Pea vs. Rice Protein: Why Neither Works Alone (And the Science Behind Blending Them)

You've probably noticed that most plant protein brands pick a lane: it's either pea protein or rice protein. Ever wondered why Heyday uses both? Turns out it's not a flavour thing or a cost-cutting thing – it's actually down to some pretty cool amino acid science. Let us explain.

Not all protein is created equal, even when the label says "20g per scoop." What actually matters is amino acid completeness – whether a protein gives your body all nine essential amino acids (the ones you can only get from food) in the right amounts. Scientists measure this using something called PDCAAS (Protein Digestibility-Corrected Amino Acid Score), a scale from 0 to 1.0. The closer to 1.0, the more "complete" the protein. Whole egg and whey usually score close to a perfect 1.0. Most single-source plant proteins? Not so much.

Pea protein's one weak spot

Pea protein genuinely earns its popularity. It's allergen-friendly, gentle on digestion, and packed with lysine – an amino acid a lot of plant proteins are short on. But it does have one gap: methionine. Pea protein is naturally a bit low in it, which is why on its own it usually scores somewhere around 0.7–0.9 on the PDCAAS scale.(1) Good, but not quite complete.

Rice protein's one weak spot

Brown rice protein has the opposite problem. It's rich in methionine (the very thing pea protein is missing), but it falls short on lysine – so short that rice protein on its own typically scores only around 0.5–0.6.(2) On its own, it's honestly one of the weaker plant proteins out there.

Put them together, and the gaps disappear

Here's the fun part: pea protein's weakness is exactly what rice protein is strong at, and vice versa. Blend them in the right ratio and the two amino acid profiles fill in each other's blanks, pushing the combined score up toward a full 1.0.(3) That's the whole reason food scientists reach for a pea-rice combo in the first place – it turns two "pretty good but incomplete" plant proteins into one that checks every box, without needing soy or any animal-derived protein to get there.

But let's be honest about the research

We're not going to oversell this one. A 2024 randomised, double-blind study out of the University of Birmingham tested a pea-rice blend against pea protein alone in real people, tracking their blood amino acid levels after drinking it.(4) The amino-acid-completeness math held up exactly as expected. But interestingly, in that particular study of ten participants, the blend didn't produce a bigger spike in blood amino acids than pea protein on its own did. That doesn't undo the case for blending pea and rice – the PDCAAS numbers are well established science – but it's a good reminder that "complete on paper" and "what your body actually does with it" are two slightly different questions, and researchers are still working out the second one. We'd rather tell you the full picture than just the flattering half of it.

So what does this mean for your shaker?

It's the actual reason Heyday protein is made with a blend of organic pea and rice protein, rather than just picking one. Two good-but-incomplete plant proteins, put together thoughtfully, get you a whole lot closer to complete than either one manages solo. Pretty neat, right?

(1) PDCAAS values for pea protein isolate vary by processing method, generally cited in the 0.7–0.9 range.
(2) Gorissen et al., 2018.
(3) Common formulation approach in plant-based protein manufacturing, used to offset single-source amino acid deficiencies.
(4) University of Birmingham, published in a peer-reviewed nutrition journal, PMC11557286.