One Bout of Exercise Alters Free-Living Postprandial Glycemia in Type 2 Diabetes

Douglas J. Oberlin; Catherine R. Mikus; Monica L. Kearney; Pamela S. Hinton; Camila Manrique; Heather J. Leidy; Jill A. Kanaley; R. Scott Rector; John P. Thyfault

Disclosures

Med Sci Sports Exerc. 2014;46(2):232-238. 

In This Article

Discussion

The current study determined if and for how many subsequent meals one bout of exercise improved PPG in individuals with T2D. Importantly, this was the first study that fed six controlled meals (same type and energy content of food at each meal) during a 2-d period, allowing us to ensure that EX-induced differences in PPG were not affected by type or quantity of food. We found that a single bout of exercise significantly lowered PPG-AUC responses to the second meal after the exercise bout by 15%, whereas a similar but nonsignificant effect was also shown at the first (15%) and third (12%) meal to also be lowered. These changes in PPG resulted in a significant 10% reduction in average blood glucose concentrations among the subjects during the first 24 h after the exercise session, similar to findings by other groups;[13,20] however, the exercise-induced effect on the average 24-h glucose concentration was no longer present on day 2. Because the effects of exercise were undetectable by the second day, these data would suggest that exercise should be performed every day to have an optimal effect on postprandial glucose responses at least at the initiation of an exercise program for individuals with T2D. Of note, a previous study from van Dijk et al.[40] did find that that one 60-min bout of exercise improved 24-h glucose values on the second day, but key differences between the studies are noted in the following paragraphs.

A recent study by van Dijk et al.[40] showed that a single exercise bout (30 min) each day was sufficient to lower the 2.5-h glucose concentration after breakfast in individuals with T2D. The same study also showed that one 60-min exercise bout lowered post 2.5-h breakfast glucose on both the day of the bout and the following morning. Thus, they showed that one exercise bout that was the same volume of ours did affect postprandial glucose values on the second day. Key differences exist between the studies, which may have affected the outcomes. First, as already mentioned, our study used controlled meals consisting of the same foods and energy content for each meal limiting the effect that macronutrient content or caloric loads could have altered glycemic responses. Differences in the glycemic control of subjects could have also affected the results. Our T2D patients had good glycemic control (HbA1c = 6.3%) compared with the subjects in the van Dijk et al.[40] study (HbA1c = 7.2%), suggesting that our subjects may have not had as much room for improvement. In addition, our subjects were primarily only taking metformin as a glucose-lowering medication, whereas the study from van Dijk et al.[40] used some individuals who were on metformin and some who were also receiving insulin therapy, which may have positively interacted with exercise responses and provided even greater improvements in glycemia. Finally, the timing of the exercise bout could have also affected the results. Our subjects performed an early morning exercise bout and then consumed meal 1 approximately 2 h later, whereas the van Dijk et al. study performed exercise after breakfast, leaving only two meals to be consumed during the postexercise period of day 1.

Although the current study demonstrates that a single exercise bout transiently improves PPG, it is not known whether prolonged exercise training would result in more persistent effects in type 2 diabetics after each exercise bout. In an acute training study, the effects observed are most likely either due to a change in energy homeostasis or directly due to skeletal muscle contractions, which result in a period of enhanced insulin sensitivity that has been documented to last for up to 48 h.[38] However, after prolonged exercise training, many other adaptations may contribute to witnessed improvements in glycemic control,[8] such as increased GLUT-4 content,[10] increased capillary density in skeletal muscle and improved vascular function,[29] and altered body composition.[8] Although these adaptations may allow a greater volume of glucose to be taken up by the working tissue, it is unknown whether they significantly change the duration of improvement in skeletal muscle insulin sensitivity and glucose uptake, resulting in prolonged reductions in PPG after an exercise bout. Furthermore, exercise training–induced adaptations that improve control of hepatic glucose output[18] and pancreatic β-cell function[36] could also play a role in potentiating the lasting effect of an exercise bout on glycemic control during subsequent postprandial periods. These concepts deserve further study.

In the current study, we attempted to control as many variables as possible without keeping subjects in a laboratory setting for the duration of the study. The number of steps per day and estimated caloric expenditure were both monitored throughout the exercise and SED conditions. The significant difference in steps per day and the estimated caloric expenditure between the first day of the exercised (including the exercise session) and the first day of the SED condition confirm that the subjects did not reduce steps or activity after the exercise bout to compensate for the exercise session in the morning. As previously stated, the diet provided in this study maintained the same food items at every meal, reducing the likelihood of differences in macronutrient content or glycemic load affecting the changes seen in glycemic control. This is a unique feature of this study, and we are not aware of a previous study utilizing this approach. Further, because the diet was provided and constant throughout the study, the observed changes that were seen in glycemic control were most likely due to the increased physical activity from the exercise session. However, a limitation of controlling the diet was that the subjects were not refed calories to compensate for the energy expended during the exercise bout. This confounds whether the effects seen in the current study were effects of muscle contraction, energy deficit, or a combination of both.[5,37]

In conclusion, the current study demonstrated that a single exercise bout is sufficient to reduce 24-h average blood glucose concentrations during the first day (day of the exercise bout) as well as PPG-AUC for a single meal (meal 2) in individuals with T2D. This study confirms other recent findings that have shown a single bout of endurance exercise or short-duration, high-intensity exercise reduces average blood glucose concentrations in individuals with T2D for 24 h and also reduces postprandial glucose concentrations.[13,40] Our findings suggest that for optimal blood glucose control, it is important for individuals with T2D to perform exercise on a daily basis, particularly at the onset of exercise training.

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