Energy can neither be created nor destroyed, but it can be changed from one form of energy to another. This principle, which is known as the Law of Conservation of Energy, is also called the First Law of Thermodynamics. The energy contained in the foods that we eat is released by chemical reactions taking place in our cells and this energy is converted to heat and work. In this sense, we can say that the chemical energy of foods is being converted in part to the mechanical energy of motion. Stored energy is also called potential energy and kinetic energy is known as the energy of motion.

The various foods that we enjoy are composed of many different molecules. Chemical energy is the energy stored in the chemical bonds of these molecules. We need calories to burn in performing daily activities and carbohydrates, proteins and fats are all potential sources of energy. Of these molecular structures, fats have the highest energy content, supplying 9 kilocalories (kcal) of energy per gram (g) of fat. Carbohydrates and proteins each give us roughly 4 kcal/g.

Energy units are generally expressed in Joules (J), named after the British scientist James Prescott Joule or in calories (cal), whose name comes from the Latin word "calor" meaning heat. A food calorie, which is sometimes represented as Cal is actually a kilocalorie (kcal) or 1000 calories.

One Joule (J) is the amount of work done when a force of 1 Newton acts over a distance of 1 meter (m): 1 J = 1 Nm, states that a Joule equals a Newton-meter. The Newton is named in honor of the great physicist Sir Isaac Newton.

One calorie is the amount of heat needed to raise the temperature of 1 g of water by 1 °C at one atmosphere of pressure. To convert from calories to Joules we can use the conversion: 1 cal = 4.184 J

One British Thermal Unit (BTU) is the quantity of heat required to raise the temperature of 1 pound of water by 1 °F.