Not all protein is equal. The protein in a steak and the protein in a plant-based burger may show similar grams on a nutrition label but produce very different anabolic responses at the cellular level. The difference is attributable to three factors: amino acid profile, particularly leucine content; digestibility and absorption rate; and the presence of other bioactive compounds that modulate protein metabolism.
Leucine as the Master Anabolic Trigger
Leucine activates mTOR, the central regulator of muscle protein synthesis, in a dose-dependent manner up to a threshold of approximately 2.5 to 3 grams per meal. Above this threshold, additional leucine provides no additional anabolic stimulus. Animal proteins including eggs, dairy, meat, and fish consistently provide this leucine threshold per 30-gram portion of protein. Plant proteins typically require larger portions to achieve the same leucine dose because they have a lower leucine concentration per gram of protein.
Protein Digestibility and DIAAS Scoring
The Digestible Indispensable Amino Acid Score is the current gold standard for protein quality assessment, developed by the FAO to replace the older PDCAAS system. Whole eggs score 1.13 (above the maximum reference of 1.0). Milk protein scores 1.14. Soy isolate scores 1.0. Pea protein scores 0.82. Rice protein scores 0.37. These scores reflect the proportion of the protein that is actually absorbed and the balance of amino acids relative to human requirements.
- Prioritise whole eggs, dairy, fish, and meat as primary protein sources for their DIAAS scores and leucine density
- Combine plant proteins to improve amino acid balance: rice and peas together provide a better profile than either alone
- Aim for at least one animal protein source per day to ensure adequate B12, zinc, and haem iron alongside protein
- Do not rely on protein bars as primary protein sources: most use low-DIAAS protein ingredients combined with high amounts of added sugar or artificial sweeteners