Julian, the amount of protein that can be used at a time varies from person to person, depending on their weight, muscle mass, and activity level. That 20g per person could very well be the figure for your average sedentary male, while a male who's blasting his muscles in the gym 3 days a week could digest much more. The whole purpose of protein is to repair and build muscle tissue, so if you’re ripping up your muscle tissue in the gym, obviously more protein would be needed to repair all that tissue and build a little more tissue so you can support that weight better next time.