I tried Glycine for a few days some years ago, not at a very high dosage but not ridiculous (can't remember). It felt bad, mood worsened, heavy legs, more difficult breathing... I stopped.
I also tried Aspartic Acid for a few days and stopped. It didn't feel right, although I might try anew. Can't remember what was going on precisely.
Glycine actually works to buffer excess methyl groups. When you are deficient in folate, this buffer system does not know when to turn off, buffering all methyl groups (including the ones you need) and in turn making you under methylated. This would make you feel worse.
The above is exactly what happened to me when I first introduced glycine as a methyl buffer for SAM-e, but I did not know that I was deficient in folate at the time. It essentially inhibited my methylation stack from working at all. Full blown POIS symptoms began pouring in, even as I was taking 400mg of SAM-e twice a day. It wasn't until I increased my folate that I could tolerate glycine properly, where it would "know" when to turn off the methyl buffering.
I believe that POIS depletes methyl groups, which would in theory deplete folate levels, as folate is a co-factor nutrient for SAM-e, and a very important methyl donor for the methylation cycle in general.
Source: Info learnt from Chris Masterjohn who has a Phd in Nutritional Science. You can learn more about this buffer system
here, which is a reddit post outlining information directly from the Masterjohn video.
