The energy provided by food to the human body is called "heat", which does not mean that the hotter the food, the higher the heat, and the unit of heat is Joule (J), for example, roasted leeks and kidneys. As a result, energy is transferred from a high-temperature object to a low-temperature object, and the energy transferred at this time is called heat.

Heat and work are two different forms of energy transfer that accompany the change of system state, and they are measures of different forms of energy transfer. They are all related to the intermediate process of state change, so they are not functions of system state.

In the process of human life activities, all life activities need energy, such as the synthetic reaction of material metabolism, muscle contraction, gland secretion and so on. And these energies mainly come from food.

Nutrients contained in animal and plant foods can be divided into five categories: carbohydrates, lipids, protein, minerals and vitamins, and water is six categories. Among them, carbohydrate, fat (fat can also store heat) and protein can release energy through in vivo oxidation. The three are collectively referred to as "productivity nutrients" or "heat sources".

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