A method of measuring a subject's insulin resistance, and a composition optimally used in said method are provided. This composition for measuring insulin resistance is converted in the body to a labeled carbon dioxide gas and discharged in the exhale breaths, and has as the active component a fatty acid of 12-38 carbon atoms formed by labeling with at least one carbon isotope； in this method of measuring a subject's insulin resistance, said composition for measuring a subject's insulin resistance is used and steps (a) and (b) are carried out: (a) a step for dosing the subject with the aforementioned composition and collecting the exhaled breath, and (b) a step for calculating the ratio of the amount of the labeled CO2 to the amount of the unlabeled CO2 or total amount of CO2 contained in the exhaled breath.
