You hear it everywhere in fitness circles. A training partner, an online coach, an Instagram post: plant-based protein has lower biological value. That's why whey. Period.
The problem with this statement isn't that it's wrong. The problem is that biological value is a measurement system from the 1950s that disadvantages plant proteins from the ground up. And when you dig deeper, the gap between plant and animal shrinks to a practically irrelevant level.
- Biological Value (BV) is a measurement system from the 1950s that tracks fecal nitrogen – with systematic bias against fibre-rich plant sources.
- Modern methods PDCAAS (FAO 1991) and DIAAS (FAO 2013) measure ileal instead of fecal digestibility and deliver a fairer picture.
- BV scores: Whey 104, Pea 65 (gap 38). DIAAS scores: Whey 1.09, Pea 0.67 (gap 0.42 – proportionally much smaller).
- Three levers neutralize the BV disadvantage of plant proteins: multi-source blends (pea+fava bean), leucine enrichment, sufficient daily dose (1.6 g/kg).
- Practice: BV as a selection criterion for athletes is outdated. Amino acid profile + leucine content drive the anabolic response.
What biological value actually measures
Biological value is simple: it measures how much nitrogen your body incorporates after eating protein versus excreting. Whole egg is the reference at 100. Whey comes in at about 104, beef at 80, soy at 74, pea at 65.1
Seems clear-cut.
But here's where the problem starts. Biological value is calculated through nitrogen measurements in stool. Gut bacteria consume amino acids that your body never absorbed – and this bacterial activity is counted as "utilized protein." With plant-based proteins that contain more fibre, this effect is stronger. They're automatically disadvantaged, regardless of how useful they actually are.
A 70-year-old measurement system leads to systematic bias against modern protein sources. The method is older than most people who do fitness training.
Biological Value (BV, 1950s) tracks fecal nitrogen – colon bacteria distort the result. PDCAAS (FAO 1991) and DIAAS (FAO 2013) measure ileal digestibility at the end of the small intestine and deliver a fairer picture for fibre-rich plant sources. DIAAS is today's gold standard for protein quality.
PDCAAS and DIAAS: Why Newer Methods Tell a Different Story
Nitrogen measurement in stool. Gut bacteria skew results. Systematic disadvantage for fibre-rich sources.
Accounts for digestibility and amino acid composition. Still stool-based measurement. Score capped at 1.0.
Digestibility measured at small intestine. Each amino acid rated individually. Uncapped. Recommended by FAO.2
PDCAAS was a step forward. But there are limits to two points: digestibility is still measured via stool, and the maximum score is capped at 1.0. That practically means high-quality proteins can't be differentiated – whether whey or potato protein, both hit the ceiling.
In 2013, the Food and Agriculture Organization of the United Nations therefore proposed a new standard: DIAAS.2 This is not just a rename. The difference is methodologically fundamental. Digestibility is measured at the end of the small intestine, before the colon and its bacteria skew the measurement. And instead of measuring total nitrogen, each essential amino acid is rated individually, without artificial caps.
The consequence: the picture becomes significantly more realistic.
Protein Sources Compared: How Big Are the Differences?
| Protein source | DIAAS | Limiting AA | Category |
|---|---|---|---|
| Pork | >100 | None | Excellent |
| Casein | >100 | None | Excellent |
| Whole egg | >100 | None | Excellent |
| Potato protein | >100 | None | Excellent |
| Whey protein | ≥75 | Histidine | High |
| Soy protein | ≥ 75 |
Met + Cys | High |
| Pea protein | <75 | Met + Cys | No claim |
| Rice protein | <75 | Lysine | No claim |
| Pea + rice (59:41) | 84 | Met + Cys | High |
DIAAS based on reference pattern for 0.5–3-year-olds (most conservative pattern). Adults receive higher scores for plant proteins. Source: Herreman et al., Food Science & Nutrition, 2020.3
The highlighted row is the turning point. Pea alone doesn't get over 75. Rice alone either. But combined in a 59:41 ratio, the DIAAS rises to 84. That's not chemical magic – it's complementarity. Pea has lots of lysine, little methionine. Rice is the exact opposite. Together they complement each other to form a nearly complete profile.
The same principle works with fava bean. It shares pea's weakness in sulfur-containing amino acids but brings a bit more methionine. Blended with rice, trainees get practically the full amino acid profile.
Why that works in the real world is another story. Before we get there: there's an article about the problem with plant protein sources that digs deeper into practical limitations. And according to Examine.com, a 70:30 blend of pea and rice protein comes remarkably close to whey's amino acid profile – at least on paper.4
Where Biological Value Reaches Its Limits
Here's the problem with all rating systems: they measure isolated protein sources on an empty stomach in controlled lab settings. But you don't eat isolated proteins.
You drink a shake with vegetables and fat, then later have another meal with carbs and a different protein source. Lab conditions and the real world aren't comparable. And that changes everything.
Mendes and colleagues tackled exactly that question in 2025: plant versus animal protein and muscle protein synthesis. 12 studies, summarized in a systematic review with meta-analysis.
In 75% of studies (9 of 12), there was no significant difference between plant and animal. In the remaining 25%, animal protein showed a slight advantage, but the effect size was negligible (Cohen's d < 0.2). Limitation: the paper is a preprint and not yet peer-reviewed.5
The evidence doesn't stop at one paper. Reid-McCann et al. (2025) found in a meta-analysis of 43 RCTs no significant difference between plant and animal protein for strength or physical performance. For muscle mass, a small but significant advantage for animal protein emerged (SMD -0.20), which was no longer detectable in adults over 60.6 And Van der Heijden et al. showed in 2024 in a randomized study that myofibrillar protein synthesis after resistance training was practically identical with a plant protein blend and whey.7
In the isolated lab test, plant protein powder scores worse. But once real people do real training and consume adequate protein, the gap becomes statistically meaningless.
Multi-source blends (pea + fava bean) complete the amino acid profile, targeted L-leucine enrichment reaches the leucine reference value discussed in research (~2.5 g, age-dependent), and adequate daily protein (1.6 g/kg) compensates for digestibility gaps. Trommelen et al. 2023 show: up to 100 g protein per meal is used for MPS11 – the disadvantages dissolve once the daily dose is right.
Three Factors That Close the Gap for Plant Proteins
When daily protein intake is at 1.6 g per kg body weight or higher, the protein source becomes secondary. Your body gets enough raw material, even if individual sources aren't optimally composed. The limiting amino acid from breakfast is compensated by lunch.
This is the critical point lab tests ignore. You constantly combine protein sources. Pea at breakfast, fish at lunch, rice at dinner. The amino acid profile adds up across the day. DIAAS measures a single meal on an empty stomach. Your body operates with multiple meals, multiple days, multiple weeks – it all adds up.
Leucine is the amino acid that gives the signal for muscle building. Research discusses a reference value of about 2.5 g per meal (age-dependent, not a universal cutoff), around which muscle protein synthesis responds strongly in studies. For details on leucine's role, check out our article on leucine and muscle protein synthesis. Plant proteins deliver less leucine per gram. But with adjusted amounts or targeted leucine supplementation, the effect difference between plant and animal narrows substantially according to current evidence.8
Muscle building depends on three things: enough total protein, sufficient leucine, and resistance training. The protein source is secondary.
Conclusion: What Biological Value Means for Your Training
body weight/day
(discussed reference value)
sources
Research doesn't say quality doesn't matter. It says quality matters far less at sufficient quantity than fitness marketing suggests.
Three concrete takeaways: first, total protein amount. With plant sources, a 10–20% increase makes sense. Pea and rice aren't as digestible as whey, and this buffer compensates. Second, leucine amount. A reference value of about 2.5–3 g per meal (age-dependent) sits in the range where muscle protein synthesis responds strongly in studies. Third, blend strategy. A mix of complementary sources – such as legume plus grain, potentially with added amino acids – covers the entire profile. What ultimately matters is the training itself: finding the right balance between high volume and high intensity maximizes results regardless of protein source.
There's one more factor assessment systems miss: digestibility in the practical sense. A protein with perfect scores on paper doesn't help if it causes bloating or discomfort in your stomach. We wrote an entire article on digestive enzymes in protein powder – how enzymes like amylase, protease, cellulase, lactase, and lipase support actual nutrient breakdown. That's precisely why SYNTYZE includes the DigeZyme® enzyme complex with these five enzymes.10
Pea protein achieves a biological value of about 65. This means: out of 100 grams of nitrogen consumed, the body incorporates 65 grams. Sounds low compared to whey (104), but the measurement method systematically overestimates animal and underestimates plant sources. Newer scoring systems like DIAAS paint a more realistic picture, especially for protein blends.
Not with sufficient quantity. Meta-analyses from 2025 show: at a daily protein intake of 1.6 g per kg body weight or more and a leucine reference value of about 2.5–3 g per meal, plant protein blends and whey show practically no difference in their effect on muscle building.56
PDCAAS measures protein quality through nitrogen in stool and caps the score at 1.0. DIAAS measures the digestibility of each individual amino acid at the small intestine, without artificial caps. The FAO has recommended DIAAS as the new standard since 2013 because it delivers more realistic results.2
Research discusses a reference value of about 2.5–3 g leucine per meal (age-dependent, not a universal cutoff), around which muscle protein synthesis responds strongly. Plant proteins deliver less leucine per gram than whey. That's why it's important to either increase total protein amount or specifically choose leucine-rich sources.89
The Bottom Line
Biological value methodically disadvantages plant proteins through stool measurement. Per current meta-analyses (2025): with enough total protein (≥ 1.6 g/kg), enough leucine (reference value ~2.5–3 g per meal), and consistent training, plant blends and whey differ in their effects on muscle building practically not at all. Total protein amount, leucine supply, and training quality are more decisive than the source itself.
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