Exercise training improves cardiovascular autonomic modulation in response to glucose ingestion in obese adults with and without type 2 diabetes mellitus
Introduction
Glucose ingestion is associated with alterations in cardiovascular autonomic regulation in lean, healthy subjects. Several authors have reported increased muscle sympathetic nervous system activity [1], [2] and plasma norepinephrine concentrations [3], and elevated ratios of low- to high-frequency components of heart rate variability (HRV) after oral glucose intake [4]. This normal response maintains arterial blood pressure, compensating for splachnic vasodilation with peripheral vasoconstriction [5]. It has been suggested that the increased activity of the sympathetic nervous system after ingestion of a glucose load is insulin mediated [4].
Previous studies suggest that cardiovascular autonomic regulation is impaired in obesity, an insulin-resistant state [4]. At rest, obesity is often characterized by cardiac sympathetic predominance [6] and high rates of muscle sympathetic firing [1]. In response to a glucose challenge, obese subjects fail to increase muscle and whole-body sympathetic activity [1], [3]. The impaired autonomic response to glucose in obesity has been previously attributed to an attenuated baroreflex response associated with chronic hyperinsulinemia and insulin resistance [4], [7].
The impaired cardiovascular autonomic regulation seen in obese and insulin-resistant individuals is consistent with an increased risk of mortality and adverse cardiac events [8]. Given this cardiovascular risk, interventions to improve cardiovascular regulation in this population would be clinically significant. Exercise training lowers sympathetic nerve traffic [9], increases baroreflex sensitivity (BRS), and improves glycemic control and insulin sensitivity [8]. It has been suggested that exercise training may improve autonomic responses to glucose ingestion by decreasing insulin resistance [3]. Weight reduction programs via caloric restriction have been shown to improve sympathetic responses to glucose intake in obese subjects [10]. However, the effects of exercise training on cardiovascular autonomic modulation in response to an oral glucose intake have not been examined in obese subjects with and without type 2 diabetes mellitus (T2D). Therefore, the main purpose of this study was to investigate the effect of aerobic exercise training on cardiac autonomic modulation (HRV), BRS, and sympathetic vasomotor modulation (systolic blood pressure variability [BPV]) after oral glucose ingestion in asymptomatic obese subjects with and without T2D. The effects of this exercise training on body composition, central adiposity, glycemic control, insulin sensitivity, fitness level, and lipid profile were also examined. We hypothesized that a 16-week exercise training program would improve autonomic responses to an oral glucose load in obese adults with and without T2D.
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Subjects
Sixty-two obese men and women (age, 40-60 years) were recruited from the local community. Fifty-nine subjects completed all aspects of testing and training. Exclusion criteria included the following: smoking, participation in regular physical activity programs during the past 6 months, irregular menstrual cycles, peripheral neuropathy, overt cardiovascular disease, electrocardiogram (ECG) abnormalities, hormonal contraceptives, uncontrolled hypertension, and β-blocker therapy. Self-reports
Results
Anthropometric, body composition, and fitness characteristics of the 2 experimental groups are shown in Table 3. At baseline, there were no significant differences in weight, BMI, percentage body fat, and fitness level between the 2 groups; but the obese subjects with T2D had greater waist circumference than the OB subjects (mean difference, OB vs T2D ± SE: 10.5 ± 3.2 cm; P = .002). The exercise intervention reduced body weight (mean difference, pre- vs posttraining: 0.82 ± 0.32 kg; P = .015),
Discussion
The main findings of this study showed that a 16-week, home-based, supervised exercise training program resulted in increased sympathetic modulation and reduced vagal modulation in response to an oral glucose load in obese adults. These alterations were seen in both obese groups, independently of diabetes status, and in the absence of remarkable changes in body weight, body composition, fitness level, and glycemic control. Furthermore, the subjects with T2D showed changes in both cardiac and
Acknowledgment
The authors thank the subjects for their tremendous amount of time and effort and acknowledge Rose Kingsbury, RN, NP, for her dedication to the study and placing all of the catheters.
Supported by a National Institutes of Health grant R21 DK063179.
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