This is an excerpt from Nutrition for Sport, Exercise, and Health by Marie Spano,Laura Kruskall & D. Travis Thomas.
Many people think that since muscle fibers are made up of protein, building and maintaining muscle must require large amounts of protein consumed from the diet. In reality, dietary protein is just one part of the equation to promote an optimal environment for muscles to adapt to physical training. The additional amount of protein required that is beyond what is already consumed to support optimal muscle adaptation is relatively small, and many athletes are already meeting their daily protein intake goals. Athletes should note that regular, well-structured training regimens combined with a proper mix of nutrients that meet their energy demands is the cornerstone for achieving their goals. Further, many other factors such as stress, frequent alcohol consumption, and inadequate rest can sabotage even the best diet and exercise plan. This is not to say that protein is not important. Research over the last decade has provided much more detail for how dietary protein works to foster muscle hypertrophy (growth) and metabolic adaptations when combined with a proper training program. The following are the four primary roles of dietary protein in an athlete's diet related to sport performance:
- Maximizing gains in muscle mass and strength
- Promoting adaptations in metabolic function (an up-regulation of oxidative enzymes)
- Preserving lean mass during rapid weight loss
- Structural benefits to other protein-containing nonmuscle tissues, such as tendons, ligaments, and bones
New guidelines now exist for daily protein needs as well as the daily distribution of protein consumed. In this section, we delve into these details and also look at the ways protein metabolism changes during and in response to exercise and how this information melds with other sections of this chapter.
Protein Metabolism and Exercise
Although carbohydrate and fat are the primary macronutrients metabolized for energy in the muscle, protein can also be used during exercise and can be oxidized directly in the muscle. Fortunately, most protein is spared for important synthetic processes, but it is important to remember from chapter 1 that the oxidation of all energy-yielding macronutrient fuel sources is always occurring in the body during exercise. The only thing that changes is the relative proportion of the macronutrients burned. At any one time during exercise, carbohydrate and fat make up more than 85 percent of the energy-yielding macronutrient fuels oxidized, but some protein is always used.
Exercise has a strong effect on protein metabolism. During exercise, the range of protein contribution to meet energy demands (ATP) is generally less than 5 to 10 percent, and in some extreme cases up to 15 percent of total energy expenditure. Many factors affect the percentage of protein oxidized during exercise, including exercise intensity, training level (new vs. experienced), and availability of other fuels (e.g., carbohydrate). The type of exercise, or mode of exercise, also has a strong influence. During strenuous resistance training, less than 5 percent of protein is oxidized as an energy source. Conversely, prolonged endurance exercise (>90 min) might result in up to 15 percent to serve as an energy source. A significant increase in protein oxidation, within the 5 to 15 percent range, occurs when muscle glycogen is depleted. As the body's most limited fuel source (carbohydrate) becomes depleted, the body must attempt to keep blood sugar stable to fuel the nervous system. For most individuals, the process of gluconeogenesis (see chapters 2 and 3) kicks in to help stabilize blood sugar levels by making new glucose from gluconeogenic precursors, including protein. For those following a low-carbohydrate diet, the production of ketones provides more fuel to the nervous system when glucose is limited. Regardless of the amount of carbohydrate in the diet, when muscle glycogen becomes depleted, BCAA oxidation within the muscle increases and contributes to the rise in protein use as a fuel source. In such a scenario, exercise intensity decreases. Gluconeogenesis and ketone production cannot keep up with the high ATP demands of high-intensity exercise.
Protein provides the building blocks (amino acids) needed to build and repair muscle.
© Human Kinetics
The most prominent gluconeogenic pathway is the glucose-alanine cycle (figure 5.16). In this metabolic pathway, the gluconeogenic amino acid alanine leaves the muscle to create new glucose in the liver, which contributes to new blood glucose. This process occurs simultaneously with muscle tissue oxidation of BCAAs. As BCAAs are liberated from muscle tissue, their catabolism results in donating their NH2 group to pyruvate in a process called transamination. The carbon skeletons from the BCAAs can then enter the TCA cycle in the mitochondria of muscle cells as TCA intermediates to contribute to ATP generation. Meanwhile, pyruvate originating from glycolysis can bind with NH2 that originated from the BCAAs deaminationto form the gluconeogenic amino acid alanine. Alanine can freely leave the muscle tissue and travel through the blood to the liver. The liver can then deaminate alanine to reform pyruvate. This liver-generated pyruvate can be further metabolized to form glucose that contributes to blood glucose or liver glycogen. Gluconeogenesis is never really turned off in human metabolism; instead it may occur only at a very low capacity and increase during long bouts of exercise, particularly when carbohydrate stores become limited. A significant limitation of this pathway is that the speed at which it can create new glucose depends on the availability of enzymes needed to drive the reaction. These enzymes must go through the protein synthesis process and are thus not capable of being made fast enough to keep up with the demands of one long endurance exercise bout. As a result, hypoglycemia can eventually occur and is the primary reason exercise halts during long events. While the glucose-alanine cycle is an integral pathway in human metabolism and contributes to exercise metabolism, it is best suited for nonexercise situations during starvation, when key enzymes have time to be up-regulated to produce glucose at the expense of muscle protein.
Figure 5.16 Alanine serves as an important gluconeogenic precursor.
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