This video will address the very significant contribution of fats to the energetics of exercise. I will begin by discussing the two main types or forms of fats that can be used by the muscles for fuel. First, free fatty acids are the energy dense molecules that can serve as an immediate fuel source for our muscles during exercise. In order to use fats for ATP production, they must be either converted to free fatty acids or already be present in the muscle in the form of free fatty acids. Shown here is the basic structure for all free fatty acids. The important take home message regarding their structure Is that free fatty acids are energy rich molecules containing a string of carbons and hydrogens which can be readily oxidized in muscle mitochondria for ATP production. Second, triglycerides or triacylglycerols are the major storage form for our free fatty acids. Similar to how the body stores its glucose in the form of glycogen, free fatty acids are stored in the form of triglycerides. Shown here is the structure for triglycerides. Basically there are three free fatty acids stored in each trigylerceride molecule that can be removed when needed for ATP production by exercising muscles. The two primary sites for triglyceride storage are in adipose or fat cells and in skeletal muscle. By far, adipose tissue contains the largest amount of triglycerides. As you can see, even in an average 154 pound individual, there is a tremendous amount of energy in the form of triglycerides stored in adipose tissue. The triglyceride stored in muscle, referred to as intramuscular triglycerides, are the second largest depo, but pales in comparison to the amount contained in adipose tissue. When we look at the total amount of energy available, from all of our fats, including free fatty acids in blood, triglycerides in blood, muscle and adipose tissue, we see there is well over 100,000 kilo calories for ATP production. As a reminder, let's compare that to the total amount of energy available from our carbohydrate stores. For the same 154 pound individual, there is only a total of approximately 2,000 kilocalories from carbohydrates stored in the body. Again, this emphasizes two important points. First, there is only a limited amount of carbohydrates stored in the body which is critical for distance and long-duration exercise. As these stores can and do deplete leading to fatigue and poor performance. Second, as there is an ample amount of fat stored in the body, we never have to worry about depleting them. A major endurance training adaptation, is the increased ability to use fats as a fuel, thereby sparing our precious carbohydrate stores. In order for the muscles to use the fat stored in adipose tissue, the free fatty acids must first be removed from the triglycerides and mobilized. The free fatty acids are then transported to the working muscles via the blood stream. Once in the muscle, the free fatty acids enter the mitochondria where they are completely oxidized via the pathway of beta oxidation, producing a large amount of ATP. Let's now look at fat metabolism during a single or acute bout of exercise. As with carbohydrate metabolism, here are the main factors influencing the extent to which fats are used during exercise. Let's look at the contribution of fats during a bout of prolonged exercise, such as running a marathon. Initially, at an exercise intensity that can be sustained over four 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 muscle glycogen, blood glucose, and even intramuscular triglycerides begin to deplete. To compensate for these declining fuel sources, the body most rely more heavily on fat stored in adipose tissue. An increase in adipose tissue triglyceride breakdown is evident as the mobilized free fatty acids enter the blood stream, thereby elevating plasma levels. Now let's examine the contribution of fat stored in very different type of exercise. In a graded exercise test, the workload, and thus the exercise intensity is increased every two to three minutes until the individual is exhausted, and has reached their VO2 max. Notice that the early workloads of the graded exercise test at the easy exercise intensity of 25% of VO2 max. Approximately 90% of the energy is being supplied by fats. On the other extreme, at the very high exercise intensity of 85% of VO2 max. The body has shifted its preference to carbohydrates as the primary fuel source. As stated in the previous video, this is known as the cross over constant. During the easy exercise intensities, at the early stages of a graded exercise test, fat is the preferred fuel for muscle. As exercise intensity increases, there is a greater reliance on carbohydrates. At some point, carbohydrates crossover to being the preferred fuel for muscles. Training adaptations associated with fat metabolism are primarily confined to endurance rather than strength training. The major adaptations are, one, an increased ability to mobilize and utilize fats at any given exercise intensity and, two, a reduction in carbohydrate utilization resulting in carbohydrate sparing. The practical significance of these training adaptations is obvious. The trained individual can use fats to a greater extent, thereby sparing their precious carbohydrate stores. This will result in an improvement in performance. Shown here is the classic training effect in fat metabolism during sub-maximal steady state exercise. As can be seen, at any given time point, the trained individual has the ability to oxidize more fat. This is a result of an improvement in the mobilization and utilization of fats. An example of this adaptation is shown here. After months of endurance training, previously sedentary individuals demonstrated increases in fat utilization and a reduction in carbohydrate use during two hours of moderate exercise. The increase in fat utilization is also observed during a graded exercise test. As the cross over point is shifted to the right. This indicates that fats continue to be the preferred fuel much longer now that you were trained. In summary, free fatty-acids are the direct fuel source for ATP production. Triglycerides are the major storage form of fat. Adipose tissue by far contains the most fat but a significant amount is contained in muscle. Fat is the preferred fuel at lower exercise intensities but also contributes at higher workloads as per the crossover concept. Endurance training results in increases in all aspects of fat metabolism including mobilization and utilization.
