Several questions regarding post ......
there are multiple reasons some i understand and some i dont. but aminos are substrates for alot of enzymatic processes and them themselves must be cleaved from proteins to peptides to free-form, there is only so much enzyme and each amino has unique function etc.. its the most complicated topic in biochem no doubt, but competition for liver enzymes, is one reason and depending on what aminos you supply the body, the body will use them differently. for example some aminos are used for hemoglobin, some for other protein complexes. taking them seperately almost gurantees no competition or co-utilization. there are multiple other reasons however, such as decreases intestinal uptake. single aminos have drug like effects, but combined it just makes it protein.
like taking tyrosine to increase catecholamines for mood enhancement is great, but it doesnt happen from eating whole protein, or taking trypthophan for similar reasons(serotonin however), but in concert it works much less so.
down stream just means after they have been converted in protein, or affects further down the biochemical line.
leucine in studies has shown to actually increase protein systhesis to appreciable amounts unlike other aminos and has research to support its use in bodybuilding unlike glutamine.
