It's your body's master antioxidant. Whereas an antioxidant like Vitamin C will donate an electron to neutralize a free radical and it becomes a free radical itself, glutathione neutralizes free radicals and recharges antioxidants that have become free radicals. High glutathione levels are a good thing to have as it will help repair oxidative damage and potentially protects against cancer. Cruciferous veggies, fish oil, blueberries, and turmeric all increase glutathione levels as does undenatured, unpasteurized whey protein. Cysteine is a precursor to glutathione synthesis but it must be double bonded cysteine. This bond is broken by heating which is why pasteurized or heat processed whey don't provide all of the benefits. I would assume raw milk is the same because it contains whey that isn't heat processed. The supplement NAC is probably the best supplemental way to get glutathione as glutathione is destroyed by acid in the stomach.