The Effect of Intensity of Aerobic Training on Insulin Sensitivity

Epidemiological studies have demonstrated that the prevalence of type 2 diabetes is increasing. It is estimated that I 0. 7% of adults over the age of 20 years and 23.1 % of adults over the age of 60 years have diabetes (NIH, 2007). Individuals with type 2 diabetes are 2-4 times more likely to die from heart disease than those without diabetes and twice as likely to die of all-cause mortality (CDC, 2005). Additionally Mokdad et al. (2000) report that the prevalence of diabetes increased 33% between 1990 and 1998. Furthermore it has been estimated that nearly 26% of the adult U.S. population has impaired glucose tolerance (NIH, 2007). While there exists a strong correlation between moderate intensity cardiovascular exercise and increased insulin sensitivity (Mayer-Davis et al., 1998; Henriksen, 2002; Al-Mahmood et al., 2007; Hasbum et al., 2006; Potteiger et al., 2003; Oshida et al., 1989), the relationship between higher exercise intensities and increased insulin sensitivity are unclear. In order to evaluate the effect of various levels of cardiovascular intensity on insulin sensitivity, college-aged males and females were recruited for this study. Forty-five healthy subjects were matched for age, gender and VO2max, and randomly assigned to a moderate-intensity, vigorous-intensity, maximal-intensity or a non-exercising control group. The weekly duration of training varied according to level of intensity in order to ensure equivalent energy expenditure across all groups. Subjects completed a six-week training protocol on a stationary bicycle ergometer. Significant increases occurred following training in VO2max in both the vigorous (15.4%) and maximal (14.2%) intensity groups. However, there were no significant changes in insulin sensitivity in any of the exercising groups or the non-exercising control group. Although there was no effect of training on insulin sensitivity, a slight trend for an effect between VO2max and insulin sensitivity was observed across all subjects in post-test data (p = 0.17) and for change of VO2max versus change in insulin sensitivity (p = 0.21 ). The lack of a significant demonstrable effect of training upon insulin sensitivity is likely attributable to the limitations of the assessment of insulin sensitivity (HOMA2) and/or the baseline level of fitness within the study population.