As constituents of polyuronides, uronic acids have a frequent and wide occurrence in nature. Much of the carbohydrate material in plants, which includes all pectic materials and plant gums, many plant mucilages, hemicelluloses and gel-forming substances and some microbial polysaccharides, belong to the group. Uronic acid residues have been shown to exist in the animal body where they may be linked wither to complex polysaccharides or to proteins. The uronic acids are reducing sugar acids formed by the oxidation of the terminal carbinol group (C atom 6) of the sugar, a process which apparently occurs readily in both plants and animals. Although a large number of uronic acids are theoretically possible, only three have been found to occur naturally, these being d-galacturonic, d-glucuronic and d-mannuronic acids.