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Medicine LibreTexts

11.4: Fuel and Nutrient Needs for Physical Activity

  • Page ID
    21173
  • Learning Objectives

    • Describe the various processes used to break down macronutrients into fuels that support physical activity.
    • Describe how nutrient needs for energy, macronutrients and micronutrients may change in response to an increase in physical activity.

    You learned in earlier chapters that the macronutrients (carbohydrate, fat and protein) are the only nutrients that give us energy in the form of the calorie. The vast majority of the energy for exercise is derived from carbohydrate and fat. If you remember the protein chapter, you will recall that protein is used for many other important roles in the body. In fact it is considered a waste of good protein/amino acids to use it for energy, especially when carbohydrate and fat do this so well!

    Next up we discuss how your body actually uses these energy-yielding nutrients. It is important to note, as you move forward, that the direct source of energy for cellular work, including muscle contraction, is a compound called adenosine triphosphate (ATP). Unfortunately very little ATP is stored by the body, so you must continually replenish ATP stores. To do this, we harness the energy from the macronutrients, again mostly carbs and fats, and an additional source known as Creatine Phosphate (CP).

    What Fuels Our Activities?

    Adenosine triphosphate (ATP; Figure \(\PageIndex{1}\)) – the energy carrying molecule in the body - provides the direct source of energy for our activities. ATP includes adenosine and 3 phosphate groups. When a phosphate is broken away/off, energy is released. The body regenerates ATP by adding a phosphate back. ATP must be generated continuously since muscles store only enough ATP for 1 – 3 seconds of activity.

    Structure of adenosine triphosphate (ATP) includes adenine, ribose and three phosphates. Energy is produced when ATP is split into adenosine diphosphate (ADP) and inorganic phosphate.
    Figure \(\PageIndex{1}\): Structure of adenosine triphosphate (ATP). (CC BY 3.0; OpenStax College via Wikimedia Commons)

    After depleting ATP stores, muscles turn to other sources. Initially, creatine phosphate (CP) is used. CP stores some energy that can be used to make ATP. Creatine phosphate stores enough energy for 3 to 15 seconds of maximal physical effort. When CP is broken down into a molecule of creatine and an independent phosphate molecule, energy is released. This energy, along with the independent phosphate molecule, can then be used to regenerate ATP.

    After creatine phosphate, carbohydrates are the next source of energy for the production of ATP as shown in Figure \(\PageIndex{2}\). Glucose is the primary carbohydrate used to generate ATP in a process called glycolysis (breakdown of glucose). For each molecule of glucose, glycolysis yields 2 ATP molecules and 2 molecules of pyruvate. Pyruvate is further metabolized in one of two ways: 

    • Anaerobic (without oxygen) breakdown of pyruvate yields lactic acid
    • Aerobic (with oxygen) breakdown of pyruvate yields 36 – 38 molecules of ATP. Aerobic metabolism supports physical activity that lasts more than 3 minutes.
    Diagram of anaerobic and aerobic metabolism. The breakdown of one molecule of glucose, or the process of glycolysis, yields two molecules of pyruvate and two ATP molecules. The further metabolism of pyruvate in the presence of insufficient oxygen (anaerobic process) results in the production of lactic acid. The metabolism of pyruvate in the presence of adequate oxygen (aerobic process) yields 36 to 38 molecules of ATP.
    Figure \(\PageIndex{2}\): Anaerobic and aerobic metabolism. (CC BY 4.0; Allison Calabrese via Human Nutrition 2020 Ed [Hawaii])

    Triglycerides (fatty acids) can also be metabolized to generate ATP. Triglyceride metabolism for energy is a relatively slow process, but is used for low intensity exercise and exercise of long duration. The amount of energy provided depends on the chain length of the fatty acid.

    For most daily activities, including exercise, we use a mixture of carbohydrate and fat for energy (see Figure \(\PageIndex{3}\)). At rest and lower exercise intensities, we rely more on fat as a fuel source. As exercise intensity increases, we rely more on carbohydrate for energy. Proteins (amino acids) are used minimally as a fuel source for exercise (about 3-6%, if needed).

    For most daily activities, including exercise, we use a mixture of carbohydrate and fat for energy. At rest and lower exercise intensities, we rely more on fat as a fuel source. As exercise intensity increases, we rely more on carbohydrate for energy.
    Figure \(\PageIndex{3}\): Fuel Sources Used During Various Exercise Intensities. (CC BY 4.0; Allison Calabrese via Human Nutrition 2020 Ed [Hawaii])

    For a review of fuel use during exercise, watch this short video:

    How Does Physical Activity Affect Nutrient Needs?

    Energy needs may be higher for athletes, it depends on body size as well as the type, intensity, and duration of physical activity. The recommended diet includes:

    • At least 45% of calories from carbohydrates. To optimize muscle glycogen stores, carbohydrate intake of 6-10 g/kg body weight is often recommended. However, more carbohydrates may be needed to support vigorous exercise. When choosing carbohydrate-rich foods athletes should focus on fiber-rich, less-processed whole grains, fruits, and vegetables. In addition to adequate quantity, timing of carbohydrate intake is key. Athletes should strive to consume carbohydrate-dense foods in the first few hours after finishing exercise to effectively replenish glycogen stores. Carbohydrate/glycogen loading for endurance activities is sometimes pursued and involves altering exercise duration and carb intake so muscle glycogen storage is maximized. However, carbohydrate loading does not always increase performance. Downsides may include water retention, diarrhea, and a heavy sluggish feeling.
    • 20-35% kcal from fat and less than 10% of total kcal as saturated fat.
    • 10-35% kcal from protein. Protein intakes range from 1.2-2.0 g/kg body weight. The majority of athletes, not to mention Americans, consume enough protein (so protein supplements are usually not necessary).

    Regular exercise increases our need for fluids. During exercise, heat is dissipated through sweating. Heat illnesses occur because our muscles and skin compete for blood flow, so maintaining water balance is critical for physically active people. Hydration recommendations include:

    • Drink fluids before, during, and after exercise.
    • Consume enough water to maintain body weight. Avoid losing more than 2-3% of body weight; even fluid loss of 1% of body weight can impair performance.
    • Training in hot environments requires careful attention to water intake – dehydration is a serious threat.
    • Do not rely on thirst as an indicator of hydration status. Clear, light-colored, and odorless urine indicates proper hydration.
    • Water is appropriate for most activities less than one hour in duration.
    • Sports beverages supply fluid, sodium, and carbohydrates and are indicated when activities last beyond one hour.

    The requirements for some vitamins and minerals may be altered in athletes. Adequate intake of these nutrients can be met with a healthy diet and should not require supplementation.

    • B vitamins - involved in energy metabolism
    • Calcium - role in muscle contraction and bone health
    • Iron - integral to oxygen transport and energy production

    Key Takeaways

    • Adenosine triphosphate (ATP) provides the direct source of energy for our activities.
    • After depleting ATP stores, muscles turn to other sources of energy including creatine phosphate (CP), glucose (carbohydrates), triglycerides, and protein (if needed).
    • For most daily activities, including exercise, we use a mixture of carbohydrate and fat for energy. At rest and lower exercise intensities, we rely more on fat as a fuel source. As exercise intensity increases, we rely more on carbohydrate for energy.
    • The recommended diet for athletes includes at least 45% of calories from carbohydrates, 20-35% of calories from fat, and 10-35% of calories from protein.
    • Maintaining water balance is critical for physically active people.