What is wrong with milk as a source of amino acids? If you think about it, it is the sole source of amino acids for one of the most important growth stages in every mammals life... It serves them perfectly welll... Milk is also an excellent source of BCAAs -
where do you think they get whey from
- Alright, I'm going to let that one slide...
Also - the rate at which your body is going to be utilising amino acids specifically for anabolism of muscle is far, far less than the intakes we are talking about. Somebody on a non-stupid diet (ie: not doing severe calorie restriction) is going to be able to maintain a positive nitrogen balance on much lower intakes. On top of this, weight lifting improves your ability to retain nitrogen as well (so, technically speaking it decreases your requirements)...
It is like filling a bucket with a very small hole in the bottom - you can keep pouring the water into the top - but the rate at which it is being drained out the bottom is not going to get any bigger as a result, and all that happens is the water spills over the top and is wasted.
Also - it is the total protein intake over the day that is going to be far more significant that that which is in one meal. And as long as there is some protein in each meal you are going to be fine. A meal (other than something very simple such as whey + dextrose) is going to stay to be processed in your intestines for quiet a few hours (depending on what you are eating, what else is in your intestines etc etc) - so the amino acids being released into your blood is not going to stop with in 20 minutes of ingestion rendering you in a catabolic state. You also have intra-muscular amino acid pools available to use if need be.