Short answer: Both are complete proteins from milk. Whey is digested fast and has more leucine; casein is digested slowly, so amino acids reach the blood more gradually.1 In single-meal studies, whey raised muscle protein building more in the hours after a drink, while casein held amino acids up longer.23 NIH notes bedtime casein studies used 27.5 or 40 g.1
| Whey | Casein | |
|---|---|---|
| Source | One of the complete proteins in milk; most protein powders are whey1 | The main complete protein in milk1 |
| Digestion | Fast1 | Slower, so amino acids enter the blood more slowly1 |
| Leucine | More than casein or soy1 | Less than whey1 |
| Blood amino acids after one meal | Sharp, short rise3 | Longer, moderate plateau3 |
| Body protein building after one meal | Up 68%3 | Up 31%; protein breakdown down 34%3 |
| Net leucine balance over 7 hours | Lower3 | Higher (more positive)3 |
| Muscle protein building after exercise | About 122% higher than casein in young men2 | Lower than whey or soy2 |
| Allergy | Milk is a major food allergen4 | Milk is a major food allergen4 |
In a 1997 study, healthy adults drank a single meal of whey or casein labeled with a tracer. Whey caused a dramatic but short rise in blood amino acids; casein gave a longer, moderate rise, probably because it leaves the stomach slowly. Body protein building rose 68% with whey and 31% with casein, but casein also cut protein breakdown by 34%, so the net leucine balance over 7 hours was more positive with casein.3
In a 2009 study of young men (6 per group), drinks with 10 g of essential amino acids from whey, casein or soy were given after leg exercise. Muscle protein building was about 122% higher with whey than casein after exercise, and about 93% higher at rest.2
Both were short, single-meal studies. They measured protein building over hours, not muscle gained over months.
NIH says several studies in people doing strength training found that protein before sleep can raise overnight muscle protein building and/or add to gains in muscle and strength. Those studies used a bedtime drink of 27.5 or 40 g of casein.1
NIH also notes that a meta-analysis found protein within an hour before or after exercise did not significantly increase strength or muscle size, and that the window after exercise when protein helps can last up to 24 hours.1
NIH cites an RDA of 0.8 g per kg of body weight a day for healthy adults, and 1.2 to 2.0 g per kg a day suggested for athletes by sports nutrition groups, with about 0.3 g per kg of high-quality protein after exercise. No upper limit was set for protein, but NIH advises caution with high intakes from foods plus supplements because data are limited.1
More detail: the Whey Protein and Casein Protein guides.