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When you rise from a chair, lift a shopping bag, or climb a flight of stairs, you depend on muscle. Its work is familiar. Its ongoing renewal is largely invisible.
Even when a muscle looks unchanged, its proteins are being made and broken down. Individual components are replaced while the tissue continues to function.
This continuing renewal is the starting point for understanding how dietary protein supports muscle.
A living structure
Muscle contains many kinds of proteins. Some generate force. Others provide structural support, participate in energy metabolism, transport substances, or help cells respond to signals.
Maintaining muscle therefore involves more than preserving its size. The tissue must maintain the machinery that allows it to contract, obtain energy, and respond to demand.
Proteins are chains of amino acids assembled in particular sequences. Within muscle cells, ribosomes join amino acids together using instructions carried by messenger RNA. Newly made proteins then undergo folding and, where required, further processing and assembly.
The incorporation of amino acids into these proteins is a physical event: amino acids become part of the tissue’s working machinery.
Building and breakdown happen together
The production of new protein is called protein synthesis. The dismantling of existing protein is called protein breakdown. Together, they constitute protein turnover.
Breakdown allows cells to remove proteins and recover their components. Some released amino acids can be used again; others leave the tissue or enter different metabolic pathways.
To understand changes in muscle protein, we must consider both sides:
Net muscle protein balance = protein synthesis − protein breakdown.
If synthesis exceeds breakdown over a period, muscle gains protein. If breakdown exceeds synthesis, it loses protein. When the two are balanced, protein content can remain stable even while substantial renewal takes place.
This balance changes with circumstances. Human tracer experiments have measured changes in both synthesis and breakdown following resistance exercise, demonstrating why either measurement alone provides only part of the picture. [1]
A person maintaining muscle is still actively making muscle proteins. Stability requires ongoing work.
New protein has several purposes
Newly synthesized protein may replace a component that has been removed, support repair, or contribute to adaptation. Maintaining a stable muscle therefore requires continuing production.
Chapter 7 examines how that production is measured, and Chapter 10 follows what is retained and recycled.
Where do the amino acids come from?
The amino acids used for synthesis come from a shared supply that includes dietary amino acids and amino acids released during the turnover of existing body proteins. The body can also produce some amino acids, while essential amino acids must be supplied by the diet.
A meal contributes to this supply after digestion and absorption. Its amino acids can serve many tissues and purposes.
Researchers can investigate the dietary contribution using proteins containing stable isotope labels. These labels allow selected amino acids to be followed through circulation and into muscle proteins.
Experiments using labeled milk protein have directly followed dietary amino acids into muscle proteins after exercise. [2]
Renewal and physical capability
For a reader concerned about aging, the eventual question is practical: can the body maintain the capacity to do the things daily life requires?
That question will guide us from the renewal of individual proteins toward the strength and physical abilities considered in Chapter 14.
Why begin here?
Before comparing protein foods or considering meal timing, we need to understand the process those choices are intended to support.
The objective is to provide for a living tissue that continually builds, dismantles, repairs, and adapts. The whole meal may influence the supply of amino acids reaching that tissue. Exercise and other conditions influence its response. The balance of synthesis and breakdown helps determine what remains.
We will follow these connections one stage at a time.
The first lesson is simple: muscle maintenance is an active process of renewal. Understanding how food contributes to that renewal begins with the meal itself.
In the next chapter, we examine the protein on our plate—and the foods that accompany it.
References
1. Phillips SM, Tipton KD, Aarsland A, Wolf SE, Wolfe RR. Mixed muscle protein synthesis and breakdown after resistance exercise in humans. American Journal of Physiology. 1997;273:E99–E107.
2. Holwerda AM, Paulussen KJM, Overkamp M, et al. Dose-dependent increases in whole-body net protein balance and dietary protein-derived amino acid incorporation into myofibrillar protein during recovery from resistance exercise in older men. Journal of Nutrition. 2019;149:221–230.