Exercise Training Effects on Insulin Sensitivity

The metabolic syndrome is conceptually the same as the insulin-resistance syndrome, and the names are essentially interchangeable. That some prefer the term insulin-resistance syndrome is due to the common understanding that an observable decrease in insulin sensitivity is the first detectable aberration in course toward metabolic syndrome. In fact, some deterioration in insulin sensitivity is generally observed prior to elevations in triglycerides levels, decreases in HDL-C concentrations, which in turn precede deterioration in fasting glucose, and glucose-tolerance measures, and perhaps even before clinically significant increases in body weight, body-mass index, and waist circumference are apparent. In fact, as a reflection of this understanding, the WHO includes a resting measure of insulin resistance in its definition of metabolic syndrome (Table 4.2).

The relationship between exercise and insulin resistance is clear. In fact, one of the most consistent, beneficial effects of exercise is a statistically and physiologically significant improvement in insulin action. This beneficial effect can be observed in a broad range of exercise conditions and models: after only a few bouts of acute exercise,6067 or with longer term exercise training68-71; with low-intensity or high-intensity exercise (Figure 4.1 from Houmard et al., 200467,72); and in exercising animals71 and humans.68-70 In a classic study, Seals et al.61 observed that even young, lean, but sedentary individuals have nearly twice the insulin response to an

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FIGURE 4.2 Relative changes (%) (after training/before training) in insulin-sensitivity index derived from the intravenous glucosetolerance test (Sj) in the control and exercise [low-volume/moderate-intensity (Low/Mod); low-volume/high-intensity (Low/High); high-volume/high-intensity (High/High)] groups. Line at 0.0 represents no change. *Significant difference from control group, P < 0.05; Alpha symbol o significant difference from the Low/High-group, P < 0.05. Reprinted with permission from J. Appl. Physiol., 96:101-106, 2004.

oral-glucose administration as young, lean, endurance athletes. Aging had no influence on this observation, as the results were the same when the young, lean, sedentary individuals were compared to older (mean age 60 years), lean, endurance athletes, demonstrating an improved insulin action in endurance athletes when compared with sedentary controls. In a study from our group,72 the data revealed that of the two groups performing the same amount of exercise (calorically equivalent to walking or jogging ~ 10 miles per week), the lower exercise-intensity group had a greater improvement in insulin sensitivity (Figure 4.2). This may specifically be due to a beneficial effect of low-intensity exercise, which is known to oxidize more fat than the same amount of more vigorous exercise. Alternatively, it may be due to the higher exercise frequency and total time of weekly exercise required by the low amount/moderate-intensity group to expend the same amount of calories through exercise. Whether this same observation holds for individuals with frank diabetes remains an open question and a potential area for future investigation.

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