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 (oxidized ascorbic acid), calcium threonate, and trace levels of xylonate and lyxonate. The manufacturer of this ingredient states that the metabolites, especially threonate, increase the bioavailability of the vitamin C in the product, and that they performed a study in humans demonstrating the increased bioavailability of vitamin C in Ester-C. 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 (Source: Medical Science Monitor, October 2007, pages 205–210). A small published study of vitamin C bioavailability in eight women and one man found no difference between Ester-C and commercially available ascorbic acid tablets with respect to the absorption and excretion of vitamin C (Source: The Bioavailability of Different Forms of Vitamin C, The Linus Pauling Institute, Oregon State University, www.orst.edu/dept/lpi/ss01/bioavailability.html). There also are studies that show no difference when comparing the effects of Ester-C with those of ascorbic acid (Source: Biochemical Pharmacology, June 1996, pages 1719–1725).