Editing Food energy (section)

== Energy usage in the human body ==
{{Main article|Bioenergetics|Energy balance (biology)}}

The food energy actually obtained by respiration is used by the human body for a wide range of purposes, including [[basal metabolism]] of various organs and tissues, maintaining the internal [[body temperature]], and exerting [[muscle|muscular]] force to maintain posture and produce motion. About 20% is used for brain metabolism.<ref name=FAO2003/>

The conversion efficiency of energy from respiration into muscular (physical) [[power (physics)|power]] depends on the type of food and on the type of physical energy usage (e.g., which muscles are used, whether the muscle is used [[Aerobic exercise|aerobically]] or [[Anaerobic exercise|anaerobically]]). In general, the efficiency of muscles is rather low: only 18 to 26% of the energy available from respiration is converted into mechanical energy.<ref name="seiler"/> This low efficiency is the result of about 40% efficiency of generating [[Adenosine triphosphate|ATP]] from the respiration of food, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overall efficiency of 20%, one watt of mechanical power is equivalent to  {{convert|4.3|kcal/h|kJ/h|order=flip|abbr=on}}. For example, a manufacturer of rowing equipment shows calories released from "burning" food as four times the actual mechanical work, plus {{convert|300|kcal|kJ|abbr=on|order=flip}} per hour,<ref name=concept2/> which amounts to about 20% efficiency at 250 watts of mechanical output. It can take up to 20 hours of little physical output (e.g., walking) to "burn off" {{convert|4000|kcal|kJ|abbr=on|order=flip}}<ref name=guyt2006/> more than a body would otherwise consume. For reference, each kilogram of body fat is roughly equivalent to 32,300 kilojoules of food energy (i.e., {{convert|3,500|kcal/lb|kcal/kg|disp=or}}).<ref name=wish1958/>