USPlabsRep
From a Lyle McDonald interview, he notes several various studies but they arent referenced, I can try to dig them uo
"As a quick note of introduction, recent studies have essentially demonstrated beyond a shadow of a doubt that it is amino acids themselves that stimulate protein synthesis (with the BCAA and leucine specifically playing the primary role). Which makes a certain sort of sense when you think about it.
In any case, what the studies did was this: infused amino acids at a high level and measured protein synthesis. What they found was bizarre: initially protein synthesis went up considerably. Then, after two hours, protein synthesis started to go down. What's interesting is that protein synthesis went down despite the maintenance of high blood amino acid levels.
As well, the researchers found increased levels of urea production (a waste product of amino acid metabolism) suggesting that the continuing influx of amino acids was simply being burnt off.
The researchers suggested that the skeletal muscle had become "desensitized" to the effect of amino acids in terms of stimulating protein synthesis. Which also suggested that allowing blood amino acids to decrease again would be necessary to "resensitize" it to the effect of amino acids.
Interestingly, this is quite similar to the pattern observed in the now famous Boirie casein/whey study. While whey protein spiked amino acid levels and promoted whole-body protein synthesis, it also increased amino acid oxidation. Perhaps this was due to the muscle being "full" and having become desensitized to the amino acid levels, causing the rest to be oxidized off.
I'd also note that Layne Norton (a natural pro-bodybuilder who is doing his PhD work in protein metabolism) has apparently shown the same with protein feeding in rats. To my knowledge, this hasn't yet been shown for protein feeding in humans.
However, bodybuilders typically try to eat protein frequently with the goal of maintaining nearly constant blood amino acid levels and the above research suggests that this may not be the best idea.
Rather, spiking amino acid levels (to promote protein synthesis) and then allowing them to decrease again (by not eating more protein too frequently) may turn out to be a better pattern for mass gains. Another possibility is that by alternating very fast acting proteins (which generate a quick spike followed by a drop) between whole-food meals may turn out to be superior."
