We go to the gas pumps and fill up our car and then just press on the pedal and the car goes. But do you really know how that gas ends up moving your car? Did you know the same thing happens in your body Dr. Sheri Colberg, Ph.D., FACSM, explains how your body’s energy system works whenever you exercise. Read Understanding Your Body‘s Energy Systems and How They Impact Your Exercise Blood Sugar Levels to learn how your engines get fuel.
The way that your muscles make and use energy during physical activities, including how fast you move, how much force your muscles produce, and how long the activity lasts, can also affect your blood sugar levels. Your body has three distinct energy systems to supply your muscles with ATP (adenosine triphosphate), which is a high-energy compound found in all cells that directly fuels muscular work. The three systems can best be considered a continuum, with one, then the next, and finally the third being recruited to produce ATP as exercise continues. If you exercise long enough (even for just a minute), you will end up using all three to some extent.
All the systems work by causing increased production of ATP, the only direct source of energy that your muscles can use; its breakdown directly fuels all contractions. When a nerve impulse initiates a muscle contraction, calcium is released within your recruited muscle cells, ATP “energizes” the muscle fibers, and they go into action. Without ATP, your muscles can’t contract and you won’t be able to exercise.
Muscle cells contain only small quantities of ATP ready for use when you start, enough to fuel any activity for about a second, at best. If you want to keep going longer, your muscles need to get ATP from another source right away. Although all the systems can supply additional ATP, the rate at which they supply it varies. The fuels used to make the ATP and the amount of time needed to produce it also differ by system. Due to differences in how your energy systems work, the type of exercise that you do can affect your blood sugar responses differently.
ATP–CP System: Short and Intense
For short and powerful activities, one energy system primarily provides all the requisite energy: the ATP–CP system. Also known as the phosphagen system, it consists of ATP that is already stored in muscle and creatine phosphate (CP), which rapidly replenishes ATP. This system requires no oxygen for energy production, making it anaerobic in nature. CP can’t fuel an activity directly, but the energy released from its rapid breakdown is used to resynthesize ATP for an additional five to nine seconds following depletion of the muscles’ initial one-second supply of ATP. In total, all of your body’s phosphagen stores (ATP and CP) can fuel an all-out effort for only about 10 seconds before being depleted. Thus, any activity that you do that lasts less than 10 seconds is fueled mainly by phosphagens, including a power lift, 40-meter sprint, pole vault, long jump, baseball pitch, or basketball dunk. Generally, these types of activities don’t lower your blood sugar levels because glucose isn’t used to produce the energy. In fact, they can raise your glucose levels due to an exaggerated release of glucose-raising hormones.
Lactic Acid System: Muscle Glycogen and Glucose Use Only
The second energy system, the lactic acid system, supplies the additional energy for activities that last longer than 10 seconds and up to about 2 minutes. The lactic acid system also produces energy anaerobically (without using oxygen) through the breakdown of muscle glycogen (a storage form of glucose in the muscle), a process called glycogenolysis. After it has been released from storage, glycogen produces energy through the metabolic pathway of glycolysis, which forms lactic acid as a by-product. When you’re resting, your muscle cells do some glycolysis, but because you are not using up much ATP, carbohydrates are processed aerobically (using oxygen) and not much lactic acid builds up.
Because of your muscles’ immediate need for additional energy when your exercise continues beyond 10 seconds, glycolysis proceeds rapidly to provide more ATP, and the system soon becomes limited by the accumulation of lactic and other acids. When large quantities are present in muscle, lactic acid drops the pH of muscle and blood, causing the associated “burn” in those muscles and fatigue. This system can make only 3 ATP from each glucose molecule derived from muscle glycogen, which is a relatively small amount compared with the 37 to 39 ATP that may be made through aerobic means. Consequently, this system can’t supply enough energy for prolonged periods of exercise. Activities that primarily depend on this energy system include 800-meter runs, 200-meter swimming events, and stop-and-start activities like basketball, lacrosse, field hockey, and ice hockey.
Aerobic System: Using Carbohydrate, Fat, and Protein With Oxygen
The other end of the spectrum is the aerobic energy system used for prolonged endurance or ultraendurance exercise. Because of their duration, these activities mainly depend on aerobic production of energy by the oxygen system. Your muscles require a steady supply of ATP during sustained activities like walking, running, swimming, cycling, rowing, and cross-country skiing, which you usually do for longer than two minutes. Running a marathon or ultramarathon, doing an Ironman triathlon, or participating in successive full days of long-distance cycling or backpacking are extreme examples of prolonged aerobic activities. The fuels for these activities are mainly a mix of carbohydrate and fat, more of the latter than the former during rest and greater carbohydrate use during exercise. Protein can be used to fuel an activity, but it usually contributes less than 5 percent of the total energy. Your body may use slightly more (up to 15 percent) protein during extremely prolonged endurance activities like running a marathon.
At rest, your diet and how recently you last exercised affect the mix of fuels that your body uses, but most people use about 60 percent fat and 40 percent carbohydrate. Your body will rapidly begin to use more carbohydrate as soon as you start to exercise, and its contribution rises further during harder exercise intensity. High-intensity or near-maximal activities use 100 percent carbohydrate. Muscle glycogen provides more—usually close to 80 percent—than blood glucose, unless you are already glycogen depleted from long-duration exercise or from being on a low-carbohydrate diet. The actual aerobic fuels that your body uses during the activity depend on your training status, your diet before and during the activity, the intensity and duration of the activity, and your circulating levels of insulin.
Circulating hormones like adrenaline mobilize fats from fat cells (adipocytes), and those fats then circulate in your blood as free fatty acids that active muscles can take up and use during less intense activities. Your body will be able to use fats more during mild and moderate activities, along with some carbohydrates. The fats stored in the muscles themselves (intramuscular triglycerides) become more important in fueling your recovery from exercise or during prolonged exercise sessions (greater than two to three hours in length).
Remember that reaching the aerobic system requires that you first use the other two. Both of your anaerobic energy systems (the phosphagens and the lactic acid system) are important at the beginning of any longer-duration exercise before your aerobic metabolism gears up to supply enough ATP (as shown in figure 2.5). These first two systems are also important whenever you have to pick up the pace or work harder, such as when you begin to run uphill or sprint to the finish line of a 10K race.
This column is excerpted from Diabetic Athlete’s Handbook (available November 6, 2008 from Human Kinetics), which contains essential exercise-related information and examples for type 1 and type 2 diabetic exercisers. Look for it in stores or find links to places to buy it online on www.shericolberg.com, along with additional information.