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Why is protein important?

This is an excerpt from Plant-Based Sports Nutrition-2nd Edition by D. Enette Larson-Meyer,Matt Ruscigno.

Dietary protein is necessary for sustaining life. It provides the amino acid building blocks necessary to synthesize (make) the wide variety of proteins in the human body, including those found in skeletal muscle, tendons, hormones, enzymes, red blood cells, and immune cells. Amino acids also serve as a source of energy, and certain amino acids can be converted into glucose through a process called gluconeogenesis (meaning new sugar formation), which is important during both prolonged exercise and starvation. Chemically, each of the 20 different amino acids is structurally distinct, but all contain at least one nitrogen-containing “amino” group attached to a central carbon atom, which is also bonded to a carbonyl acid group (COOH), a hydrogen atom, and a unique side chain; the side chain distinguishes the different amino acids.

Our body’s requirement for protein is fundamentally a requirement for amino acids and nitrogen. As humans, we cannot produce nine of these amino acids—called the essential amino acids—but can produce the remaining 11 or so as long as enough nitrogen is consumed in the diet (see table 5.1).3 Together, the 20 essential and nonessential amino acids are arranged in various combinations to form the many proteins required for structure, function, and regulation throughout the body. The arrangement of these amino acid building blocks differs for each protein in the body as well as in the foods we eat. Closely related species, such as humans and animals, tend to produce proteins with a similar spectrum of amino acids, whereas those of very different species, such as plants and mammals, tend to have different spectrums. Protein synthesis is initiated after a cellular signal—typically from a hormone or growth factor—is communicated to the cell’s genetic material (DNA) to turn on gene transcription and ultimately protein synthesis. In the case of exercise-induced increases in skeletal muscle protein building (or synthesis), a key signal is a molecule called mammalian target of rapamycin (or mTOR), which is triggered not only by resistance exercise but also by ingestion of dietary protein.4,5 Amino acids serve both to supply the necessary building blocks and also provide a signal to maximally activate mTOR and promote muscle protein synthesis.

Although the body can synthesize nonessential amino acids, essential amino acids must be obtained through the diet. If the mixture of the essential amino acids consumed in the diet does not match that required by the body, the amino acids found in the shortest supply, relative to the amount needed for protein synthesis, are referred to as the limiting amino acids. In general, diets based on a single plant protein do not support optimal growth because the single plant sources do not typically supply enough of all the needed building blocks for human protein synthesis. For example, maize is low in tryptophan, rice is low in lysine and threonine, wheat is low in lysine, and most legumes, except soy, are low in methionine or tryptophan or both.6 Diets based on combinations of different plant proteins,6,7 as well as soy, dairy, egg, fish, or meat protein, however, can easily supply enough of the right amino acid building blocks for human growth and muscle development.

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