What is the difference between whey and casein?
Whey protein digests relatively quickly and is rich in essential amino acids. Casein protein digests more slowly and can feel more filling for some people. Both can support muscle gain and fat-loss diets when total protein is adequate.
The practical difference is use case. Whey is easy when you want a quick shake. Casein can work well before a longer gap between meals or before bed if it sits well. Neither is mandatory, and neither outperforms a whole diet that already hits protein targets.