Casein is the other milk protein, and it clots in the stomach instead of emptying quickly, so amino acids trickle into the blood over several hours. That makes it the usual choice before a long gap without food — most often at night. It is high in calcium because the mineral travels with the casein micelle. Per gram of protein it is slightly lower in leucine than whey, which matters for the meal right after training and much less for the daily total.