I've read through this thread again and pulled out some interesting bits for me:
Selenium supplementation, via the metabolite through hydrogen selenite, breaks down collagen. On the plus side this may tackle (liver) fibrosis. The drawback is broken down collagen (hair, skin) and a therefore probably significantly increased demand for resynthesis from glycine, ascorbic acid, prolin (via ornithine/arginine/citrullin).
The control feed + cysteine studies in rodents and the GlyNAC studies in humans sort of tell the tale:
If not doing MetR + CysR then a regular diet supplemented with cysteine actually consistently looks better in several aspects including body composition, fat pads, strength, inflammation. Peculiar.
The control feed + cysteine in rodents resulted even in better looking markers than MetR + cysteine. Strange.
I surmise there must be crucially overlooked specific discrepancies in the transfer of mice study results to humans. Perhaps not least because glutathione is much more crucial for redox homeostasis and substance conjugation in non-ascorbic-acid-synthesizing humans in comparison to mice. Bridging to 4):
Regularly increased glycine and citrulline intakes to both balance the relative ratio to and the absolute uptake of Met and to scavenge methyl groups in the metabolism after absorption do appeal.
Very obviously, large protein intakes are bad for longevity not merely because of mTOR activation. Eating a typical protein composition but in low to moderate amounts only as in the previously quoted low-protein diets enhanced by extra glycine and citrulline (+proline) appeals. The latter will also benefit the maintenance and rebuilding of collagen structures and allow for use of selenium supplementation to reap to a large extent the quoted increase of fgf21.
@Mauritio said:
Glycine also lowers its absorption. Although it's more true to say that methionine lowers glycines absorption , IIRC up to 86% !
"...l-Met uptake is very potently inhibited by l-arginine, l-citrulline, l-lysine and l-isoleucine in the rat intestine 53. "
So if Met inhibits Gly absorption, and Cit inihibits Met absorption,
then Gly+Cit is ought to bring about win-win conditions.