Whether you are engaging in high intensity sprint exercises or endurance type activities, carbohydrates play a critical role in the energetics sustaining working muscles. Carbohydrates play such a key role that I will devote two videos to their metabolism and contribution. In this video I will address where and how carbohydrates are stored in the body. Second, I will discuss the very important topic of how much carbohydrate is stored in the body. And finally, I will examine the major factors that influence when and to what extent carbohydrates are used during exercise. The two major sites for carbohydrate storage in the body are muscle and liver. Carbohydrates are stored in the form of glycogen. Glycogen is basically many strings of glucose molecules attached to one another. Glucose of course is the form of carbohydrate that can be directly used by all cells of the body for energy and ATP production. Thus, when the body needs to call upon its carbohydrate stores for energy production, individual glucose units are removed from the parent glycogen molecule for this purpose. The major function of muscle glycogen is to supply glucose units for muscle energetics. The major function of liver glycogen is to maintain blood glucose levels, which is critical as during exercise muscle extracts glucose from the blood for fuel. From an energy standpoint, notice that there is very little glucose in the blood. Finally the total amount of energy available in the form of carbohydrate in the body is approximately 2000 Kilocalories in an average 154 Pound individual. This brings up an extremely important point, this is not a lot of energy. There is only a limited amount of carbohydrate stored, and thus, carbohydrates can and will deplete during prolonged distance exercise, as well as high-intensity exercise of sufficient duration. Let's compare the amount of carbohydrate stored in the body with that for fats. In the same 154 pound individual, there is well over 100,000 kilocalories of energy in our fat stores. That's 50 times more than that found for carbohydrates. There is no threat of depleting our fat stores during a single bout of exercise. As we will see a key endurance or aerobic training adaptation is the ability to use fats as fuel to a greater extent, thereby preserving our precious carbohydrate stores. There are many factors that influence the degree and extent to which we use carbohydrate store in a single bout of exercise. Of these, the exercise intensity and duration, have the greatest impact. Shown here is a graded exercise test to maximal oxygen uptake or VO2 max. Notice at the early, low exercise intensity work loads, faster clearly the preferred fuel for muscles. As the exercise intensity increases during the course of the test, the reliance on carbohydrates as a fuel source also increases. At some point, carbohydrates become the preferred fuel. This is known as the crossover concept. The preferred fuel for muscles crosses over from fats to carbohydrates. When we take a closer look at exactly where the fuel is coming from, we see that early in the graded exercise test. At 25% of VO2 max, approximately 90% of the energy for the muscles is coming from circulating free fatty acids, and fat or triglycerides stored in muscle. At the very high intensity of 85% of VO2 max carbohydrates account for approximately 75% of the energy coming from circulating plasma glucose and muscle glycogen. A major factor contributing to the greater reliance on carbohydrates at higher excercise intensities is the characteristics of the muscle fiber type being recruited. We will discuss these characteristics in depth in the next module. But for now understand that at high workloads, we must use our type two muscle fibers, to generate the necessary force required. These Type II muscle fibers rely more on carbohydrates than fats for fuel. As the bulk of carbohydrate used by the muscles comes from our glycogen stores, let's take a quick look at exactly how these stores are mobilized during exercise. Individual glucose units are systematically removed from the main glycogen molecule in the form of glucose 6-phosphate. In muscle, these glucose-6 phosphate units can now enter the pathway of glycolysis, eventually resulting in ATP production. The breakdown of glycogen during exercise, known as glycogenolysis, is activated by the enzyme phosphorylase. Phosphorylase can be turned on during exercise either by an increase in the adrenal hormone, epinephrine, or by increase in the intracellular calcium levels in the contracting muscle. Both of these mechanisms play a role in the activation of phosphorylase and muscle glycogen breakdown during exercise. As stated earlier, the Glucose 6 phosphate now form from Glycogenolysis will enter the ATP generating pathway of glycolysis. Key enzymes in the pathway of glycolysis will be activated during exercise. As covered in the video on ATP, ATP generating pathways such as glycolysis are turned on when the energy charge of muscle drops below resting levels. The greater the excercise intensity, the greater is the rate of ATP utilization, resulting in a large drop in the energy charge in working muscles. Basically the greater rate of ATP utilization, the greater the activation of ATP-generating pathways such as Glycolysis. Let's examine more closely the relationship between the exercise intensity and glycogen breakdown. Shown here are individuals exercising at five distinct intensities. Notice that at the highest exercise intensity, 150% of max. Muscle glycogen is being broken down and depleted at a very high rate. There are two main reasons for this response. First, the rate of ATP utilization is extremely high at this exercise intensity. And thus, ATP production must attempt to keep pace. Given that this excercise intensity can only be maintained for a very short period of time, the majority of ATP produced must come from glycogen already stored in the muscle. There is insufficient time to call upon our fat stored in adipose tissue, or even the glycogen stored in liver. Second, at high exercise intensities, as mentioned earlier in this video, we will predominantly be recruiting our Type II muscle fibers, which rely more heavily on carbohydrates than fats for fuel. Now let's look at the other extreme. The easy workload at approximately 31% of max. Notice that the rate of glycogen muscle depletion is significantly lower. This can be explained by one, a lower rate of ATP utilization at this workload. And two, a greater reliance on fats as a fuel source as per the crossover concept. And three, a greater improvement of our Type I muscle fibers which have the capacity to use glycogen and glucose aerobically, thereby slowing the rate of carbohydrate utilization. Let's briefly revisit that concept of anaerobic versus aerobic carbohydrate utilization. When glycogen and glucose units are broken down anaerobically via glycolysis, only two ATP are produced. However, when the exact same glycogen and or glucose units are broken down aerobically, 30 ATP are produced. Thus the aerobic breakdown of these units in the mitochondria allows for 15 times more ATP production per unit. As a result of this greater ATP production, the muscles can afford to use carbohydrates at a much slower rate. It is worth noting that whether glucose is broken down anaerobically or aerobically, the initial 10 steps in the pathway of glycolysis are identical. In summary, carbohydrates are a major fuel used by working muscles. The excercise intensity and fiber recruitment are major factors determining carbohydrate utilization. There is a limited amount of carbohydrates stored in the human body. The activation of glycogenolysis and glycolysis are regulated by multiple factors including hormones, intramuscular calcium levels, and the energy charge.
