But think about what’s actually happening during digestion. you’ve just eaten 40 grams of protein and it’s being digested at some rate (see part 2). It appears in the bloodstream where it can be taken up into other tissues such as the liver, skeletal muscle, etc. Eventually the protein in the gut will be fully digested and blood levels will then drop; more amino acids are being taken up into tissues than are being released into the bloodstream (b/c digestion is finished).

See
“Gastric emptying, gastric secretion and enterogastrone response after administration of milk proteins or their peptide hydrolysates in humans.”

Boris, leucine is just one of the amino acids that they often track in studies like this, I don’t remember the reason why offhand. But it’s used as a surrogate marker for overall amino acid appearance, utilization, etc. It’s probably just easy to measure, it’s also one of the aminos that is utilized heavily in skeletal muscle. So it is a useful marker in this regards.

The current fascination with leucine, like most supplements, has an element of truth (in that leucine is a key player in turning on protein synthesis by activating something called mTOR) but some really piss-poor studies are being taken out of context to sell product (as usual).

With appropriate protein intake, any athlete will get more leucine than they need. The average protein has 15-25% branch chain amino acids, with whey having up to 25% and casein around 20% and about 1/3rd of that will be leucine so an athlete consuming 1.5 g/lb protein is getting tons of leucine already. There is absolutely no reason to supplement it or add it to anything unless, for some reason, you were consuming inadequate protein in the first place.