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The Body’s Fuel Sources

Seeking top performance or just sneak in some extra fatty carbs? Check out the difference between fat, protein and carbs!

The body’s ability to perform activities like running, cycling, and swimming depends on its capacity to extract energy from consumed food. Carbohydrates, fats, and proteins in food function as potential fuel sources. These macronutrients undergo various metabolic pathways within the body, ultimately generating water, carbon dioxide, and a crucial energy molecule called adenosine triphosphate (ATP).

  • ATP acts like high-energy batteries, storing energy readily available for various functions, from breathing to strenuous exercise. It’s the sole molecule capable of powering muscle contractions.
  • Creatine phosphate (CP) also functions as a high-energy compound, similar to ATP, but stored in smaller quantities within cells. It provides quick energy for short, intense activities. To sustain physical exertion, however, cells require constant replenishment of both CP and ATP.

Daily Food Choices and Energy Stores

The choices we make regarding food directly impact our body’s fuel reserves. The human body utilizes three primary forms of energy:

  • Carbohydrates
  • Fats
  • Proteins

Some of these fuels can be stored in a way that offers muscles an immediate energy source. Carbohydrates, for example, are easily broken down into glucose, the body’s preferred fuel source. Glucose can be used directly for energy or sent to the liver and muscles for storage as glycogen. During exercise, muscles convert glycogen back into glucose, the only fuel they can utilize. The liver also converts its glycogen stores back into glucose, but releases it into the bloodstream to maintain blood sugar levels. Muscles utilize both their own glycogen stores and this circulating blood glucose during exercise. Notably, the brain relies heavily on blood sugar (glucose) for energy, both at rest and during physical activity. The body constantly utilizes and replenishes its glycogen stores. The amount and type of carbohydrates consumed, along with training patterns, influence the size of these glycogen stores.

Energy Stores and Depletion During Exercise

Our bodies hold limited reserves of carbohydrates (glycogen) for immediate energy, enough for approximately 90 to 120 minutes of vigorous activity. Muscle glycogen depletion can lead to a feeling of hitting a wall during exercise. As exercise continues, blood glucose becomes increasingly important for fueling the body. Liver glycogen stores are rapidly used to meet this elevated demand. When liver glycogen is depleted, blood sugar levels drop, resulting in hypoglycemia (low blood sugar) and a significant slowdown. Consuming carbohydrates during exercise can delay muscle glycogen depletion and prevent hypoglycemia.

Fat as a Fuel Source

Fat is the body’s primary source of stored energy, offering more than double the potential energy compared to carbohydrates or protein. During exercise, the body breaks down stored fat (triglycerides in adipose tissue) into fatty acids that are transported to muscles for fuel. This process is slower compared to using carbohydrates for energy. However, fat stored within muscle fibers is readily accessible during exercise. Unlike limited glycogen stores, body fat provides a virtually unlimited energy source for athletes. Even lean individuals have enough fat reserves to fuel over 100 hours of marathon running!

Efficiency of Fat as Fuel

Fat is a more weight-efficient fuel source compared to carbohydrates. Storing carbohydrates requires additional water weight. Our body weight would double if we stored the same amount of energy as glycogen (with its associated water) compared to fat. Most individuals have sufficient fat stores, and the body efficiently converts excess calories from any source (fat, carbohydrate, or protein) into body fat. However, using fat for exercise requires sufficient oxygen intake.

Protein and Energy Needs

Our bodies do not maintain protein reserves specifically for energy. Protein’s primary function is building, maintaining, and repairing tissues, along with synthesizing enzymes and hormones. Typically, protein fulfills only 5% of the body’s energy needs. However, in situations like low daily calorie intake, insufficient carbohydrate intake, or later stages of endurance exercise with depleted glycogen stores, muscle tissue can be broken down and used as fuel. This breakdown provides access to specific amino acids (protein building blocks) that can be converted into glucose. Notably, the brain requires a constant supply of glucose for optimal function.

Carbohydrates: Exercise and Overall Health

Carbohydrates are a crucial fuel source for the body, offering several benefits:

  • Enhanced Efficiency
    Carbohydrates require less oxygen to convert into energy compared to protein or fat. This makes them the body’s preferred fuel source, particularly during high-intensity exercise when oxygen availability is limited.
  • Brain and Nervous System Support
    Low blood glucose levels can lead to irritability, disorientation, and lethargy. Consuming carbohydrates helps maintain optimal blood sugar levels, ensuring clear thinking and proper nervous system function.
  • Fat Metabolism
    The body requires some carbohydrates to effectively burn fat for energy. Limited dietary carbohydrates can hinder this process, despite abundant fat reserves.
  • Preserving Muscle Mass
    Adequate carbohydrate intake prevents the body from using protein (from muscle or dietary sources) as fuel. This allows protein to perform its primary functions: building, maintaining, and repairing tissues, along with synthesizing vital molecules like hormones, enzymes, and neurotransmitters.

Fat: A Valuable Energy Source for Fitness and Beyond

Fat plays a significant role in supporting an active lifestyle:

  • Energy Density
    Fat offers more than double the potential energy per gram compared to protein and carbohydrates. This concentrated source of energy can be beneficial for various activities.
  • Fueling Low-Intensity Exercise
    During rest and low to moderate intensity exercise (below 65% of aerobic capacity), fat contributes significantly to meeting muscle fuel needs.
  • Endurance Enhancement
    As exercise duration increases and intensity tends to decrease (allowing for more oxygen availability), fat becomes the primary fuel source. This spares stored carbohydrates (glycogen) and delays fatigue, allowing for longer exercise sessions.

Protein: Essential for Muscle and Beyond

While not a primary fuel source, protein plays a crucial role during exercise and overall health:

  • Energy Source During Extended Exercise
    As prolonged exercise depletes muscle glycogen stores, typically in later stages of endurance activities, the body utilizes amino acids from skeletal muscle protein. These converted amino acids can provide up to 15% of the energy required.
  • Energy Source in Calorie or Carbohydrate Deficient Diets
    When daily calorie or carbohydrate intake falls short of the body’s needs, protein is used for energy. This breakdown can lead to a loss of lean muscle mass.


  • Our body uses carbohydrates, fats, and proteins for energy. These are broken down and turned into ATP, the fuel that powers our muscles and organs.
  • Carbohydrates are the preferred fuel source, especially during intense exercise. They are stored as glycogen in the liver and muscles.
  • Fat is a more concentrated energy source than carbohydrates and protein. Our body has nearly unlimited fat stores, but it takes more oxygen to burn fat for energy.
  • Protein is typically not used for energy, but in cases of low calorie intake or depleted glycogen stores, muscles can be broken down to provide energy.
  • Consuming a balanced diet with adequate carbohydrates is important to avoid muscle breakdown and optimize endurance exercise.