get the glucose you need to restore glycogen stores right after you've exhausted them.
Include protein to help with muscle repair. Examples of appropriate post-workout snacks include a turkey breast sandwich with lettuce and tomato, a glass of chocolate milk or plain yogurt with fresh berries. At other meals, minimize your carb intake to keep your daily intake relatively low.