To assess the correctness of power you must know both the nutritional needs of man is the nutrient content of various foods, that is, both the food needs of man, both averages the chemical composition of food.
The composition of foods from a nutritional point of view is summarized in tables made by the National Institute of nutrition, that are intended to give useful information for rational nutritional needs. Foods are divided into homogeneous groups (cereals, vegetables, etc.) and for each food related percentage values of the edible portion and energy content, proteins, lipids, carbohydrates, fiber and key vitamins and minerals.
Analyzing the data of food composition tables looking at the clear distinction between foods of plant origin and foods of animal origin. Not only that, you may also notice a wax homogeneity in the chemical composition of food of animal origin, that in fact all come mainly from muscle tissue, and instead, some differences in the food of plant origin, which have very different characteristics of the organism plant (seeds, tubers, stems, leaves, flowers).
Nutritionists have proposed a classification of all foods available in groups, each of which contains a wide range of foods with a composition equivalent (but not equal) in nutritional principles. The groups vary from country to country, according to availability and local food traditions.
In Italy it has adopted a subdivision into seven groups, justified by some of the characteristics of our food tradition: the great variety of fruits and vegetables available all year has led to devote two groups in these foods, depending on which are the source of vitamin A or vitamin C; the considerable weight of the vegetables in our diet has led to regard a group of their own, complementary to that of cereals; a separate group was also dedicated to fat seasoning into account their energy content and their consistent use in the preparation of many traditional dishes from our kitchen.