THE SHORT VERSION

  • Ultrafiltered milk and whey protein both come from milk. Ultrafiltration is a process; whey is one of milk’s two main protein families.
  • Milk protein is about 80% casein and 20% whey. Ultrafiltered milk generally keeps that mix, while a whey ingredient supplies the whey portion.
  • In one study of older men, whey produced a higher muscle protein synthesis rate than casein. Separately, lab models showed that blends with at least 50% casein formed persistent curds.
  • Both are high-quality complete protein sources. These small, short-term comparisons help explain how they differ, but do not establish one category of drink as better for everyone.

Two drinks sit side by side on a shelf. Each offers 25 grams of protein. One is made with ultrafiltered milk. The other names whey protein as its protein source. Both come from milk, so it is fair to wonder whether the difference is mostly wording.

The difference starts before either drink reaches your glass. Ultrafiltration is a separation process. Whey is one of the two main protein families in milk. Conventional ultrafiltration concentrates the milk proteins together, while making a whey ingredient means separating the whey from the casein.1,2

What the words on the label mean

Cow’s milk protein is roughly 80% casein and 20% whey. Both supply all nine essential amino acids, the ones your body cannot make enough of, in adequate amounts. That is what “complete protein” means.3 The two families still differ in their amino acid makeup and in how they behave during digestion.3

In ultrafiltration of skim milk, a membrane holds back the proteins while water and smaller dissolved molecules pass through. A further washing step, called diafiltration, can remove more lactose and other small molecules.2 The protein that remains generally keeps milk’s original mix: about four parts casein to one part whey.2,3

That is why milk protein concentrate and milk protein isolate contain both families, while whey protein concentrate and whey protein isolate come from the whey portion. Different filtration steps can be used to make either kind, so the word “filtered” by itself does not tell you which proteins are inside.1,2

On the ingredient list What it tells you
Ultrafiltered milk Milk concentrated by membrane filtration, generally keeping both casein and whey.
Milk protein concentrate or isolate Ingredients with both major milk proteins, typically near milk’s original mix.
Whey protein concentrate or isolate Ingredients taken from the whey portion of milk.

What 25 grams can be made of

A simple example makes the difference easier to see. Suppose a drink’s 25 grams of protein came entirely from milk that kept its 80-to-20 mix. That would be about 20 grams of casein and 5 grams of whey protein.2,3

A serving with 25 grams of whey protein would have a different makeup. The arithmetic assumes milk’s natural mix was kept; adding another protein ingredient would change it.

An illustration of the arithmetic above, not an analysis of any product. Milk’s approximate protein ratio is described in references 2 and 3.

An illustration of the arithmetic above, not an analysis of any product. Milk’s approximate protein ratio is described in references 2 and 3.

That difference can matter in the stomach. Casein can form curds that slow how fast protein leaves the stomach, while whey generally stays more soluble. Processing and the structure of the finished food can change these patterns. Part 6 looks at how.4

Mulet-Cabero and colleagues tested mixtures with different casein-to-whey ratios using simulated digestion and an absorption model built on isolated intestinal tissue.

Mixtures with at least 50% casein formed persistent curds and showed slower emptying of nutrients and slower leucine absorption in those models.5 This laboratory work helps explain how combining proteins can change their digestive behavior. It does not tell us how a particular milk-based drink will behave in a person.

What human studies add

Human studies show why matching grams of protein leaves part of the story open. Pennings and colleagues gave 48 older men 20 grams of whey, casein, or casein hydrolysate, which is casein already cut into smaller pieces.6

Whey produced a higher rate of muscle protein synthesis, the process of building muscle protein, over the measured period after the drink. The researchers pointed to whey’s digestion and absorption characteristics, together with its higher leucine content.6

Leucine is an essential amino acid that helps signal muscle to build protein. Part 5 covers it in detail. Whey is relatively rich in leucine, and a milk-protein blend supplies it too. Matching total protein grams does not guarantee a matching amount of leucine or the same delivery pattern.3,6

The finished food complicates the comparison. Horstman and colleagues gave ten older adults several milks, yogurt, cheese, whey protein concentrate, and micellar casein, each supplying 25 grams of protein. This was a crossover study, meaning each person tried every product on a different day.7

Whey concentrate and yogurt produced the largest rises in blood essential amino acids, while cheese and micellar casein produced more prolonged responses.7

A food that contains casein cannot be given an exact digestive timetable from its ingredient name alone. The arrangement of nutrients inside the food, often called the food matrix, plays a part. This study looked at the combined effect of the whole food, not at ultrafiltration by itself.7

Does this matter for most people?

Both protein sources score well on measures of protein quality. In a pig study, whey protein isolate, whey protein concentrate, milk protein concentrate and skimmed milk powder all scored at least 100 on the digestible indispensable amino acid score, or DIAAS, using the adult scoring pattern.8

DIAAS rates how well a protein supplies digestible essential amino acids relative to people’s needs. A score of 100 or above is classed as excellent quality. Pigs are used because aspects of their digestion are similar to ours, but the study remains an animal model.8

Quantity matters too. In a meta-analysis of 49 trials with 1,863 healthy adults, protein supplementation produced a small additional gain in fat-free mass, which includes muscle, during resistance training.9

The researchers estimated that gains in fat-free mass leveled off around 1.6 grams of protein per kilogram of body weight per day. That estimate was uncertain and does not set a fixed limit for every person.9

Total protein across the day matters. The comparisons discussed here do not establish that choosing one milk-protein source over another matters more than total intake.

What we do not know yet

These experiments did not compare every ultrafiltered-milk drink with every whey-based drink. The protein-source trial enrolled older men, the food-matrix study had only ten participants, and the ratio experiment used laboratory models.5,6,7

Their findings explain plausible differences. They do not show that one whole category of drink will outperform the other for every person or purpose.

An ingredient list also leaves manufacturing details open. Heating conditions, protein form, and interactions within a formulation can change what happens in the stomach. A prediction based on whey or casein alone remains a prediction until the finished food has been examined.4

Back to the two drinks

When the protein totals match, the next question is what those totals contain. Ultrafiltered milk generally brings both casein and whey. A whey ingredient brings whey. Knowing the source gives you a clearer starting point for understanding the amino acids supplied, how they are digested, and what evidence exists for the finished drink.

Both are high-quality complete protein sources. The research helps explain their differences, but does not make one category the better choice for everyone.

Related reading

References

Educational content. The Fountain is written to explain, not to make health claims about any product. This article summarizes published peer-reviewed research. Nothing here is medical advice.

  1. Carter BG, Cheng N, Kapoor R, Meletharayil GH, Drake MA. Invited review: microfiltration-derived casein and whey proteins from milk. Journal of Dairy Science. 2021;104(3):2465–2479. https://doi.org/10.3168/jds.2020-18811
  2. McSweeney DJ, Maidannyk V, Montgomery S, O’Mahony JA, McCarthy NA. The influence of composition and manufacturing approach on the physical and rehydration properties of milk protein concentrate powders. Foods. 2020;9(2):236. https://doi.org/10.3390/foods9020236
  3. Auestad N, Layman DK. Dairy bioactive proteins and peptides: a narrative review. Nutrition Reviews. 2021;79(Suppl 2):36–47. https://doi.org/10.1093/nutrit/nuab097
  4. Horstman AMH, Huppertz T. Milk proteins: processing, gastric coagulation, amino acid availability and muscle protein synthesis. Critical Reviews in Food Science and Nutrition. 2023;63(30):10267–10282. https://doi.org/10.1080/10408398.2022.2078782
  5. Mulet-Cabero AI, Torcello-Gómez A, Saha S, Mackie AR, Wilde PJ, Brodkorb A. Impact of caseins and whey proteins ratio and lipid content on in vitro digestion and ex vivo absorption. Food Chemistry. 2020;319:126514. https://doi.org/10.1016/j.foodchem.2020.126514
  6. Pennings B, Boirie Y, Senden JMG, Gijsen AP, Kuipers H, van Loon LJC. Whey protein stimulates postprandial muscle protein accretion more effectively than do casein and casein hydrolysate in older men. The American Journal of Clinical Nutrition. 2011;93(5):997–1005. https://doi.org/10.3945/ajcn.110.008102
  7. Horstman AMH, Ganzevles RA, Kudla U, Kardinaal AFM, van den Borne JJGC, Huppertz T. Postprandial blood amino acid concentrations in older adults after consumption of dairy products: the role of the dairy matrix. International Dairy Journal. 2021;113:104890. https://doi.org/10.1016/j.idairyj.2020.104890
  8. Mathai JK, Liu Y, Stein HH. Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS). British Journal of Nutrition. 2017;117(4):490–499. https://doi.org/10.1017/S0007114517000125
  9. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine. 2018;52(6):376–384. https://doi.org/10.1136/bjsports-2017-097608