Prioritize high-leucine protein sources for maximal MPS stimulus

Not all protein is equal for muscle synthesis — leucine content is the critical variable.

Why it works

Leucine is the primary mTORC1-activating amino acid in muscle. Plant proteins often have lower leucine concentrations per gram and lower digestibility (reducing bioavailability) compared to animal proteins. This does not mean animal protein is required, but it does mean plant-based eaters need to pay attention to leucine density and may need higher total protein intakes to achieve the same MPS stimulus. The threshold dose of leucine (~2.5 g per meal) can be reached with plants, but requires deliberate selection.

How to do it

1. High-leucine animal sources (per 30 g protein): whey (~2.7 g leucine), chicken/fish/beef (~2.1–2.4 g), eggs (~2.0 g).
2. High-leucine plant sources (per 30 g protein): soybeans (~2.3 g), edamame (~2.2 g), lentils are lower (~1.6 g).
3. If primarily plant-based, aim for the higher end of the protein target (1.0 g/lb) and emphasize soy, edamame, and legume combinations.
4. Consider adding leucine powder (available as a supplement) to lower-leucine plant-protein meals if consistently falling short.

Evidence

Research from Phillips’ lab and others establishes leucine as the critical amino acid for MPS stimulation via mTOR. Comparative studies consistently show whey protein produces greater acute MPS than soy protein per gram, largely explained by leucine content and digestion speed. (rct)

Acute MPS differences between protein sources may be less important over long-term training than daily totals. Long-term RCTs comparing plant vs. animal protein for muscle hypertrophy show more similar outcomes than acute studies suggest.

Sources

• Tang et al. (2009), "Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis," Journal of Applied Physiology

Common mistake

Assuming equal grams of protein means equal muscle-building potential. A 30 g portion of lentils provides fewer leucine-available grams than 30 g of chicken, requiring a larger plant-protein portion to achieve the same MPS signal.