According to their functions how can nutrients are classified?
One possible and utile functional classification for nutrients is the one that divides them into energetic, structural and regulatory.
Energetic nutrients are those used as energy source for the metabolism; mostly they are the carbohydrates (but fats and proteins can also be changed into acetyl-CoA and "cycle" the Krebs cycle). Structural nutrients are those used in the support and structure of cells and tissues; they are the amino acids that variety structural proteins, like collagen, the membrane proteins, the cytoskeleton proteins, the contractile proteins of the muscle tissue, etc. Regulatory nutrients are those that form enzymes and coenzymes of the homeostasis, metabolites of the osmotic and electrolytic equilibrium of cells and hormones; some amino acids, vitamins and mineral salts are part of this group.