Whether you’re a young athlete engaged in high-intensity physical activity or an older adult who thinks of exercise training as a weekend tangle with the weed wacker, your body relies on muscle protein synthesis. But what is muscle protein synthesis, and why is it so important? In this article, we’re going to explore this topic and uncover everything you need to know about this vital function.
What Is Muscle Protein Synthesis?
Muscle protein synthesis is the process whereby human skeletal muscle is built (protein anabolism). This so-called mixed muscle protein synthesis can be further broken down into myofibrillar protein synthesis, which refers to protein synthesis involving muscle fibers, and mitochondrial protein synthesis, which involves the synthesis of proteins necessary for energy production in muscle cells.
The muscle-building of protein synthesis is balanced by muscle breakdown (protein catabolism) in an ongoing process called protein metabolism. And the muscle protein turnover that results from the interaction between protein synthesis and breakdown is controlled by amino acids.
The Practical Handbook of Biochemistry and Molecular Biology lists more than 300 common amino acids, but only 20 of these are incorporated into the body's proteins. These 20 amino acids—the building blocks of life—build human muscle by combining in long chains to create thousands of different proteins.
But because the entire body needs proteins to function, these same proteins—even those used in skeletal muscle protein synthesis—will eventually be broken down as they’re used by the cells.
Moreover, 11 of these 20 different amino acids are considered nonessential because they can be created by the body, but 9 are considered essential because they must be obtained from dietary sources.
The 11 nonessential amino acids are:
- Aspartic acid
- Glutamic acid
And the nine essential amino acids are:
Amino acids are vital for helping to maintain the body’s protein balance as it moves through the never-ending process of breaking down and synthesizing new proteins. But muscle protein synthesis rates must remain in balance with rates of breakdown or more muscle will be lost than created.
This feat is accomplished by ensuring a steady supply of all 20 amino acids. And because the nonessential amino acids can be created by the body—and also make up at least 50% to 60% of most dietary proteins—we generally consume more than enough of these amino acids via the foods we eat.
However, because we must obtain essential amino acids from food, diets lacking in high-quality protein sources can limit the supply of necessary amino acids.
To avoid a situation in which our bodies don't have enough essential amino acids to keep protein synthesis and breakdown in balance, we must be sure we're getting plenty of high-quality protein sources in our diet.
Amino Acids and Muscles
The body’s main reservoir of protein is muscle tissue, so if we’re not giving our bodies the fuel they need—and if we’re not engaging in regular resistance exercise, which has been shown to increase the response of muscle protein synthesis—one of the first areas we may notice the imbalance is in our muscles.
In other words, without the amino acids the body needs for muscle tissue growth, maintenance, and repair, both muscle mass and muscle strength will be lost and exercise performance—and even basic functioning—will suffer.
Let’s break this down and see why amino acids are so important for muscle health.
As we stated earlier, if the balance between protein synthesis and breakdown is tilted in favor of protein breakdown, the amount of protein in the body will decrease.
For muscles, less protein being synthesized means less muscle growth (muscle hypertrophy) and more muscle breakdown. And because muscles are the body’s main reservoir of amino acids, if the daily intake of amino acids can’t keep up with the demands of protein synthesis, the muscles will actually release some of their amino acids for use by other parts of the body.
Over time, this emphasis on protein degradation instead of protein synthesis can have implications that go far beyond simple losses in exercise performance.
In fact, excessive muscle wasting can cause a cascade of negative reactions that may first be noticeable as changes in body composition but can eventually affect everything from wound healing to heart function. And in older adults, loss of muscle tissue can lead to a more serious condition called sarcopenia, which can have life-threatening consequences in the face of serious illness, injury, or surgery.
What the Research Has to Say
A number of studies can help us illustrate the various ways muscle protein synthesis may be affected by age, exercise, and nutritional status.
For example, researchers including M. Drummond, B. Pennings, and B. Rasmussen conducted a study in 2008, published in the Journal of Applied Physiology (J Appl Physiol), that compared anabolic signaling in both young and elderly men. And they found that the activation of mammalian target of rapamycin (mTOR)—a key initiator of the MPS response (muscle protein synthesis)—occurs faster in young men after both resistance exercise and ingestion of essential amino acids.
Similarly, the authors (V. Kumar, P. Atherton, K. Smith, and M. Rennie) of another study from 2009, published in the same journal, found that the muscle protein synthetic response decreases even further in older women—a finding also noted in a 2019 study conducted by J. Trommelen, M. Betz, and L. van Loon, published in Sports Medicine (Sports Med).
Both of these studies back up an earlier study from 2005, conducted by researchers including C. Katsanos, M. Sheffield-Moore, A. Aarsland, and R. Wolfe and published in the American Journal of Clinical Nutrition (Am J Clin Nutr), which found that the anabolic response to dietary protein decreases with age.
And a 2013 study conducted by researchers including L. Breen and T. Churchward-Venne, published in the Journal of Clinical Endocrinology and Metabolism (J Clin Endocrinol Metab), found that as little as 14 days of reduced activity can induce a state of anabolic resistance in even healthy elderly adults.
In addition, a 2008 study by researchers that included D. Moore, M. Tarnopolsky, and S. Phillips, also published in the American Journal of Clinical Nutrition, demonstrated that 20 grams of intact protein—the whole protein found in food sources, with its strings of individual amino acids connected to one another—was needed to achieve the maximum protein dose response after resistance exercise.
And a 1997 study by researchers G. Biolo, K. Tipton, S. Klein, and R. Wolfe, published in the American Journal of Physiology (Am J Physiol), found that the effect of amino acids on muscle protein synthesis was enhanced by prior exercise—a finding that was thought to be due in part to increased blood flow.
Finally, a number of studies have found that ingestion of whey protein results in a rapid but short-lived postprandial increase in amino acid concentrations in the blood, while casein protein is more slowly digested and results in a more moderate and prolonged postprandial increase.
Whey protein also tends to stimulate muscle protein synthesis to a greater extent than both casein and its predigested form, casein hydrolysate—a fact that’s been attributed to (among other factors) the higher leucine content in whey protein.
Branched-Chain Amino Acids
To correct imbalances in muscle protein turnover and shift the focus heavily toward muscle protein synthesis, the world of bodybuilders and endurance athletes has long been interested in the benefits of branched-chain amino acids, or BCAAs.
The BCAAs—leucine, isoleucine, and valine—are touted by fitness junkies for their ability to preserve muscle stores of glycogen (the primary fuel used by muscles during exercise), minimize protein breakdown during exercise, and reduce post-exercise muscle soreness and muscle recovery.
On the face of it, this sounds great.
However, as we’ve learned, muscle protein synthesis involves all 20 amino acids working together—linking together—in specific ways to build the protein our bodies need. And if any essential amino acid is in short supply—the so-called limiting amino acid—protein synthesis will stop as soon as its supply has been exhausted.
Because making a complete protein requires sufficient quantities of each of the 20 amino acids, there’s no single best amino acid or group of amino acids for optimal muscle growth and muscle repair, as the body needs a balanced supply of all of them to produce protein.
However, essential amino acid supplements that emphasize specific amino acids to target specific areas—while also supplying a balanced supply of every other essential amino acid—can still be helpful.
The Importance of Protein Synthesis for Tissues and Organs
Amino acids are known as the building blocks of life because the proteins they create are involved in almost every biochemical process that occurs inside the body—which means that protein synthesis is important not just for skeletal muscle but for whole-body health as well.
Every organ undergoes protein synthesis. Muscle protein synthesis simply refers specifically to the building of new muscle protein. However, the body’s various tissues and organs depend on the process of muscle protein synthesis as well.
Tissues and organs are able to sustain a balance between protein synthesis and breakdown—even if protein ingestion doesn’t occur in sufficient quantities—because they can draw from amino acids circulating in the blood.
These amino acids are released into the bloodstream during muscle protein breakdown and held at stable levels so the body's tissues and organs can take them up as needed—whether for fighting infections, repairing wounds, controlling vascular function, balancing metabolic processes, or any number of other tasks.
What’s more, muscle is the only tissue in the body that can lose some of its mass without negatively impacting health. However, in the absence of dietary protein intake, the result of this catabolic response is a net loss of skeletal muscle.
But when you again ingest dietary protein, the situation is reversed, creating an anabolic response in muscle tissue whereby dietary amino acids are absorbed and sufficient skeletal muscle protein is produced to offset the muscle protein lost during the postabsorptive state (when the gastrointestinal tract is empty and energy comes from the breakdown of the body’s reserves).
The importance of muscle as a source of amino acids in the blood, in the absence of dietary intake, can be illustrated by the Irish Republican Army (IRA) hunger strike that took place in 1981, when a number of protestors starved themselves to death while imprisoned in Northern Ireland.
During the strike, the protestors requested that samples of their blood be taken daily so that some medical benefit might also be derived from their sacrifice. All of the protestors were of normal weight at the outset of the strike, and they remained in reasonably good health throughout, with normal blood concentrations of amino acids.
However, when their muscle mass eventually became so depleted that insufficient amino acids were left to sustain normal blood concentrations, it served as a signal that death was imminent. For most of the protestors, this wasting process took an average of 60 days.
By contrast, morbidly obese people generally have both increased fat mass and increased muscle mass (with associated greater stores of amino acids)—so much so that there are documented cases of obese individuals surviving for more than a year on only micronutrients (vitamins and minerals) and water.
While these examples depict extreme circumstances, they clearly highlight the central role of muscle mass in maintaining normal amino acid availability during periods in which nutrients from food are not being absorbed.
They also illustrate the importance of dietary amino acids for maintaining muscle protein and overall health and offer graphic proof that the optimal diet is one that emphasizes both protein and amino acid nutrition, regardless of whether the individual engages in regular resistance training or is a sedentary person and unconcerned about physical function.