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 C12-38 fatty acid labeled with at least one isotope of C, wherein the C12-38 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 CO2 amount to unlabeled CO2 amount contained in the expired air or the ratio of labeled CO2 amount to total CO2 amount contained in the expired air.
