K. Sugimoto et al. Correlation between the serum fructose concentrations in the portal and cardiac samples during the sucrose-tol- erance test (Study 1). Individual fructose concentrations in the portal and cardiac samples in both the control and eucalyptus leaf extract (ELE) rats (open circles). The co- efficient of correlation was 0.825 (n = 31). Fig. 3. 208 Fig. 2. peaked at 30 min, and then decreased. These changes were consistent with the changes in the SFC in the samples derived from cardiac blood, except that the SFC in the cardiac samples at 30 min was considerably lower than that in the portal samples. In the ELE group, the increase in the SFC in the portal samples was significantly lower than that in the control group (p<0.05), and the inhibition ratio at 30 min was 53%. The SFC in the cardiac samples was also lower than that in case of the control group, but not signifi- cantly so. The correlation between the SFCs in the portal and cardiac samples is shown in Fig. 2. The coefficient of correlation between these variables (n = 31) was 0.825. The changes in the serum glucose concentrations in both portal and cardiac samples during the sucrose-tolerance test were similar in the 2 groups (Fig. 1C and D). Effect of ELE on changes in the postprandial fructose concentrations in the serum samples derived from peri- pheral blood (Study 2) The time course of the peripheral SFCs during the sucrose-tolerance test is shown in Fig. 3. The fasting SFCs in the 2 groups almost equaled the SFC in the cardiac samples in Study 1 (approximately 20 μmol/L). After the sucrose load, the peripheral SFC in the control group progressively increased and reached 146 ± 29 μmol/L at 60 min. Although the peripheral SFCs in the ELE group also increased, the increment was significantly lower, and the inhibition ratios were 90% and 65% at 30 and 60 min, respectively (p<0.05). Eucalyptus leaf extract (ELE) significantly suppresses the postprandial elevation of the peripheral serum fructose concentration after the sucrose-tolerance test in rats (Study 2). The mean (SD) peripheral serum fructose concentrations in the control (open circles) and ELE (closed circles) groups are shown. Fasting rats (n = 10) were orally administered water (control group) or ELE at a dose of 1 g/kg body weight (ELE group) prior to the ingestion of sucrose (2 g/kg body weight). The fructose concentrations in the peripheral serum sampled from the tail were determined. *p<0.05 compared to the control group. Discussion The results of Studies 1 and 2 show that during the sucrose-tolerance test, ELE suppressed the increase in postprandial SFCs in not only portal samples but also cardiac samples. Changes in the fructose concentration in the peripheral blood after oral fructose administration have been examined in rats by using an enzymatic method [16] and in human subjects by using a colorimetric method [17–21]. In the case of these methods, other reducing substances in the blood complicate the interpretation of the fructose analysis; for example, high glucose content interferes with the measure- ment of fructose concentrations. Thus, these substances reduce the sensitivity and accuracy of these methods. In the case of the enzymatic method used by Sugimoto et al. [13], the calibration curve was deflected at low fructose concen- trations because of coexisting reducing substances (data not shown). It is necessary to measure blood fructose concentra- tions by using highly sensitive measurement methods because the fructose concentration in peripheral blood is generally low [18, 19]. We developed a highly sensitive method for the measurement of SFCs: complete exclusion of blood glucose by using a normal-phase high-performance liquid chromato- graphy (HPLC) column followed by GC/MS, with 13C6- J. Clin. Biochem. Nutr.