Sheri R. Colberg, PhD, and Michael C. Riddell, PhD
Physical Activity: Regulation of Glucose Metabolism, Clinical Management Strategies, and Weight Control
Engaging in regular physical activity is an effective measure to protect against a number of health risks across all ages. Physical activity is known to reduce the risk of coronary heart disease, stroke, osteoporosis, and some cancers in the general population.1 Moreover, increased physical activity reduces the likelihood of developing obesity, osteoarthritis, and low back pain, while improving mental health.1 Considerable evidence exists for the health benefits of regular physical activity for people with type 1 diabetes (T1D).2 These health benefits include….
- Increased cardiovascular and cardiorespiratory fitness
- Muscle strength
- Maintenance of insulin sensitivity
- Weight control
- Reduced cardiovascular risk profile
- Improved sense of well-being
- Reduced morbidity and mortality
It is also critical to note that increased physical activity is strongly associated with psychological and social well-being in adolescents with T1D.3 Unfortunately, older patients and females with diabetes tend to be less physically active, spending only ~3.5 days/week doing 60 min of physical activity.3
Although regular aerobic activity or resistance training may not necessarily improve overall glycemic management, as measured by glycated hemoglobin (A1C) or fasting glycemia, it appears to reduce all-cause mortality in those with T1D.2,4 One seminal cohort study of people with T1D found that 7-year mortality was 50% lower in those reporting ~2,000 kilocalories (kcal) of weekly physical activity (equivalent to 7 h/week of brisk walking) compared to those reporting <1,000 kcal of activity.4 The estimated increase in longevity with regular exercise is estimated to be ~10 years, the same value by which diabetes is estimated to lower life expectancy.5 The reason why glycemic control is not always improved, in spite of the fact that glycemia acutely decreases with most types of exercise, may be related to rebound hyperglycemia or inappropriate modifications to diet and insulin regimens in anticipation of exercise and in recovery, or both. The reduction in insulin requirements resulting from regular exercise appears to range between 6% to over 15%.6,7 In theory, regular physical activity should help prevent diabetes-related complications through beneficial effects on insulin sensitivity, blood pressure, lipid levels, endothelial function, and possibly glycemic control. In one large epidemiological study of 628 individuals with T1D, males (but not females) who reported higher levels of historical physical activity had a significantly lower prevalence of nephropathy and neuropathy, but not retinopathy.8 Regular aerobic exercise increases cardiorespiratory fitness in both T1D and type 2 diabetes (T2D) and may limit the development of diabetic peripheral neuropathy and perhaps microvascular and macrovascular disease, although definitive evidence is lacking in this regard.9–11
THE MAJOR CLASSIFICATIONS OF ACTIVITY AND TYPES OF FITNESS
Physical activity is a broad term that encompasses all forms of body movement that substantially increase energy expenditure.12 Exercise may be considered one form of physical activity that is often structured and performed for improved physical fitness or pleasure. Sedentarism is a term used to characterize the behavior of people accumulating less than 30 min of activity daily (or <1.5 Metabolic Equivalents of Tasks [METs]-h/day).13 Sedentary behavior has been associated with poor glycemic management in both adult and pediatric populations of T1D.3,14,15 As mentioned, however, regular exercise training has not been shown to universally improve glycemic control in the vast majority of studies in patients with T1D, perhaps because of the difficulties in making appropriate insulin or carbohydrate adjustments for exercise.
The terms aerobic and anaerobic are commonly used to define the two major forms of physical activity that can be performed. These two types of activity can have very divergent effects on glycemia in patients with T1D.16 In brief, aerobic activities may be defined as ones that produce rhythmic, repeated, and continuous movements of the same large muscle groups for at least 5–10 min at a time. Aerobic fitness refers to the body’s ability to transport and use oxygen during prolonged strenuous exercise or work.12 In contrast, anaerobic or resistance activities use muscular strength to move a weight or work against a resistant load; a sprinting activity may be primarily fueled through anaerobic metabolic pathways as well. Anaerobic fitness refers to the body’s ability to produce energy without the use of oxygen.12
Physical fitness refers to a physiologic state of well-being that allows one to meet the demands of daily living (health-related physical fitness) or that provides the basis for sports performance (performance-related physical fitness) or both.12 Health-related physical fitness involves the components of physical fitness related to health status, including cardiovascular fitness, musculoskeletal fitness, body composition, and metabolism.12 All these components of fitness are important for patients with T1D for optimal health and performance.17
Physical fitness can be assessed through well-established appraisal protocols, used by qualified exercise professionals, from organizations like the American College of Sports Medicine (http://www.acsm.org/). These assessments are designed to evaluate the individual components of health-related physical fitness, including body composition, aerobic fitness, and musculoskeletal fitness (muscular strength, muscular endurance, power, and flexibility). Exercise and fitness assessment is beyond the scope of this review, but interested readers may refer to a recent text that includes sections on exercise stress testing in patients with diabetes.18 Table 11.1 lists general terms and definitions of fitness activities, while Table 11.2 gives some metabolic equivalents of a variety of common physical activities.
In general, the overall fitness of an individual with T1D may be somewhat impaired compared to age-matched controls, but patients have achieved all levels of competitive stature including national, professional, and Olympic status.19-21 Poor glycemic control in T1D is associated with reduced fitness.21,22 Highly trained individuals with T1D who are in good control can achieve the same cardiopulmonary exercise responses as trained subjects without diabetes, but these responses are clearly reduced by poor glucose control.23,24 Importantly, all forms of exercise are deemed reasonably safe for anyone with T1D, particularly if there is no evidence of underlying micro- and macrovascular disease. However, appropriate pre-exercise screening is warranted using available physical activity clearance algorithms.25
Infants, toddlers, and preschoolers (0–4 years). Common sense more than empiric research supports the recommendations for physical activity for children from birth to age 5 years with T1D. In general, infants’ activity is based upon their interactions with caregivers who provide the infants with opportunities to explore movement and their surroundings. The physical activity of infants supports their development; active play, along with interactions with adults, enhances the infants’ well-being and physical and emotional development. Toddlers should participate in at least 30 min of physical activity daily, often with unstructured play lasting more than 30 min at a time. In fact, toddlers should not be sedentary for more than 60 min at one time (except during sleep) in order to encourage their motor skill development. Toddlers will likely engage in indoor and outdoor activities that promote development of their large muscles and gross motor skills, utilizing either structured or unstructured physical activities. Preschoolers should have at least 60 min of structured physical activity daily, along with several hours of unstructured play each day. As with toddlers, preschoolers should generally not be sedentary for more than 60 min at a time (except for sleep). The physical activity of preschoolers continues to help them develop motor skills and enhances their socialization.
The caregivers and parents of infants, toddlers, and preschoolers contribute to the overall health and well-being of these young children by promoting physical activity with both structured and unstructured play. Just like older children, for every 30 min of physical activity, the young child with T1D will require additional carbohydrate intake in order to prevent severe hypoglycemia. It is difficult to adjust insulin in advance of activity in young children due to the unpredictable nature of the exercise. In general, 5–10g/30–60 min of physical activity will be needed for young children, depending upon the child’s initial blood sugar and intensity of the exercise. It is important for the supervising adults to check the young child’s blood glucose levels frequently as these young children are unable to convey symptoms of hypoglycemia. Thus, it is recommended to check blood glucose levels both before and after physical activity and to generally begin periods of exercise with glucose levels of 150–200 mg/dl in toddlers and preschoolers.
Young children (5–11 years) and adolescents (12–17 years). For health benefits, it is generally recommended that all children (aged 5–11 years) and youth (aged 12–17 years) accumulate at least 60 min of moderate- to vigorous-intensity mixed aerobic and anaerobic physical activity daily (420 min/week).1,26,27 This recommendation appears appropriate for youth with T1D, although it can clearly increase the risk for hypoglycemia.28 More physical activity likely provides greater health benefits, helping to limit high blood cholesterol, high blood pressure, metabolic syndrome, obesity, low bone density, and depression, although the risk for injury does increase.27 Being physically active with T1D, compared with being sedentary with the disease, is expected to improve cholesterol levels, blood pressure, body composition, bone density, cardiorespiratory and musculoskeletal fitness, and various aspects of mental health and well-being.2 These recommendations appear particularly suitable for children and adolescents with T1D since physical activity patterns in youth track into adulthood. As cardiovascular disease (CVD) is the major cause of early mortality and morbidity in diabetes it is important to begin these activity patterns in childhood and adolescence.29,30 To summarize, the recommendations for daily exercise in all children and adolescents with T1D are expected to include:
- At least 60 min of accumulated physical activity every day
- Vigorous-intensity aerobic activities at least 3 days/week
- Activities that strengthen muscle and bone at least 3 days/week each
Children and youth should be physically active daily as part of play, games, sports, transportation, recreation, physical education, or planned exercise in the context of family, school, and community activities (e.g., volunteer, employment). This should be achieved above and beyond the incidental physical activities accumulated in the course of daily living. Reducing sedentary time is convincingly associated with a favorable cardiovascular profile, and several expert panels recommend limiting leisure screen time to <2 h/day.26
For school-aged children and youth who are physically inactive, doing amounts below the recommended levels (such as 30 min/day instead of 60 min/day) likely provides some health benefits, compared with being completely sedentary. The benefits of exercise are highlighted by the finding that sedentary youth with T1D have higher A1C levels compared with active youth.3,15,27 Interestingly, extensive media consumption is a significant risk factor for poor metabolic control in youth with diabetes, irrespective of socioeconomic status and physical activity.31 For sedentary youth, it may be appropriate to start with smaller amounts of physical activity and gradually increase duration, frequency, and intensity as a stepping stone to meeting the guidelines.
These guidelines for daily physical activity, which should include vigorous aerobic and muscle-strengthening activities, may be considered ambitious for young persons with T1D, given their potential fear of hypoglycemia and sometimes sedentary nature.3,32 A large sample of children and adolescents with T1D (23,251 youth ages 3–18 years) found that ~45% of this cohort was generally sedentary (i.e., <30 min of continuous physical activity per week, excluding school sports), with only 37% of the cohort having regular physical activity for 30 min 1–2 times/week.33 However, this study also found that A1C levels were lower in patients with higher frequencies of physical activity and that blood lipid profiles were more favorable compared to those who were sedentary. Interestingly, multiple regression analysis revealed that regular physical activity was one of the most important factors influencing A1C levels.33 Other studies34–36 have found that more frequent exercise is associated with lower A1C levels, although a number of exercise training studies have found no relationship,7,37–40 perhaps because appropriate insulin and carbohydrate adjustments for exercise were not in place.
To achieve health benefits, adults ages 18–64 years should accumulate at least 150 min of moderate- to vigorous-intensity aerobic physical activity per week, in bouts of 10 min or more.26,41 It is also beneficial to add muscle- and bone-strengthening activities that use major muscle groups, at least 2 days/week. Although the total energy expenditure should be ~1,000 kcal/week of physical activity, the American College of Sports Medicine has stated that health benefits occur with energy expenditures as low as 500 kcal/week, with additional benefits occurring at higher levels. 41 In other words, according to the ACSM guidelines, some is good and more is better in most cases.41
Adults with T1D can meet recommended guidelines through planned exercise sessions, transportation, recreation, sports, or occupational demands, in the context of family, work, volunteer, and community activities. This should be achieved above and beyond the incidental physical activities accumulated in the course of daily living.42 Following these guidelines can reduce the risk of premature death, coronary heart disease, stroke, hypertension, colon cancer, breast cancer, and osteoporosis, and improve fitness, body composition, and indicators of mental health.26 The potential benefits far exceed the potential risks associated with physical activity, even in people with T1D.25 Physical activity appears to lower cardiovascular mortality risk at all levels of glycemic control.43 For those who are initially physically inactive, doing amounts below the recommended levels can provide some health benefits. For these adults, it is appropriate to start with smaller amounts of physical activity and then gradually increase the duration, frequency, and intensity as a stepping stone to meeting these guidelines.
To achieve health benefits and improve functional abilities, adults ages 65 years and older should accumulate at least 150 min of moderate- to vigorous-intensity aerobic physical activity per week, in bouts of 10 min or more.1,45 It is also beneficial to add muscle- and bone-strengthening activities that use major muscle groups, at least 2 days/week. 1,45
While the exercise guidelines for adults also apply to older adults, there are some additional ones that apply only to older adults (with or without T1D): 1,45
- When older adults cannot do 150 min of moderate-intensity aerobic activity a week because of chronic conditions, they should be as physically active as their abilities and conditions allow.
- Recommend exercises that maintain or improve balance, particularly if they are at risk of falling.
- Determine their level of effort for physical activity relative to their level of fitness.
- Those with chronic conditions should understand whether and how their conditions affect their ability to do regular physical activity safely.
Older adults can meet these guidelines through the same means as younger adults (i.e., increased activities of daily living) to reduce the risk of comorbid disease and premature death, maintain functional independence and mobility, as well as improve fitness, body composition, bone health, cognitive function, and indicators of mental health.1,45 The potential benefits exceed the potential risks associated with physical activity, even for older adults with diabetes.25 These guidelines may be appropriate for older adults with frailty or other comorbid conditions; however, individuals with health issues should consult a health professional to understand the types and amounts of physical activity appropriate for them based on their exercise capacity and specific health risks or limitations.25
Anne Peters, MD, and Lori Laffel, MD, MPH, Editors
Jane Lee Chiang, MD, Managing Editor
Used with permission by the American Diabetes Association. Copyright © 2013 American Diabetes Association.
Please note: We are proud to have Dr. Anne Peters as a member of our Advisory Board member for Diabetes In Control, Inc.