Ester-C Best Antioxidant Anti-Ageing Ester-C opis Trade name for a combination form of vitamin C that contains mainly calcium ascorbate, but in addition contains small amounts of the vitamin C metabolites dehydroascorbic acid (oxidised ascorbic acid), calcium threonate, and trace levels of xylonate and lyxonate. The manufacturer of this ingredient states that the metabolites, especially threonate, increase the effectiveness of the vitamin C in the product. However, this study has not been published in a peer-reviewed journal. There is a small, in vitro study that supports the notion that Ester-C is more potent than ascorbic acid by itself.