Fast and slow proteins: does digestion speed matter?
A glass of milk holds two kinds of protein, and they reach your bloodstream at different speeds. The fast ones are the whey proteins, which arrive quickly and leave quickly. The slow one is casein, which comes in gradually and stays for hours. Here you'll see how scientists found this out, which of the two is "better" (spoiler: it depends) and why the difference almost disappears in real food.
In short
- In 1997, French scientists showed that whey protein raises blood amino acids sharply and briefly, while casein raises them moderately and keeps them up for hours.
- About 80% of the protein in milk is casein. It thickens in the stomach, which is why it digests more slowly.
- There's no permanent winner. In young people drinking the protein on its own, slow casein helped the body hold on to more protein. In older men eating a mixed meal, the fast one did better.
- Egg white is among the faster proteins, but with the yolk and other food on the plate, the pace slows down.
- Fat and carbohydrates on the plate slow down even the fast proteins. Varied sources through the day are the simplest answer.
Where the idea comes from: the 1997 experiment
Carbohydrates have long been divided into fast and slow. In 1997, a team in Clermont-Ferrand, France, set out to check whether the same applies to protein. The study appeared in PNAS, the journal of the US National Academy of Sciences.
The study involved 16 healthy young people. Some drank 30 g of whey protein, others 43 g of casein. The amounts were chosen so that both drinks contained the same amount of leucine. Leucine is one of the nine essential amino acids, and we cover it in our article on complete protein. The scientists then tracked the participants' blood for seven hours.
Whey raised blood amino acids sharply and briefly. Casein raised them moderately but held them on a plateau. Leucine from whey made a high peak that faded within 4 to 5 hours. Leucine from casein stayed on a plateau for 6 to 7 hours.
After 300 minutes, the picture flipped. With casein, leucine was still 61% above the starting level; with whey, 29%. After whey, most of the other amino acids had already gone back to where they started.
Why casein is slow
About 80% of the protein in cow's milk is casein. The rest are the proteins that stay in the whey when cheese is made.
The two behave differently from the stomach onward. The 2017 position stand of the International Society of Sports Nutrition (ISSN) puts it this way: whey protein dissolves in water and digests quickly, while casein is insoluble, clots and digests more slowly. The 1997 authors put the slow pace down mainly to casein clotting in the stomach, so the stomach empties more slowly.
You can see the same thing in the kitchen. When milk curdles, the thick part, the curds, is mostly casein. The yellowish liquid, the whey, carries off most of the other proteins. You can read more about the two in our article on milk protein.
Fast or slow: which is better?
The 1997 answer is a surprise. Whey increased protein building across the whole body by 68%, casein by 31%. Casein, however, reduced protein breakdown by 34%, while whey barely changed it.
In the end, the seven-hour tally came out better for casein. The leucine balance, meaning how much of it the body held on to, was clearly positive. With whey, it was around zero.
In 2001, the same group ran a clever test. They gave people the same 30 g of whey, once all at once and once as several small servings spread over time. In small servings, whey behaved like a slow protein, and the balance jumped: 87 versus 6 micromoles per kilogram. The authors concluded that digestion speed on its own affects how much protein the body holds on to.
In 2003 came a twist. In a study of 9 older men, average age 72, and 6 young men, this time with a mixed meal containing carbohydrates and fat, fast whey helped the older men hold on to more protein than slow casein did. The authors concluded that with age, a "fast" protein may be more useful than a "slow" one for limiting protein loss. There's more about protein after 50 and 65 in our article on protein and aging.
Myth
"Slow protein is always better." It depends on who eats it, when and with what. In young people drinking the protein on its own, casein came out ahead. In older men eating a mixed meal, whey did better. The experiments are small, with 5 to 16 people each, so read them as a general direction, without turning them into a rule.
Which proteins are fast and which are slow
The ISSN position stand gives a rough ranking based on experiments with single proteins:
| Group | Examples |
|---|---|
| Faster | whey, egg white, soy, very lean meat (over 95% lean) |
| Slower | casein, fatty cuts of meat (under 80% lean) |
Notice the meat. It digests at different speeds depending on how much fat it contains. That's the first hint that speed depends on the whole food.
Where eggs fit in
The ISSN puts egg white among the faster proteins. But we rarely eat egg white on its own.
In a 2017 study, 10 young men ate either whole eggs (18 g of protein and 17 g of fat) or egg whites alone with the same 18 g of protein and no fat. They ate them after a strength-training session. Leucine from the egg whites reached the blood faster. Over 300 minutes, though, the total amount of leucine that reached the blood was almost the same: 68% from the whole eggs and 66% from the whites.
So the yolk slows the pace, while the amount that arrives in the end stays about the same. We didn't find a study that puts eggs on the same chart as whey and casein. The most honest summary: egg white on its own is on the fast side, while a whole egg, especially on a plate with bread and cheese, arrives more gradually. There's more about eggs in our article on egg protein.
In real food, the differences even out
Almost all of these experiments gave the protein on its own, stirred into water, on an empty stomach. The 1997 authors themselves write that in a more complex meal the effect may be weaker, because fat slows stomach emptying and carbohydrates affect insulin.
The 2003 study confirms it. Fat and carbohydrates slow stomach emptying, and when they're present, whey digests more slowly.
Protein blends do something similar. In a 2013 study of 19 young people, a blend of soy and milk proteins raised blood amino acids less at first than pure whey, but kept them higher for longer.
Why this caveat matters
Most studies on fast and slow proteins use powders stirred into water, with few people. So the numbers show the mechanism. Your lunch of beans, bread and salad behaves in its own way.
What this means for your plate
You don't need to measure how fast your protein digests. Here's what follows from the research:
- Amount first. Check that you're eating enough protein, and only then think about speed. The math is in our article on how much protein you need a day.
- Mix your sources. Yogurt, cheese, eggs, beans and meat across different meals give you both fast and slow proteins with no planning at all.
- Spread protein through the day. Put some into breakfast, lunch and dinner. Read how in our article on spreading protein through the day.
- Don't sweat the details. On a real plate with fat, bread and vegetables, the differences between fast and slow get smaller.
As for muscle, the EU allows us to say that protein contributes to the maintenance of muscle mass. There's no authorized claim about digestion speed.
The protein in MEALUP comes from two sources: egg white and milk protein concentrate. We have no data on how fast MEALUP digests, so we won't promise you "fast" or "slow". You sprinkle it over the food you're eating anyway, and its protein becomes part of your ordinary meals.
Frequently asked questions
Is whey better than casein?
It depends on the situation. In the 1997 experiment, whey increased protein building more, while casein reduced protein breakdown more. In young people drinking the protein on its own, casein helped the body hold on to more protein. In older men eating a mixed meal, it was the other way around.
Are yogurt and cheese fast or slow?
They contain milk protein, which is mostly casein. We didn't find direct speed measurements for Bulgarian yogurt and cheese. And in a meal with bread, fat and vegetables, the differences even out anyway.
Are eggs a fast protein?
According to the ISSN, egg white is among the faster proteins. With the yolk, leucine reaches the blood more slowly, but over 5 hours the total amount is almost the same as with the whites alone.
Does speed matter for older people?
In a 2003 study of men around 72 years old, the fast protein helped them hold on to more protein than the slow one. That's one small study. If you have a health condition or follow a special diet, ask your doctor.
Sources
- Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrère B, "Slow and fast dietary proteins differently modulate postprandial protein accretion", Proceedings of the National Academy of Sciences, 1997
- Dangin M et al., "The digestion rate of protein is an independent regulating factor of postprandial protein retention", Am J Physiol Endocrinol Metab, 2001
- Dangin M et al., "The rate of protein digestion affects protein gain differently during aging in humans", The Journal of Physiology, 2003
- Jäger R et al., "International Society of Sports Nutrition Position Stand: protein and exercise", Journal of the International Society of Sports Nutrition, 2017
- van Vliet S et al., "Consumption of whole eggs promotes greater stimulation of postexercise muscle protein synthesis than consumption of isonitrogenous amounts of egg whites in young men", American Journal of Clinical Nutrition, 2017
- Reidy PT et al., "Protein blend ingestion following resistance exercise promotes human muscle protein synthesis", The Journal of Nutrition, 2013
