An oxidized derivative of a monosaccharide. There are two biologically important types of sugar acid, the aldonic acids and the *uronic acids
. In aldonic acids the aldehyde group of a monosaccharide is oxidized to a carboxylic acid. The aldonic acid of glucose, gluconic acid, is an intermediate in the pentose phosphate pathway. Ascorbic acid (vitamin C) is also an aldonic acid.