Humans get energy by releasing the stored chemical energy in the foods they eat. Foods are made up of different types of macromolecules, each of which holds a different amount of energy. When humans break down food to power their biological functions, it is called catabolic metabolism.
Humans primarily engage in a metabolic pathway called aerobic metabolism. Aerobic metabolism uses oxygen as an electron acceptor, which enables the “burning” of food. This is one of the reasons that humans require oxygen to live. However, during times of intense exercise, aerobic metabolism may require more oxygen than the body can process. At such times, the body shifts to anaerobic metabolic pathways. Anaerobic pathways are much less efficient than aerobic metabolism, and anaerobic metabolic functions produce undesirable waste products – primarily lactic acid.
The amount of energy in a particular food is measured in kilocalories, often written as “Calories.” Calories are also used to quantify the amount of energy consumed by an organism. In order for an organism to maintain homeostasis, it must ingest and expend roughly equivalent amounts of calories. If a human ingests more calories than he uses, he gains weight. Conversely, someone who does not ingest as many calories as he uses, loses weight.