Egg Protein or Casein: What's the Difference

Egg protein and casein are rarely compared directly, although both belong to animal proteins with high biological value and both are absorbed more slowly than whey. Yet the mechanisms of this "slowness" and their areas of use differ. The editorial team examined how these proteins differ from the standpoint of physiology and the evidence base.

What exactly is in the jar

Egg protein in sports nutrition is, as a rule, dried pasteurized egg white (albumin). The yolk with its fats and cholesterol is removed, so the product contains mostly protein with a minimum of fats and carbohydrates. Whole-egg powders are less common.

Casein is the main protein of cow's milk, accounting for about 80% of all milk proteins. The most common form on sale is micellar casein, obtained by microfiltration of milk with preservation of the natural micelle structure, as well as caseinates - chemically modified forms that dissolve better.

So the first difference is the raw material: egg versus milk. It determines the allergen profile, the taste, and the production technology. Egg protein contains no lactose, whereas casein may contain traces of it.

The second difference is physical behavior in liquid. Micellar casein forms a thick drink and even a "pudding" with less water, whereas egg white makes a thinner shake prone to foaming.

• Egg protein:pasteurized egg white, lactose-free, a light shake.
• Casein:milk protein, micellar form or caseinates, thick texture.

Protein fractions and amino acids

Egg white consists of several fractions: the main share is ovalbumin, followed by ovotransferrin, ovomucoid, and lysozyme. This protein historically served as the reference standard for assessing protein quality - it was the one other sources were compared with before the PDCAAS scale appeared.

Casein is represented by alpha, beta, and kappa fractions, which together with calcium phosphate form micelles. This structure also makes casein a source of calcium and phosphorus as part of dairy products.

On the PDCAAS scale both proteins receive the maximum score of 1.0. Both contain the full set of essential amino acids. The differences in leucine between them are small compared with the difference given by, for example, plant proteins.

Egg white is also rich in sulfur-containing amino acids - methionine and cysteine, which makes it a good complement to plant proteins that are deficient in these amino acids.

Absorption kinetics

Casein is considered the classic "slow" protein. In the acidic environment of the stomach the micelles curdle into a clot that is digested gradually. The study by Boirie and co-authors (1997) showed that after casein intake amino acids enter the blood slowly and over a long time, suppressing protein breakdown in the body.

Egg white occupies an intermediate position: it is absorbed more slowly than whey but faster than casein. Pasteurization matters not only for safety - heat treatment increases the digestibility of egg white compared with raw.

Different kinetics dictate different roles. A short, pronounced peak of amino acids stimulates muscle protein synthesis more strongly, while a prolonged supply more strongly inhibits its breakdown.

In real life, absorption rate is also affected by the composition of the meal. Casein as part of milk or cottage cheese with fats and fiber will be absorbed even more slowly than powder in water.

What the research shows

For casein the strongest evidence base concerns intake before sleep. Res and co-authors (2012) showed that 40 g of casein before sleep after an evening workout increased muscle protein synthesis overnight. In a 12-week study by Snijders and co-authors (2015), nighttime protein intake was associated with greater gains in muscle mass and strength.

For egg protein there are fewer specialized training studies. Interesting data were obtained by van Vliet and co-authors (2017): consuming whole eggs after a workout stimulated muscle protein synthesis more strongly than a protein-equivalent amount of egg whites, which points to a role for yolk components.

Direct comparisons of "egg white powder versus casein" in training programs are practically nonexistent. So conclusions have to be drawn on the basis of physiology and general data on protein quality.

The meta-analysis by Morton and co-authors (2018) reminds us that against a background of sufficient daily protein intake, the protein source plays a secondary role compared with the training load.

Allergens, tolerability, and technology

Egg and milk are both among the main food allergens under EU regulations. For people with an allergy to cow's milk protein, casein is contraindicated, and egg protein may become an alternative. And vice versa - with an egg allergy, casein is suitable.

Lactose intolerance is rarely a problem for low-lactose micellar casein, though sensitive people may feel discomfort. Egg protein contains no lactose.

Raw egg white contains avidin, which binds biotin, and can be a source of salmonella. Industrial pasteurization neutralizes microorganisms and largely inactivates avidin, so a quality powder does not carry these risks.

In cooking the differences are noticeable: egg white whips up and "holds" the structure of baked goods, while casein makes dough and creams thicker. Both proteins are widely used in recipes for homemade bars and pancakes.

Editorial conclusions

Egg protein and casein are complete animal proteins with the maximum quality score but different kinetics: egg is absorbed moderately fast, casein slowly and over a long time.

Casein has the better evidence base for intake before sleep, while egg protein is valuable as a lactose-free animal alternative to milk proteins.

Tolerability and allergies play the key role - they often determine the choice.

We also recommend our materials on protein intake before sleep, on milk protein fractions, and on food protein allergy in athletes.

References

1. Boirie Y, Dangin M, Gachon P, et al. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci USA. 1997;94(26):14930–14935.
2. Res PT, Groen B, Pennings B, et al. Protein ingestion before sleep improves postexercise overnight recovery. Med Sci Sports Exerc. 2012;44(8):1560–1569.
3. Snijders T, Res PT, Smeets JS, et al. Protein ingestion before sleep increases muscle mass and strength gains during prolonged resistance-type exercise training in healthy young men. J Nutr. 2015;145(6):1178–1184.
4. van Vliet S, Shy EL, Abou Sawan 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. Am J Clin Nutr. 2017;106(6):1401–1412.
5. Hoffman JR, Falvo MJ. Protein – which is best? J Sports Sci Med. 2004;3(3):118–130.
6. 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. Br J Sports Med. 2018;52(6):376–384.
7. Regulation (EU) No 1169/2011 of the European Parliament and of the Council on the provision of food information to consumers. Official Journal of the European Union. 2011;L304:18–63.