Muscle Protein Synthesis

The biological process of building new muscle protein, elevated by resistance training and protein intake, and the basis of muscle growth.

Muscle protein synthesis (MPS) is the process by which your body builds new muscle protein from amino acids. Muscle growth happens when MPS exceeds muscle protein breakdown over time, a positive net protein balance sustained across days and weeks, not any single meal or workout.

Both resistance training and protein intake independently stimulate MPS, and the effect is larger when they’re combined. A training session elevates MPS for roughly 24 to 48 hours afterward, which is part of the biological basis for training each muscle group multiple times per week rather than once.

Getting enough total daily protein, spread across several meals, matters more for MPS than the exact timing of a post-workout shake. See our protein intake calculator to find your daily target, then use the meal calorie distribution tool to check that each meal clears a practical per-meal leucine threshold.

The per-meal leucine threshold

MPS responds to protein intake in a threshold-like pattern: a meal needs roughly 2 to 3g of leucine (found in about 25 to 40g of a complete protein source, depending on the food) to maximally stimulate synthesis, and eating more than that in one sitting doesn’t proportionally extend or amplify the response as much as spreading the same total protein across more meals does. This is the mechanism behind advice to distribute daily protein across 3 to 5 meals rather than concentrating it in one or two, since more meals crossing the threshold means more separate MPS-stimulating events across the day.

Frequently Asked Questions

How does the muscle full effect limit protein utilization in a single meal?
The 'muscle full' effect occurs when muscle protein synthesis peaks approximately 90 to 120 minutes after amino acids enter circulation, after which MPS returns to baseline regardless of elevated amino acid levels. Consuming more than 30 to 45 grams of high-quality protein in one meal does not further amplify acute MPS, which is why distributing protein across 3 to 5 meals across the day is more effective.
What is the leucine threshold required to trigger muscle protein synthesis?
To initiate translation initiation via the mTORC1 pathway, a meal must provide approximately 2.5 to 3.5 grams of leucine (roughly 0.05 g/kg of body weight). In whole foods, this corresponds to approximately 25 to 35 grams of whey protein, beef, poultry, or eggs, or 35 to 45 grams of isolated plant-based protein sources.
How long remains the muscle protein synthesis window elevated after resistance training?
Following resistance training, muscle protein synthesis remains significantly elevated for 24 to 48 hours, peaking between 12 and 24 hours post-workout in trained individuals. Because this elevation is sustained for up to two days, consistent daily protein intake and overall calorie balance matter far more than consuming a protein shake immediately post-workout.