although my friend, a qualified doctor, is now telling me it doesn't really work like that
Reading the posts on this subject here so far it seems that many people are under the impression that the proteins taken in in the diet are the same ones which end up improving those enormous fit muscles ! Nothing could be further from the truth ! A similar lay misconception is about castor oil (or similar) being good for arthritis because it "lubricates" the joints. Unfortunately, in neither case does it work anything like so simply.
When food is ingested it is DIGESTED in the stomach, by strong hydrochloric acid and numerous powerful enzymes. Large molecules, particularly the proteins and lipids, are broken down to their smaller component parts in order to be absorbed into the blood stream for transport to the cells and tissues. The component parts of proteins, for example, are the much smaller amino acids. Once in the cells and tissues these component parts are once again recombined, to be formed back into proteins. But note, this time HUMAN proteins !
If, as seems to be believed by many, foreign ( ie plant or animal) protein was actually directly absorbed into the blood, then some fairly catastrophic allergic or anaphylactic reactions would be likely to occur.
There's a good account of this subject here, for anyone who is interested :
Protein Digestion and Absorption