Fuel Sources – Human Nutrition: 2020 Edition (2023)

The human body uses carbohydrates, fats and proteins from food and body stores for energy. These essential nutrients are needed regardless of the intensity of your activity. Whether you lie down and read a book or run the Honolulu Marathon, your body 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 fuel energy source, which can be produced with the presence of oxygen known as 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 metabolism takes place in the cytosol of muscle cells. As can be seen in Figure 16.2, Anaerobic vs. Aerobic Metabolism, a small amount of ATP is produced in the cytosol without the presence of oxygen. Anaerobic metabolism serves 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 cell's mitochondria and can use carbohydrates, proteins, or fats as a fuel source. Aerobic metabolism is a much slower process than anaerobic metabolism, but it produces more ATP.

Figure 16.3 Anaerobic versus aerobic metabolism

It plays an important role in the uptake and delivery of oxygen to muscle cells throughout the body. Oxygen is taken in by the lungs and transferred from the lungs to the blood, where the oxygen-rich blood is circulated 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. During physical activity, however, the muscles' energy and oxygen requirements increase. 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.

During the early stages of exercise, your muscles are the first to respond to a change in activity level. However, your lungs and heart don't respond as quickly and don't begin to increase their oxygen supply 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 called ADP (adenosine diphosphate) to make ATP. After about 10 seconds, the creatine phosphate stored in the muscle cells is also used up.

The stored ATP and creatine phosphate in the muscles are used up within about 15 seconds after exercising. 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 obtained from muscle blood. 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 increase to deliver more oxygen to your muscles. Aerobic metabolism is the most efficient way to produce ATP, producing 18 times more ATP for each molecule of glucose 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.

Figure 16.4 The effect of exercise duration on energy systems

Fuel sources for anaerobic and aerobic metabolism change depending on the amount of nutrients available and the type of metabolism. Glucose can come from blood glucose (which comes from carbohydrates in the diet or from the synthesis of glycogen and glucose in the liver) or from muscle glycogen. Glucose is the main source of energy for anaerobic and aerobic metabolism. Fatty acids 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 exercising muscles. Although protein is not considered a primary source of energy, small amounts are used at rest or during activity. The amount of amino acids used for energy metabolism increases when total dietary energy intake falls short of nutrient requirements or when you engage in long-term resistance training. When amino acids are broken down by removing the nitrogenous amino acid, the remaining carbon molecule can be broken down into ATP by aerobic metabolism or used to produce glucose. When training lasts for many hours, the use of amino acids for energy and glucose synthesis increases.

Figure 16.5 Fuel sources for aerobic and anaerobic metabolism

Training intensity determines the contribution of the type of fuel source used to ATP production (see Figure 16.4 “The Effect of Training Intensity on Fuel Sources”). 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 level 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.

During low-intensity activity, the body uses aerobic metabolism over anaerobic metabolism because it is more efficient at producing large 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. Eventually, when glycogen stores are depleted, fatigue sets in.

Figure 16.6 The effect of exercise intensity on energy sources

The fat burning zone

The fat burning zone is a 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, your body burns a higher percentage of calories from fat during low-intensity aerobic activity, but there's more to it than that. 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, when looking at the actual number of calories burned, the higher intensity activity burns the same amount of fat and a much higher total calorie count.

(Video) Nutrition Overview (Chapter 1)

If you are familiar with endurance sports, you may have heard of "hitting the wall" or "hitting the wall". These slang terms refer to the extreme fatigue that manifests itself after about 120 minutes of an endurance sport, such as a marathon or long-distance cycling. The underlying physiology of "hitting the wall" means that the muscles have used up all their stored glycogen and are therefore dependent on other nutrients to meet their energy needs. Fatty acids are transported from fat storage cells into muscle to compensate for nutrient deficiencies. However, fatty acids take longer to convert to energy than glucose, which reduces performance. To avoid "hitting the wall" or "crashing", endurance athletes perform a carbohydrate load, called a carbohydrate load, a few days before competition. This maximizes the amount of glycogen stored in an athlete's liver and muscle tissue. It's important not to assume that carb loading works for everyone. Without accompanying resistance training, the amount of glucose stored will not increase. If you plan on running a five mile race with your friend for fun and decide to eat a large amount of carbs in the form of a big spaghetti dinner the night before, the excess carbs will be stored as fat. So unless you're an endurance athlete training for more than 90 minutes, there's no benefit to loading carbs, and there may even be some downsides. Another way athletes "hit the wall" is by consuming carbohydrate-rich foods and beverages during an endurance sport. In fact, during the Tour de France, a 22-day, 24,000-mile race, the average cyclist consumes over 60 grams of carbs per hour.