The human body uses energy from food to fuel movement and essential body functions, but the body cells don't get energy directly from food. After food is digested, the carbohydrates, protein and fat break down into simple compounds -- glucose, amino acids and fatty acids -- which are absorbed into the blood and transported to various cells throughout the body. Within these cells, and from these energy sources, adenosine triphosphate (ATP) is formed to provide fuel. The body uses 3 different systems to supply cells with the necessary ATP to fuel energy needs. Most of the body's activities use a continuum of all three energy systems, working together to ensure a constant supply of energy.
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The body needs a continuous supply of ATP for energy -- whether the energy is needed for lifting weights, walking, thinking or even texting. It's also the unit of energy that fuels metabolism, or the biochemical reactions that support and maintain life. For short and intense movement lasting less than 10 seconds, the body mainly uses the ATP-PC, or creatine phosphate system. This system is anaerobic, which means it does not use oxygen. The ATP-PC system utilizes the relatively small amount of ATP already stored in the muscle for this immediate energy source. When the body's supply of ATP is depleted, which occurs in a matter of seconds, additional ATP is formed from the breakdown of phosphocreatine (PC) -- an energy compound found in muscle.
Lactic Acid System
The lactic acid system, also called the anaerobic glycolysis system, produces energy from muscle glycogen -- the storage form of glucose. Glycolysis, or the breakdown of glycogen into glucose, can occur in the presence or absence of oxygen. When inadequate oxygen is available, the series of reactions that transforms glucose into ATP causes lactic acid to be produced -- in efforts to make more ATP. The lactic acid system fuels relatively short periods -- a few minutes -- of high-intensity muscle activity, but the accumulation of lactic acid can cause fatigue and a burning sensation in the muscles.
The most complex energy system is the aerobic or oxygen energy system, which provides most of the body's ATP. This system produces ATP as energy is released from the breakdown of nutrients such as glucose and fatty acids. In the presence of oxygen, ATP can be formed through glycolysis. This system also involves the Krebs or tricarboxylic acid cycle -- a series of chemical reactions that generate energy in the mitochondria -- the power plant inside the body cells. The complexity of this system, along with the fact that it relies heavily on the circulatory system to supply oxygen, makes it slower to act compared to the ATP-PC or lactic acid systems. The aerobic system supplies energy for body movement lasting more than just a few minutes, such as long periods of work or endurance activities. This system is also the pathway that provides ATP to fuel most of the body's energy needs not related to physical activity, such as building and repairing body tissues, digesting food, controlling body temperature and growing hair.
Putting It All Together
Three energy systems work in the body to provide energy. While these systems are well known for their role in fueling athletic performance, ATP is essential for every energy need in the body -- including all the automatic body processes of growth, development and maintaining vital body functions. These energy systems do not work independently and do not function in isolation. Rather, all systems operate at all times, but some may predominate based on the body's activities, including the type, intensity and duration of physical activity as well as a person's fitness level.
Reviewed by: Kay Peck, MPH, RD