In this video, we will continue on with the two part mini-series on carbohydrate metabolism. In the last video, we discussed the importance of muscle glycogen during exercise. We will begin this video examining the important role for liver glycogen. I remind you that the main function of muscle glycogen is to provide glucose units for glycolysis in the exercising muscle. Thus the glucose units produced from breakdown of muscle glycogen stay in the muscle providing fuel for ATP production. However, the main function of liver glycogen is to provide glucose to maintain blood glucose levels during exercise. During aerobic activities, the working muscles rely on blood-borne fuel sources, such as glucose and free fatty acids. As the muscles extract glucose from the blood for subsequent ATP production, blood glucose levels would fall dangerously low if the body had no way to immediately replace them. Fortunately, liver glycogen can be broken down for this purpose, thereby preventing exercise induced hypoglycemia. Let's examine to what extent the muscles rely on blood glucose as a potential fuel during mild, moderate, and intense exercise. Shown here is the rate of blood glucose uptake by the legs during 40 minutes of cycling. Not surprisingly, as discussed in the previous video, the greater the exercise intensity, the greater dependance on carbohydrates for fuel. Thus exercising at 75 to 90% of your maximal oxygen uptake elicits the greatest increase in blood glucose uptake by the legs. As long as glycogen stores hold out, the liver does a remarkable job of maintaining blood glucose levels, preventing exercise induced hypoglycemia. The breakdown of liver glycogen is primarily regulated by the pancreatic hormone glucagon. Similar to epinephrine regulating muscle glands glycogenolysis, glucagon activates the enzyme phosphorylase initiating the breakdown of liver glycogen to glucose. The newly formed glucose will now diffuse into the blood, maintaining blood glucose levels as the active muscles continue to extract glucose for fuel. Let's take a step back and look at the big picture for carbohydrate utilization during prolonged exercise, such as a marathon. Initially, at an exercise intensity that can be sustained for 4 hours, approximately 50% of the fuel for the working muscles is coming from carbohydrates, while the remaining 50% is coming from fats. Notice that as the individual gets deep into the exercise session, both muscle and liver glycogen stores begin to deplete. This emphasizes the point that there is only a limited amount of carbohydrates stored in the body to meet energetic needs during exercise. As will be discussed in a later module, the depletion of carbohydrates coincides with a decrease in performance and the onset of fatigue. This figure underscores that point. When test subjects were asked to exercise to exhaustion at 75% of their VO2 max, they could exercise significantly longer when they began the exercise session with higher amounts of glycogen stored in muscle. By manipulating one's diet in advance, the amount of glycogen stored in the muscle can also be manipulated. This is known as carbohydrate loading and will be discussed in detail in our nutritional video in module three. For now, understand that the amount of glycogen stored in muscle prior to a big endurance event will allow you to exercise for a longer period of time at your chosen race pace. A second technique commonly used by endurance athletes to offset carbohydrate depletion and delay the onset of fatigue is known as carbohydrate feeding during exercise. Basically, the ingestion of a very dilute carbohydrate drink during exercise can help maintain blood glucose levels, thereby sparing liver glycogen. As indicated here, when subjects exercise at 70 to 75% of their VO2 max to exhaustion, without carbohydrate feeding, they fatigue, on average at the 3 hour time point. When a dilute carbohydrate drink is given every 30 minutes, after the onset of exercise, notice that these same subjects could exercise an additional hour before fatiguing. That's the ingestion of the carbohydrates drink allowed for additional source of carbohydrate, thereby sparing liver glycogen. Now let's look at how carbohydrate metabolism is altered after a period of endurance training. Shown here is the classic muscle cell adaptation to training. A previously sedentary individual can expect a 2 fold or a 100% increase in mitochondrial content after months of regular training. This will affect carbohydrate utilization in several ways. First, as fats can only be used for ATP production in mitochondria, having twice as much mitochondria will allow for an increase in the ability to use fats for fuel. This will result in less carbohydrate utilization, also known as carbohydrate sparing. Second, having twice as much mitochondria will improve our ability to use the carbohydrates that we do burn via the aerobic, or the oxidative pathway, versus the anaerobic pathway. We get 15 times more ATP from glucose when it's used aerobically versus anaerobically. Or to put it another way, we can use 15 times less carbohydrate for the same rate of ATP production if we can use the aerobic pathway thus sparing carbohydrate. Evidence of greater fat use and carbohydrate sparing is demonstrated here. A lower respiratory exchange ratio in any given work load clearly indicates that fats that are being used to a greater extent after training. Remember the respiratory exchange ratio for fat is 0.70. Further support for carbohydrate sparing is shown here. There is a clear shift to the right in the workload eliciting the crossover point during a graded exercise test after training. This indicates that fats are the preferred fuel for muscle for an extended period of time. In review, endurance training increases the number of mitochondria and skeletal muscle. This adaptation contributes to one's ability to use fats to a greater extent, and to burn the carbohydrate that is being used aerobically. Both of these training adaptations will lead to carbohydrate sparing. In summary, liver glycogenolysis is responsible for maintaining blood glucose levels during exercise. Carbohydrate loading, prior to a competition, can increase muscle glycogen stores resulting in improved performance. Consuming dilute carbohydrate drinks during exercise can help maintain blood glucose levels resulting in improved performance. Endurance training will result in carbohydrates sparing.
