The University of New Mexico explains that heat production is a byproduct of metabolism, or the sum of the chemical processes in the body. When the body breaks down food molecules, the energy in the chemical bonds of the food is released, which powers the body. However, the human body is only about 25 percent efficient, meaning that 75 percent of the energy from the food is released as heat.
Some of this excess heat is allowed to escape from the body, while some of it is conserved to maintain the ideal human body temperature. This process is called thermoregulation, and, according to the University of New Mexico, the hypothalamus is primarily responsible for this process. For example, if too much heat has been produced or retained, the hypothalamus accelerates the production of sweat. Conversely, if the body is allowing too much heat to escape, the hypothalamus may signal the body to constrict the blood vessels of the skin to reduce heat loss.
Different types of food contain different amounts of energy. The amount of energy contained in a given food is usually measured in calories or, more commonly, kilocalories. By convention, the term “calories” with a capital “c” usually refers to kilocalories. Fats have nine calories per gram, while carbohydrates and proteins contain four calories per gram, according to the University of Texas at Arlington.