This is an excerpt from Nutrition for Sport, Exercise, and Health 2nd Edition With HKPropel Access by Marie Spano,Laura Kruskall & D. Travis Thomas.
Whether you train to increase strength or endurance, your body will adapt to accommodate or get used to the repeated stimulus, which ultimately will improve performance. The concept of training to improve performance encompasses the three key principles outlined in chapter 1. Although the magnitude and specificity of these changes depend on the individual and the characteristics of the training program, any program that improves aerobic capacity will have a significant effect on fuel utilization and the energy systems. Because many athletes adopt a training regimen that promotes improvements in strength, power, and endurance, metabolic adaptations geared toward improving mitochondrial oxygen availability are prominent.
Figure 2.17 describes some key metabolic adaptations. Structural and biochemical adaptations to endurance training include increased mitochondrial number and size and increased concentration of oxidative enzymes involved in beta-oxidation, the TCA cycle, and the ETC. These changes improve aerobic energy system efficiency in the muscle. The NADH shuttling system and a change in LDH protein structure are also observed. These adaptations improve electron delivery to the ETC and increase the ability of the muscle to oxidize lactic acid. A host of cardiovascular changes also take place to support adaptation. The most significant changes are related to improvements in the heart’s stroke volume and angiogenesis, which increases capillary density and capacity to transport fatty acids from the plasma to the muscle cell. Again, these adaptations are important in promoting aerobic system efficiency and, in this case, improving oxygen delivery to working muscle. Unfortunately, the principle of reversibility still holds true when training ceases. Most of these adaptations are lost after approximately 5 weeks of detraining, and about half of the increase in muscle mitochondrial content is lost after just 1 week of detraining. Even more troubling to consider is that it takes about 4 weeks of retraining to regain many positive endurance-related skeletal muscle adaptations lost in the first week of detraining (21). The rapid rate of reversibility is one of the fundamental reasons why exercise is recommended as part of a healthy lifestyle that is done daily, in some form, as a key strategy to promote health, longevity, and chronic disease prevention.
Exercise training stimulus is undoubtedly instrumental in promoting a multitude of cardiovascular system, muscle tissue, and cellular changes to improve energy system efficiency. Although these adaptations have a clear benefit to metabolic health that should not be understated, other outcomes related to fuel substrate utilization offer key benefits to the competitive athlete and anyone else starting an exercise program to improve health. These benefits are best described by understanding the crossover concept of carbohydrate and fat utilization during exercise. By definition, the crossover is the point on the graph (see chapter 11) at which the body starts using more carbohydrate than fat as its energy source. Recall that exercise intensity is directly proportional to carbohydrate use because of the anaerobic environment created at high exercise intensities. Also recall that a person’s ability to oxidize fat during exercise with increasing intensity depends on their aerobic capacity. If someone were to draw a simple figure to represent the crossover concept for a sedentary friend and compare it with an athlete friend, the figure, especially where the crossover occurs on the percent V̇O2max axis, would look very different. Consistent with the cardiovascular, muscle tissue, and cellular changes that occur with training, the crossover point also shifts to the right with training. This phenomenon has huge metabolic significance, for both athletes and people trying to become healthier through exercise. For those starting an exercise program, the shift to the right that occurs with chronic training adaption means they will be able to work out longer and harder using a more efficient aerobic energy system to primarily oxidize fatty acids. Although this finding may be initially interpreted as a direct strategy to lose body fat by directly oxidizing fat, this result is not the key outcome of this adaptation and is not the key player that promotes loss of body fat. The key benefit is that this adaptation allows for higher-intensity exercise capacity, an acquired advantage to training that is of primary importance for increasing energy (calorie) expenditure and thus promoting weight loss and weight maintenance. Think of trading in your four-cylinder car for a car with a V-8 engine. Although in the car world, this trade is clearly not advantageous for fuel economy, the shift in the crossover with training allows you to aerobically burn fuels more efficiently and burn more calories using large muscle groups with a reduced reliance on the lactic acid–producing anaerobic system. By continuing to push yourself with repeated training, you can further augment this adaption and increase muscle mass, which will burn more fuel (calories) at rest. Consider how long it takes someone to run a mile before training compared with how long it takes after training. Regardless of the time difference in running the mile, the number of calories burned will be similar. The major difference is the energy-yielding macronutrient distribution of fuels oxidized, the energy systems used, and the calories burned divided by the amount of time to run the mile. The trained person will oxidize more fat, feel better during the run because of less lactic acid production, and burn more calories per unit of time. The result is usually an enjoyable experience during which mileage and calorie expenditure is increased, thus improving metabolic health and assisting in the maintenance of a healthy body weight.
When continued training promotes metabolic adaptation that shifts the crossover point to the right, the athlete benefits from the same metabolic advantages as the nonathlete but to a greater degree. These adaptations support the athlete’s high-intensity efforts and allow fat, as an endless fuel source, to be used earlier in the activity and at much greater intensities and power outputs than in the nonathlete. But the most important metabolic benefit of the crossover adaptation for the athlete is not weight and body composition management—the most important benefit is the ability to preserve and protect limited carbohydrate stores until the highest-intensity effort is required to compete in an athletic event. Because carbohydrate stores in the body are limited in muscle tissue, and occur in a relatively small amount in the liver, the preservation of this fuel source for anaerobic system activity to fuel ATP generation is crucial to high-intensity performance. Think of a trained distance runner metabolically capable of tapping primarily into fat stores to fuel much of the ATP demand for most of a race. When it is important for the runner to run up hills or overtake other runners in the final portion of the race, the increased intensity (and increased ATP demand) will need to be met anaerobically with the only fuel that the anaerobic system can use—carbohydrate. If this runner did not adequately prepare for the race by consuming enough carbohydrate leading up to the race, their capacity to produce ATP anaerobically will be compromised and will negatively affect performance.
Putting It Into Perspective
Metabolic Effect of Dietary Nitrate
Consuming nitrate-rich vegetables such as spinach, arugula, and beetroot juice or beetroot supplements may enhance athletic performance. Several studies have documented performance enhancement and mechanisms related to metabolism. For some athletes, nitrate significantly improves skeletal muscle oxygen uptake and mitochondrial use of oxygen. Nitrate may also reduce the amount of oxygen needed to generate ATP during submaximal aerobic exercise and reduce ATP demand for muscles to produce force (11). Because these benefits may improve exercise tolerance and aerobic system efficiency, dietary nitrate supplementation may be beneficial for both trained athletes and novice athletes who do not have a highly adapted aerobic energy system (22). To learn more about the dietary nitrate supplementation, refer to chapter 9.