The energy supplied by carbohydrates is less than the energy you get from fat. In the same manner, the amount of energy stored in your body as carbs -- glycogen -- is significantly less than the amount of energy converted into fat and stored in adipose tissue. You could survive about one day on stored glycogen, while stored fat could carry you for weeks. But both types of stored calories, even a small amount of fat, are essential to keep you healthy and support muscle activity.
Storage of Carbs for Quick Energy
When carbohydrates are digested, they’re broken down into single molecules of sugar, then absorbed into the bloodstream. Blood sugar, or glucose, goes to cells throughout the body that need it to produce energy. If you consume more carbs than the body needs, the extra glucose goes to the liver, where it’s converted into glycogen. Glycogen is the storage form of carbohydrates.
Glycogen is stored in the liver and skeletal muscles, which have a limited amount of storage space. The liver holds 75 to 100 grams of glycogen, which equals 300 to 400 calories. Skeletal muscles generally contain an additional 300 to 400 grams of glycogen -- 1,200 to 1,600 calories' worth -- but if you follow an intense training schedule, they might store more. Depending on the amount of exercise, diet, muscle fiber type, and body weight, muscles may hold up to 700 grams of glycogen, reported Nutrition and Metabolism in December 2015.
The glycogen stores in the liver are used to bring blood sugar back to normal when levels drop. Muscles don’t release glycogen back into the bloodstream. It stays in muscles until it’s needed to fuel an increase in activity, which makes glycogen essential for optimal performance during long-lasting or intense exercise.
Carbs Stored as Body Fat
After glycogen stores are full, sugar that arrives in the liver is converted into triglycerides, which are stored as fat. While a relatively low limit to the amount of carbs can be kept as glycogen, you can store an unlimited amount of excess energy as fat. Fat cells, or adipocytes, expand in size to hold fat; when they reach maximum capacity, new fat cells are produced to create the storage space needed. Fat stores can be as high as 70 percent of body weight in people who are morbidly obese, reported Methods in Enzymology in January 2015.
Some fat stores are essential to keep your body working normally. In men, about 2 to 5 percent of total body weight should consist of essential fat, while the amount of essential fat storage in women is 10 to 13 percent of body weight, reports the American Council on Exercise. Average body-fat percentage is 25 to 31 percent in women and 18 to 24 percent in men, which represents approximately 50,000 to 100,000 calories of stored energy, according to Montana State University. Obesity is defined as fat stores of 25 percent or higher in men and 32 percent or above in women.
Energy for Muscles From Stored Carbs
Carbs stored as glycogen and fat are important sources of energy for active muscles. When you begin to exercise, your fat stores start breaking down and releasing fatty acids into your bloodstream. During exercise, most of the fatty acids are used to energize muscles. You can also store a small amount of fat -- about 2 percent of your body fat -- as small fat droplets within your muscle cells.
Glycogen is the major fuel for muscles during moderate-to-intense exercise, while prolonged low-to-moderate exercise burns more fat. According to studies cited in the November 2015 issue of Sports Medicine, in which stored adipose tissue was measured, a large release of fat occurs during low-intensity exercises, but only a small or moderate of stored fat breaks down during high-intensity exercise. While several factors influence the breakdown of stored fat during exercise, increased blood flow due to activity plays a crucial role.
Intake Recommendations for Carbs
Carbohydrates, including glycogen, provide 4 calories for every gram. When carbs are converted into fats for storage, they become a more concentrated source of energy, because each gram of fat supplies 9 calories. Of course, carbs aren't stored as fat unless the total calories you consume exceed the amount of calories used every day. You’ll have plenty of carbs for energy if 45 to 65 percent of your daily calories comes from healthy carbs such as fruits, whole grains, beans and starchy vegetables.
If you’re engaged in endurance activities or athletic training, you may need to consume more carbs and get extra carbs during and right after exercise to refill glycogen that was depleted. The carb requirements for endurance and resistance training vary -- and your needs may be different depending on your level of training -- but, as a general guideline, athletes should consume 3 to 5 grams of carbohydrates for every pound of body weight each day, recommends Colorado State University.
- California State University San Bernardino: Carbohydrate
- Nutrition and Metabolism: Glycogen Availability and Skeletal Muscle Adaptations with Endurance and Resistance Exercise
- Methods in Enzymology: Quantifying Size and Number of Adipocytes in Adipose Tissue
- American Council on Exercise: What are the Guidelines for Percentage of Body Fat Loss
- Montana State University: Energy: How and Where is it Stored?
- University of New Mexico: The Physiology of Fat Loss
- Sports Medicine: The Physiological Regulation of Skeletal Muscle Fatty Acid Supply and Oxidation During Moderate-Intensity Exercise
- Institute of Medicine: Dietary Reference Intakes: Macronutrients
- Colorado State University Extension: Nutrition for Athletes