Muscle Growth explained: MPS, MPB, and Whole-Body Balance

Introduction: What Actually Determines Muscle Growth 

Muscle growth is governed by a simple but powerful equation:  Net Protein Balance = MPS - MPB. Muscle protein synthesis (MPS) builds new proteins, while muscle protein breakdown (MPB) removes old or damaged ones. Your muscle mass doesn’t change based on MPS alone it changes based on the difference between how much you build and how much you break down over time.

Resistance training raises both MPS and MPB, but feeding protein after training shifts the balance positive. When this repeated positive balance accumulates over weeks and months, the result is hypertrophy. 

Muscle Protein Synthesis (MPS): The Builder 

MPS is the process your body uses to create new muscle proteins. It responds strongly to loading (training) and to feeding (protein intake). After a workout, MPS increases, peaking around 2-4 hours post-training. When you consume ~20-40 grams of high-quality protein, MPS rises again, creating a window where your body is repairing and building.

MPS has a ceiling. Once you reach about 20-30 grams of high-quality protein or ~10 grams of essential amino acids, adding more protein does not raise MPS further instead, excess amino acids are oxidized or used for other bodily needs. This is known as the muscle-full effect, where muscle temporarily stops responding even if amino acids are still elevated in your bloodstream.

MPS is the primary mechanism for hypertrophy, but it is episodic it rises, peaks, and returns to baseline. This is why meal spacing (every 3-4 hours) and hitting repeated MPS spikes across the day work better than one massive protein dose. 

Muscle Protein Breakdown (MPB): The Cleaner and Recycler 

MPB is not the enemy. It rises after training because your muscle must remove damaged, misfolded, or oxidized proteins before new ones can be built. MPB also recycles amino acids internally when dietary protein is not available, preventing muscle from stalling during repair.

Trying to suppress MPB is counterproductive. MPB is essential for adaptation because it clears out damaged tissue and sets the stage for MPS to rebuild stronger fibers. When you consume protein post-exercise, MPB decreases modestly, but the real advantage of feeding is the rise in MPS, not the suppression of breakdown.

In other words:  MPB is a necessary part of the process, and hypertrophy comes from increasing MPS at a rate higher than MPB. 

Whole-Body Protein Balance: The Bigger Picture Beyond Muscle 

Muscle makes up about 40% of your body’s protein, but it’s also the reservoir your body draws from when whole-body needs increase. Immune function, hormone production, enzyme turnover, repair of other tissues, and energy production during stress all require amino acids.

Once MPS has reached its ceiling, additional protein still contributes to whole-body protein balance by: 

• Increasing whole-body protein synthesis 
• Suppressing whole-body protein breakdown 
• Improving nitrogen balance 
• Preventing muscle from being cannibalized for systemic needs 

This means higher protein intakes can still support muscle retention and recovery even when MPS is maximized, especially during stress or calorie deficits

This concept is the missing link in most explanations of muscle growth. Muscle isn’t just competing with MPB it’s competing with the entire body’s protein demands. 

Training + Protein: How Muscle Actually Grows 

Muscle growth follows a repeatable four-step cycle: 

1. Training creates micro-damage 
   Mechanical tension and strain increase MPB to clear damaged proteins. 
2. Protein feeding elevates MPS 
   Amino acids supply the building blocks and leucine triggers the switch. 
3. Whole-body protein balance is supported 
   Adequate dietary protein prevents the immune system, liver, and other tissues from siphoning amino acids from muscle.
4. Repeated over time = hypertrophy 
   When daily net balance stays positive, muscle grows. 

This cycle explains why nutrient timing matters, but why total daily protein intake matters most. Consistent daily intake ensures repeated MPS spikes, controlled MPB, and protected whole-body status. 

Daily Protein Intake: The Primary Driver 

To optimize MPS, MPB, and overall protein balance, the evidence converges on these targets: 

• Daily protein: 1.6–2.2 g/kg/day 
• Per meal:  20–40 g protein (2–3 g leucine) 
• Spacing:  Every 3–4 hours 
• Pre-sleep:  30–40 g casein or equivalent 

During a calorie deficit, protein needs rise to 2.2–3.0 g/kg/day, with leaner individuals requiring the highest intake to prevent muscle loss. 

Protein timing is useful. Protein quality matters. But total daily intake remains the most important factor for long-term muscle growth. 

Final Summary 

• MPS builds new muscle proteins  
• MPB removes damaged proteins  
• Whole-body balance determines whether the body drains muscle or preserves it. 
• Daily protein intake is the strongest predictor of long-term muscle gain and retention. 

This simple framework MPS, MPB, and whole-body protein balance explains everything from leucine thresholds to meal spacing to why high protein intake is more protective during stress or calorie deficit. 

- Conrad RN

 ❯ ❯ ❯ ❯ ❯

References 

Stokes, T., Hector, A. J., Morton, R. W., McGlory, C., & Phillips, S. M. (2018).  Recent perspectives regarding the role of dietary protein for the promotion of muscle hypertrophy with resistance exercise training.  Nutrients, 10(2), 180.  https://doi.org/10.3390/nu10020180 

Gwin, J. A., Church, D. D., Wolfe, R. R., Ferrando, A. A., & Pasiakos, S. M. (2020).  Muscle protein synthesis and whole-body protein turnover responses to ingesting essential amino acids, intact protein, and protein-containing mixed meals with considerations for energy deficit.  Nutrients, 12(8), 2457.  https://doi.org/10.3390/nu12082457

Tipton, K. D., Hamilton, D. L., & Gallagher, I. J. (2018).  Assessing the role of muscle protein breakdown in response to nutrition and exercise in humans.  Sports Medicine, 48(S1), S53–S64.  https://doi.org/10.1007/s40279-017-0845-5