Both D-serine and L-serine (the commonly available form of serine) help multiple processes we need, including manufacture of glycine and cysteine in our bodies. However, L-serine conversion to D-serine is lacking in older people.
Since you can't easily find D-serine and L-serine appears to not work correctly, you can at least supplement glycine and cysteine, which are also required in our bodies and also are no longer being properly made in our bodies. Supplementing glycine and cysteine has already been proven to greatly benefit older people.
So you can get a good amount of benefit from what is commonly available, even if you can't find what is not commonly available.
God did give man wisdom, and part of the wisdom. Was the abi,ity to study and find cures and help for certain diseases to increase life expectancy, but God for sure knows the day and hour, we don’t, and must do all we can to try to prolong our stay here, lewv8ng the inevitable in God’s hands. God provided the wisdom and such for mankind to benefit from, and possibly live longer and healthier, from discoveries. The end comes at our appointed times, but our appointed times might be based on the fact that we ate healthier, and too, Care of ourselves
However, L-serine conversion to D-serine is lacking in older people.
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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.