The Real Thing Ester-C is a 100% natural, esterified form of vitamin C making it more potent and bioavailable. Ester-C uplifts the immune system and also has anti-ageing effects because it's an antioxidant that protects the body against the free-radical damage that leads to premature ageing. It functions like vitamin C biologically, but the Ester-C version is absorbed around 400 times quicker and won't irritate the stomach.