Calorimetry is the measurement of heat. By means of animal calorimetry (the term ‘animal’ here includes humans) we can estimate the energy costs of living. All life processes including growth, work and agricultural production (milk, eggs, wool, etc.) use energy, the source of the energy being food. The energy content of food is metabolised (i.e. changed) in the body into other energy forms, only some of which are useful in the sense of growth or production. Much of the wasted energy is given off from the body in the form of heat – hence the name calorimetry.

The heat may be measured directly by physical methods (Direct Calorimetry) or it may be inferred from quantitative measurement of some of the chemical by-products of metabolism (Indirect Calorimetry). These alternatives are possible because of the natural constraints imposed on energy transformations by the laws of thermodynamics. Of fundamental importance are the Law of Conservation of Energy (energy cannot be created or destroyed, only changed in form) and the Hess Law of Constant Heat Summation (the heat released by a chain of reactions is independent of the chemical pathways, and dependent only on the end-products). In effect these laws ensure that the heat evolved in the enormously complex cycle of biochemical reactions that occur in the body is exactly the same as that which is measured when the same food is converted into the same end-products by simple combustion on a laboratory bench or in a calorimeter.

The partition of the gross energy of food into its major energy sub-divisions is illustrated in Fig. 1.1. Some food is undigested resulting in a loss of energy as faeces.