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Chapter 5 Aerobic Exercise Rx for Type 2 Diabetes
Although physical activity is a key element in type 2 diabetes (T2D) management, many people with this chronic disease do not become or remain regularly active (Morrato 2007). Many high-quality studies done to date have established that participation in regular physical activity improves blood glucose control, blood lipids, blood pressure, cardiovascular risk, mortality risk, and quality of life. Acute and chronic improvements in insulin action with aerobic exercise training are primarily responsible for the enhanced glycemic control (King 1995; Boulé 2001, 2005; O’Gorman 2006).
The treatment goal for individuals with T2D is to achieve and maintain optimal blood glucose, lipid, and blood pressure levels to prevent or delay chronic complications of diabetes (American Diabetes Association 2013). Many people with T2D can achieve blood glucose control using the combination of a nutritious meal plan, regular exercise participation, modest weight loss, and medication use (U.S. Department of Health and Human Services 2011). Lifestyle changes that include dietary improvements and regular physical activity are central to diabetes management. When medications are used to control T2D, they should augment lifestyle improvements, not replace them.
Case in Point: Aerobic Exercise Rx for a Typical Older Adult with T2D
DG is a 58-year-old man who has had T2D for at least a decade. Although he golfs for 2–3 h most weekends and does some yard work on occasion, it has been at least 5 years since he has engaged in any structured physical activity, even just regular walking. His motivation to start doing more physical activity comes from the fact that his A1C levels have started creeping up over time, along with his body weight. His doctor put him on exenatide within the past year (in addition to metformin, which he has been on since being diagnosed), but it has only helped him to lose ~10 lb since he started using it. He admits that his job is stressful and that he often puts in long hours that are mostly sedentary in nature (due to lots of meetings and document preparation).
Resting Measurements
Height: 70 inches
Weight: 245 lb
BMI: 35.1 (obese)
Heart rate: 77 beats per minute (bpm)
Blood pressure: 135/85 mmHg (on medication)
Fasting Labs
Plasma glucose: 108 mg/dl (controlled with metformin and exenatide)
A1C: 6.9%
Total cholesterol: 155 mg/dl (on medication)
Triglycerides: 85 mg/dl
High-density lipoprotein cholesterol: 52 mg/dl
Low-density lipoprotein cholesterol: 86 mg/dl
Questions to Consider
1. What type of aerobic exercise should DG consider doing to help lower his blood glucose and his body weight?
2. What exercise frequency, intensity, and duration should DG focus on?
3. How should his exercise training progress over time?
4. Are any precautions needed for DG when he exercises?
(Continued)
ACUTE AND CHRONIC METABOLIC AND PSYCHOLOGICAL EFFECTS OF PHYSICAL ACTIVITY
Engaging in physical activity facilitates glucose uptake, improves insulin sensitivity, and aids in glucose homeostasis, with effects that lower blood glucose levels for 2–72 h after the last bout of activity, depending on exercise duration, intensity, and subsequent food intake (King 1995; Boulé 2001, 2005; O’Gorman 2006). Exercised skeletal muscles continue to take up more blood glucose during the ensuing rest period, with the contraction-mediated pathway persisting for several hours (Ivy 1981, Garetto 1984) and insulin-mediated uptake for longer (Richter 1982, Cartee 1989, King 1995, Bajpeyi 2009). Given that these effects are short in duration, remaining physically active is an essential component of diabetes self-management behavior for all individuals with T2D.
Acute Effects of an Exercise Session
Low to moderate physical activity. In individuals with T2D exercising moderately, muscular uptake of blood glucose usually rises more than hepatic glucose production, and blood glucose levels decline over the course of the activity (Minuk 1981). At the same time, plasma insulin levels fall, making the risk of exercise-induced hypoglycemia low, as long as someone is not taking insulin or insulin secretagogues (Koivisto 1984). Muscular contractions increase blood glucose uptake to supplement muscular glycogen use (Ploug 1984, Richter 1985). As this uptake pathway is contraction induced and distinct from the one triggered by the binding of insulin to a cell surface receptor (Khayat 2002), glucose uptake into working muscle is normal even when insulin-mediated uptake is impaired (as it usually is in T2D) (Colberg 1996, Zierath 1996, Braun 2004). The glucose-lowering effects of acute moderate aerobic exercise are similar whether the physical activity is performed in a single session or multiple bouts with the same total duration (Baynard 2005). Glucose production also shifts from hepatic glycogenolysis to enhanced gluconeogenesis as exercise duration increases (Suh 2007, Wahren 2007).
Blood glucose reductions during any physical activity are related to the duration and intensity of the exercise, pre-exercise control, and state of physical training (Colberg 1996, Boulé 2001, Boulé 2005, Sigal 2007). Although prior physical activity of any intensity generally enhances uptake of circulating glucose for glycogen synthesis (Christ-Roberts 2003, Galbo 2007) and stimulates fat