Diabetes and Exercise

Anatomy and Physiology

Acute Effects of Exercise

Physical activity increases blood glucose uptake in contracting muscles. Blood glucose levels are regulated through hepatic glucose production via gluconeogenesis and glycogenolysis, along with the utilization of free fatty acids and, rarely, amino acids. Exercise intensity and duration are the primary factors influencing glucose metabolism. Initially, glycogen, a multibranched polysaccharide of glucose, serves as the primary energy source for working muscles. As exercise continues and glycogen stores are depleted, muscles increasingly absorb circulating blood glucose and free fatty acids from adipose tissue. At this stage, hepatic glucose production shifts from glycogenolysis to gluconeogenesis.[6]

Glucose uptake in muscles occurs through 2 distinct pathways. During rest and the postprandial state, glucose uptake relies on an insulin-dependent pathway. In contrast, exercise activates a non-insulin-dependent pathway, facilitating glucose transport into contracting muscles to supplement intramuscular glycogenolysis.

Glucose transporter (GLUT) proteins facilitate glucose uptake, with GLUT4 serving as the primary insulin-dependent transporter responsible for insulin-regulated glucose absorption in muscle and fat cells. However, in type 2 diabetes mellitus, insulin-mediated glucose uptake via GLUT4 is impaired. Both aerobic and resistance exercises increase the number of GLUT4 proteins, enhancing blood glucose uptake despite insulin resistance.[7][8]

In individuals with type 2 diabetes mellitus who engage in moderate-intensity exercise, muscle glucose uptake surpasses hepatic glucose production via glycogenolysis, reducing blood glucose levels overall. However, plasma insulin levels also decline during exercise, significantly lowering the risk of exercise-induced hypoglycemia unless exogenous insulin or an insulin analog is used.[9]

Long-Term Effects of Exercise

The primary long-term benefits of exercise are improved insulin sensitivity and better blood glucose regulation. Aerobic exercise remains a cornerstone of managing type 2 diabetes mellitus, with studies showing that even a week of aerobic training can enhance both blood glucose levels and insulin sensitivity. Sustained moderate-intensity exercise increases skeletal muscle responsiveness to insulin by upregulating GLUT4 transporters.

Long-term training also enhances fat oxidation due to improved insulin sensitivity. While exercise in individuals with diabetes mellitus is typically associated with greater carbohydrate metabolism and reduced fatty acid oxidation, prolonged training shifts this balance. Aerobic exercise promotes lipid oxidation, conserving muscle glycogen and preventing acute drops in blood glucose levels.

Resistance training improves blood glucose regulation and insulin sensitivity in individuals with type 2 diabetes mellitus by increasing skeletal muscle mass. A 16-week randomized controlled trial in which participants performed resistance training twice weekly demonstrated a 7.1% reduction in fasting blood glucose, a 46.3% increase in insulin action, and significant visceral fat loss compared to baseline. Improved blood glucose control results from the muscle mass gains associated with long-term resistance training.[10][11]

Physical Activity and Prevention of Diabetes Mellitus

Regular exercise helps prevent the onset of type 2 diabetes mellitus. Several prospective and cross-sectional studies indicate that higher levels of physical activity, regardless of the specific type, are linked to a reduced risk of developing the disease. Both moderate physical activity, defined as at least 2.5 hours per week, and vigorous physical activity have been associated with a lower risk of type 2 diabetes mellitus in high-risk individuals.[12][13]

Indications

Exercise regimen recommendations come from the American Heart Association (AHA), the American Diabetes Association (ADA), and the American College of Sports Medicine (ACSM). Patients with type 2 diabetes mellitus are generally encouraged to engage in 30 to 60 minutes of moderate-intensity aerobic activity daily. Resistance training at least twice per week is also recommended. However, individuals with moderate-to-severe proliferative retinopathy should avoid resistance training. For physically fit patients, a shorter duration of more vigorous aerobic exercise is an alternative option.

Aerobic Exercise

Extensive literature highlights the positive effects of aerobic exercise on glycemic control. Aerobic exercise involves continuous rhythmic movements engaging large muscle groups, such as jogging, cycling, and Zumba. The most recent ADA guidelines recommend at least 30 minutes of aerobic activity 3 to 7 days per week or 150 minutes weekly. Beginners in moderate-intensity exercise can start with 10 minutes of stretching and warm-up, followed by 15 to 20 minutes of aerobic activity such as walking, running, swimming, dancing, cycling, or rowing. Maintaining consistency and performing exercise at the same time relative to meals and insulin injections is important. Gradual increases in duration and intensity should be planned based on patient tolerance, with the goal of achieving 150 minutes of moderate-intensity aerobic exercise per week.[14]

For vigorous aerobic exercise, patients with diabetes mellitus who are generally fit, young, and able to exercise regularly and have greater aerobic capacity may engage in 75 minutes per week of more intense activity.[15] The preferred regimen is jogging at 9.6 km per hour. Alternatively, low-volume, high-intensity training involves brief, intense efforts, such as cycling at 85% to 90% of the individual's maximal heart rate for 60 seconds, followed by 60 seconds of rest, repeated 10 times. As with moderate-intensity exercise, gradual increases in duration and intensity should be planned based on patient tolerance.

Resistance Training

Growing evidence in recent years highlights the positive health outcomes of resistance training. This form of strength training enhances muscle strength and endurance through various techniques, including weightlifting, bodyweight exercises, resistance bands, isometrics, and plyometrics. Free weights and weight machines are commonly used. In the absence of contraindications such as proliferative retinopathy, patients should engage in resistance training at least twice per week, focusing on major muscle groups, including the core, upper body, and lower body.

Combining Aerobic Exercise and Resistance Training

The ADA recommends a combination of aerobic exercise and resistance training as the most effective approach for maintaining optimal glycemic control. A randomized controlled trial involving 28 postmenopausal women with diabetes mellitus assigned participants to either a combined training program (resistance and aerobic exercise) or an aerobic exercise-only program for 16 weeks. The results demonstrated that participants in the combined training group experienced greater improvements in glucose sensitivity and muscle mass compared to those in the aerobic exercise-only group.[16]

Contraindications

Relative contraindications for an exercise regimen include vascular conditions such as angina. Uncontrolled diabetes mellitus increases the risk of coronary artery disease (CAD), and affected individuals may experience silent myocardial infarction with exertion.[17] Patients with symptomatic CAD should initiate an exercise program under supervision, such as cardiac rehabilitation.[18]

Individuals with diabetes mellitus and proliferative retinopathy should avoid exercises involving straining, head-down positions, jumping, or Valsalva-like maneuvers. These activities may elevate blood and intraocular pressures along with the risk of retinal bleeding.[19]

Weight-bearing exercises pose a relative contraindication for individuals with severe diabetic neuropathy, as lower extremity sensory loss increases the likelihood of injury or ulceration. Patients with foot ulcers should avoid weight-bearing exercises on the affected foot, though they may try non-weight-bearing activities.[20]

Preparation

Exercise provides significant benefits, and adverse events remain uncommon. Nevertheless, individuals with diabetes mellitus must be properly prepared for exercise. Preparation requires structured planning and an assessment of physical abilities to minimize injury risk. Younger patients typically tolerate moderate-intensity exercise and resistance training without issues, whereas older individuals require careful screening to prevent structural damage, falls, or worsening of macrovascular and microvascular complications.

Age-related arthritis may necessitate structured physical therapy before initiating an exercise regimen for individuals with joint pain. Screening for chest pain, shortness of breath, lower extremity sensory loss, foot ulcers, and retinopathy is essential to identify CAD, severe neuropathy, diabetic foot ulcers, and vision-threatening conditions. Patients interested in vigorous physical activity may require a graded exercise test based on age, diabetes duration and severity, and cardiovascular risk factors.[21][22]

In general, an electrocardiogram may be indicated for individuals who meet 1 or more of the following criteria:

• Age over 40 years, with or without CAD risk factors other than diabetes mellitus
• Age over 30 years with any of the following:
  • Diabetes mellitus for more than 10 years
  • Hypertension
  • Cigarette smoking
  • Dyslipidemia
  • Proliferative or preproliferative retinopathy
  • Nephropathy, including microalbuminuria
• Any of the following, regardless of age:
  • Known or suspected CAD, cerebrovascular disease, or peripheral artery disease
  • Autonomic neuropathy
  • Advanced nephropathy with renal failure [23]

The above criteria do not rule out the need for a cardiac stress test when clinically indicated.

A 5- to 10-minute session of warm-up exercises, such as walking or low-intensity cycling, is generally recommended before starting physical activity, followed by 5 to 10 minutes of stretching that targets large muscle groups. After completing the main exercise, a 5- to 10-minute cool-down period similar to the warm-up helps gradually lower the heart rate.

Technique or Treatment

Maintaining adequate fluid intake before, during, and after exercise is essential to compensate for fluid loss due to sweating. Hydration during exercise helps prevent dehydration and supports optimal performance.

Monitoring blood glucose levels before, during, and after exercise is also recommended.[24] If blood glucose falls below 100 mg/dL, consuming fast-acting carbohydrates such as glucose tablets or juice is advised. Hypoglycemia is generally uncommon in individuals with type 2 diabetes mellitus who are not treated with insulin or insulin analogs. However, patients on insulin or insulin analogs may benefit from ingesting 15 to 30 grams of rapidly absorbed carbohydrates 15 to 30 minutes before exercise to reduce the risk of hypoglycemia.[25]

For individuals using insulin, administration should be done at least 60 to 90 minutes before exercise and at a site away from the primary muscles involved. For example, if cycling, insulin should be injected into the arms, whereas for exercises involving both arms and legs, the abdomen is a preferable injection site to prevent increased insulin absorption.[26]

Vigorous exercise should be avoided when blood glucose levels exceed 250 mg/dL due to the risk of worsening hyperglycemia. Proper footwear is crucial to prevent trauma and blisters. Polyester blend socks, silica gels, or air midsoles can help keep feet dry and reduce the risk of foot complications.[27]

Clinical Significance

The health benefits of well-controlled diabetes mellitus are widely recognized. Regular exercise plays a significant role in improving various metabolic and cardiovascular outcomes.

Glycemic control improves in individuals who engage in regular physical activity.[28] Enhanced carbohydrate metabolism and increased insulin sensitivity contribute to a 10% to 20% reduction in glycated hemoglobin (HbA1c) from baseline in those who participate in 30 to 60 minutes of moderate-intensity exercise 3 to 7 times per week.

Regular exercise also helps prevent cardiovascular disease, as insulin resistance, in combination with risk factors such as hypertension, hyperlipidemia, central obesity, and smoking, accelerates the development of CAD.[29] Physical activity lowers plasma insulin levels and enhances insulin sensitivity, which in turn improves several CAD risk factors.

Hyperlipidemia is another area where exercise demonstrates benefits. Regular physical activity lowers very low-density lipoprotein (VLDL) levels, and studies indicate that it can also improve high-density lipoprotein (HDL) levels.[30]

Weight management is supported through regular exercise, which promotes weight loss and long-term weight maintenance when combined with an appropriate diet. Notably, central adipose tissue reduction has been observed in individuals who exercise consistently, contributing to a lower risk of metabolic syndrome.

Hypertension also improves with consistent physical activity. Increased insulin sensitivity and reduced plasma insulin levels play a role in lowering blood pressure, as elevated blood pressure has been associated with insulin resistance and hyperinsulinemia.

Exercise serves as an important preventive measure against diabetes mellitus.[31] Substantial evidence suggests that regular physical activity delays the onset of diabetes in high-risk individuals, including those with prediabetes, gestational diabetes, or a strong family history of the disease.[32]

Enhancing Healthcare Team Outcomes

Maintaining an exercise program in patients with type 2 diabetes is essential, as it has been linked to improved glycemic control and a reduction in both microvascular and macrovascular complications, ultimately lowering morbidity and mortality. While the benefits of exercise for these patients are well established, implementing and sustaining an exercise regimen can be challenging.[33] An interprofessional team approach is often necessary to achieve successful outcomes. Endocrinologists, primary care physicians, diabetes educators, nursing professionals, obesity specialists, and physical therapists all play a critical role in educating patients on the importance of exercise as a therapeutic strategy for managing diabetes and reducing cardiovascular risk factors.[34]

Close monitoring of blood glucose levels and HbA1c by primary care physicians or endocrinologists is recommended. Fluctuations in blood glucose, whether high or low, may prevent patients from engaging in physical activity, even though exercise is essential in managing diabetes mellitus. Some studies suggest that simple behavioral counseling by clinicians and nurse educators during clinic visits can improve adherence to exercise. Educating various healthcare professionals on delivering practical counseling techniques can further support patient engagement.[35]

A physical therapy consultation can be particularly beneficial for patients who have been sedentary or suffer from joint conditions. Many individuals with type 2 diabetes prefer walking as their primary form of physical activity, making a pedometer-based intervention a motivating and effective strategy for increasing daily movement.[36][37] Since overweight and obesity are common in patients with type 2 diabetes, involving an obesity specialist to develop a tailored exercise regimen and, if appropriate, prescribe weight-loss medications may enhance patient outcomes.

Sustaining an exercise regimen for more than 3 months can be challenging.[38] A 10-year study of 255 patients with diabetes mellitus enrolled in a diabetes education program emphasizing exercise showed a decline in compliance rates from 80% at 6 weeks to less than 50% at 3 months. By 1 year, the compliance rate had dropped to less than 20%.

In contrast, certain factors have been associated with higher levels of physical activity, including confidence in one's ability to exercise, social support, and access to facilities and safe walking areas.[39] An individualized approach incorporating behavior change strategies may improve long-term adherence. A coordinated interprofessional effort, with educators and counselors working alongside clinicians, can help optimize patient engagement and sustain exercise participation.