Question: How does the body provide energy?

Mark Burnley answered on 13 Jun 2011:

All the energy in you body is provided by the food you eat. In the cells, the energy is stored in chemical bonds in sugars (glucose and stored glycogen), fat and in “high energy phosphates”. These “high energy phosphates” are mainly adenosine triphosphate (ATP) and phosphorylcreatine (PCr). ATP is the key energy source, and all other energy sources are used to maintain the level of ATP in the cell. For example, as soon as exercise starts the muscles break down ATP to adenosine diphosphate (ADP) and inorganic phosphate, so ATP levels drop. The energy this liberates is used to produce movement (and heat, which is why exercise makes you hot). This immediately causes PCr to be broken down to creatine and inorganic phosphate, and the energy released in that reaction is used to “stick” the phosphate back on ATP. The powerhouses in the cell, the mitchondria notice that PCr levels are dropping, so they start to break down fats and sugar and consume oxygen to restore the PCr that is broken down. The restored PCr can then be used to break down even more ATP and so it goes on. When exercise gets harder, oxygen consumption might not occur fast enough to keep PCr levels from falling, and so the cell breaks down sugar to lactate for a faster release of energy, and some of this lactate enters the blood. We can measure this with a fingertip blood sample. The fitter you are, the less lactate you’ll produce during exercise, so it’s a good measure of fitness to find out the point at which lactate starts to increase in the blood. So there are several sources of energy, all working together to keep ATP levels where they should be. If these systems don’t keep up, then PCr levels become very low and inorganic phosphate and ADP levels get very high. We think ADP and inorganic phosphates can cause muscle fatigue, which causes you to slow down. By slowing down, these energy systems can catch up, allowing you to exercise again.