Eating and the Energy Pathways for Exercise

Runners running along a river

Edwin Jimenez/Getty Images

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 fuel through several different energy pathways and having a basic understanding of these systems can help you train and eat more effectively, and boost your overall sports performance.

Macronutrients and ATP

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 the exercise. These nutrients get converted to energy in the form of adenosine triphosphate or ATP. Energy released by the breakdown of ATP allows muscle cells to contract. 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 exercise of a moderate to high intensity.
  • Fat can fuel low-intensity exercise for long periods of time.
  • Proteins are generally used to maintain and repair body tissues and are not normally used to power muscle activity.

Metabolic Pathways to Create ATP

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, with the intensity and duration of the exercise determining which method gets used when.


The ATP-CP energy pathway (sometimes called the phosphate 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) and 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 an anaerobic system that creates ATP exclusively from carbohydrates, with lactic acid being a by-product. Anaerobic glycolysis provides energy by the (partial) breakdown of glucose without the need for oxygen.

Glycolosis is an anaerobic pathway that 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). This threshold is characterized by muscle pain, burning, and fatigue making it difficult to maintain such 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.

If this level is surpassed, the body cannot deliver oxygen quickly enough to generate ATP and anaerobic metabolism kicks in again. Since this system is short-lived and lactic acid levels rise, the intensity cannot be sustained and the athlete will need to decrease intensity to remove lactic acid build-up.​

Fueling the Energy Systems

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 the 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 percent 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 the exercise intensity increases, carbohydrate metabolism takes over. It is more efficient than fat metabolism but has limited energy stores. This stored carbohydrate (glycogen) can fuel about 2 hours of moderate to high-level exercise. After that, glycogen depletion occurs (stored carbohydrates are used up) and 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).

With appropriate training, these energy systems adapt and become more efficient and allow greater exercise duration at a higher intensity.

Was this page helpful?
Article Sources
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, J.H., and Costill, D.L. Physiology of Sport and Exercise: 3rd Edition. 2005. Human Kinetics Publishing.