Basics on How Energy Works in Your Body

Crash Course Thermodynamics: Your body releases energy stored within the chemical bonds of foods you intake, and temporarily traps it within the chemical bonds of special molecules known as ATP and creatine phosphate (PCr). The energy  stored in these special bonds is ultimately used to fuel muscle activity when needed, with the help of oxygen, and produces heat energy.

Carbohydrates (the best source of energy) can be stored in skeletal muscle and the liver in a starchlike substance called glycogen. Glycogen can become depleted after about 60 minutes of intense physical activity. Fatigue occurs about the same time that glycogen stores become low, for basic reasons; Active muscle that have now been depleted of their carbohydrate stores are forced to now rely on fat for fuel. Fat is not able to be “burned” as fast and efficiently as carbohydrates, so your body runs out of fuel. Also, the liver is depleted of its glycogen stores and is no longer able to store blood sugar, therefor it must struggle to maintain blood sugar levels by converting protein (from amino acids) sources to blood sugar. This process is slow and can not keep up with the pace at which muscle takes up sugar in the body. This can cause low blood sugar and result in lightheadedness and fatigue.