The fundamental nutrition bases
From the physical point of view, every living organism is a thermodynamic system that exchanges matter and energy with the environment: the material they need to grow, to replace worn parts and to reproduce; the energy it needs to feed the chemical processes that take place within it and, if necessary, to do work.
While the matter present in the natural environment is used by all organisms (although in different ways and to varying degrees), with regard to the possibility of using the energy present in nature things are very different and between bodies exist a clear differentiation. There are in fact organisms that are able to directly use the energy available in nature (autotrophic organisms) and organisms which can only indirectly use the energy already stored by other bodies in the form of matter (heterotrophic organisms).
In nature, the only usable energy in direct form is solar energy and the only organisms that are able to use it are green plants, which thanks to the chlorophyll, the green pigment of their leaves, they are able to activate the photosynthesis process. The animals, however, unable to use the solar energy to produce matter, are forced to obtain energy and matter through the transformation of matter accumulated in the body of other organisms: herbivorous animals use for this purpose the material accumulated by green plants, while carnivorous animals use the accumulated matter by herbivores and other carnivores.
The man, omnivorous animal, using for this purpose both the material accumulated by green plants is the accumulated matter from animals (herbivores and carnivores).
From the point of view of the organism transformer this process defines power and define food substances that the body uses for energy (heat and work) and plastic (growth and renewal of body tissues).
Foods, which are mostly of natural origin (vegetable and animal tissues) also contain protective substances (vitamins, minerals) that regulate fundamental processes (growth, reproduction, etc.). Only if they enter into the process on Food also synthetic substances used as supplements and, in special cases, as actual synthetic foods.