Many physiological and nutritional demands occur within the body during exercise. As muscles contract, the demand for oxygen, hydrogen and other key nutrients increases. The human body requires a continuous supply of energy to perform its many functions. As energy demands increase with exercise, additional energy must be supplied or the exercise will end. Show Factors of performance Whether a recreational athlete or an elite athlete, many factors influence performance including, but not limited to, diet, hydration, fitness level, intensity and duration. There are many factors that predict what source of fuel will be used. Proteins, fats and carbohydrates are all possible sources of fuel for exercise and muscle contraction. During moderate-intensity exercise, roughly half of the energy is derived from glycogen, while the other half comes from glucose in the blood and fatty acids. Carbohydrates (glucose/glycogen) serve as the primary source of fuel as duration and intensity increase. If exercise continues for a significant period of time, fatty acids will serve as the fuel source when glycogen stores are nearly depleted. It must be noted that fat metabolism cannot occur without the presence of glucose, and thus muscle glycogen and blood glucose are the limiting factors in performance. Protein or, more specifically, amino acids, will only be used as an energy source if other calories are insufficient. Food choices A person’s diet will influence which source of fuel is used and therefore, performance level. If a person consumes a high-carbohydrate diet, more glycogen will be used for fuel. If the diet is high in fat, fat will be used as the fuel source. A high-fat diet is not recommended as even the leanest person has plenty of stored fat for long endurance exercise. A high-fat, low-carbohydrate diet can lead to poor performance due to low glycogen stores. As a guideline for endurance athletes, roughly 60–70 percent of calories should come from carbohydrates, 10–15 percent protein and 20–30 percent from fat. You should consume a well-balanced diet containing carbohydrates, protein and fat during training periods. Carbohydrate intake before, during and after exercise is crucial. A high-carbohydrate pre-exercise meal not only prevents hunger pangs during exercise, it also provides optimal blood glucose levels for endurance exercising and increases glycogen stores. Avoid high-fat foods in a pre-exercise meal as it delays stomach emptying and takes longer to digest. This meal should be three to four hours before an event. Marathon runners talk about “hitting a brick wall.” This refers to the time when fuel sources have been drained and not replaced. When glycogen and blood glucose levels are low, the body is out of fuel and cannot keep going no matter how fast an athlete wants to go. For exercise lasting longer than an hour, you should ingest carbohydrates to fuel the brain and muscles. You can maintain a sufficient supply of energy by consuming 26–30 grams of carbohydrates every 30 minutes during exercise. Most sports drinks provide 15–20 grams of carbohydrate, so consuming 8–12 ounces every 15–30 minutes is recommended. As for protein, only a few amino acids can actually be used directly as energy. Thus, protein consumption during exercise is not advantageous. Fluid intake Muscle glycogen stores must be replaced after endurance exercise. Resynthesis of muscle glycogen is promoted when carbohydrates are consumed immediately after exercise. Unfortunately, due to an elevated body temperature, appetite is usually depressed and many athletes have difficulty consuming foods immediately after exercise. Drinking carbohydrates via a sports drink or shake provides carbohydrates and promotes rehydration. Adequate fluid intake is also crucial for any athlete. You should weigh yourself before and after an endurance event, especially during hot weather. For each pound lost during exercise, drink three cups of fluid. Fluids should not be restricted before, during or after an event. Athletes should not rely on thirst as a sign of fluid loss. Consume roughly 14–22 ounces of fluid before an event, 6–12 ounces every 15–30 minutes during an event, and after the event, 16–24 ounces for every pound of body weight lost. Anne Harguth is a registered dietitian with Mayo Clinic Health System in Waseca.
The human body uses carbohydrate, fat, and protein in food and from body stores for energy to fuel physical activity. These essential nutrients are needed regardless of the intensity of the activity you are doing. If you are lying down and reading a book or running a marathon, these macronutrients are always needed in the body. However, in order for these nutrients to be used as fuel for the body, their energy must be transferred into the high energy molecule known as adenosine triphosphate (ATP). ATP is the body’s immediate fuel source and can be generated either with in the presence of oxygen or without the presence of oxygen. The type of metabolism that is predominately used during physical activity is determined by the availability of oxygen and how much carbohydrate, fat, and protein are used. Anaerobic and Aerobic MetabolismAnaerobic metabolism occurs in the cytosol of the muscle cells. As seen in Figure 10.1., a small amount of ATP is produced in the cytosol without the presence of oxygen. Anaerobic metabolism uses glucose as its only source of fuel and produces pyruvate and lactic acid. Pyruvate can then be used as fuel for aerobic metabolism. Aerobic metabolism takes place in mitochondria of the cell and is able to use carbohydrates, protein, or fat 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 the majority of the ATP in the body is generated. Figure 10.1. Anaerobic vs aerobic metabolism. Note that carbohydrate is the only fuel utilized in anaerobic metabolism, but all three macronutrients can be used for fuel during aerobic metabolism. Physical Activity Duration and Fuel UseThe respiratory system plays a vital role in the uptake and delivery of oxygen to muscle cells throughout the body. Oxygen is inhaled by the lungs and transferred from the lungs to the blood, where the cardiovascular system circulates the 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 are able to supply the muscles with adequate amounts of oxygen to meet the energy needs for aerobic metabolism. However, during physical activity, your muscles need more energy and oxygen. In order to provide more oxygen to the muscle cells, your heart rate and breathing rate will increase. The amount of oxygen that is delivered to the tissues via the cardiovascular and respiratory systems during exercise depend on the duration, intensity and physical conditioning of the individual.
Figure 10.2. Energy systems used to fuel exercise change with duration of exercise. The ATP-creatine phosphate system is used up within seconds. The short-term and long-term systems kick in and provide energy for exercise as the duration of the workout goes on. The fuel sources for anaerobic and aerobic metabolism will change depending on the amount of nutrients available and the type of metabolism.
Figure 10.3. Fuel sources for anaerobic and aerobic metabolism. Both dietary sources and body storage of carbohydrates, fat, and protein can all be used to fuel activity. Amount varies depending on duration and intensity of the activity. Physical Activity Intensity and Fuel UseExercise intensity determines the contribution of different fuel sources used for ATP production. Both anaerobic and aerobic metabolism combine during exercise to ensure that the muscles are equipped with enough ATP to carry out the demands placed on them. The contribution from each type of metabolism depends on the intensity of an activity. During low-intensity activities, aerobic metabolism is used to supply enough ATP to muscles. However, during high-intensity activities, more ATP is needed, so the muscles must rely on both anaerobic and aerobic metabolism to meet the body’s demands.
Table 10.2. Summary of fuels used for activities of different intensities and durations. During low-intensity activities, the body will use aerobic metabolism over anaerobic metabolism, because it is more efficient and produces larger amounts of ATP. Fatty acids are the primary energy source during low-intensity activity. With fat reserves in the body being almost unlimited, low-intensity activities are able to continue for a long time. Along with fatty acids, a small amount of glucose is used as well. Glucose differs from fatty acids, because glycogen storages can be depleted. As glycogen stores are depleted, the glucose supply becomes depleted, and fatigue will eventually set in. Figure 10.4. The effect of exercise intensity on fuel sources. Anaerobic exercise utilizes only glucose for fuel. As activities become more aerobic, the body can utilize fatty acids and, to a small extent, amino acids, for energy production. One important clarification about exercise intensity and fuel sources is the concept of the fat-burning zone. Many people think that in order to lose body fat, they should exercise at a lower intensity so that fat is the primary fuel source. The fat-burning zone is typically referred to as a low-intensity aerobic activity that keeps your heart rate between 60 and 69 percent of maximum heart rate. The cardio zone, on the other hand, is a high-intensity aerobic activity that keeps the heart rate between about 70 and 85 percent of maximum heart rate. So which zone do you burn the most fat in? Technically, your body burns a higher percentage of calories from fat during a low-intensity aerobic activity. When you begin a low-intensity activity, about 50% of the calories burned come from fat, whereas in the cardio zone only 40% come from fat. However, this isn’t the whole story. High-intensity activity burns more total calories per minute. At this higher rate of energy expenditure, you can burn just as much or more total fat and more total calories as during a lower intensity activity. If weight loss is one of your goals, high-intensity activities will burn more total calories, helping to shift to negative energy balance, and will promote a greater level of fitness. However, the best exercise program is one that is enjoyable, sustainable, and safe for you; if you’re just starting out, it’s wise to begin with low- to moderate-intensity activities and work your way up from there. Figure 10.5. The fat-burning zone. While a greater percentage of calories burned in lower intensity exercise come from fat, the overall total calorie burn is greater in higher intensity exercise. Attributions:Image Credits:
|