How Dietary Protein Builds Muscle: The Biology of Protein Synthesis

Every time you eat protein, a signaling cascade starts in your muscle cells. Within an hour or two, the rate at which those cells build new proteins goes up. This is muscle protein synthesis. And it’s not a vague, motivational concept. It’s a specific, measurable biochemical process.

Understanding how it works changes how you think about both protein intake and training.

The mTOR Pathway: The Master Switch

The central player in muscle protein synthesis is a protein kinase called mTOR (mechanistic target of rapamycin). It acts as the cell’s sensor for growth conditions. When conditions are right, mTOR activates and kicks off the synthesis of new muscle proteins (Saxton and Sabatini, 2017, Cell).

Two main inputs activate mTOR in muscle. The first is mechanical tension from resistance exercise. Lifting heavy things sends a mechanical signal through integrin proteins in the cell membrane, eventually reaching mTOR. The second is amino acids, particularly leucine.

When both inputs are present simultaneously, the anabolic response is substantially greater than either input alone. This is why protein timing around training has any relevance at all. Both signals converge on mTOR, amplifying each other.

From Amino Acids to New Protein: Transcription and Translation

Once mTOR is activated, it phosphorylates downstream targets including S6K1 and 4E-BP1. These proteins regulate translation, the process of reading messenger RNA to build proteins.

Muscle cells don’t spontaneously create new contractile proteins from nothing. They follow instructions encoded in DNA. When the cell needs more of a specific protein, gene expression changes. The relevant segment of DNA gets transcribed into messenger RNA. Ribosomes then read that mRNA and assemble the corresponding protein from amino acids, one at a time.

This is translation. And it’s where dietary amino acids become structural muscle tissue.

The amino acids you eat don’t go directly into muscle. They enter the free amino acid pool in blood and cells, get directed to tissues with the highest demand, and then get incorporated into proteins by ribosomes. Muscle cells competing for amino acids after a training session will pull heavily from that pool if it’s adequately supplied.

Leucine: The Key Trigger

Not all amino acids signal mTOR equally. Leucine, one of the three branched-chain amino acids (BCAAs), is the primary trigger for mTOR activation from a nutritional standpoint (Churchward-Venne et al., 2012, Nutrition and Metabolism).

Leucine is sensed by a complex of proteins inside the cell (the Ragulator complex and the GATOR1 complex) that gate mTOR activation based on leucine availability. When intracellular leucine rises above a threshold, these sensors give mTOR the green light.

This is why protein sources with higher leucine content tend to produce a stronger muscle protein synthesis response per gram. Whey protein has among the highest leucine content of any protein source. Eggs, milk, and meat are also high. Most plant proteins have lower leucine per gram, which is one reason plant-based athletes often need more total protein to achieve equivalent anabolic signaling. This is covered in more depth in the leucine threshold guide.

The Protein Breakdown Side

Muscle protein synthesis only tells half the story. Muscles are in a constant state of turnover: old, damaged proteins get broken down (muscle protein breakdown) and new ones get built. Net muscle balance is synthesis minus breakdown.

At rest and in a fasted state, breakdown slightly exceeds synthesis. Muscle mass is slowly being lost and must be replenished. After eating protein and exercising, synthesis exceeds breakdown, and net muscle protein is accrued. Over 24 hours, the balance of these cycles determines whether you’re gaining, maintaining, or losing muscle.

This is why spreading protein across multiple meals matters more than a single large dose. Each meal stimulates a synthesis response that lasts roughly 3-5 hours before returning to baseline. A single 150g protein meal at dinner does not keep synthesis elevated all day. Research by Moore et al. (2009, American Journal of Clinical Nutrition) found diminishing returns above about 20g of protein per meal for muscle protein synthesis in young men, with the optimal dose around 20-40g per meal depending on body mass and training status.

The Recovery Window

Resistance exercise creates microscopic damage to muscle fibers. The repair and rebuilding process is what produces stronger, larger muscle over time. This process requires protein.

The period of elevated muscle protein synthesis after training extends for 24-48 hours in trained individuals (less in beginners, who show a more exaggerated and prolonged response). During this window, amino acid availability has an outsized impact on net muscle gain.

Protein intake before sleep has gotten attention in this context. The overnight period is the longest protein-free window most people have. Research by Trommelen and van Loon (2016, Nutrients) demonstrated that casein protein taken before sleep increased overnight muscle protein synthesis. Casein digests slowly, providing a sustained amino acid release through the night.

The practical implication: total daily protein spread across the day, including near training and before sleep, is the most evidence-based approach to maximizing muscle protein synthesis. No single meal or timing trick replaces consistent intake at adequate total doses.

How Training Changes the Equation

Without a training stimulus, elevated amino acid availability alone produces a modest and transient rise in synthesis. The muscle has no reason to grow. With training, the demand signal amplifies the protein signal substantially.

This is why sedentary people don’t build muscle just by eating more protein. And why athletes need more protein than sedentary people. More training means more total protein turnover, more demand for amino acids, and more opportunity for net protein gain.

The upper limit of how fast muscle can be built is also set by training volume, recovery, and genetics, not just protein intake. Once protein intake is adequate (roughly 1.6-2.2g per kg of body weight), adding more doesn’t accelerate growth. The training stimulus, not the protein supply, is usually the limiting factor.

This article is for educational purposes only. It’s not medical advice. Talk to your doctor or a registered dietitian before making significant changes to your diet.