Protein absorption rates: comparing common sources

Key takeaways

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Protein absorption rate describes how quickly amino acids from a protein appear in the blood after it is eaten. This page looks at digestion speed for whey, casein, egg, soy and pea, and where it can be a consideration when choosing a product.

Here are the numbers, where they come from, and the situations where digestion speed can be a consideration when choosing a product.

What “protein absorption rate” actually means

When a study reports an absorption rate in grams per hour, it is not measuring how much protein your gut is capable of accepting. It is measuring how quickly amino acids from that protein appear in your bloodstream after you drink it on an empty stomach. Two things drive that speed: how fast your stomach empties, and how quickly the protein is broken down once it reaches the small intestine.

That distinction matters, because a common question is whether “you can only absorb 30 g of protein in one sitting”. A review of per-meal protein distribution (Schoenfeld & Aragon, 2018) describes the idea of a limit as “a long-held misperception in the lay public” and notes that, on the nutritional definition of absorption (the passage of nutrients from the gut into systemic circulation), “the amount of protein that can be absorbed is virtually unlimited”.

So absorption rate is really a question about timing: how sharp the amino acid peak is, and how long it lasts.

Protein absorption rates compared: whey, casein, egg, soy and pea

Treat the figures below as a map, not a spec sheet — different studies, doses and processing methods shift them.

These numbers come from shakes drunk on an empty stomach. Taking protein with a meal is a different situation, which the table does not describe.

Whey and casein compared

The two proteins come from the same milk. The ISSN position stand (Jäger et al., 2017) describes whey protein as water soluble, easy to mix and rapidly digested, and casein as water insoluble, coagulating in the gut and digested more slowly than whey protein.

This was examined in the 1997 tracer study (Boirie et al.). Whey produced a sharp, high spike in blood amino acids that faded within a few hours. Casein produced a lower, flatter curve that lasted much longer. The study reports that whole-body protein breakdown was inhibited by 34% after casein but not after whey.

Our casein vs whey guide goes deeper on choosing between them, and the research on protein taken before sleep has more detail on the before-bed setting.

PDCAAS and DIAAS: the quality scores behind the label

Speed is only half the story. The other half is quality — how complete the amino acid profile is, and how much of it you actually absorb rather than pass through.

The older scoring system, PDCAAS, runs from 0 to 1.0 and caps out at 1.0. Whey, casein, egg and soy isolate all hit that ceiling, which makes the score useless for telling them apart. Pea typically lands around 0.8-0.9.

The newer system, DIAAS, measures digestibility further up the gut and does not truncate at 1.0, so it separates the proteins that PDCAAS lumps together:

• Whey and milk proteins — commonly reported above 1.0, often around 1.1-1.2.
• Egg and casein — usually just above 1.0.
• Soy protein isolate — roughly 0.9-1.0.
• Pea protein isolate — typically in the 0.7-0.8 range.

The practical translation: animal proteins are generally digested at above 90% efficiency, well-refined plant isolates at roughly 80-90%, and less-processed plant flours lower still. A long-standing review comparing the major protein sources covers how these scoring systems developed and why they disagree.

The plant protein difference is leucine, not just digestibility

If you only look at digestibility percentages, you will underestimate the gap — because the more meaningful difference is amino acid composition, and specifically leucine.

Leucine is one of the nine essential amino acids, and the ISSN position stand (Jäger et al., 2017) reports that whey and casein routinely contain the highest leucine content of all other protein sources, at 11% and 9.3% respectively. An analysis of commercially available plant-based protein isolates found that most contain meaningfully less leucine per gram than dairy proteins, alongside shortfalls in specific essential amino acids — lysine in grain proteins, methionine and cysteine in legumes like pea.

Some people who use plant powders take a larger serving or blend complementary sources. Two approaches are common:

• Increase the serving. Roughly 30-45 g of a plant isolate is usually suggested where 20-25 g of whey would do.
• Blend complementary sources. Pea is low in methionine and high in lysine; rice is the reverse. Combining them produces a far more complete profile than either alone — and research on dairy-and-vegetable protein blends shows blends can deliver amino acid availability that single sources struggle to match.

Our whey vs plant protein comparison works through this trade-off in more detail if you are choosing between them.

How much protein per meal is actually useful?

Here is where the absorption obsession usually loses the plot. The ISSN position stand on protein and exercise (Jäger et al., 2017) notes that recommendations for protein per serving, for athletes aiming to maximise muscle protein synthesis, are mixed and depend on age and recent resistance exercise, with general recommendations of 0.25 g of a high-quality protein per kg of body weight, or an absolute dose of 20–40 g.

Beyond that per-meal figure, there is also a daily figure. The International Society of Sports Nutrition position stand on protein and exercise states that, for building and maintaining muscle mass, an overall daily protein intake of 1.4 to 2.0 g per kilogram of bodyweight is sufficient for most exercising individuals.

The EU-authorised claim is shown below.

If you want to work out your own daily target, our how much protein per day guide does the arithmetic.

Practical choices, without overthinking it

Strip away the numbers and the decision tree is short:

• Around training: whey concentrate or isolate. Mixes thin.
• Before bed or a long gap: casein, or a casein-dominant blend.
• Dairy-free: egg protein if you tolerate it, or soy isolate as a single plant option.
• Fully plant-based: a pea-and-rice blend at a slightly larger serving is one option.
• If you struggle with dairy: isolate is lower in lactose than concentrate — see our guide to whey and lactose.

When comparing two tubs, absorption rate is one of several things people look at, alongside protein per 100 g, price per serving, and whether the label discloses its actual amino acid content. Our cost calculator handles the price-per-serving maths, and our whey protein reviews score products on declared composition alone.

The honest summary

The ISSN position stand (Jäger et al., 2017) describes whey protein as rapidly digested and casein as digested more slowly than whey protein. Whey and milk proteins score highest on digestibility and leucine; plant isolates score lower.

The ISSN position stand (Jäger et al., 2017) reports that total protein and calorie intake appears to be the most important consideration when it comes to promoting positive adaptations to resistance training. The shake after training and the one before bed are options for fitting protein into the day.

Sources: EU Register of nutrition and health claims, ISSN Position Stand: protein and exercise, How much protein can the body use in a single meal?

Related: Casein vs whey – Whey vs plant protein – How much protein per day – Best time to take whey

Frequently asked questions

How fast is whey protein absorbed?

Schoenfeld & Aragon (2018) describe whey as a “fast-acting” protein and note that its absorption rate has been estimated at about 10 grams per hour. In the 1997 tracer study (Boirie et al.), whey produced a dramatic but short increase in plasma amino acids.

Can your body only absorb 30g of protein at once?

Schoenfeld & Aragon (2018) describe the idea that there is a limit to how much protein can be absorbed as "a long-held misperception in the lay public" and note that, on the nutritional definition of absorption, "the amount of protein that can be absorbed is virtually unlimited". The ISSN position stand (Jäger et al., 2017) notes that recommendations for protein per serving, for athletes aiming to maximise muscle protein synthesis, are mixed and depend on age and recent resistance exercise, with general recommendations of 0.25 g of a high-quality protein per kg of body weight, or an absolute dose of 20–40 g.

Is casein better than whey?

CapyFuel does not rank them. The ISSN position stand (Jäger et al., 2017) describes whey protein as rapidly digested and casein as water insoluble, coagulating in the gut and digested more slowly than whey protein. Casein is commonly used before bed or between widely spaced meals, and whey around training.

Is plant protein worse than whey?

Plant isolates are somewhat less digestible and lower in leucine than dairy proteins, which is why they score lower on DIAAS. Some people take a slightly larger serving (roughly 30-45 g where 20-25 g of whey would do) or use a complementary blend such as pea with rice.

What is the difference between PDCAAS and DIAAS?

PDCAAS scores protein quality on a 0 to 1.0 scale and caps at 1.0, so whey, casein, egg and soy isolate all look identical at the top. DIAAS measures digestibility further up the digestive tract and is not capped, so it separates them - dairy proteins commonly score above 1.0 while pea isolate sits closer to 0.7-0.8.

Does protein absorption rate actually affect results?

No effect claim is made for absorption rate on this page. The ISSN position stand (Jäger et al., 2017) reports that total protein and calorie intake appears to be the most important consideration when it comes to promoting positive adaptations to resistance training. Absorption rate is one of the things people consider when choosing which powder to use after training versus before bed.

Sources & further reading

1. EU Register of nutrition and health claims
2. ISSN Position Stand: protein and exercise (PMC)
3. How much protein can the body use in a single meal? (PMC)
4. Slow and fast dietary proteins differently modulate postprandial protein accretion (PubMed)
5. Protein content and amino acid composition of commercially available plant-based protein isolates (PMC)
6. Protein - Which is Best? (PMC)