2.9 Exercise and type 2 diabetes (T2DM)
2.9.5 Chronic effects of exercise training
2.9.5.1 Pre-exercise evaluation
Safe exercise participation can be complicated by the presence of diabetes-related health complications like CVD, hypertension, neuropathy, or micro-vascular changes.60 For individuals desiring to participate in low-intensity PA like walking, health care providers should use clinical judgment in deciding whether to recommend pre-exercise testing.60 Conducting exercise stress testing before walking is unnecessary.55 There is no evidence to suggest that it is routinely necessary as a CVD diagnostic tool, and requiring it may create barriers to participation.55 For exercise more vigorous than brisk walking or exceeding the demands of everyday living, sedentary and older diabetic individuals will likely benefit from being assessed for conditions that might be associated with risk of CVD, contraindicate certain activities, or predispose to injuries, including severe peripheral neuropathy, severe autonomic neuropathy, and preproliferative or proliferative retinopathy. Before undertaking new higher-intensity PA, such patients are advised to undergo a detailed medical evaluation and screening for BG control, physical limitations, medications, and macrovascular and microvascular complications.61 This assessment may include a graded exercise test depending on the age of the person, DM duration, and the presence of additional CVD risk factors.61,63 The prevalence of symptomatic and asymptomatic CAD is greater in individuals with T2DM and maximal graded exercise testing can identify a small proportion of asymptomatic persons with severe coronary artery obstruction.62,63
Most young individuals with a low CAD risk may not benefit from preexercise stress testing.55 In the Look AHEAD trial, although exercise-induced abnormalities were present in 1303 (22.5%) participants, only older age was associated with increased prevalence of all abnormalities during maximal testing.63 A systematic review of the US Preventive Services Task Force concluded that stress testing should not be routinely recommended to detect ischaemia in asymptomatic
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individuals with a low CAD risk (<10% risk of a cardiac event more than 10 yrs) because the risks from invasive testing done after a false-positive test outweigh the benefits of its detection.64 The lower the CAD risk, the higher the chance of a false positive.55 Current guidelines attempt to avoid automatic inclusion of lower-risk individuals with T2DM, stating that exercise stress testing is advised primarily for previously sedentary individuals with DM who want to undertake activity more intense than brisk walking. The goal is to more effectively target individuals at higher risk for underlying CVD.55
In general, ECG stress testing may be indicated for individuals matching one or more of these criteria: 55
Age >40 yr, with or without CVD risk factors other than diabetes
Age >30 yr and
o Type 1 or 2 diabetes of >10 yr in duration o Hypertension
o Cigarette smoking o Dyslipidaemia
o Proliferative or preproliferative retinopathy o Nephropathy including microalbuminuria
Any of the following, regardless of age
o Known or suspected CAD, cerebrovascular disease, and/or peripheral artery disease (PAD)
o Autonomic neuropathy
o Advanced nephropathy with renal failure
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Use of these criteria does not exclude the possibility of conducting ECG stress testing on individuals with a low CAD risk or those who are planning to engage in less intense exercise.55 In the absence of contraindications to maximal stress testing, it can still be considered for anyone with T2DM. Although clinical evidence does not definitively determine who should undergo such testing, potential benefits should be weighed against the risk associated with unnecessary procedures for each individual.55 In individuals with positive or nonspecific ECG changes in response to exercise, or with nonspecific ST and T wave changes at rest, follow-up testing may be performed. There is no evidence available to determine whether preexercise evaluation involving stress testing is necessary or beneficial before participation in anaerobic or resistance training.55 Therefore, before undertaking exercise more intense than brisk walking, sedentary persons with T2DM will likely benefit from an evaluation by a physician. ECG exercise stress testing for asymptomatic individuals at low risk of CAD is not recommended but may be indicated for higher risk.55
2.9.6 RECOMMENDED PA PARTICIPATION FOR PERSONS WITH T2DM 2.9.6.1 AEROBIC EXERCISE TRAINING:
a. Frequency. Aerobic exercise should be performed at least 3 days per week with no more than two consecutive days between bouts of activity because of the transient nature of exercise-induced improvements in insulin action.55 Most clinical trials evaluating exercise interventions in T2DM have used a frequency of three times per week, but current guidelines for adults generally recommend five sessions of moderate activity.55
b. Intensity. Aerobic exercise should be at least at moderate intensity, corresponding approximately to 40%–60% of VO2 max (maximal aerobic capacity). For most people with T2DM, brisk walking is a moderate-intensity exercise. Additional benefits may be gained from vigorous exercise (96% of VO2 max).55 A meta-analysis showed that exercise
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intensity predicts improvements in overall BG control to a greater extent than exercise volume, suggesting that those already exercising at a moderate intensity should consider undertaking some vigorous PA to obtain additional BG (and likely CV) benefits.55
c. Duration. Individuals with T2DM should engage in a minimum of 150 minute per week of exercise undertaken at moderate intensity or greater.55 Aerobic activity should be performed in bouts of at least 10 minutes and be spread throughout the week. Around 150 minutes per week of moderate-intensity exercise is associated with reduced morbidity and mortality in observational studies in all populations.55 Recent College of Sports Medicine/American Heart Association guidelines recommended 150 minutes of moderate activity (30 minutes, 5 days per week ) or 60 minutes of vigorous PA (20 minutes on 3 days) for all adults, whereas recent US federal guidelines65 recommended 150 minutes of moderate or 75 minutes of vigorous activity, or an equivalent combination, spread throughout each week.55
d. Mode. Any form of aerobic exercise (including brisk walking) is recommended, provided that it uses large muscle groups and causes sustained increases in HR. It is likely to be beneficial.55
e. Rate of progression. At present, no study on individuals with T2DM has compared rates of progression in exercise intensity or volume. Gradual progression of both is advisable to minimize the risk of injury, particularly if health complications are present, and to enhance compliance.55
2.9.6.2 RESISTANCE EXERCISE TRAINING
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a. Frequency. Resistance exercise should be undertaken at least twice weekly on nonconsecutive days, but more ideally three times a week, as part of a PA program for individuals with T2DM, along with regular aerobic activities.55
b. Intensity. Training should be moderate (50% of 1-repetition maximum, or 1-RM) or vigorous (75%–80% of 1-RM) for optimal gains in strength and insulin action.55
c. Duration. Each training session should minimally include 5–10 exercises involving the major muscle groups (in the upper body, lower body, and core) and involve completion of 10–15 repetitions to near fatigue per set early in training, progressing over time to heavier weights (or resistance) that can be lifted only 8–10 times. A minimum of one set of repetitions to near fatigue, but as many as three to four sets, is recommended for optimal strength gains.55
d. Mode. Resistance machines and free weights (e.g., dumbbells and barbells) can result in fairly equivalent gains in strength and mass of targeted muscles. Heavier weights or resistance may be needed for optimization of insulin action and BG control.55
e. Rate of progression. To avoid injury, progression of intensity, frequency and duration of training sessions should occur slowly. In resistance exercise, increases in weight or resistance should be gradual. Once a target number of repetitions per set can be consistently exceeded, a greater weight or resistance can be used. The frequency of training can then be increased. Progression from twice-weekly to thrice-weekly sessions over 6 months using three sets of weights of 8–10 repetitions done at 75% to 80% of 1-RM (1-repetition maximum) may be beneficial.55
2.9.6.3 COMBINED AEROBIC, RESISTANCE AND OTHER TYPES OF
57 TRAINING:
Inclusion of both aerobic and resistance exercise training is recommended. Combined training thrice weekly in individuals with T2DM may be of greater benefit to BG control than either aerobic or resistance exercise alone.55 However, the total duration of exercise and caloric expenditure was greatest with combined training in all studies done to date, and both types of training were undertaken together on the same days.55
2.9.6.4 DAILY MOVEMENT (UNSTRUCTURED ACTIVITY)
Individuals with T2DM are encouraged to increase their total daily, unstructured PA to gain additional health benefits. Non-exercise activity thermogenesis (i.e., energy expending for activities of daily living (ADL)) can create a large daily caloric deficit to prevent excessive weight gain.55 Examples of such non-exercise activity thermogenesis can occur in ADL such as bathing, cooking, house cleaning, laundry, and shopping.66
2.9.6.5 FLEXIBILITY TRAINING
Flexibility training may be included as part of a PA program, although it should not substitute for other training. Older adults are advised to undertake exercises that maintain or improve balance, which may include some flexibility training, particularly for many older individuals with T2DM with a higher risk of falling.55 Although flexibility exercise (stretching) has frequently been recommended as a means of increasing joint range of motion (ROM) and reducing risk of injury, two systematic reviews found that flexibility exercise does not reduce risk of exercise-induced injury.55
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2.9.6.6 EXERCISE WITH NON-OPTIMAL BG CONTROL
Hyperglycaemia: While hyperglycaemia can be worsened by exercise in T2DM individuals who are insulin deficient and ketotic (due to missed or insufficient insulin), very few persons with T2DM develop such a profound degree of insulin deficiency.55 Therefore, individuals with T2DM generally do not need to postpone exercise because of high BG, provided that they are feeling well.
If they undertake strenuous physical activities with elevated glucose levels (9300 mg/dL or 16.7 mmol/L), it is prudent to ensure that they are adequately hydrated.67 If hyperglycaemic after a meal, individuals with T2DM will still likely experience a reduction in BG during aerobic work because endogenous insulin levels will likely be higher at that time.55
Hypoglycaemia: Of greatest concern to many exercisers is the risk of hypoglycaemia. In individuals whose diabetes is being controlled by lifestyle alone, the risk of developing hypoglycaemia during exercise is minimal, making stringent measures unnecessary to maintain BG.55 Glucose monitoring can be performed before and after PA to assess its unique effect.
Activities of longer duration and lower intensity generally cause a decline in BG levels but not to the level of hypoglycaemia. While very intense activities can cause transient elevations in BG, intermittent high intensity exercise done immediately after breakfast in individuals treated with diet only reduces BG levels and insulin secretion.55 In insulin or insulin secretagogues users, who frequently have the effects of both exercise and insulin to increase glucose uptake, PA can complicate DM management.For pre-exercise BG levels of less than 100 mg/dL (5.5 mmol/L), the ADA recommends that carbohydrate be ingested before any PA. 68 This applies only to individuals taking insulin or the secretagogues more likely to cause hypoglycaemia (e.g., sulfonylureas like Glyburide, Glipizide, and Glimepiride, as well as Nateglinide and Repaglinide).55 If controlled with diet or other oral medications, most individuals will not need
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carbohydrate supplements for exercise lasting less than an hour. Insulin users should likely consume up to 15 g of carbohydrate before exercise for an initial BG level of 100 mg/dL or lower, with the actual amount dependent on injected insulin doses, exercise duration and intensity, and results of BG monitoring. Intense, short exercise requires less or no carbohydrate intake.68 Later-onset hypoglycaemia is a greater concern when carbohydrate stores (i.e., muscle and liver glycogen) are depleted during an acute bout of exercise. In particular, high-intensity exercise (e.g., repeated interval or intense resistance training) can result in substantial depletion of muscle glycogen, thereby increasing risk for post exercise hypoglycaemia in users of insulin or insulin secretagogues.55 In such cases, the consumption of 5–30 g of carbohydrate during and within 30 minutes after exhaustive, glycogen-depleting exercise will lower hypoglycaemia risk and allow for more efficient restoration of muscle glycogen.55
2.9.6.7 MEDICATION EFFECTS ON EXERCISE RESPONSES
Current treatment strategies promote combination therapies to address the three major defects in T2DM: impaired peripheral glucose uptake (liver, fat, and muscle), excessive hepatic glucose release (with glucagon excess), and insufficient insulin secretion.55 Medication adjustments for PA are generally necessary only with use of insulin and other insulin secretagogues.55 To prevent hypoglycaemia, individuals may need to reduce their oral medications or insulin dosing before (and possibly after) exercise.55 Before planned exercise, short-acting insulin doses will likely have to be reduced to prevent hypoglycaemia. Newer, synthetic, rapid-acting insulin analogs (i.e., lispro, aspart, and glulisine) induce more rapid decreases in BG than regular human insulin. Individuals will need to monitor BG levels before, occasionally during, and after exercise and compensate with appropriate dietary and/or medication regimen changes, particularly when exercising at insulin peak times. If only longer-acting insulins like glargine, determir, and Neutral Protamine
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Hagedorn (NPH) are being absorbed from subcutaneous depots during PA, exercise induced hypoglycaemia is not as likely, although doses may need to be reduced to accommodate regular participation in PA.55 Doses of select oral hypoglycaemic agents (Glyburide, Glipizide, Glimepiride, Nateglinide, and Repaglinide) may also need to be lowered in response to regular exercise training if the frequency of hypoglycaemia increases. Diabetic individuals are often prescribed a variety of medications for comorbid conditions, including diuretics, β-blockers, ACE inhibitors, aspirin, lipid-lowering agents, and more. These medications generally do not affect exercise responses, with some notable exceptions. β-blockers are known to blunt HR responses to exercise and lower maximal exercise capacity to ~87% of expected via negative inotropic and chronotropic effects.55 They may also block adrenergic symptoms of hypoglycaemia, increasing the risk of undetected hypoglycaemia during exercise. However, β-blockers may increase exercise capacity in those with CAD, rather than lowering it, by reducing coronary ischaemia during activity. Diuretics, however, may lower overall blood and fluid volumes resulting in dehydration and electrolyte imbalances, particularly during exercise in the heat. Statin use has been associated with an elevated risk of myopathies (myalgia and myositis), particularly when combined with use of fibrates and niacin.55
2.9.6.8 ADOPTION AND MAINTENANCE OF EXERCISE BY PERSONS WITH DIABETES MELLITUS
Most American adults with T2DM or at highest risk for developing it do not engage in regular PA;
their rate of participation is significantly below national norms.69 Additional strategies are needed to increase the adoption and maintenance of PA. One of the most consistent predictors of greater levels of activity has been higher levels of self-efficacy, 55 which reflect confidence in the ability to exercise.Social support has also been associated with greater levels of PA.55 Physicians vary in
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counselling their patients to exercise: on average, advice or referral related to exercise occurred at 18% of office visits among diabetic patients,69 and 73% of patients reported receiving advice at some point to exercise more.55 The availability of facilities or pleasant and safe places to walk may also be important predictors of regular PA.71 When prescribing PA for the prevention or control of T2DM, the effects of the dose of the prescription on adherence are small.72 Therefore, practitioners are encouraged to use factors such as choice and enjoyment in helping determine specifically how an individual would meet recommended participation. Affective responses to exercise may be important predictors of adoption and maintenance, and encouraging activity at intensities below the ventilatory threshold may be most beneficial. Many individuals with, or at risk of developing T2DM prefer walking as an aerobic activity, 55 and pedometer-based interventions can be effective for increasing aerobic activity. Efforts to promote PA should focus on developing self-efficacy and fostering social support from family, friends, and health care providers. Encouraging mild or moderate PA may be most beneficial to adoption and maintenance of regular PA participation. Lifestyle interventions may have some efficacy in promoting PA behaviour.55
In summary, exercise plays a major role in the prevention and control of insulin resistance, prediabetes, GDM, T2DM, and DM related health complications.55 Both aerobic and resistance training improve insulin action, at least acutely, and can assist with the management of BG levels, lipids, blood pressure(BP), cardiovascular risk, mortality, and QOL, but exercise must be undertaken regularly to have continued benefits and likely include regular training of varying types.55 Most persons with T2DM can perform exercise safely as long as certain precautions are taken. The inclusion of an exercise program or other means of increasing overall PA is critical for optimal health in individuals with T2DM.55