Every sprint, jump, and long run depends on stored chemical energy inside the body. The two main energy stores for exercise are glycogen, a carbohydrate stored mostly in muscles and the liver, and fat, stored mostly in adipose tissue and within muscle cells. Glycogen is like a small, fast-access fuel tank, while fat is a large, slower-burning fuel supply.
Understanding these fuels helps athletes plan training, pacing, and nutrition.
During exercise, muscles break down ATP to power contraction, but ATP stores are tiny and must be constantly rebuilt. Glycogen can be broken down quickly to make ATP, especially during high-intensity exercise, while fat provides lots of energy during lower-intensity and longer-duration activity. The body usually uses a mixture of carbohydrate and fat, with the balance changing as intensity and duration change.
Training can improve how efficiently the body stores glycogen, uses fat, and delays fatigue.
Key Facts
- ATP is the immediate energy source for muscle contraction, but stored ATP lasts only a few seconds.
- Glycogen is stored mainly in skeletal muscle and the liver: glucose units linked together form glycogen.
- Fat stores much more total energy than glycogen: fat provides about 9 kcal/g, while carbohydrate provides about 4 kcal/g.
- During high-intensity exercise, the body relies more on glycogen because it can produce ATP quickly.
- During low to moderate-intensity endurance exercise, the body uses a larger share of fat for ATP production.
- Energy balance can be summarized as stored energy change = energy intake - energy expenditure.
Vocabulary
- Glycogen
- Glycogen is the storage form of glucose found mainly in muscles and the liver.
- Adipose tissue
- Adipose tissue is body fat tissue that stores triglycerides for long-term energy use.
- ATP
- ATP is the molecule that directly supplies energy for muscle contraction and many cell processes.
- Aerobic metabolism
- Aerobic metabolism is the process of making ATP using oxygen, mainly from carbohydrates and fats.
- Fat oxidation
- Fat oxidation is the breakdown of fatty acids with oxygen to produce ATP during exercise.
Common Mistakes to Avoid
- Thinking fat is only used after glycogen runs out is wrong because the body usually burns both fuels at the same time, with the ratio depending on intensity and duration.
- Assuming glycogen is stored only in the liver is wrong because most usable exercise glycogen is stored directly inside skeletal muscles.
- Believing fat produces ATP faster than carbohydrate is wrong because fat stores more total energy but is slower to break down, especially during intense exercise.
- Ignoring exercise intensity when predicting fuel use is wrong because sprinting and distance jogging place very different demands on glycogen and fat metabolism.
Practice Questions
- 1 A cyclist stores about 500 g of glycogen. If carbohydrate provides about 4 kcal/g, how many kilocalories of energy are stored as glycogen?
- 2 A runner uses 600 kcal during a training session. If 40 percent of the energy comes from fat, how many kilocalories come from fat and how many come from carbohydrate or other sources?
- 3 Explain why a 100 m sprinter depends more on glycogen than fat, while a marathon runner depends on a greater mixture of glycogen and fat.