Proteins are composed of 20 different amino acids that are linked together via peptide bonds between their carboxyl and amino terminals to form polypeptide strands of varying amino acid sequences and lengths. The specific sequence of the amino acids is determined by messenger RNA templates and ultimately dictates the final shape and function of the protein. Proteins have a wide variety of structural and regulatory functions that are crucial to normal cellular metabolism and function and would be critically important for the athlete. These could include, for example:
contractile myofibrillar proteins (e.g. actin and myosin), which convert chemical energy to mechanical work to allow for contractions of skeletal and cardiac muscles
structural proteins (e.g. collagen), which provide the physical scaffolding for the muscle cell to transit the forces to the bone levers
energetic mitochondrial proteins, which collectively generate the adenosine triphosphate (ATP) energy that is vital for essentially all cellular function
regulatory enzymes, which catalyse the metabolism of a plethora of chemical reactions essential for cellular function (e.g. ATP hydrolysis and synthesis)
haematological proteins, which are important for oxygen delivery (e.g. red blood cells) and plasma volume (e.g. plasma albumin).
While it is essential for all proteins within the body (e.g. immune system, cardiovascular system) to be present in the proper proportion and be of high quality and function for an individual to excel at their chosen sport, the proteins within skeletal muscle are of primary importance to the athlete, given that the composition of skeletal muscle typically determines athletic success (e.g. strength and/or endurance capacity). Therefore, while it is important to optimise whole-body protein balance, this chapter focuses primarily on the nutritional requirements to enhance skeletal muscle protein repair and remodelling that contribute to improved athletic performance.
Proteins within the body are incredibly dynamic; they are continually being broken down into their constituent amino acids and resynthesised from amino acids within the free amino acid pool. Proteins that are damaged (e.g. through oxidation, nitrosylation and/or mechanical forces) are targeted for degradation primarily by the ubiquitin—proteasome system. The amino acids liberated from the breakdown of proteins (especially from skeletal muscle, the body’s primary amino acid storage depot) can be: (1) reutilised as substrates for the synthesis of new muscle proteins, which happens to an underappreciated extent after exercise; (2) exported from the cell to sustain other vital body functions (e.g. synthesis of circulating proteins or for gluconeogenesis); and/or (3) transaminated and utilised as a source of fuel (e.g. the branched-chain amino acids leucine, isoleucine, valine) or to replete the tricarboxylic acid cycle intermediates within the mitochondria of the muscle.
Beyond childhood growth, sustaining muscle and lean body mass is a matter of balancing the rates of protein synthesis and breakdown day to day. If an athlete has the goal of gaining muscle, however, a shift towards a net positive muscle protein balance ...