A composition for use in a method for measuring a sugar/fatty acid combustion ratio in a test subject, the composition comprising, as an active ingredient, a C 12-38 fatty acid or a salt thereof labeled with at least one isotope of C, the method comprising determination of an area under a Δ-labeled C(‰) -expired air collection time t curve obtained by a glucose metabolism ability measurement method having steps (i) and (ii) below (hereinafter referred to as "AUC t [labeled C-glucose]"), determination of an area under a Δ-labeled C(‰) -expired air collection time t curve obtained by the method having steps (a) and (b) below (hereinafter referred to as "AUC t [labeled C-fatty acid]"), and using a value (AUC t [labeled C-glucose]/AUC t [labeled C-fatty acid]) obtained by dividing the AUC t [labeled C-glucose] by the AUC t [labeled C-fatty acid] as an index for the sugar/fatty acid combustion ratio: Method for determining AUC t [labeled C-glucose]: (i) intravenously administering a composition to a test subject and collecting expired air, the composition comprising, as an active ingredient, glucose labeled with at least one isotope of C, so that the glucose is converted in the body into labeled carbon dioxide that is excreted in expired air; and (ii) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in expired air; Method for determining AUC t [labeled C-fatty acid]: (a) intravenously administering the composition comprising the C 12-38 fatty acid or a salt thereof labeled with at least one isotope of C to a test subject and collecting expired air, so that the C 12-38 fatty acid or a salt thereof is converted in the body into labeled carbon dioxide that is excreted in expired air; and (b) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in expired air. Method for determining AUC t [labeled C-glucose]: (i) intravenously administering a composition to a test subject and collecting expired air, the composition comprising, as an active ingredient, glucose labeled with at least one isotope of C, so that the glucose is converted in the body into labeled carbon dioxide that is excreted in expired air; and (ii) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in expired air; (i) intravenously administering a composition to a test subject and collecting expired air, the composition comprising, as an active ingredient, glucose labeled with at least one isotope of C, so that the glucose is converted in the body into labeled carbon dioxide that is excreted in expired air; and (ii) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in expired air; Method for determining AUC t [labeled C-fatty acid]: (a) intravenously administering the composition comprising the C 12-38 fatty acid or a salt thereof labeled with at least one isotope of C to a test subject and collecting expired air, so that the C 12-38 fatty acid or a salt thereof is converted in the body into labeled carbon dioxide that is excreted in expired air; and (b) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in expired air. (a) intravenously administering the composition comprising the C 12-38 fatty acid or a salt thereof labeled with at least one isotope of C to a test subject and collecting expired air, so that the C 12-38 fatty acid or a salt thereof is converted in the body into labeled carbon dioxide that is excreted in expired air; and (b) determining the ratio of labeled CO 2 amount to unlabeled CO 2 amount contained in expired air or the ratio of labeled CO 2 amount to total CO 2 amount contained in expired air.