The timing of moderate aerobic exercise around a meal can affect the glycemic effect of this activity when done by individuals with Type 2 diabetes. For instance, postprandial exercise of moderate intensity decreases glycemia after breakfast in Type 2 diabetic patients, but this effect does not persist during and after the following lunch meal. Moreover, 1 hour of aerobic exercise has a minimal impact on plasma glucose level when performed in fasted moderately hyperglycemic Type 2 diabetic men, but induces an important decrease in plasma glucose level when performed 2 hours after breakfast.
To our knowledge, the effects of undertaking physical activity before or after the evening meal in individuals with Type 2 diabetes have not been reported, but would have practical implications for the prescription of physical activity aimed at providing the greatest benefit to glycemic control at that time of day. Thus, the purpose of this study was to examine the differing effects of a single bout of pre- or postprandial exercise done at a moderate pace on the glycemic response to a standardized evening (dinner) meal in older individuals with Type 2 diabetes.
In prior studies of exercise done before or after breakfast and lunch, postprandial activity generally reduces glycemia more than pre-meal. This study sought to examine the effects of exercise before or after an evening meal.
The study examined the differing effects of a single bout of pre- or postprandial moderate exercise or no exercise on the glycemic response to an evening (dinner) meal in individuals with Type 2 diabetes. Twelve men and women subjects (mean age of 61.4±2.7 years) with Type 2 diabetes were treated with diet and/or oral medications.
Three trials were conducted on separate days consisting of a rest day when subjects consumed a standardized dinner with a moderate glycemic effect and 2 exercise days when they undertook 20 minutes of self-paced treadmill walking immediately before or 15 to 20 minutes after eating.
Blood samples were taken every 30 minutes over a 4-hour period and later assayed for plasma glucose; from these data both absolute and relative changes in glucose levels were determined, as well as the total glucose area under the curve (AUC) of the 4-hour testing period. Initial samples were additionally assayed for glycated hemoglobin and lipid levels.
Twenty minutes of self-paced walking done shortly after meal consumption resulted in lower plasma glucose levels at the end of exercise compared to values at the same time point when subjects had walked pre-dinner. Total glucose AUC over 4 hours was not significantly different among trials.
The current study examined the glycemic effects of 20 minutes of self-paced, mild to moderate walking done either immediately before or shortly after eating the same dinner. Walking after meal consumption resulted in lower plasma glucose levels at the end of exercise compared to values at the same time point when subjects had walked pre-dinner.
The blunting effect of postprandial exercise on blood glucose elevations has been well established. Moderate intensity exercise done 2 hours after breakfast decreases glucose levels more than during fasting conditions in Type 2 diabetic subjects, but the effect does not persist after lunch without additional exercise. For diabetic subjects in this study, none of whom were being treated with exogenous insulin, consumption of a meal with a moderate glycemic effect likely resulted in a greater release of endogenously released insulin that lowered their post-meal glycemic responses further with the addition of exercise. The binding of insulin to its cellular receptors in muscle and adipose tissues recruits GLUT4 transport proteins to the cell surface that facilitates glucose transport. Muscular contractions themselves are known to stimulate glucose transport into muscle cells without the need for insulin through an independent mechanism, but in an additive manner, thereby potentiating the effects of post-meal exercise.
The findings in our study also concur with Poirier and colleagues, who found that moderate cycle exercise after any meal (breakfast, lunch, or dinner) results in a significant decrease in glucose levels, again likely resulting from the natural release of insulin stimulated by food intake. Others have reported that both postprandial high-intensity exercise and longer bouts of walking (i.e., 2 hours versus 1) reduce glucose levels and insulin secretion, suggesting that the effect of exercise is related more to total energy expenditure rather than to peak exercise intensity. Thus, it is possible that the short duration of the exercise bout in this study (20 minutes) might have had a greater impact on absolute and relative glycemia and total glucose AUC if either its intensity or its duration had been increased.
Aerobic exercise releases glucose-raising hormones, such as epinephrine and norepinephrine, in response to exercise intensity, with large amounts generally only being elicited by intense activities (greater than 75% of maximal aerobic capacity). Given the mild or moderate pace chosen by the subjects in this study (40% of HRR), it is unlikely that excessive amounts of these catecholamines were released. Although differing amounts may have been released during exercise at opposing times around the evening meal, others have shown that postprandial exercise releases more of these hormones and increases fat use compared with preprandial, making it unlikely that varying catecholamine release can explain the lower blood glucose values following post-meal exercising observed in the current study.
Exercise timing and intensity aside, individuals with diabetes will likely experience improved glycemic control from simply engaging in regular training at any time of day, although the acute glycemic impact of a single bout of moderate activity may vary with the timing, as was demonstrated in the present study. When moderate aerobic exercise is undertaken regularly, this type of training has a more chronic effect in that it increases whole-body insulin-mediated glucose disposal in obese Type 2 diabetic patients, independently of alterations in the insulin-signaling cascade, likely as the result of a greater GLUT4 protein content. The subjects in this study varied in their self-reported regular exercise participation; however, exercise done outside the study was limited on test days and was not a factor in the current findings.
From the results of the study it was concluded that postprandial walking may be more effective at lowering the glycemic impact of the evening meal in individuals with Type 2 diabetes compared with pre-meal or no exercise and may be an effective means to blunt postprandial glycemic excursions.
And it appears that 20 minutes of self-paced mild to moderate intensity walking may be more beneficial for controlling postprandial glycemia in Type 2 diabetic individuals when undertaken shortly after an evening meal rather than immediately beforehand. Postprandial hyperglycemia is an established cardiovascular risk factor and oxidative damage resulting from such glycemic excursions is a factor in the development of diabetic complications that may be moderated by exercise. Accordingly, older diabetic individuals are advised to undertake aerobic exercise after meals, including the evening one, to blunt the glycemic response resulting from meal consumption and reduce the likelihood of negative health consequences associated with postprandial glucose excursions.
American Medical Directors Association Volume 10, Issue 6, Pages 394-397 (July 2009)
Sheri R. Colberg,