What is casein? Casein vs whey, and when to take it

Casein is most of the protein in cow's milk; whey is the rest. Both come from milk, but they digest at different speeds. This page covers what casein is, how it differs from whey, when there is evidence for taking it, and who should avoid it.

What is casein?

Agriland's article on milk proteins says casein is about 80% of the protein in cow's milk and whey the other 20%. Casein is separated in one of two ways: lowering skim milk to pH 4.6 so the casein precipitates, or adding rennet so the milk sets into a gel that is cut and stirred until the liquid whey drains off, leaving casein to be dried. The whey sold in tubs comes from that liquid.

How is casein different from whey?

An article in The Physiological Society's Physiology News summarises Boirie and colleagues (1997): whey stays liquid in the stomach and empties quickly, so its amino acids reach the blood fast; casein clots in stomach acid and empties more slowly, so its amino acids arrive slower, at lower levels, but for longer. In that work whey raised protein synthesis briefly, while casein held back protein breakdown for longer, which is why the researchers called whey a fast protein and casein a slow one.

When to take casein

The International Society of Sports Nutrition (ISSN) nutrient timing paper says about 30–40 g of casein before sleep can raise muscle protein synthesis overnight. Getting enough protein over the whole day matters more than timing; the method is in how and when to take whey and how many scoops a day.

Who should avoid casein?

Cow's milk allergy: casein is milk protein, so avoid it just like whey. Mayo Clinic warns that milk allergy can cause anaphylaxis. The alternative is plant protein
Lactose intolerance: a different problem from a milk protein allergy; check the lactose on the label, more in lactose intolerance and whey