As mentioned, it is able to neutralize the reactive oxygen species that damage cells
Sulfur-Rich Foods: Garlic, onions, and cruciferous vegetables provide the sulfur molecules necessary for your body to synthesize its own glutathione
This process seems to not only increase intercellular levels of Glutathione and precursors at the time of ingestion, but also builds up stores of these substances within the cells which lasts for a substantial post-ingestion time interval." Fidelus RK, Tsan MF
doi: 10.1111/bph.13336
For some people, the change is pretty subtle, maybe just an even and glowing look
Hum Brain Mapp 41:194217 Mandal PK, Shukla D, Govind V, Boulard Y, Ersland L (2017) Glutathione conformations and its implications for in vivo magnetic resonance spectroscopy