[nagant]

However, L-serine conversion to D-serine is lacking in older people.
~~~~~

I am under the impression that only left-folded supplements (L-serine) are digestible. That would mean that supplementing with a right-folded amino acid (D-serine) would do no good.

[ConservativeMind]

Complain about that with the authors, because they disagree with you.

From above:

“Remarkably, similar benefits on cognition, though not on the peripheral signs of aging, could be induced by three weeks of dietary supplementation with D-serine.”