Egg Protein or Casein: What's the Difference

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

What exactly is in the tub

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

Casein is the main protein of cow's milk, accounting for about 80% of all milk proteins. The most common on sale is micellar casein, obtained by microfiltration of milk with preservation of the natural structure of the micelles, 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 have residues of it.

The second difference is physical behaviour in liquid. Micellar casein forms a thick drink and even a "pudding" with a smaller amount of water, whereas egg white gives a thinner shake prone to foaming.

• Egg protein:pasteurised 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 benchmark for assessing protein quality - it was precisely with it that other sources were compared before the appearance of the PDCAAS scale.

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 within dairy products.

On PDCAAS both proteins receive the maximum score of 1.0. Both contain a 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 sulphur-containing amino acids - methionine and cysteine, which makes it a good complement to plant proteins deficient in these amino acids.

The kinetics of absorption

Casein is considered the classic "slow" protein. In the acidic environment of the stomach the micelles coagulate into a clot that is gradually digested. The study by Boirie and colleagues (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. Pasteurisation is important not only for safety - heat treatment increases the digestibility of egg white compared with raw.

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

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

What research shows

For casein, the strongest evidence base concerns intake before sleep. Res and colleagues (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 colleagues (2015), night-time protein intake was associated with a greater increase in muscle mass and strength.

As for egg protein, there are fewer specialised training studies. Interesting data were obtained by van Vliet and colleagues (2017): the intake of whole eggs after a workout stimulated muscle protein synthesis more strongly than an amount of egg whites equivalent in protein, which points to a role for the components of the yolk.

There are practically no direct comparisons of "egg white powder versus casein" in training programmes. Therefore conclusions have to be drawn on the basis of physiology and general data on protein quality.

The meta-analysis by Morton and colleagues (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, tolerance and technology

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

Lactose intolerance is rarely a problem for micellar casein with a low lactose content, but 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 pasteurisation neutralises microorganisms and largely inactivates avidin, so a quality powder does not carry these risks.

In cooking the differences are noticeable: egg white whips 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 a better evidence base for intake before sleep, egg protein is valuable as a lactose-free animal alternative to milk proteins.

Tolerance and allergies play a key role - it is precisely they that often determine the choice.

We also recommend our articles on protein intake before sleep, on the fractions of milk protein and on allergy to food proteins 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.