Casein Protein

Casein is the predominant protein fraction of milk, comprising approximately 80% of total bovine milk protein. Its distinguishing physical property is isoelectric precipitation near gastric pH (~4.6), producing a slowly digested curd that releases amino acids over many hours — the canonical "slow" protein in Boirie's fast/slow framework.

Molecular organization

Casein exists in milk as micelles — colloidal particles 50-500 nm in diameter composed of four main casein proteins (alpha-s1, alpha-s2, beta, and kappa) stabilized by calcium phosphate nanoclusters. Kappa-casein forms the hydrophilic outer surface that keeps micelles in suspension. Chymosin (rennet) cleavage of kappa-casein destabilizes the micelle and drives curd formation in cheesemaking. At gastric pH in vivo, acidic denaturation accomplishes the same coagulation, slowing transit and enzymatic access.

Digestion kinetics

Boirie et al. (1997) reported that a 30 g casein meal elevated plasma amino acids for over 7 hours, compared to approximately 3 hours for an equivalent whey dose. Leucine appearance rate peaked later and lower but integrated area under the curve over 7 hours was comparable. Net whole-body protein balance was more positive following casein due to suppression of proteolysis rather than superior stimulation of synthesis — the complementary mechanism to whey's acute-anabolic profile.

Pre-sleep casein

A research program led by Luc van Loon at Maastricht University demonstrated that 30-40 g casein ingested 30 minutes before sleep is digested and absorbed overnight, elevating muscle protein synthesis during the sleep period by approximately 22% over placebo. Chronic training studies incorporating pre-sleep casein have shown modest additive effects on muscle mass and strength beyond daytime protein intake alone, though effect sizes are modest and contingent on total daily protein adequacy.

Protein quality

Casein scores PDCAAS 1.00 (truncated) and DIAAS approximately 1.18, placing it among the high-quality proteins. Its leucine content (~8.5% of amino acids) is slightly lower than whey's but still above the saturation threshold for meals in the 30-40 g range. All nine essential amino acids are present in adequate proportion.

Allergy and tolerance

Casein is a major milk allergen distinct from whey proteins; approximately 30-50% of milk-allergic individuals react primarily to casein. Casein is heat-stable and not denatured by pasteurization or cooking, unlike whey proteins. Lactose-intolerant individuals typically tolerate casein protein concentrates well due to low residual lactose.

Commercial forms

Micellar casein preserves the native micellar structure and retains the slowest digestion kinetics. Calcium caseinate and sodium caseinate are processed forms with somewhat faster digestion. Casein hydrolysates provide rapid amino acid delivery without the curd-forming behavior. Cottage cheese, Greek yogurt (after whey straining), and hard cheeses are whole-food casein sources.