Characteristics of protids
Proteins are constituted with amino acids (elements with nitrogen). There is about twenty amino acids and eight of them can not be produced by our body (isoleucin, leucin, lysin, methionin, phenylalanin, theonin, tryptophan, valin).
A food constituted with only animal or only vegetable proteins is unbalanced because the lack of a single amino acid can prevent the assimilation of the others.
Ideal for our body is to get 1/3 from meats, 1/3 from dairies, 1/3 from vegetables.