Altered Cardiac Autonomic Function May Precede Insulin Resistance in Metabolic Syndrome

Background: Previous studies on the change of cardiac autonomic function and insulin resistance in metabolic syndrome recruited subjects with cardiovascular-related disease and defined metabolic abnormality with a more severe cutoff. We explored the alteration of cardiac autonomic function and insulin resistance in predisease community dwellers with different numbers of metabolic abnormalities. Methods: A total of 1298 subjects were classified as none (n = 539), one (n = 378), 2 (n = 218), and 3 or more metabolic abnormalities (n = 154). Insulin resistance was calculated by homeostatic model assessment. Cardiac autonomic function included 5-minute standard deviation of R-R interval, low- and high-frequency power spectrum, and low-/high-frequency power spectrum ratio, the ratio of the longest R-R interval around the 30th beat and the shortest R-R interval around the 15th beat after standing, and the ratio of the longest expiratory R-R interval to the shortest inspiratory R-R interval during deep breathing. Results: Subjects with a single metabolic abnormality or more had a lower standard deviation of R-R interval and expiratory/inspiratory ratio than subjects without metabolic abnormality in multivariate analysis. Subjects with 3 or more metabolic abnormalities had a higher low-/high-frequency power spectrum ratio, but a lower high-frequency power. Insulin resistance was higher in groups with 2 metabolic abnormalities or more, but not in the group with one metabolic abnormality, than those without metabolic abnormality. Conclusions: Cardiac autonomic function altered in predisease subjects with one or more metabolic abnormalities, while insulin resistance existed in subjects with 2 or more metabolic abnormalities. Thus, autonomic function change may precede insulin resistance in the initiation of metabolic syndrome.