The advice to "just hit your total daily protein target" misses the second half of the equation: not all protein provides equivalent amino acid availability.
For omnivorous athletes eating mixed diets, this nuance rarely matters — animal proteins uniformly score high on amino acid completeness. For plant-based athletes, it is the central variable that determines whether their protein intake actually supports training adaptation or simply fills a macronutrient number.
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Why PDCAAS Was Replaced
Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the dominant protein quality metric from 1989 until 2013. It scores protein sources against human essential amino acid requirements, corrected for digestibility.
The fundamental flaw: PDCAAS truncates scores at 1.0. Any protein with an amino acid profile that exceeds human requirements is scored identically to one that barely meets them — obscuring the true quality advantage of high-quality complete proteins.
The FAO/WHO replaced it with Digestible Indispensable Amino Acid Score (DIAAS) in 2013. DIAAS improvements:
- Uses ileal digestibility (measured at the end of the small intestine) rather than faecal digestibility — which overstates plant protein digestibility because colonic bacteria ferment unabsorbed amino acids that appear "digested" in faecal measurements
- Does not truncate at 1.0 — whey protein scores 1.09–1.25 on DIAAS, reflecting its genuine superiority
- Uses individual amino acid digestibility rather than a single crude protein digestibility value
DIAAS Scores for Key Protein Sources
| Protein Source | DIAAS Score | Limiting Amino Acid | Notes |
|---|---|---|---|
| Whey protein isolate | 1.09–1.25 | None | Reference standard |
| Whole egg | 1.13 | None | High bioavailability |
| Chicken breast | 1.08 | None | — |
| Soy protein isolate | 0.90–0.98 | Methionine | Only complete plant protein |
| Pea protein isolate | 0.82–0.89 | Methionine | Low methionine content |
| Brown rice protein | 0.59–0.70 | Lysine | Severely limiting |
| Wheat protein | 0.40–0.50 | Lysine | Very low quality score |
| Lentils (whole food) | 0.52 | Methionine + cysteine | |
| Black beans (cooked) | 0.68 | Methionine |
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The Complementation Myth That Isn't Quite a Myth
The 1970s advice to combine proteins within each meal to achieve "complete" amino acid profiles was later revised — research established that the body maintains an amino acid pool over several hours, meaning strict within-meal complementation is not required.
However, the revision was over-interpreted. Complementation over the full day remains important for plant-based athletes. A vegan athlete eating rice-based meals for breakfast and lunch, with legume protein only at dinner, may not fully leverage the lysine availability from legumes at the times when muscle protein synthesis is most responsive (i.e., post-exercise).
For optimal muscle protein synthesis stimulation, the leucine threshold (~0.05g leucine per kg body weight per meal) must be met at each feeding. Plant proteins contain less leucine per gram of protein than animal sources:
| Source | Leucine per 30g protein |
|---|---|
| Whey | ~3.0g |
| Chicken breast | ~2.5g |
| Soy protein isolate | ~2.3g |
| Pea protein isolate | ~2.0g |
| Brown rice protein | ~2.2g |
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Practical Framework for Plant-Based Athletes
Protein targets:
- General athletic population: 1.6–2.0g/kg
- Plant-based athletes: 1.8–2.2g/kg to account for lower DIAAS scores and leucine density
Distribution: Target 0.4g protein per kg body weight per meal, prioritising post-exercise timing with higher leucine density sources (soy or pea+rice blend rather than wheat-based protein).
For athletes calculating plant protein needs adjusted for training load, target body composition, and meal distribution, the protein intake calculator at winsport.uk/tools/nutrition/protein-calculator generates targets with meal-level breakdowns.
Are you currently adjusting protein targets upward for plant-based athletes — or applying the same g/kg recommendation regardless of dietary pattern?