ABSTRACT
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Protein as a Macronutrient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Classification of Amino Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Digestion and Absorption of Protein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Protein Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Nitrogen Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Protein Turnover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Clinical Evaluation of Protein Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Excess Protein Intake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Protein Deficiencies — Underlying Causes . . . . . . . . . . . . . . . . . . . . . . . . . 102 Protein and Exercise Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Protein and Skeletal Muscle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Protein and Bone Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Protein and Immune Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Protein and Red Blood Cell Production . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Protein and Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Amino Acids as Therapeutic Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Musculoskeletal health depends on dietary protein. The word protein derives from the Greek word proteos meaning “primary — most important.” Indeed the structure of the human body is built on a foundation of structural proteins, which are derived from dietary proteins. Each cell’s genome codes for specific proteins that are used for the structure, regulation, and communication particles necessary for survival. These include muscle, enzymes, hormones, contractile proteins, immunoglobulins, neurotransmitters, bone structure proteins, oxygen, and carbon dioxide carrying molecules, to name a few. Almost half the total protein content
in the body is present in just four proteins, myosin, actin, collagen, and hemoglobin.1 Myosin and actin comprise muscle and collagen comprises bone and connective tissues. Protein is indeed proteos in the musculoskeletal system.
