Plant protein is often described as “incomplete” or “lower quality” than animal protein — a claim that tends to spark real debate. A detailed amino acid analysis out of Maastricht University gives us actual numbers to look at, instead of just opinions.

What the researchers did

Researchers analyzed the essential amino acid (EAA) content of 10 commercially available plant-based protein isolates — oat, lupin, wheat, hemp, microalgae, soy, brown rice, pea, corn, and potato — and compared them against animal-based proteins (whey, milk, casein, egg) and human skeletal muscle protein itself. Using a precise lab method (UPLC-MS/MS), they measured the full amino acid profile of each source, with particular attention to leucine, lysine, and methionine — the three amino acids most closely tied to triggering muscle protein synthesis after a meal.

The head-to-head comparison

Across the board, plant-based protein isolates contained less total EAA than animal-based proteins. Oat, lupin, and wheat isolates came in at just 21–22% EAA content, compared with 43% for whey, 39% for milk, 34% for casein, and 32% for egg — and 38% for human muscle protein itself.

But total EAA is only part of the story — the mix matters as much as the amount. Here’s where it gets genuinely important to understand: a protein’s quality for muscle-building is capped by whichever essential amino acid is scarcest relative to what the body needs, not by whichever one is most abundant. This is called the limiting amino acid principle, and it’s the reason a single “leucine content” ranking can be actively misleading.

Take corn. It had the highest leucine content of any source in the entire study — 13.5%, ahead of milk (9.0%), egg (7.0%), and even human muscle itself (7.6%). Looking at leucine alone, corn looks like a standout. But corn’s lysine content is just 1.5%, well under the WHO/FAO/UNU requirement of 4.5% — the lowest lysine level of any source tested. Corn’s high leucine can’t compensate for that shortfall; the lysine ceiling still limits what the protein can do. The same pattern shows up elsewhere: soy, microalgae, and pea are relatively strong on lysine (4.6–5.9%) but run low on methionine, while corn, hemp, and brown rice do reasonably well on methionine (1.7–2.5%) but are low on lysine. Oat, lupin, and wheat fall short on both. Potato was the one plant source in the study that cleared the requirement bar on every essential amino acid measured.

Why animal protein ranked higher

The gap comes down to biology, not marketing. Animal proteins — whey, milk, egg, casein — are built from tissues that are structurally similar to human muscle, so their amino acid ratios naturally line up closer to what our bodies need to rebuild muscle. Plants, by contrast, evolved their proteins for their own purposes: storing energy for seeds, building cell walls, defending against pests — not matching human muscle composition. Because of the limiting amino acid principle above, that mismatch matters more than total protein content alone: it’s why plant proteins tend to run short on one or two specific amino acids (most often lysine or methionine) rather than being uniformly “weaker” across the board.

This is also why there’s no single plant protein that ranks worst across every measure. Corn and brown rice are strong on methionine but short on lysine — and hemp shares that same lysine gap, while also having the lowest leucine content of any source tested, plant or animal. Soy, microalgae, and pea sit on the opposite side: comfortable on lysine, short on methionine. Oat, lupin, and wheat fall short on both. “Plant protein” isn’t one weakness; it’s several different, source-specific ones — which is exactly what makes combining sources such an effective fix.

What this doesn’t mean

This is a compositional analysis, not a trial measuring actual muscle growth in people, so it can’t tell us exactly how much these amino acid gaps change real-world outcomes over months of training. It’s also not one verdict for “plant protein” as a single category — corn beats every animal source on leucine and still under-delivers overall because of its lysine gap, which is a good reminder that no single amino acid tells the whole story, plant or animal. Lumping all plant proteins together misses what this data is actually showing: composition varies enormously by specific source, and the “incomplete protein” label oversimplifies a more nuanced picture.

Best strategies if you’re plant-based: how to get enough complete EAA

The researchers’ own conclusion points to a practical fix, and it doesn’t involve eating more animal protein: combining different plant protein sources can close the amino acid gaps almost entirely. A few ways to put that into practice:

1. Pair complementary sources at the same meal or across your day. Sources tend to be low in different amino acids, so combining them fills in each other’s gaps — classic pairings like rice + beans, hummus + whole wheat pita, or a pea + rice protein powder blend cover more ground than either source alone.
2. Match a high-lysine source with a high-methionine source, not just a high-leucine one. Soy, microalgae, and pea are relatively strong on lysine but light on methionine; corn, hemp, and brown rice are the reverse. Pairing one from each group covers more ground than picking a single source for its leucine number alone — a high-leucine source like corn still falls short on its own if lysine is the limiting factor.
3. Eat more total plant protein than you would animal protein for the same effect. Because plant sources are less concentrated in EAAs, matching the anabolic impact of 25 g of whey may require a larger total amount of a single plant source — or, more efficiently, a blend.
4. Use protein blends over single-source isolates when possible. A blend engineered to combine multiple plant proteins (rather than relying on just pea, or just rice, alone) is more likely to approximate a complete amino acid profile in one product.
5. Pay attention to your per-meal dose, not just the percentage. The leucine threshold that reliably triggers muscle protein synthesis is roughly 2.5–2.7 g per meal. Because plant sources vary so much in concentration, that means very different serving sizes — the study found it took as little as 20 g of corn protein but around 54 g of hemp protein to hit that same leucine dose. Check the serving size on your protein powder, not just its ingredient list.

Why this matters for you

“Plant protein is incomplete” isn’t quite the full story, and neither is “all protein is the same.” The real picture: individual plant sources each have specific, identifiable gaps, and those gaps are very fixable through variety and smart combining — not by writing off plant protein altogether. If you’re building meals or shakes around a single plant source, this is worth a second look, not because plant protein doesn’t work, but because one isolate alone may be leaving amino acids on the table that a bit of variety would easily cover.

If you’d like help building a protein approach — plant-based, animal-based, or a mix — that actually supports your training and your goals, book a free 25-minute introductory call and let’s talk through it.

Citation: Gorissen SHM, Crombag JJR, Senden JMG, Waterval WAH, Bierau J, Verdijk LB, van Loon LJC. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids. 2018;50(12):1685–1695.