This invention provides a method for measuring insulin resistance of a test subject, and a composition suitably used for the method. A method for measuring insulin resistance of a test subject comprising steps (a) and (b) below, the method using a composition for measuring insulin resistance comprising, as an active ingredient, a C 12-38 fatty acid labeled with at least one isotope of C, wherein the C 12-38 fatty acid is converted in the body into labeled carbon dioxide that is excreted in expired air, (a) intravenously administering the composition to a test subject and collecting expired air； and (b) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in the expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in the expired air.
