We age and die as a result of our body accumulating oxidative damage. For example, smoking creates oxygen radicals that damage lung cells. This damage reduces the function of these cells and when enough cells have been damaged or die, lung tissue fails to function and we die from lung failure.
The best way to live your healthiest life is to reduce future oxidative damage as much as possible. The key to doing this is to restore and maintain high glutathione levels. Glutathione is the main antioxidant in your body that quenches the radicals that damage cells.
Healthy glutathione levels have been associated with longevity. Studies have shown that centenarians have similar glutathione levels to that of young people. In contrast, people who die at a younger age almost always have a low level of glutathione, which almost certainly contributes to the degenerate condition.
Can glutathione levels be restored? Evidence suggests they can. Studies on middle age people showed that taking supplements restored their glutathione levels to that of their younger counterparts in 24 weeks. Studies on mice taking similar supplements found those who received supplements lived 24% longer than those who didn't. Furthermore, biochemical markers of good health improved with supplementation suggesting restoring these biochemical markers is probably a useful predictor of lifespan.
Does this mean humans taking supplements that restore glutathione levels will live longer lives? As humans aren't mice, we can't extrapolate this data to human longevity, but the results so far are promising.
What's the best way to restore glutathione levels? Glutathione is made up of cysteine, glycine and glutamic acid. In these studies, NAC and glycine are the two amino acids that improved health and longevity in mice and improved health biochemical markers in people (roughly 7 g per day in people). Note that NAC and glycine levels were low in these middle aged people whereas glutamic acid levels were normal, and that NAC is converted into cysteine before becoming a component of glutathione.
Can Vitamin U be substituted for NAC? Quite possibly, although this has not been tested directly. Vitamin U is an amino acid (S-methylmethionine) that is produced naturally by all flowering plants and is found in all produce, especially stalky and cruciferous vegetables. Vitamin U is integrated into the human body by specific enzymes in the liver and kidney. One advantage of taking Vitamin U over NAC is that these enzymes in the liver and kidney specifically regulate the integration of Vitamin U, converting it into cysteine-->glutathione when needed without any of the potential side effects of NAC dosing.
Note that while restoring glutathione levels will minimize further damage from oxidative stress, it won't directly reverse existing damage. However, restoring healthy glutathione levels will allow your body's natural repair systems to heal your body the best they can.