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Exercise & Glycemic Control

iStock-914575174An excellent review of the importance of exercise for people with type 2 diabetes was recently published in the American Journal of Medicine Open, and highlights not only the benefits of exercise but also points to the optimal type, dosing, and timing for maximum impact. As noted in the article, 10.4% of adults in the U.S. have type 2 diabetes, with over 1/3 of those affected unaware of the diagnosis. Additionally, nearly ½ of all U.S. adults have prediabetes, characterized by beta cell dysfunction and insulin resistance, which may progress to diabetes if insulin secretion can no longer keep up with the glucose burden. Thus, this review is applicable to many in the U.S. that fall somewhere on the spectrum of impaired glucose regulation.

It's well-established that dietary and lifestyle factors improve glycemic control and reduce the risk of progression to diabetes, as observed in the Diabetes Prevention Program trial and its 58% reduction in the risk of developing diabetes among those with prediabetes. It’s worth noting that the number of study participants needed to treat with metformin to prevent 1 case of diabetes over 3 years was approximately 14, versus only 7 with the lifestyle intervention. This intervention included 150 minutes per week of exercise and a targeted 7% reduction in body weight, with those losing the most weight lowering their risk by 90%. Yet this recent review suggests that how weight loss is achieved may be as important as how much weight is lost, and that exercise reduces the risk of progression to diabetes both via increasing weight loss and by mechanisms independent of weight loss (such as improved pancreatic function, endothelial health, etc.).

One example of this relates to the type of exercise and the type of weight loss. A focus only on total weight lost ignores an important distinction between the composition of the loss; losing muscle mass, for instance, is likely to be detrimental in most cases, while loss of fat (particularly visceral and hepatic fat) is associated with greater benefit, even if the total weight loss is marginal. Data from the STRRIDE/AT/RT trial (Studies Targeting Risk Reduction Interventions Through Defined Exercise-Aerobic Training and/or Resistance Training) suggest that aerobic exercise may offer an advantage in terms of visceral and hepatic fat reduction, as resistance training alone was not found to improve either parameter, though it did reduce subcutaneous abdominal fat. Aerobic exercise specifically has been shown to favorably shift the composition of hepatic fat (e.g., a greater production of polyunsaturated fatty acids vs. saturated fats), an effect associated with improved insulin sensitivity.

This recent review cites a number of studies that suggest overall exercise volume may be key, though there is a fair degree of uncertainty about the optimal approach. High-interval intensity training (HIT), for example, appears to provide the same benefits as moderate aerobic activity over a shorter duration, and is appealing to those with limited time. Among participants with diabetes, only 6 sessions of HIT over a 2-week period (a total of 75 minutes per week, characterized by short high-intensity bursts followed by short rest periods), was associated with significant reductions in 24-hour average blood glucose concentrations. Additionally, HIT increased markers of skeletal muscle mitochondrial capacity, known to be impaired among people with diabetes.

Research regarding the timing of exercise is also summarized in this review. While research focus on the timing of calorie intake suggests that earlier intake is associated with greater weight loss and improvements in insulin sensitivity (many circadian-based rhythms, including body temperature, circulating insulin, adiponectin secretion, etc., become more unfavorable in the afternoon and evening) less attention has been given to the optimal timing of exercise. This review highlights data suggesting (with some uncertainty) that because glucose control deteriorates in the afternoon and evening, these may be more ideal times to exercise, to mitigate the loss of glycemic control. Because diabetics tend to have a worsening in insulin sensitivity overnight (in contrast to non-diabetics), evening exercise may have even more benefit, at least in some individuals with diabetes. A recent review also suggests that postprandial exercise may be more beneficial than exercise in the fasting state, mitigating the glucose spikes following a meal as well as postprandial lipemia. This is consistent with a recent statement from the American College of Sports Medicine advocating for postprandial exercise to limit postprandial hyperglycemia. Certainly, the use of glucose monitoring during adjustments to exercise routines may help personalize therapy, as there is substantial interindividual variation.

The review also makes recommendations to break up sedentary time with activity, noting that each hour of sedentary time (above 8 hours per day) has been associated with a 1 to 22% increase in risk for diabetes and an 8% increased risk for cardiovascular mortality. Thus, even short and moderate interventions, such as walking, light resistance exercise, standing vs. sitting, etc., have been associated with improvements in 24-hour glucose control. Interestingly, while even a 6-minute break every hour improved glycemic control among diabetics, 3 minutes every 30 minutes was associated with better nocturnal glucose control among participants with medication-controlled diabetes. Recommendations around medications are less clear; while exercise still has benefits among people taking metformin, for example, metformin does limit these benefits. This suggests that among prediabetics, the full benefits of exercise will not be realized once metformin is started, potentially obscuring the possibility for diet/exercise to help reverse progression.

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