10.3: Practice Fuel Sources (2023)

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    The human body uses carbohydrates, fats, and proteins from food and body stores to obtain energy as fuel for physical activity. These essential nutrients are needed no matter how intense your activity is. Whether you lie down and read a book or run a marathon, these macronutrients are always needed by the body. However, in order for these nutrients to be used as fuel for the body, their energy must be transferred to the high-energy molecule adenosine triphosphate (ATP). ATP is the body's immediate source of fuel and can be made from it.aerobic metabolismin the presence of oxygen oranaerobic metabolismwithout the presence of oxygen. Which type of metabolism is predominantly used during physical activity depends on the availability of oxygen and the amount of carbohydrates, fats and proteins consumed.

    Anaerobic and aerobic metabolism

    Anaerobic metabolism takes place in the cytosol of muscle cells. As shown in Figure 10.1, in the absence of oxygen, a small amount of ATP is produced in the cytosol.Anaerobic metabolism uses glucose as the sole source of fuel.and produces pyruvate and lactic acid. Pyruvate can be used as a fuel for aerobic metabolism.Aerobic metabolism takes place in the mitochondria of the cell and can use carbohydrates, proteins or fats as fuel sources.. Aerobic metabolism is a much slower process than anaerobic metabolism, but it can produce much more ATP and is the process by which most ATP is created in the body.

    10.3: Practice Fuel Sources (2)
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    Duration of physical activity and fuel consumption

    The respiratory system plays an important role in taking up and delivering oxygen to muscle cells throughout the body. Oxygen is inhaled into the lungs and transported from the lungs into the blood, where the cardiovascular system circulates oxygen-rich blood to the muscles. The oxygen is then taken up by the muscles and can be used to generate ATP. When the body is at rest, the heart and lungs can supply the muscles with enough oxygen to meet the energy needs of aerobic metabolism. However, during physical activity, your muscles need more energy and oxygen. To deliver more oxygen to your muscle cells, your heart and breathing rates are increased. The amount of oxygen delivered to tissues by the cardiovascular and respiratory systems during exercise depends on the duration, intensity and physical condition of the person.

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    • During the first steps of the exercise., your muscles are the first to respond to a change in activity level. Your lungs and heart respond more slowly and are unable to increase oxygenation during these early stages. In order for our body to obtain the energy necessary to take the first steps, the muscles depend on a small amount of ATP stored in the resting muscles. Stored ATP can only provide energy for a few seconds before it runs out. Once the stored ATP is nearly depleted, the body turns to another high-energy molecule known ascreatinephosphateto convert ADP (adenosine diphosphate) to ATP. After about 10 seconds, the creatine phosphate stored in the muscle cells is also used up.
    • About 15 seconds of exerciseStored ATP and creatine phosphate are used in the muscles. The heart and lungs have not yet adapted to the increased demand for oxygen, so the muscles must start producing ATP through anaerobic metabolism (without oxygen). Anaerobic metabolism can produce ATP at high rates, but it only uses glucose as a fuel source. Glucose is derived from muscle glycogen. Around 30 seconds, the anaerobic pathways are working at full capacity, but as glucose availability is limited, it cannot continue for a long period of time.
    • When your workout hits two to three minutes, your heart rate and breathing rate increased to deliver more oxygen to your muscles. Aerobic metabolism is the most efficient way to produce ATP; produces significantly more ATP for each glucose molecule than anaerobic metabolism. Although the main source of ATP in aerobic metabolism is carbohydrate, fatty acids and proteins can also be used as fuel to generate ATP.
    10.3: Practice Fuel Sources (3)

    Fuel sources for anaerobic and aerobic metabolism change depending on the amount of nutrients available and the type of metabolism.

    • Glucoseit can come from blood sugar (derived from dietary carbohydrates, liver glycogen, and glucose synthesis) or from muscle glycogen. Glucose is the main source of energy for anaerobic and aerobic metabolism.
    • fatty acidsThey are stored as triglycerides in muscle, but about 90% of the stored energy is in adipose tissue. As low to moderate intensity exercise continues to utilize aerobic metabolism, fatty acids become the predominant energy source for muscle training.
    • AlthoughproteinNot considered a primary source of energy, small amounts of amino acids are consumed at rest or during activity. The amount of amino acids used for energy metabolism increases when total dietary energy intake falls short of nutritional requirements or when you engage in long-term resistance training. When the amino acids are broken down and the nitrogenous amino group is removed, the remaining carbon molecule can be broken down into ATP through aerobic metabolism or used to produce glucose. When training lasts for many hours, the use of amino acids for energy and glucose synthesis increases.
    10.3: Practice Fuel Sources (4)

    Intensity of physical activity and fuel consumption

    Exercise intensity determines the contribution of the various fuel sources used to produce ATP.Both anaerobic and aerobic metabolism are combined during exercise to ensure that the muscles receive enough ATP to meet the demands placed on them. The contribution of each metabolic type depends on the intensity of an activity. During low-intensity activities, aerobic metabolism is used to supply enough ATP to the muscles. However, high-intensity activity requires more ATP, so muscles must rely on both aerobic and anaerobic metabolism to meet the body's demands.

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    activity intensity

    activity duration

    preferred fuel

    Is oxygen necessary?

    example of activity

    Very high

    30 sec. - 3 minutes


    no - anaerobic



    3 minutes - 20 minutes


    and, aerobic


    low to moderate

    >20 minutes


    and, aerobic


    Table 10.2. Summary of fuels used for activities of varying intensity and duration.

    During low-intensity activities, the body uses aerobic metabolismin anaerobic metabolism because it is more efficient and produces greater amounts of ATP.Fatty acids are the main source of energy in low-intensity activities.As the body's fat reserves are almost unlimited, low-intensity activities can be continued for a long time. In addition to fatty acids, a small amount of glucose is also used. Glucose differs from fatty acids in that it can deplete glycogen stores. When glycogen stores are depleted, the glucose supply is depleted and eventually fatigue sets in.

    10.3: Practice Fuel Sources (5)

    An important clarification on exercise intensity and energy sources is the concept of the fat burning zone. Many people think that in order to lose body fat, they need to exercise at a lower intensity, so fat is the main source of fuel.The fat burning zone is commonly known as low-intensity aerobic activity that keeps your heart rate between 60 and 69% of your maximum heart rate. The cardio zone, on the other hand, is a high-intensity aerobic activity that keeps your heart rate between 70 and 85 percent of your maximum heart rate.So which area do you burn the most fat in? Technically, during low-intensity aerobic activity, your body burns a higher percentage of calories from fat. When you start a low-intensity activity, about 50% of the calories you burn come from fat, whereas in the cardio zone, only 40% come from fat. However, this is not the whole story.High-intensity activities burn more calories per minute overall. At this higher rate of energy expenditure, you can burn as much or more total fat and more total calories than with lower intensity activities.If weight loss is one of your goals, high-intensity activity will burn more calories overall, helping to put you in a negative energy balance and promoting higher levels of fitness. However, the best exercise program is one that is fun, sustainable and safe for you; If you're just starting out, it's a good idea to start with low to moderate intensity activities and work your way up from there.

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    10.3: Practice Fuel Sources (6)


    • Human Nutrition and Food Sciences Program at the University of Hawaii at Manoa. (2018). performance nutrition.human nutrition. pressbooks.oer.hawaii.edu/humannutrition/chapter/introduction-11/

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