Science and everyday application, v. 1.0 (2023)

The human body uses carbohydrates, fats and proteins from food and from body stores as energy for physical activity. These essential nutrients are needed no matter how intense your activity. Whether you're lying down reading a book or running a marathon, your body always needs these macronutrients. 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 produced from itin the presence of oxygen or without 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 its sole fuel source.and produces pyruvate and lactic acid. Pyruvate can be used as a fuel for aerobic metabolism.Aerobic metabolism takes place in the cell's mitochondria 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.

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Figure 10.1. Anaerobic vs. aerobic metabolism. Remember that carbohydrates are the only fuel used in anaerobic metabolism, but all three macronutrients can be used as fuel during aerobic metabolism.

Duration of physical activity and fuel consumption

The respiratory system plays an important role in taking in and delivering oxygen to muscle cells throughout the body. Oxygen is breathed into the lungs and transferred from the lungs to 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. In order to get more oxygen to the muscle cells, your heart rate and breathing rate 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.

  • During the initial phase of the exercise., your muscles are the first to respond to a change in activity level. Your lungs and heart don't respond as quickly, and you can't increase your oxygenation at this early stage. In order for our body to get the energy it needs during these early stages, muscles rely on a small amount of ATP stored in resting muscles. Stored ATP can only provide energy for a few seconds before running out. Once the stored ATP is almost depleted, the body turns to another high-energy molecule known as to convert ADP (adenosine diphosphate) into 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 up 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 (without oxygen) metabolism. Anaerobic metabolism can produce ATP at high rates but uses only glucose as a fuel source. Glucose is derived from muscle glycogen. At around 30 seconds, the anaerobic pathways are running at full capacity, but since glucose availability is limited, it cannot continue for a long period of time.
  • When your workout reaches two to three minutes, heart rate and breathing rate increased to deliver more oxygen to the 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.

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Figure 10.2. The energy systems used to power the exercise change with the duration of the exercise. The ATP creatine phosphate system is used in seconds. The short-term and long-term systems are activated and provide energy for the exercise for the duration of the training.

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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 in muscle as triglycerides, 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 great source of energy, small amounts of amino acids are used during rest or during activity. The amount of amino acids used for energy metabolism increases when the total energy intake from your diet does not meet your nutritional requirements or when you engage in long-term resistance exercise. When 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 consumption of amino acids for energy and glucose synthesis increases.

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Figure 10.3. Fuel sources for anaerobic and aerobic metabolism. Both food sources and body stores of carbohydrates, fats, and proteins can be used to fuel activity. The amount varies depending on the duration and intensity of the activity.

Intensity of physical activity and fuel consumption

The intensity of exercise determines the contribution of the various fuel sources used to produce ATP.Both anaerobic and aerobic metabolism are combined during exercise to ensure muscles are supplied with 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 deliver enough ATP to the muscles. However, high-intensity activity requires more ATP, so muscles must rely on aerobic and anaerobic metabolism to meet the body's demands.

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activity intensity

duration of activity

preferred fuel

Do you need oxygen?

activity example

Very high

30 Sec. - 3 min


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 metabolismon anaerobic metabolism because it is more efficient and produces larger amounts of ATP.Fatty acids are the main source of energy in low-intensity activities.With the body's almost unlimited fat stores, low-intensity activities can be sustained for long periods of 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 supply of glucose becomes depleted and eventually fatigue sets in.

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Figure 10.4. The effect of exercise intensity on fuel sources. Anaerobic exercise only uses glucose as fuel. When activities become more aerobic, the body can use fatty acids and, to a lesser extent, amino acids for energy.

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An important clarification regarding exercise intensity and energy sources is the concept of a fat burning zone. Many people think that to lose body fat they need to exercise at a lower intensity, so fat is the main source of energy.The fat burning zone is commonly known as low-intensity aerobic activity that keeps your heart rate between 60 and 69 percent of your maximum heart rate. The cardio zone, on the other hand, is high-intensity aerobic activity that keeps your heart rate between 70 and 85 percent of your maximum heart rate.So, in which area do you burn the most fat? Technically, during low-intensity aerobic activity, your body burns a higher percentage of calories from fat. When you start out with a low-intensity activity, about 50% of the calories you burn come from fat, while in the cardiovascular area, only 40% come from fat. However, this is not the whole story.High-intensity activities burn more calories per minute overall. With this increased energy expenditure, you can burn as much or more total fat and more total calories than you would with lower-intensity activities.If weight loss is one of your goals, high-intensity activity burns more total calories, helps you shift into negative energy balance, and promotes 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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Figure 10.5. The fat burning zone. While a higher percentage of calories burned comes from fat during lower-intensity exercise, total calorie burn is greater during higher-intensity exercise.


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