Interesting article. Together with Trimethlglycine-intake I already supplement 5.6g glycine per day, plus 2.8g from diet.
Can't agree with everything. Especially this had me really stunned:
Glutathione is synthesised from the amino acids glutamate, cysteine, and glycine, but studies have shown that the rate of synthesis is primarily determined by levels of glycine in the tissue. If there is insufficient glycine available the glutathione precursor molecules are excreted in the urine. Vegetarians excrete 80% more of these precursors than their omnivore counterparts indicating a more limited ability to complete the synthesis process. [McCarthy et al, 2018]
Usually cysteine is considered the rate-limiting component. Indeed if reading the linked actual study it says:
Hence, joint supplementation with NAC and glycine could be expected to collaborate in stimulating glutathione production. In the previously cited clinical study,9 the elderly subjects received 100 mg/kg of both NAC and glycine for 14 days, and their red blood cell total glutathione levels (reduced glutathione plus diglutathione) rose from 1.26 mmol/L at baseline to 2.23 mmol/L, a concentration slightly higher than that measured in young subjects who did not take supplements.9
100 mg/kg of NAC and glycine amounts to 7g of each. While I already do get more than that glycine for almost 13 years, that much NAC has never been tested long term. Usual long-term doses go up to 1.8g. There is the well-known theoretical concern from rodent studies of pulmonary hypertension at about double that (not yet replicated in humans).
Additonal NAC also has bio-film disrupting properties. About which I'm not concerned about much at the usual doses, additionally the mucous membranes of the intestines are one of the fastest regenerating cells in the human body. But constant repeatedly that high doses? - I wouldn't be sure about its harmlessness and its impact on the microbiome again.
Since I only got about 0.8g of cysteine from my diet per day. NAC above 1.8 g really not long-term safe, its clearly still cysteine which is the foremost rate-limiting component in glutathione synthesis. Long before the bottleneck due to glycine appears.
In this case I would rather add up more supplemental cysteine and cystine (the form usually found in diet) before increasing NAC above its known safe upper long-term intake.