Lowering dietary carbohydrate intake has demonstrated benefits for insulin resistance, the underlying cause of T2DM, by independently promoting both weight loss and a reduction in insulin levels.
-
6. Exercise in Managing T1DM
The ability to adjust the therapeutic regimen (insulin administration and timing, type and quantity of food ingestion before and after physical exercise) that allows safe participation and high performance has recently been recognized as an important management strategy in individuals with T1DM. In particular, the important role played by the patient in collecting self-monitored blood glucose data related to physical exercise responses to improve performance and enhance safety is now fully accepted
[1][2][43][1,2,43].
When amateurs exercise at 50–60% of VO
2max, which is below their anaerobic threshold, glucose levels rise following the increased uptake of skeletal muscle. The level of glycemia is generally reduced 30/45 min after PA, and it is preserved within the physiological range by two processes: a rapid increase in endogenous glucose production and a reduction in systemic insulin levels
[41]. Patients should reduce or stop insulin infusion before exercise. This is to avoid the hypoglycemic effect due to the increase in glucose uptake at the skeletal muscle and to suppress endogenous glucose production.
Different is the situation in which the subject is involved in physical exercise above the anaerobic threshold and close to the VO
2max. In this condition, the body produces a higher adrenergic activation that facilitates, beyond cardiovascular response, an endogenous increase in glucose level, exceeding the metabolic need at the peripheral level. This creates a state of moderate, transient hyperglycemia that does not exceed 140 mg/dL in healthy people. In T1DM, given that insulin cannot be secreted in response to this hyperglycemic response, the hyperglycemia value often continues to increase after exercise, sometimes becoming dangerous (>400 mg/dL)
[41].
Hypoglycemia, which can occur during, immediately after, or many hours after PA, can be avoided. Indeed, during physical exercise, the body requires approximately 30–50% less insulin than in resting conditions to transport glucose across the cell membrane of myocytes. This is due to a considerable fraction of exercise-stimulated transmembrane glucose transport that occurs via noninsulin-dependent mechanisms
[44]. In this case, it is important that the patient has both an adequate knowledge of the metabolic and hormonal responses to PA and well-tuned self-management skills. The increasing use of intensive insulin therapy has provided patients with the flexibility to make appropriate insulin dose adjustments for various activities. Moreover, in T1DM, after 2–4 years of disease onset, the ability to increase glucagon secretion (the main counterregulatory hormone) in response to hypoglycemia is permanently and completely abolished. However, in T1DM, hypoglycemia is caused by excessive exogenous insulin injection, involving low glycemia and high insulinemia that blocks glucagon release
[41].
Aerobic performance is reduced in T1DM because of cardiovascular, muscular, and metabolic impairments. When compared to their nondiabetic counterparts, young patients with T1DM showed a reduction in VO
2max despite insulin therapy
[45], a difference exacerbated in adults with neuropathic complications or sedentary lifestyles
[46]. Different is the situation in which athletes with T1DM are compared to their nondiabetic counterparts, where VO
2peak was found to be similar
[47]. Moreover, in T1DM, various cardiovascular parameters, such as end-diastolic volume and left ventricular ejection fraction, do not show a normal increment due to exercise, meaning that exercise can lead to normal aerobic and cardiovascular parameters
[41].
The general recommendations for PA in adults with T1DM, free of complications, are the same for children, considering that kids are more subjected to a greater variability in glycemic level. For this reason, attention should be focused on glycemic fluctuation during exercise so that parents, teachers, and athletic coaches are properly trained. When dealing with adolescents, hormonal variations can increase the difficulty in managing glycemic levels. Notwithstanding these additional difficulties, it is out of the question that following the useful recommendations to avoid hypoglycemia, PA is a safe and satisfying practice for most children and adolescents with T1DM
[42].
7. Exercise in Managing T2DM
A standard recommendation for people with and without diabetes is to start PA with a warm-up and end with a cool-down phase. The warm-up involves 5–10 min of aerobic exercises (walking, cycling, etc.) at low intensity. The aim is to prepare the skeletal muscles, heart, and lungs for a progressive increase in exercise intensity. Then, muscles can be gradually stretched for an additional 5–10 min to maintain a good range of motion in the joints
[48]. After the main training session, a 5–10 min cool-down should be organized similarly to the warm-up. The aim of the cool-down is to gradually bring the heart rate back to its pre-exercise value. There are some recommendations that are particularly important and specific for people with T2DM. Moderate weight training exercises with light weights and high repetitions can be suggested for maintaining or enhancing upper body strength in people with diabetes. Different long-term studies have established long-lasting beneficial effects of regular PA on carbohydrate metabolism and insulin sensitivity, which can be maintained for at least 5 years. They suggested PA programs with an intensity in the range of 50–80% of VO
2max, three to four times a week, for 30–60 min a session. Aerobic exercise should be suggested, considering safety measures for PA involving the feet, which are essential for many patients with diabetes
[1][2][48][49][50][51][1,2,48,49,50,51]. High-resistance exercise using weights may be acceptable for young individuals with diabetes but not for older individuals with long-standing diabetes.
Good hydration is also crucial, as dehydration can adversely affect glycemic levels and heart function. Through PA, fluid should be frequently consumed to compensate for losses in sweat reflected in body weight loss or the maximal amount of fluid tolerated (e.g., 0.5 L of fluid consumed 2 h before PA)
[52]. Precautions should be taken when exercising in extremely hot or cold environments.
It has been shown that improvements in glycated hemoglobin (HbA1c) are generally 10–20% of the baseline and are most marked in patients with mild T2DM and in those who are likely to show insulin resistance
[53]. It remains true, unfortunately, that most of these studies suffer from inadequate randomization and controls and are confounded by associated lifestyle changes. The general agreement is that regular exercise should not be expected to dramatically affect HbA1c values; other variables, such as increased food intake or reduced insulin dosages, compensate for any increases in glucose disposal. Nonetheless, epidemiological evidence confirmed that being physically active, rather than sedentary, can lower mortality and morbidity for any given level of HbA1c
[54].
It seems that long-lasting programs of PA are good enough and demonstrate a higher rate of adherence in patients with prediabetes or uncomplicated T2DM. In this kind of study, researchers started their training programs with initial supervision, followed by home-training programs with follow-up to assess the level of adherence
[4]. A lot of them have shown good results in the maximum oxygen consumption with few complications for patients
[55].
8. Psychological Profile, Muscular, and Cardiovascular Evaluations
Other important aspects to consider when prescribing an exercise program are the evaluation of motor responses and the consideration of the psychological profile. Evaluations represent a fundamental moment for understanding the real capabilities of an individual, setting up protocols and assessing results over time. People with diabetes show both impaired exercise tolerance and an excessive risk of developing heart failure, which are not entirely explained by known cardiovascular risk factors or coronary artery disease
[56]. The risk for cardiovascular disease and other diabetes-related complications, including neuropathy, retinopathy, and nephropathy in persons with long-standing disease, is high, and care should be taken to properly screen individuals before recommending a new exercise program. Caution is warranted for those with advanced disease complications and medical screening; before initiating any new vigorous exercise program, a graded exercise stress test with ECG and blood pressure monitoring should be performed. The main assessments that should be made before carrying out any type of physical exercise are, in order: (i) the Health Assessment Questionnaire; (ii) assessment of balance level and risk of falls; (iii) cardiopulmonary exercise testing; (iv) the assessment of muscle strength.
Health assessment questionnaires
[57]. While several tools are available to measure health-related quality of life (HRQoL) for patients with diabetes, the design and, therefore, duration of these measurements may limit their feasibility in the daily routine of a sports facility. Furthermore, these measures do not distinguish items for diabetes-specific quality of life. Thus, a specific questionnaire on the diabetics’ quality of life (DMQoL) was developed with only 10 questions, sensitive to the change related to the progression of diabetes compared to the initial stages (e.g., glycemic changes). The combination of the DMQoL and the WHOQOL-BREF (the shortened version of the quality of life questionnaire designed by WHO) provides a comprehensive picture of overall health-related quality of life in patients with diabetes and improves the ability to detect changes clinically significant to the pathology.
Tests that determine the level of balance and the risk of falls. Patients with T2DM, particularly those ≥65 years old, exhibit an increased rate of falls. It is, therefore, important to assess the risk of falling before the prescription of an exercise intervention. The most used tests are the Timed Up and Go Test (TUG), the Functional Reach Test, (FRT), the Berg Balance Scale (BBS), and the Dynamic Gait Index (DGI). Among those tests, the TUG showed the greatest sensitivity (90%) and specificity (88%) to the phenomenon
[58]. In this assessment, the patient gets up from a chair, walks 3 m, turns around, returns to the chair, and sits down again. This task must be completed within 10.6 s. Times between 11 and 20 s are within the normal range for frail elderly and disabled patients; times ≥20 s indicate that the person needs external assistance. A score ≥30 s predicts a higher risk of falling.
Cardiopulmonary exercise testing (CPET). Measurements of ventilation, gas exchange, and electrocardiography during an incremental exercise test are noninvasive protocols that provide an assessment of pulmonary, cardiovascular, and muscle function during exercise (see
Table 1). The addition of echocardiographic monitoring (“imaging-CPET”), mainly used in patients with heart failure, may provide further insight into different aspects of cardiac function during exercise and their impact on exercise intolerance
[59]. Even though guidelines recommend the application of objective exercise prescriptions using CPET data
[59], it is common to find programs without CPET information or with limited resources to establish exercise intensity on the basis of resting heart rate (e.g., exercising heart rate threshold set 20 or 30 beats/min above the resting heart rate). This simple method has been criticized and demonstrated to be inadequate by several researchers. Moreover, it should be noted that performing this test routinely and its cost mean that specific criteria must be defined regarding patients for whom it would be imperative to perform the test.
Table 1.
Physiological variables measured during CPET.