This invention provides a method for measuring glucose metabolism ability of a subject and a composition that is suitably used in the method. The method for measuring glucose metabolism ability of the present invention is as follows: a method for measuring glucose metabolism ability of a subject, using a composition for measuring glucose metabolism ability, the composition comprising, as an active ingredient, glucose labeled with at least one isotope of C, wherein the glucose is converted in the body into labeled carbon dioxide that is excreted in expired air, the method comprising steps (a) and (b) below: (a) administering the composition to the subject and collecting expired air； and (b) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount or the ratio of labeled CO 2 amount to total CO 2 amount.
