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Lecture 6

Fuel Utilization During Exercise,
Aerobic and Anaerobic Metabolism,
Control of Muscle Protein Metabolism/Anabolism

Guest Lecturers: Dr. A. Scott Connelly, William H. Carpenter, M.S.

I. Fuel Utilization During Exercise
Under most circumstances, fat and carbohydrate are the fuels utilized during exercise. The degree to which each fuel acts as the primary or secondary source of energy and the efficiency with which energy is utilized depends on the prior nutrition of the athlete and the intensity and duration of the exercise. At low levels of prolonged exercise most energy needs come from fat and lesser energy needs come from carbohydrate. At higher intensity carbohydrate plays a greater role but is limited in its duration of action. Protein plays only a minor role at very high levels of energy utilization, but adequate protein intake is critical for maintenance of lean body mass to enable exercise performance.

Energy is extracted from foods in the body by converting the chemical energy stored in chemical bonds to high energy phosphate bonds in ATP (Adenosine Triphosphate). This high energy bond can be used in a number of biochemical reactions as a fuel with the conversion of ATP to ADP (adenosine diphosphate). If ADP begins to accumulate in muscle then an enzyme is activated in muscle to break down phosphocreatine (PCr) in order to restore ATP levels (PCr + ADP ® ATP + Cr). The creatine released from this reaction is converted to creatinine and excreted in the urine. The stores of PCr are extremely limited and could only support muscle ATP levels for about 10 seconds if there were no other sources of ATP. Since ATP is provided from other sources, PCr ends up being a major energy source in the first minute of strenuous exercise. PCr has the major advantage of being localized in the muscle so that it can rapidly restore and maintain ATP levels for intense exercises such as sprinting, jumping, lifting, and throwing.

II. Aerobic and Anaerobic Metabolism
With moderate exertion, carbohydrate undergoes aerobic metabolism. Under these conditions, oxygen is used and the carbohydrate goes through both the Embden-Meyerhoff pathway of anaerobic metabolism in which glucose is converted to lactate, but, prior to the conversion of pyruvate to lactate, pyruvate enters the Krebs Cycle in mitochondria where oxidative phosphorylation results in a maximum extraction of energy from each molecule of glucose. If there is plenty of oxygen available and the exercise is of low to moderate intensity, then the pyruvate from glucose is converted to carbon dioxide and water in the mitochondria. Approximately 42 ATP equivalents can be produced from a single glucose molecule compared to only 4 ATP with anaerobic metabolism. Aerobic metabolism supplies energy more slowly than anaerobic metabolism, but can be sustained for long periods of time up to 5 hours. The major advantage of the less efficient anaerobic pathway is that it more rapidly provides ATP in muscle by utilizing local muscle glycogen. Other than PCr, it is the fastest way to resupply muscle ATP levels. Anaerobic glycolysis supplies most energy for short term intense exercise ranging from 30 seconds to 2 minutes. The disadvantages of anaerobic metabolism are that it cannot be sustained for long periods, since the accumulation of lactic acid in muscle decreases the pH and inactivates key enzymes in the glycolysis pathway leading to fatigue. The lactic acid released from muscle can be taken up by the liver and converted to glucose again (Cori Cycle), or it can be used as a fuel by the cardiac muscle directly or by less active skeletal muscles away from the actively contracting muscle.

Muscle glycogen is the preferred carbohydrate fuel for events lasting less than 2 hours for both aerobic and anaerobic metabolism. Depletion of muscle glycogen causes fatigue and is associated with a build-up of muscle lactate. Lactate production increases continuously but physiologists have defined a point at which breathing changes as a result of acid-base imbalance called the anaerobic threshold. Both the nutrition and conditioning of the athlete will determine how much work can be performed in a specific exercise before fatigue sets in. This can be measured directly or indirectly. An indirect measurement uses an exercise treadmill or stairway according to standard protocols and pulse is measured. The more conditioned athlete can produce the same amount of work at a lower pulse rate. This indirect determination assumes that pulse rate is proportional to oxygen consumption. On the other hand, oxygen consumption can be measured directly during exercise. A motorized treadmill is usually used to increase the intensity of exercise until fatigue occurs. The amount of oxygen consumed just before exhaustion is the maximal oxygen uptake or VO2max.

Exercise intensity can be expressed as a percentage of VO2max. Low intensity such as fast walking would be 30 to 50% of VO2max. Jogging can demand 50 to 80 % of VO2max depending on the intensity, and sprints can require from 85% to 150% of VO2max (with the added 50% coming from short term anaerobic energy production).

It is possible to build up glycogen stores prior to exercise to improve performance. With exercises lasting for more than 20 to 30 minutes, blood glucose becomes important as a fuel to spare muscle glycogen breakdown. Both aerobic and endurance training lead to increases in glycogen stores, triglycerides, oxidative enzymes, and increased number and size of mitochondria. Both the oxidative enzymes involved in the Krebs cycle oxidation of glucose and the lipoprotein lipase needed to convert triglycerides to fatty acids are increased through training. This is not a general effect, but is specific to the muscle and muscle fiber type being used for the exercise. Slow twitch muscle fibers provide for prolonged aerobic activity, while the fast-twitch muscle fibers are used for short intense activities.

The fatigue that develops with intense exercise can be related to specific fiber types. In prolonged exercise at 60 to 75 percent of VO2max Type I fibers (red, slow twitch) and Type IIa ( red, fast twitch) are recruited during the early stages of exercise, but as the intensity increases Type IIb fibers (white, fast twitch) must be recruited to maintain the same intensity. It requires more mental effort to recruit Type IIb fibers and they produce lactic acid. As the glycogen levels drop in the red muscle fibers, they will rely more on fat. Since fat is less efficient than carbohydrate, intensity will decrease ( pace will slow).

At the other end of the spectrum, during mild exercise such as a brisk walk muscles burn fat for fuel because the supply of ATP provided from fat is adequate to maintain intensity. As mentioned earlier in this course, fatty acids are readily available from stored fat and the rate of lipolysis is three times the rate of fatty acid release at rest so that fatty acids can be supplied at an increased rate rapidly during the onset of low levels of exercise. So while fat is not very useful for short term, intense exercise, it is a great advantage for increasingly prolonged exercise especially when it is maintained at a low or moderate level of intensity.

The advantage of fat as a fuel is that it provides extensive stores of calories in a easily portable form. Since fat is not hydrated it weighs much less per unit calorie than protein or carbohydrate (9 Cal/gm of fat vs. 4 Cal/gm of carbohydrate or protein). When you compare the number of ATP produced per carbon atom, fat is also more efficient. A 6-carbon glucose molecule produces 36 to 38 ATP on average providing a ratio of 6 ATP/Carbon, while an 18 carbon fatty acid produces 147 ATP providing a ratio of 8.2 ATP/Carbon. However, carbohydrate is more efficient than fat when the amount of ATP produced per unit of oxygen consumed is considered. Six oxygen molecules are required to metabolize six-carbon glucose producing 36 ATP (ratio = 6 ATP/oxygen molecule), while 26 oxygen molecules are required to produce 147 ATP from an 18 carbon fatty acid (5.7 ATP/oxygen molecule). Therefore, for a performance athlete it is important to maintain the efficiency edge provided by carbohydrate as long as glycogen is available in the muscles. Under usual exercise conditions, protein only provides about 6% of energy needs. With high intensity endurance exercise, the production of glucose from amino acids can be significant up to about 10 or 15% of total energy needs. The only food that provides energy for short-term fast-paced exercise is carbohydrate, while slow steady aerobic exercise uses all three primary fuels but primarily fat and carbohydrate.

The Exercise Prescription: How Much Exercise Is Enough ?
The practical application of the above knowledge falls into two categories: first, the prescription of adequate amounts of exercise to optimize performance, and second, the use of dietary, hormonal and pharmacological ergogenic aids to improve performance. The second topic will be covered later in the course, but this brief introduction to exercise prescription is provided as a background to your upcoming self-assessment exercise.

Cardiovascular Training
A gradual incremental exercise program emphasizing cardiovascular fitness is the basis of all exercise programs. Vigorous exercise involves minimal risks for healthy individuals but can be risky for couch potatoes or the dedicated sedentary. These individuals should check with their physician first as should all those over 35, or with medical conditions such as arthritis, hypertension, shortness of breath, diabetes, obesity, or a family history of heart disease.

A basic prescription involves a stretching session and a ten minute low intensity warm-up, to increase blood flow and minimize risk of injury. Then exercises to increase muscular strength, endurance, and flexibility are done. These should be performed at an intensity adequate to increase heart rate into a training zone which is 60 to 90 % of maximum age-adjusted heart rate (MHR = 220 - age ). I usually start individuals at 50 to 60% of MHR, and then keep them in the training zone. For weight loss, prolonged sessions at 70% of MHR are effective at burning fat, while increased levels of exercise induce muscle to hypertrophy. A ten minute cool-down is important to minimize cramping and muscle injury at the end of each session.

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Lecture 1
:Introduction to Nutrition in Western Civilization
Lecture 2:
Dietary Macronutrients, Body Fat, and Blood Lipids
Lecture 3:
Digestion and Absorption of Macronutrients
Lecture 4:
Basic Principles of Nutrient Metabolism
Lecture 5:
Lecture 6:
Fuel Utilization During Exercise
  Lecture 7:Biochemistry of Oxidant Stress in Health and Disease Antioxidants
Lecture 8:Nutrition for the 21st Century






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