The body stores energy in a variety of places and in different forms for use. Some forms of energy can be stored for longer periods than other types of energy. The energy source used will vary depending on the demands placed on your body. Your body uses multiple energy sources so your brain always has energy. Knowing where the energy stores are located and how they are used helps ensure you have enough energy for your activities.
Liver Glycogen
In the body, excess carbohydrates from food are stored as glycogen. One storage place for glycogen is in the liver. During times of fasting, such as between meals, your liver breaks down the stored glycogen to provide energy in the form of glucose for your working tissues.
Muscle Glycogen
When your muscles contract, they use glucose. This glucose comes from breaking down glycogen stores within the muscles. This energy source is first used by muscles when they begin working.
Adipose Tissue
When you consume excess calories, your body stores the extra energy as adipose tissue, or fat. In addition to being stored under your skin, or subcutaneously, fat is also stored within muscle tissue. After glycogen stores are depleted or during prolonged times of exercise, your body will use adipose tissue in the muscles and subcutaneously for energy. This is because your body can store more energy in adipose tissue than in your muscles and liver as glycogen. With training, you can increase adipose tissue in your muscles, increasing your capacity for endurance activities because there is less distance between the working tissue and the energy source.
Lactic Acid
Your muscles produce lactic acid normally during exercise. Lactic acid does not cause muscle soreness and in fact can be used by your body as energy. Your body converts lactic acid glucose, which can be used by working tissues such as muscles for energy.
Tissues
Your body does not prefer to use protein for energy, however, it can. During times of low energy stores, such as calorie restriction, your body may not have enough calories to fuel the demands placed on it. When this occurs, your body will break down tissues, including muscles and organs, to supply the energy it needs.
Consuming excess protein does not increase protein stores or muscle size. In fact, excess protein intake increases calorie intake. When you consume more calories than your body needs, it will store those calories as adipose tissue, no matter the original source of those calories.
Creatine Phosphate
The first few seconds of energy is fueled by creatine phosphate. This energy source is quickly used up, and energy then is supplied by other sources. Creatine phosphate energy is used during many activities, but increasing its storage has been shown to improve explosive activities, such as sprinting.
