Eating and the Energy Pathways for Exercise

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What you eat really does have an impact on how effectively and efficiently you can provide energy to your working muscles. The body converts food into adenosine triphosphate (ATP) for fuel through several different energy pathways. Understanding these systems can help you train and eat more effectively, and boost your overall sports performance.

Energy Pathways in the Human Body

Because the body cannot easily store ATP (and what is stored gets used up within a few seconds), it is necessary to continually create ATP during exercise. In general, the two major ways the body converts nutrients to energy are:

  • Aerobic metabolism (with oxygen)
  • Anaerobic metabolism (without oxygen)

These two pathways can be further divided into three main energy systems (listed below). Most often it's a combination of energy systems that supply the fuel needed for exercise. The intensity and duration of the exercise determine which method gets used when.


The ATP-CP energy pathway (sometimes called the phosphagen system) is an anaerobic pathway because it doesn't require any oxygen to create ATP. The "CP" stands for creatine phosphate, a naturally occurring compound that enables short bursts of energy.

The ATP-CP pathway supplies about 10 seconds worth of energy and is used for short bursts of exercise, such as a 100-meter sprint.

This pathway first uses up any ATP stored in the muscle (about 2 to 3 seconds worth). Then it uses creatine phosphate (CP) to recycle ATP until the CP runs out (another 6 to 8 seconds). After the ATP and CP are used, the body will move on to either aerobic or anaerobic metabolism (glycolysis) to continue to create ATP to fuel exercise.


Glycolysis is both an anaerobic and anaerobic system which creates ATP exclusively from carbohydrates, with lactic acid being a byproduct. Anaerobic glycolysis provides energy by the (partial) breakdown of glucose without the need for oxygen.

Glycolosis is considered both an aerobic and anaerobic pathway. This process produces energy for short, high-intensity bursts of activity lasting no more than several minutes.

After several minutes, the lactic acid build-up reaches a threshold known as the lactate threshold (LT). When you reach this threshold, you experience muscle pain, burning, and fatigue, making it difficult to keep exercising at this intensity. However, training can increase the threshold.

Aerobic Metabolism

Aerobic metabolism fuels most of the energy needed for long duration activity. It uses oxygen to convert macronutrients (carbohydrates, fats, and protein) to ATP. This system is a bit slower than the anaerobic systems because it relies on the circulatory system to transport oxygen to the working muscles before it creates ATP.

Aerobic metabolism is used primarily during endurance exercise, which is generally less intense and can continue for long periods of time.

During exercise, an athlete will move through these metabolic pathways. As exercise begins, ATP is produced via anaerobic metabolism. With an increase in breathing and heart rate, there is more oxygen available and aerobic metabolism begins and continues until the lactate threshold is reached and anaerobic metabolism kicks in again.

Fueling the Energy Systems

Sports nutrition is built upon an understanding of how macronutrients, such as carbohydrates, fat, and protein, contribute to the fuel supply needed by the body to perform. Macronutrients contribute to the process in different ways.

Each macronutrient has unique properties that determine how it gets converted to ATP.

  • Carbohydrate is the main nutrient that fuels moderate to high intensity exercise.
  • Fat can fuel low-intensity exercise for long periods of time.
  • Protein is generally used to maintain and repair body tissues and is not normally used to power muscle activity.

Because your body uses different pathways to create energy, and each pathway relies on different macronutrients, it's important to consume fat, carbohydrates, and protein in your diet.

Nutrients get converted to ATP based on the intensity and duration of activity, with carbohydrate as the main nutrient fueling exercise of a moderate to high intensity, and fat providing energy during exercise that occurs at a lower intensity.


Fat is a great fuel for endurance events, but it is simply not adequate for high-intensity exercises such as sprints or intervals. If exercising at low intensity (or below 50% of max heart rate), you have enough stored fat to fuel activity for hours or even days, as long as there is sufficient oxygen to allow fat metabolism to occur.


As exercise intensity increases, carbohydrate metabolism takes over. It is more efficient than fat metabolism but has limited storage capacity. Stored carbohydrate (glycogen) can fuel about two hours of moderate to high-level exercise. After that, glycogen depletion occurs (stored carbohydrates are used up). If that fuel isn't replaced, athletes may hit the wall or "bonk."

An athlete can continue moderate to high-intensity exercise for longer by simply replenishing carbohydrate stores during exercise. This is why it is critical to eat easily digestible carbohydrates during moderate exercise that lasts more than a few hours. If you don't take in enough carbohydrates, you will be forced to reduce your intensity and tap back into fat metabolism to fuel activity.

In fact, carbohydrates can produce nearly 20 times more energy (in the form of ATP) per gram when metabolized in the presence of adequate oxygen than when generated in the oxygen-starved, anaerobic environment that occurs during intense efforts (sprinting).

Frequently Asked Questions

Which are the 3 energy systems used to create ATP?  

The three main energy systems the body uses to create ATP are: the ATP-CP energy pathway (or phosphagen system), glycolysis, and aerobic metabolism.

What is the ATP-CP energy system? 

The ATP-CP energy system powers very short bursts of exercise, and supplies up to 10 seconds of power and energy to your body.

How does the ATP-CP energy system work? 

The ATP-CP energy system works by using ATP and creatine phosphate (CP) to give your body fuel. While ATP provides about 2 to 3 seconds of energy, the CP provides 6 to 8 seconds. Together, they can provide enough energy for a quick 10-second sprint.

Which is the first energy pathway?

The ATP-CP, or phosphagen, system is the first energy pathway that is used during exercise. This energy pathway is quickly depleted and allows for a quick burst of fuel to lift heavy weights or perform a short sprint.

A Word from Verywell

Energy pathways in the body can adapt as you increase your fitness. With appropriate training, these energy systems become more efficient and allow you to exercise at a higher intensity for longer periods of time.

Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  • Wilmore JH, Costill DL. Physiology of Sport and Exercise 3rd Edition. Human Kinetics Publishing, 2005.

By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.