If guys who already lift weights drink a 20g protein shake after working out for 9 weeks, they’ll gain more muscle and get stronger—even if the protein is a mix of fast- and slow-digesting types.

If you're a guy who lifts weights, drinking a protein shake after your workout with different mixes of fast- and slow-digesting milk proteins — like whey and casein — gives you about the same muscle and strength gains, even if your body processes the amino acids differently.

Young men who lift weights for muscle growth and eat more than 2.0 grams of protein per kilogram of body weight each day tend to get stronger on average than those who eat less than that amount, based on improvements in their squat, bench press, and deadlift numbers over 8 weeks.

In young men who lift weights recreationally, eating more than 2.0 grams of protein per kilogram of body weight per day led to slightly greater increases in strength over 8 weeks compared to eating less, even though muscle size increased similarly in both groups. This suggests that strength gains may come from improvements in how the nervous system controls muscles or in lifting technique, rather than just from muscle growth.

For young guys lifting weights, eating 1.8 grams of protein per kilo of body weight each day is enough to gain muscle and get stronger in 10 weeks — it doesn’t matter if they take a 35g casein protein shake during the day or before bed.

If you're a guy who lifts weights, getting a bigger spike in amino acids after your workout doesn't actually help you gain more muscle or get stronger over time.