In biology, energy homeostasis, or energy balance, is an aspect of bioenergetics concerning the energy flow through living systems, by the process of metabolism. Energy homeostasis involves the human body using chemical and neural signals to adjust the amount of energy flows; and to regulate caloric intake by signaling the brain to regulate the sensation of hunger. Fifty percent of the energy from glucose metabolism is immediately converted to heat.
In the US, biological energy is expressed using the energy unit Calorie with a capital C (i.e. kilocalorie), which equals the energy needed to increase the temperature of 1kilogram of water by 1°C (about 4.18kJ).
Energy intake (food) = Energy expended (heat + work) + Energy stored.
The first law of thermodynamics states that energy can be neither created nor destroyed. But energy can be converted from one form of energy to another. So when a calorie of food eaten enters a body, ultimately 100% of that calorie will be converted to heat, resulting in three particular short-term effects: a portion of that calorie is either stored as fat, transferred to the body's cells as chemical energy, by Adenosine triphosphate (ATP), a coenzyme and used for mechanical work or chemical synthesis, or immediately dissipated through heat.