Casein is a protein found in milk, making up about 80% of milk's total protein content (whey makes up the other 20%). Unlike whey, which is rapidly absorbed, casein forms a gel-like substance in your stomach that digests slowly over 5 to 7 hours. This slow digestion creates a sustained, steady release of amino acids into your bloodstream — the opposite of whey's sharp spike and quick return to baseline.
Why It Matters
Casein's slow-release property makes it uniquely suited for periods when you won't be eating for many hours — most commonly overnight. Research has shown that consuming 30 to 40 grams of casein before bed supports overnight muscle preservation, reducing the breakdown that naturally occurs during a 7 to 8 hour fasting window. Like whey, casein is a complete protein with a strong amino acid profile, but its sustained delivery means amino acids are available for a longer period.
What to Look For
Micellar casein is the least processed and slowest-digesting form — look for it as the primary ingredient. Calcium caseinate is a more processed form that digests slightly faster. If you're choosing between whey and casein, they serve different purposes: whey is better around exercise and at meals when you want fast absorption; casein is better before bed or when you know a long gap between meals is coming. Many people use both. Casein mixes thicker than whey and has a different texture — some people enjoy it as a pudding-like consistency with less liquid. If you have lactose intolerance, note that casein products may contain more lactose than whey isolate. However, if you have a true milk protein allergy (IgE-mediated), casein itself is the allergen — no amount of lactose removal makes it safe. Milk protein allergy and lactose intolerance are different conditions; consult your allergist if you are unsure which applies to you.
For GLP-1 Medication Users
Casein's slow digestion is a double-edged sword for GLP-1 users. On one hand, it provides amino acids over a longer window — useful when you're eating less frequently. On the other hand, the slow gastric emptying caused by GLP-1 medications combined with casein's already slow digestion can lead to prolonged fullness or discomfort. If you want to try casein, start with a half serving and see how your stomach responds before committing to a full scoop.
Related Terms
Whey
A dairy-based protein derived from cheese production. Fast-absorbing, high in leucine, and rated 1.0 on the PDCAAS scale — widely considered the gold standard for muscle preservation.
Muscle Protein Synthesis
The process by which your body builds new muscle tissue. It requires both adequate protein intake and a stimulus like resistance training to work effectively.
Protein Timing
The practice of distributing protein intake across meals throughout the day rather than consuming most of it in one sitting. Spreading protein out produces a stronger muscle-building response.
Isolate
A protein powder processed to contain 90% or more protein by weight, with most fat, lactose, and carbohydrates removed. The purest common form of protein powder.