We will conclude module one by discussing the key concepts of protein metabolism. As you will see, proteins play a small yet significant role in muscle energetics during exercise. The main contribution of proteins come after an exercise bout and involves the early stages of training adaptations. This may include the synthesis of key structural, enzymatic, and muscular proteins depending upon the training stimulus. Proteins are composed of their amino acid building blocks. Essential amino acids cannot be made by the body, thus they must come from dietary sources. Nonessential amino acids can be synthesized in the body so no dietary sources required. In order to use proteins for fuel, they must be broken down into their amino acid form. Shown here is the essential amino acid pool. Notice that amino acids from this pool, can be used to synthesize body proteins. And body proteins that are degraded can release their amino acids building blocks into this central pool. Importantly, amino acids from this central pool can be converted to carbohydrate and fat, thereby serving as a potential fuel source for the exercising muscles. As nitrogen is a fundamental component for all amino acids, researchers and clinicians typically measure nitrogen balance to determine an individual's dietary protein needs, as well as their general whole body protein metabolism. Simply stated, the amount of nitrogen entering the body through the diet is compared to the amount being excreted. If an individual nitrogen input is equal to their nitrogen output, there said to be in nitrogen balance indicating that they do not need to alter their dietary protein intake. Most healthy adults with an adequate diet will be in nitrogen balance. You'll be in positive nitrogen balance if your dietary intake of nitrogen exceeds that of output. This will suggest that the body is holding on to more nitrogen for the synthesis of body proteins. Children will be in positive nitrogen balance as the variety of proteins must be synthesized for growth and development. An individual can also enter into nitrogen balance when they engage in intense strength training coupled with a high protein diet. Body builders taking anabolic steroids magnify this response. Negative nitrogen balance occurs when output exceeds input. This would suggest that the body is breaking down or degrading proteins for fuel. This can happen during starvation or illness and even with the best aging. The recommended daily allowance for sedentary adult men and women is 0.8 grams of protein per kilogram of body weight per day. As shown here the protein requirement will be greater than intense in endurance training as well as during heavy strength training. These individuals should pay close attention to their protein intake to ensure their remain in nitrogen balance. Let's look at how a single bout of exercise affects protein metabolism. Muscle protein synthesis actually decreases during a bout of aerobic exercise. Remember, ATP utilizing pathways such protein synthesis are inhibited as the energy charge drops during exercise to preserve ATP for mechanical work. However, notice the very significant increase in the rate of protein synthesis during the period immediately following exercise. This is a crucial time for the initiation of training adaptations as will be discussed shortly. Proteins can actually be broken down or degraded during a single bout of exercise with their resulting amino acids being used for fuel. The carbon skeletons of certain amino acids can directly be used for ATP production in the mitochondria. The extent to which protein are degraded will be dependent on exercise intensity and duration as well as the availability of carbohydrates. When carbohydrates stores are low there will be greater dependence on protein for fuel. All things considered, proteins can contribute approximately five to 15% of the fuel required for exercise. Now, let's examine the basic adaptations related to protein metabolism associated with both endurance and strength training. We will begin by looking at the protein response when previously sedentary individuals initiate a regular exercise program. Notice that at the onset, an individual dependent upon the exercise intensity of training, can enter into negative nitrogen balance, indicating that during this period, protein breakdown exceeds that for synthesis. Such an individual would benefit by increasing their protein intake above the recommended daily allowance. We will thoroughly discuss the mechanisms responsible for both endurance and strength training adaptations in module three. For now it is important to realize that individuals involved in heavy endurance training will demonstrate adaptations specifically related to mitochondrial protein metabolism. This includes increases in mitochondrial number, size and related enzymes, all requiring an increase in protein synthesis. With intense strength or resistance training, the muscle can increase in size, known as hypertrophy. This increase in muscle mass can only occur with an elevation in the rates of muscle protein synthesis. This emphasizes the need for such individuals to have a dietary protein intake significantly above that for the recommended daily allowance. Intense strength training, when coupled with the use of anabolic steroids, will result in even greater gains in muscle mass and thus the need for increased dietary protein intake. Finally, as a reminder, the rates of protein synthesis are immediately elevated after a single bout of exercise. For strength training, ingestion of proteins and amino acids during this approximately one hour window will further stimulate and enhance muscle protein synthesis. In summary, athletes as well as individuals initiating a training program need to be sure to maintain nitrogen balance. Protein degradation increases during exercise and certain amino acids can be used directly as fuel for ATP production. Muscle protein synthesis decreases during exercise but increases during recovery. Training adaptations related to protein metabolism are task specific and occur primarily post exercise and may require an adjustment in the recommended daily allowance. This concludes module one. In module two, I will discuss how many key physiological systems respond and adapt to exercise. I hope to see you there.
