Table 1.

Functions of proteins

•	Proteins are the major components of muscles, required for muscle dynamics and function
•	Enzymes are proteins. Therefore, proteins are essential for intermediary metabolism and energy production. Similarly, all cell carriers are proteins
•	Some proteins are involved in specific immunity (that is, immunoglobulins) and in nonspecific immunity (for example, opsonins)
•	Proteins contribute to the architecture and structure of organs and tissues. A typical example is collagen, which has a major architectural role, for example, in bone and skin
•	Proteins secreted into the blood by the liver are carriers of lipid-soluble molecules: hormones (for example, transthyretin for thyroxin), vitamins (for example, retinol binding protein for vitamin A), nutrients (for example, albumin for free fatty acids and tryptophan), and a number of drugs
•	Proteins in the blood, especially albumin, are involved in the control of oncotic (colloid osmotic) pressure
•	Proteins contribute physiologically to energy expenditure (12 to 15% of total daily expenditure) in the postabsorptive state, through release of amino acids following proteolysis. This may occur directly (for example, branched-chain amino acids in the muscles) or indirectly (through glucose (gluconeogenesis) or ketone body (ketogenesis))