It is with liposomal supplementation. This means that it has a surrounding lipid so microscopic balls containing the difficult to absorb substance (especially any thiol-containing molecular compounds) like Glutathione, can make it past without other peptides inhibiting its absorption.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6389332/

Thiols science: This research is informative for the topic of the role thiol compounds plays in the antioxidant system, of which, Glutathione is scientifically considered the body's master antioxidant, it recycling (think of "guarding") SOD (Super Oxide Dismutase) and Glutathione itself being recycled by Vitamin C. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7041647/

I'm forming a hypothesis tonight, as a "citizen scientist", that the very reason why thiols are hard to absorb is because thiols are often helping to get oxidized lipids and oxidizing chemicals OUT of the body in the two ways that can physically happen. It seems interesting to me.

NAC raises glutathione in serum and is extremely available.