Muscle protein synthesis, explained in plain English

By Aali Qureshi, founder of Deadly Company · Published October 24, 2026 · Updated October 31, 2026

Short answer: Muscle protein synthesis (MPS) is how your body builds new muscle protein from amino acids; it runs opposite breakdown, and the balance between the two decides whether you gain, keep, or lose muscle. A feeding switches MPS on when leucine crosses roughly 2.5–3g, about the leucine in five eggs, and the nine essential amino acids supply the material. Muscle Water Aminos delivers 16g of EAAs with 4g of leucine per serving.

The construction site you carry around

Strip the biochemistry away and muscle is a building under permanent renovation. Every day, work crews demolish a fraction of it (breakdown) and rebuild a fraction of it (synthesis). In a healthy adult who is neither gaining nor losing, the two crews work at the same rate and the building holds its shape. Tip the balance toward breakdown for weeks — eat too little, move too little, lose weight fast — and the building shrinks. Tip it toward synthesis, repeatedly, and it grows. There is no third possibility and no shortcut around the arithmetic. There is only the running score.

That’s why “muscle loss on a GLP-1” is not a mystery or a moral failing. The medication suppresses appetite, intake falls, and the building crew gets neither its starting signal nor its materials at the old rate — while demolition, which needs no permission, continues on schedule. In the STEP 1 body-composition substudy, lean mass was roughly 25–39% of the total weight lost: of every 10 lb lost, up to ~4 lb isn’t fat. That number is the running score, measured. Everything in this Journal — the protein targets, the training, the way our own product is formulated — is an attempt to move that score. MPS is the name of the scoreboard’s left-hand column.

The switch: leucine

Construction crews do not start on a rumour; they start on an order. In muscle, the order is issued by leucine — one of the nine essential amino acids, and the one the cell’s signalling machinery (a pathway centred on a protein called mTOR, if you want the name to search) treats as the announcement that building materials have arrived. Below a certain amount of leucine in a feeding, the order is never really issued: synthesis ticks along at its background rate and the feeding’s potential is mostly wasted. Cross the threshold — roughly 2.5–3g in a single feeding for most adults, more like the top of that range or beyond it for older muscle — and synthesis rises sharply. We work through the leucine trigger, in full elsewhere; the short version is that 2.5–3g of leucine is about the leucine in five eggs, eaten at once. A real quantity of food, not a dusting — which is precisely the point the pixie-dose end of the supplement industry hopes you never do the maths on.

The bricks: all nine EAAs

An order with no materials builds nothing. MPS assembles new muscle protein from the nine essential amino acids — essential meaning your body cannot manufacture them, so every one must arrive from food or not arrive at all. This is the second place products fail: branched-chain amino acids (BCAAs) are three of the nine — leucine among them, which is why BCAA products can flip the switch — and the remaining six are simply absent, at any dose. The switch flips, the crew assembles what it can until the scarcest missing brick runs out, and synthesis stalls. Ignition without bricks. (It’s also why BCAAs alone cannot finish the job.) The mirror failure is the blend with bricks but a hidden, inadequate leucine dose: materials without the order.

A complete feeding needs both, which is why the dose conversation in this Journal always has two numbers, not one — leucine against the trigger, and total EAAs against the response range: synthesis keeps rising as EAA dose climbs toward roughly 15–18g, a range conceded even in the educational pages of brands selling 5g servings.

Part of the mechanism	What it does	The dose that matters	In food terms
The switch	Leucine signals the cell to start building	~2.5–3g per feeding	About the leucine in five eggs
The bricks	All nine EAAs supply the material	Response rises toward ~15–18g EAAs	16g of EAAs ≈ the whole ~138g steak

The window, the refractory period, and why feedings are plural

A feeding’s effect is not permanent. Synthesis rises after eating, peaks, and subsides back toward baseline within a few hours — even if amino acids are still circulating, the muscle becomes temporarily refractory, as if the site accepts one delivery per shift. Two practical rules fall out of that physiology. First, the day is built from several adequate feedings rather than one heroic one: a giant protein load at dinner cannot retroactively switch on the morning. Second, each feeding has to clear the bar on its own — a day of five sub-trigger feedings is five non-events wearing a good total.

This is the architecture behind the 1.2–1.6 g/kg daily target in the 2025 joint advisory (ACLM/ASN/OMA/TOS) — at 180 lb, roughly a chicken breast’s worth of protein at every meal — and behind placing formulated servings where meals fall short. Muscle Water Aminos is a protein supplement, not a meal replacement. Food first — this covers the protein on the days food isn’t happening; its 16g of EAAs are the whole ~138g steak’s worth of building blocks, delivered as one complete, trigger-clearing feeding among the day’s several.

Exercise: the foreman who doubles the crew

One input changes the whole system’s responsiveness: contraction. Exercised muscle becomes markedly more sensitive to amino acids — the same feeding produces a larger, longer synthesis response — and the sensitivity lasts a day or more after the session. In construction terms, training doesn’t just place an order; it expands the site and keeps it open longer. This is why every protein argument in this Journal arrives holding hands with a training argument, and why the two halves are always presented here as one protocol: feedings switch synthesis on, training decides how much the switching is worth. It is also merciful physiology for this audience — the sensitising signal is effort, not load, and light loads taken close to your limit produced comparable muscle outcomes to heavy loads in the major research syntheses (Schoenfeld 2017; Lopez 2021).

What MPS is not

Three honest limits, because an explainer that oversells its concept is an advertisement. MPS is a rate, not a result: a single measured spike after a meal is not muscle gained — only the weeks-long balance between synthesis and breakdown decides that. MPS is not the only thing eating does: food carries energy, micronutrients, and everything else a body runs on; an article series about amino acids is about one column of the ledger, and we keep the meal-replacement boundary in print for exactly that reason. And MPS numbers from studies are averages over hours, in specific populations — young men in many of the classics, older and female muscle less often — which is why this Journal states thresholds as ranges and aims its dosing advice at margins rather than minimums.

The bottom line

Muscle protein synthesis is the building process; breakdown is the demolition; your muscle is the running score. Leucine places the order — roughly 2.5–3g per feeding, about five eggs’ worth — the nine EAAs supply the bricks, the response tops out toward 15–18g of them, and training multiplies whatever the feeding achieves. Every recommendation in this Journal is that paragraph wearing different clothes. When a product, a diet, or a headline makes a muscle claim, translate it into this mechanism and ask the only two questions that matter: does it flip the switch, and does it bring the bricks?

This explainer is published by Deadly Company, the 1° Deadlier lifestyle company — the same company behind the GEAR supplement system Muscle Water Aminos belongs to, the apparel, and Body English, the training side of the protocol — because we’d rather teach you the mechanism and let it grade our label than ask you to take the label on faith. The mechanism sits inside a larger framework: the full GLP-1 muscle-preservation protocol is where the switch, the bricks, the training, and the daily target are assembled into a plan, and What is Deadly Company? is where the company behind the Journal explains what it is. Learn the scoreboard first. Everything else in this series is commentary on it.

FAQ

Can I measure my own MPS? Not directly outside a research lab — measuring synthesis rates requires tracer methods no consumer device replicates. What you can measure are its long-run consequences: strength on repeat efforts, measurements, DEXA lean-mass trends over months. Those are the scoreboard; MPS is the play-by-play you infer from it.

Is more leucine always better? No. Leucine is a switch, and switches are binary past their threshold: once synthesis is fully on, extra leucine adds little without the other EAAs to build with — and the response plateaus toward the 15–18g EAA range regardless. That is why formulated servings pair a leucine margin with a full EAA dose rather than chasing leucine alone.

Does fasting switch MPS off completely? Breakdown continues and synthesis idles at its background rate during a fast — the balance runs negative, which is normal physiology across a night and a problem only when deficits stretch across weeks. The system is built for oscillation; it is chronic one-sidedness, not a skipped breakfast, that costs muscle.

Why do studies keep using young men if the audience is older? Convenience and history — and it is a genuine limitation, which is why this Journal treats young-muscle thresholds as floors for older readers, not targets. Older muscle needs the top of the leucine range and the same full EAA supply, as the anabolic-resistance article in this series details.

Sources: Mechanism, threshold, plateau, refractory behaviour, and exercise sensitisation: the muscle-protein-metabolism literature as consolidated in this Journal’s MPS–leucine angle file, 2026-10-21 (per-feeding leucine trigger ~2.5–3g, higher with age; EAA response plateau ~15–18g, per the dose literature and the brands’-own-education concessions documented there). 2025 joint advisory on nutritional priorities during GLP-1 therapy (ACLM/ASN/OMA/TOS): 1.2–1.6 g/kg adjusted body weight/day. STEP 1 body-composition substudy (n=140, 68 weeks): lean mass ≈25–39% of total weight lost. Load research: Schoenfeld et al. (2017); Lopez et al. (2021). Translation Bank, verified 2026-10-21 (trigger = the leucine in five eggs; whole ~138g steak = 16g EAAs; of every 10 lb lost, up to ~4 lb isn’t fat; a chicken breast at every meal at 180 lb).

This article is education, not medical advice. It does not diagnose or treat any condition. Talk to your clinician about your medication and your protein target before changing your intake. These statements have not been evaluated by the FDA.