Where Does Our Energy Come From?

Endurance training and competition require significant amounts of energy over long periods of time. In addition, when we are not training, and even during sleep, moderate amounts of energy are needed to maintain fitness and health. This energy is generated by the body’s metabolism with the help of various foods and specific nutrients.

The energy we use for endurance takes a long wondrous journey before we access it. All the energy we use to work out originates as light energy from the sun. Plants absorb this energy, convert it (through photosynthesis), and store it as chemical energy. We eat the plants—and the animals that feed on plants—to obtain this chemical energy through the consumption of carbohydrates, proteins, and fats. Then, we convert it to mechanical energy used for movement. Converting foods into usable energy for muscle action is the job of the body’s metabolism. High-quality food and the efficient metabolism potentially produce tremendous amounts of energy for endurance.

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Carbohydrates, proteins, and fats are three of our key macronutrients used to obtain energy. When consumed, they are chemically broken down in the intestine and absorbed as glucose, amino acids, and small fat particles, respectively. Eventually, on a molecular level, these substances are converted to ATP (adenosine triphosphate). The energy used for all our body’s needs comes from ATP. Different macronutrients can provide varying amounts of energy, with fats contributing more potential energy than carbohydrates and proteins combined.

Traditionally, exercise physiology textbooks discuss three different energy-producing systems in the body. These include:

 

 

 
  • The creatine phosphate system for very short-term energy needs
  • The anaerobic system for short-term energy needs
  • The aerobic system for long-term energy needs

The creatine phosphate (CP) system is the most immediate energy source, but is very limited. For example, at the beginning of a 100-meter sprint, an athlete will use CP for some energy, but it’s limited to only about five to ten seconds worth of energy. (This system is only mentioned here because it is emphasized less and its benefits minimized in endurance training relative to the other two systems.)

For energy to be maintained beyond the time limits of CP, the anaerobic system is utilized. Energy from this system is derived from sugar (glucose) in the anaerobic muscles. This energy is used during sprint, power, and is very useful at the end of endurance competition, where a final kick is needed. It may also be used throughout competition, such as while running a steep hill, a breakaway during cycling, or at the end of a basketball game or tennis match. But the maximum amount of energy the anaerobic system can generate is about three minutes worth of all-out effort, longer if it’s used sparingly. If you use up this energy too early in competition, you can adversely and sometimes tragically affect performance at the end, a time when most athletes need a burst of energy.

Since the body has strict limitations on energy gained from the anaerobic system, a third source of energy is available, the main focus for endurance athletes. This long-term energy source comes from the aerobic system, which converts fat in the aerobic muscles to energy. The aerobic system, with its use of fat, may have up to 75,000 kcal of energy available, which is enough abundant energy to maintain training or competition for many hours, or even days.

Triglycerides are the specific type of fat used for energy by aerobic muscles. When chemically broken down in our body’s fat stores (they originally enter these stores mostly from carbohydrates and fat from the diet), they enter the bloodstream as free fatty acids, where they are carried to the aerobic muscles and burned in the mitochondria for energy. This process of burning fat is called beta-oxidation. The utilization of some glucose by the aerobic system is vital because it maintains the fat-burning process. During training or competition, if you deplete your sugar and sugar reserves (glycogen stores), you’ll also cease burning fat.

Herein lies the endurance game; as the time of your activity increases, more fat and aerobic energy must be generated. But as the intensity increases, less oxygen is delivered for fat burning, so aerobic metabolism is diminished, forcing the body to use more anaerobic energy, which is very limited. If you use up all your sugar, even fat burning stops, along with the rest of your body. Do you refrain from going faster so you can go farther? Can you really go farther and faster?

You can have the best of both; the answer is in proper training. By properly programming your aerobic system to burn more fat for energy, you can develop both endurance and aerobic speed.

The table below indicates how much aerobic and anaerobic energy are used during competition.

Table Indicating How Much Aerobic
and Anaerobic Energy Is Used During
Competition of Varying Length
  Time (minutes)     % Anaerobic     % Aerobic  
  1     70     30  
  2     50     50  
  4     35     65  
  10     15     85  
  30     5     95  
  60     2     98  
  120     1     99  
  >120     <1     >99  

 

 

This is an extremely brief view of the body’s metabolism. The chemical activity that takes place in the body every moment during training and competition, and at rest, is very complex. For example, there are at least nine hundred enzymes known to be involved with this metabolism. Just the breakdown of sugar to energy requires nineteen different chemical reactions. And it’s not just the metabolism that is complex but all of the body’s systems that interrelate with it.

It’s also possible to observe the inter-relationships between different aspects of the body. As I’ve watched athletes and measured their improvement during a particular training period, I’ve seen the brain, muscles, and metabolism respond to progress in slightly different ways. You may experience this too. For example, your perceived exertion at the same intensity may not always feel the same: when you first begin training the aerobic system, the biggest complaint is typically that you don’t feel much of a workout because of the relatively slow pace. During that phase, the muscles are more developed than the metabolic components. But in time this situation changes. By developing more aerobic speed, you may now find it’s more of an effort to train that fast, even though the heart rate is the same. In this case, the muscular body has not kept pace with the progressing metabolism.