Clinical Considerations and Practical Advice for People Living With Type 2 Diabetes Who Undertake Regular Exercise or Aim to Exercise Competitively Free

As a foundation, it is useful to understand the general physical activity/exercise guidelines. People with diabetes are encouraged to adopt and maintain a regular physical activity routine, with specific recommendations varying depending on their health status. The most recent consensus statements from the American Diabetes Association (ADA) (11,26) are summarized below.

• Adults should engage in at least 150 minutes/week of moderate- to vigorous-intensity aerobic activity, spread over at least 3 days/week, with no more than 2 consecutive days without activity. Shorter durations (minimum 75 minutes/week) of vigorous-intensity or interval training may be sufficient for younger and more physically fit individuals.

• Adults with type 1 or type 2 diabetes should engage in two to three sessions per week of resistance exercise on nonconsecutive days.

• Flexibility training and balance training are recommended two to three times per week for older adults with diabetes.

• All adults, and particularly those with type 2 diabetes, should decrease the amount of time they spend daily in sedentary behavior.

• Prolonged sitting should be interrupted with bouts of light activity every 30 minutes for blood glucose benefits, at least in adults with type 2 diabetes.

The risks associated with exercise in type 2 diabetes can be minimized with planning and preparation (Figure 1). The following sections provide practical tips on initial medical assessment and pre-exercise screenings, glucose monitoring and nutritional considerations, and the combined effects of exercise and medications. As previously noted, all guidance should be individualized and modified as needed based on patients’ needs and the type of exercise they undertake.

Over time, the hyperglycemia associated with type 2 diabetes can damage the heart, blood vessels, eyes, kidneys, and nerves. For this reason, there is often concern about the safety of exercise for people with diabetes. For most people with type 2 diabetes, the benefits of exercise will outweigh the risks. Before starting exercise for the first time or when beginning a program of vigorous physical activity, people with diabetes should be assessed for conditions that might increase risks associated with certain types of exercise or predispose them to injury. Table 1 provides guidance on what screening or adaptions should be made before starting exercise.

There is no evidence that screening asymptomatic people with type 2 diabetes for heart disease is of benefit (27). Therefore, in line with ADA consensus guidelines, we only recommend pre-exercise cardiovascular testing for patients with symptoms of heart disease or individuals with autonomic neuropathy (11). Individuals who have recently had a stroke or heart attack should restart exercise in a supervised rehabilitation program. If blood pressure is high, high-intensity exercise should be avoided until blood pressure has lowered. People with peripheral artery disease can perform all forms of activity, with low-intensity walking (28) and resistance training both improving symptoms.

Individuals with autonomic neuropathy are at higher risk of hypoglycemia and should exercise with caution in hot or cold environments, as they are at higher risk of becoming dehydrated. If they have postural hypotension, they should avoid activities with rapid postural or directional changes to avoid fainting or falling. These individuals should consider using heart rate reserve and ratings of perceived exertion to monitor their exercise intensity, as often they will have a blunted heart rate response to exercise (29).

Exercise does not increase the risk of foot ulcer or reulceration in people with peripheral neuropathy (30). In fact, regular exercise may prevent the onset or delay the progression of peripheral neuropathy (31). All people with neuropathy should check their feet regularly, wear appropriate shoes, and keep their feet dry. All forms of exercise are fine except with active Charcot foot or a foot ulcer, when weight-bearing exercises should be avoided.

All forms of activity can be undertaken by people with diabetic kidney disease. Regular aerobic exercise may also prevent the onset or delay the progression of diabetic nephropathy (32,33). For people with end-stage renal disease, we recommend starting exercise at a low intensity and gradually building up. Electrolytes should be monitored if the activity is done during a dialysis session.

Most people with diabetic retinopathy can do all forms of activity, but there are some restrictions for certain eye conditions. People with type 2 diabetes who have a vitreous hemorrhage should not exercise until they receive clearance from their ophthalmologist. People with severe nonproliferative or unstable proliferative retinopathy should avoid vigorous-intensity activity and powerlifting (i.e., using very heavy weights).

In competitive sports, the importance of meeting nutrient and energy needs to support the amount of exercise being performed is well recognized (34,35). For individuals with type 2 diabetes who exercise on a regular basis, combining evidence-based diabetes and sports nutrition principles into a nutrition plan is key to supporting exercise performance, weight management, and glucose management. Beyond affecting the glucose response, types, amounts, and timing of nutrients affect fuel availability during and after exercise (35). The principle of nutrition periodization recognizes that amounts of exercise and training vary from day to day and week to week and that nutrient and energy intake must be flexible and adaptable to meet—and not exceed—goals for the specific types and amounts of exercise being performed (36). Ideally, people with type 2 diabetes should have access to a registered dietitian nutritionist (RDN) who can help them develop a nutrition plan.

The nutrition plan for someone with type 2 diabetes who is exercising on a regular basis is similar to that recommended for the general public, with energy intake divided into 45–65% from carbohydrate, 15–25% from protein, and 20–30% from fat. The eating plan should be sufficient to meet the demands of training and help with adaptation and recovery between exercise training sessions. It should also contain all essential vitamins and minerals. As exercise levels increase, daily intake of carbohydrates and protein will also need to increase. This means that tables generated for athletes without type 2 diabetes should be used (Table 2). Supplements will only be of any benefit if the nutrition plan is inadequate or there is a diagnosed deficiency such as a low iron or calcium level. There is no evidence that extra doses of vitamins improve sports performance (37,38).

Some individuals may want to lose weight—particularly fat mass—to improve sports performance, make the team in a weight-class sport, or achieve an aesthetically pleasing body shape. When these individuals want to lose weight, it is imperative to minimize the risk of introducing disordered eating behaviors (39). Successfully losing weight while doing a significant amount of exercise and without introducing nutrient deficiencies such as dehydration, inadequate protein and carbohydrate intake, and low micronutrient intakes can be difficult. Thus, these individuals should have the support of an RDN.

A variety of eating patterns are recognized to be appropriate and healthful for individuals with diabetes (40). Although the macronutrient composition of these eating patterns may vary, key aspects of health-promoting eating patterns are their focus on whole foods while minimizing highly processed foods, limiting refined grains and added sugars, and incorporating nonstarchy vegetables as a core part of meals and snacks. Eating plans for active individuals with type 2 diabetes should be personalized, flexible, and sustainable to support exercise and lifestyle goals, manage glycemia, improve insulin sensitivity, maintain a healthy body weight/body composition, and reduce cardiovascular disease risk. For detailed information beyond this article, interested readers are referred to a recent ADA consensus report on nutrition and diabetes (40).

As insulin resistance, driven in part through chronic hyperglycemia, is a key underlying mechanism for type 2 diabetes, lowering carbohydrate intake has been suggested to improve glycemia and other outcomes of type 2 diabetes (41,42). Several carbohydrate-restrictive eating plans have been described in the literature (43) and are commonly grouped into three main categories based on carbohydrate intake of 1) 20–50 g/day or <10% of the 2,000 kcal/day total nutrition intake that is generally sufficient to induce ketosis; 2) <130 g/day or <26% of a 2,000 kcal/day total nutrition intake; and 3) <45% of the 2,000 kcal/day total nutrition intake (43,44). Very-low-carbohydrate eating plans result in a so-called metabolic shift toward greater reliance on fatty acids and less reliance on glucose as a primary energy source and thus have received much attention in both diabetes and sports nutrition. However, these eating plans carry potential risks with exercise, including dehydration, electrolyte imbalances, nutrient deficiencies, and bone loss (45). Evidence is lacking that these eating plans are suitable or offer performance benefits, including in athletes with type 2 diabetes, and they may contribute to reduced exercise capacity, especially at high exercise intensities (46).

The timing of meals and snacks before, during, and after exercise is a key part of nutrition plans. Daily carbohydrate intake should relate to the fuel cost of the exercise training, with the aims of avoiding weight gain or reducing body fat mass and preventing hypoglycemia, particularly for people taking insulin or an insulin secretagogue. With respect to blood glucose management, thinking about timing of exercise in relation to meals and snacks and adjusting insulin and other medication dosages in anticipation of the amount of planned exercise is essential to keeping glucose in the target range.

Accounting for intake of both the types and amounts of carbohydrates consumed is an essential focus because this nutrient is central to fueling muscles and keeping blood glucose from falling or rising excessively, especially as exercise intensity and duration increase. A useful resource that interested readers are referred to is the Athlete Plates (47), which provide a simple illustration for how food intake can be adjusted for easy, moderate, and hard exercise days to meet nutrient and energy needs for different levels of activity. During events lasting >1 hour, carbohydrate intake may be necessary to maintain optimal performance, although it is most needed for long-endurance events or if the glucose level is falling. A general starting guideline for carbohydrate intake is 30–60 g/hour spread out in three or four equal amounts across the hour (48).

An individualized nutrition plan should be created alongside a RDN (39). The nutrition plan should be personalized to consider requirements for the individual’s chosen sport and the impacts on glycemia and weight management. An RDN with specialty knowledge of diabetes and sports is the most qualified to assist active people with type 2 diabetes. Readers are referred to a joint position statement on nutrition and athletic performance from the American College of Sports Medicine and its partner organizations for a comprehensive review of evidence-based nutrition recommendations to enhance exercise performance and recovery (48).

Having a hydration (drinking) plan that outlines the types and amounts of fluids to consume before and during exercise is also important for performance and glucose management (Figure 2A). Fluid requirements vary with duration of exercise, type of event, and ambient air temperature. During exercise, water is lost through sweat and breathing. Electrolytes (salt, potassium, and chloride) are also lost in sweat. Thus, if sufficient water and electrolytes are not taken when exercising, dehydration and electrolyte deficiency will occur. Even mild dehydration can affect performance (49). Because of the osmotic effect of glucose, people with type 2 diabetes are at greater risk of dehydration. In addition, some diabetes drugs (e.g., sodium–glucose cotransporter 2 [SGLT2] inhibitors) can cause further dehydration (discussed further below).

At the start of exercise, individuals should be well hydrated. The urine color chart in Figure 2B can be used to monitor hydration status. Urine the color of lemonade suggests adequate hydration, whereas urine the color of apple juice suggests that more fluid is needed. In the hour before exercise, 200–300 mL water with a small amount of low-glycemic or diluted no-sugar-added fruit juice for flavor should be drunk. This will help with hydration, as well as helping the body absorb glucose during activity. Water is adequate for hydration for any exercise lasting up to 90 minutes. If the exercise lasts >90 minutes or additional carbohydrate is needed, an isotonic drink that also supplies electrolytes should be consumed. Water is the best fluid for rehydrating. It can be difficult to know how much fluid was lost during exercise. A practical solution to this is to weigh before and after exercise and take in 500 mL of fluid for every 0.5 kg of weight loss. If weight loss is not known, then a rough estimate for how much water is needed after exercise would be 500 mL for every 1 hour of exercise.

People who are not taking diabetes medications (i.e., managing type 2 diabetes with dietary and lifestyle adaptations only) can exercise normally and do not need to monitor their glucose for exercise. Individuals who are on insulin therapy should check their glucose before starting an exercise session so they know their starting glucose level and the direction in which the blood glucose is trending. This information will allow them to determine whether any other measures should be taken before starting exercise (Figure 3). For people taking insulin or a sulfonylurea, if glucose falls below 70 mg/dL (4.0 mmol/L) or below 118 mg/dL (6.6 mmol/L) with a continuous glucose monitoring (CGM) down arrow during exercise, carbohydrates should be consumed. Regardless of whether individuals are taking any medications, if glucose falls below 63 mg/dL (3.5 mmol/L) or 70 mg/dL (4.0 mmol/L) on CGM, exercise should be stopped and carbohydrates should be consumed. Exercise should not resume until glucose reaches 90 mg/dL (5 mmol/L).

Exercise should be delayed for individuals taking insulin or sulfonylureas if the glucose level is <90 mg/dL (5 mmol/L) because of the risk that glucose will drop during exercise. If the glucose concentration is 90–270 mg/dL (5–15 mmol/L), it is safe to start exercise. If the glucose level is >270 mg/dL (>15 mmol/L), whether people can exercise will depend on whether they have eaten in the past 2 hours (Figure 3). For individuals who take a biguanide, dipeptidyl peptidase 4 (DPP-4) inhibitor, SGLT2 inhibitor, glucagon-like peptide 1 (GLP-1) receptor agonist, or thiazolidinedione, exercise should not be performed if glucose is <70 mg/dL (4.0 mmol/L); if glucose is >270 mg/dL (>15 mmol/L) and they feel well, exercise can be started, but glucose should be closely monitored and fluids increased. A hypoglycemic event within the previous 24 hours is a contraindication to performing exercise because of a substantially increased risk of a more serious hypoglycemic episode during the exercise (50,51).

CGM allows for real-time monitoring of interstitial glucose concentrations, is becoming more affordable (i.e., reimbursable), and has evidence of decreased A1C in people with type 2 diabetes who take insulin (52). CGM provides information unattainable by capillary blood glucose checks. It features instantaneous real-time display of both the current glucose level and the rate of glycemic change and provides alerts and alarms for actual and impending hypoglycemia and hyperglycemia, with both around-the-clock monitoring and the ability to check glycemic data retrospectively. Commercial CGM systems are becoming progressively more accurate, precise, and user-friendly and are small, reasonably unobtrusive, and comfortable. These systems can inform, educate, motivate, and alert people with diabetes about their glycemic management.

Clinicians and people with diabetes must be aware, however, that a lag time exists between the glucose value in the vasculature and the interstitial fluid glucose measured with CGM and can influence the accuracy of CGM readings compared with capillary blood glucose values (53). At rest, there is a lag time of ∼5 minutes (54); however, in situations of rapid glucose change such as exercise or after a meal containing carbohydrates, this lag time can increase to 12–24 minutes or even longer, as seen in people with type 1 diabetes (53,55). Therefore, it is important to preempt changes in glucose with exercise using the CGM trend arrows and to adjust insulin or consume rapid-acting carbohydrates in advance to impending hypoglycemia or hyperglycemia. Figure 3 shows how to adapt exercise advice for individuals who use CGM.

Some pharmacological agents used in management of type 2 diabetes (insulin and insulin secretagogues) may confer risk of hypoglycemia or ketoacidosis around exercise (with some evidence also with SGLT2 inhibitors), especially if exercise sessions are prolonged. With metformin, there is some evidence that its use with exercise training can blunt exercise-induced improvements in cardiorespiratory fitness (56,57). Evidence on common pharmacological therapies that are likely to influence glycemia during exercise is discussed below and summarized in Table 3. However, the body of literature identifying interactions between exercise and pharmacological therapies for type 2 diabetes is sparse, and more research is needed in this area (58,59).

Individuals who use insulin will be at greater risk of hypoglycemia during exercise and therefore will need to make adaptations to their exercise plans. This risk can be minimized by exercising fasted before breakfast, when the insulin levels are lowest, or at least 2 hours after a meal, when fast-acting mealtime insulin will have mostly cleared.

Glycemic responses to physical activity depend on the type, duration, and intensity of the exercise bout, which means it is difficult to make uniform recommendations for insulin management. However, for people exercising within 2 hours of a meal and who are on a regimen of fast-acting and basal insulin, we recommend starting with a 30% reduction of insulin given at the meal before exercise. For people taking twice-daily mixed insulin, we recommend a 15% reduction of insulin if taken with that meal (Figure 3). Consuming extra carbohydrates is an alternative to reducing insulin. Based on our experience, a good starting amount is 30 g/hour, split up evenly across the hour with some taken every 20 minutes (Figure 3). Adaptations should then be made based on results. As mentioned in the discussion on glucose monitoring above, additional glucose may be needed before starting the exercise session. Glucose checks should be performed before and during exercise in individuals who take insulin.

Metformin is the first-line glucose-lowering therapy and most frequently prescribed drug for type 2 diabetes (60), with many years of established safety and efficacy and a low cost (61,62). In the acute setting, metformin has been shown to lower glucose by a small amount but is not associated with hypoglycemia (58). Therefore, adaptations do not need to be made for exercise for people taking metformin alone (Figure 3). Because metformin decreases the blood glucose concentration primarily by reducing hepatic gluconeogenesis (63) and exercise results in metabolic adaptations that also include improvements in insulin sensitivity in both skeletal muscle and adipose tissue, it is logical to think that combining metformin with exercise training could lead to additive improvements in glucose homeostasis. However, both preclinical animal and clinical human studies have shown that this is not the case (64–68).

A study by Malin et al. (65) found that, in people with prediabetes, exercise training resulted in improved whole-body insulin sensitivity, but adding metformin blunted these adaptations. Other randomized controlled trials have found that there are no additional benefits of adding metformin to an exercise intervention in people with type 2 diabetes (66–68). To summarize, for people with type 2 diabetes, the addition of metformin to a lifestyle modification or supervised exercise training program does not appear to alter glycemia more than either metformin or physical training alone.

SGLT2 inhibitors are a newer class of oral diabetes medications that are emerging as an effective means to improve glucose homeostasis (69,70), with additional cardiovascular and cardiorenal health benefits (71–74). These drugs work by preventing the reuptake of glucose in the proximal convoluted tubule of the kidney, resulting in an increase in urinary glucose excretion (75,76).

Because of the osmotic effect of glucose, this process results in significantly more urine being produced over 24 hours—in some cases, an increase of just under 500 mL. SGLT2 inhibitors also reduce plasma insulin concentrations and increase plasma glucagon levels, which increases the rate of lipolysis. During exercise, lipolysis is also increased relative to when at rest (77), and there is an increase in fluid loss (as sweat and via exhalation). Therefore, using SGLT2 inhibitors may increase the risk of euglycemic ketoacidosis, and individuals with β-cell deficiency or longer use of SGLT2 inhibitors may be at higher risk (78–80).

No studies have looked at how SGLT2 inhibitors affect glucose during exercise, but there is no reason to presume that they would lower glucose (Figure 3). Because of an increased risk of ketoacidosis when using an SGLT2 inhibitor, additional caution should be taken in patients treated with insulin when insulin doses are reduced by too much or not administered at all, which may happen during exercise. Administration of an SGLT2 inhibitor should be carefully considered, and patients should be informed of the possible risks and how to prevent them.

Glucagon like peptide 1 (GLP-1) is an incretin hormone released from the intestinal cells that stimulates insulin and inhibits glucagon release, therefore having an important role in glucose homeostasis (81). GLP-1 receptor agonists have emerged as a promising treatment for type 2 diabetes, with evidence of beneficial cardiovascular effects and improved glucose homeostasis (82).

There is some evidence that exercise can increase GLP-1 in healthy, overweight, and obese individuals (83–85), although less is known about the effects of exercise on GLP-1 in people with type 2 diabetes (86). Mensberg et al. (87) found that, in people with type 2 diabetes who engaged in endurance and resistance training, treatment with a GLP-1 receptor agonist resulted in greater improvements in A1C, body weight, fasting blood glucose, and blood pressure compared with exercise alone.

GLP-1 receptor agonists do not increase the risk of hypoglycemia in general, but that risk may be increased when they are combined with insulin, especially during exercise, where insulin sensitivity is increased. Therefore, an appropriate reduction in insulin will be crucial for exercising in individuals on combination therapy with a GLP-1 receptor agonist and insulin.

Both sulfonylureas and meglitinides act by increasing insulin release from pancreatic β-cells (88). Exercising while taking sulfonylureas has been shown to result in a greater drop in glucose concentration because of higher endogenous insulin levels (89). The increased risk of hypoglycemia during exercise is higher when undertaking low- to moderate-intensity continuous exercise (90), and the starting glucose level is comparatively low. Therefore, increased caution is advised during exercise for people who are taking certain sulfonylureas, and these individuals are advised to consume carbohydrates if their glucose is low before exercise (Figure 3) and to carry rapid-acting carbohydrate sources during exercise to treat hypoglycemia (12). Little is known about either the acute or chronic interactions of meglitinides and exercise.

Athletes and other individuals who rigorously exercise with type 2 diabetes represent a unique population whose diabetes care requires a multidisciplinary approach. When undertaking an exercise training program, individuals with type 2 diabetes must take additional factors into consideration compared with those without diabetes, including the effects of medications, and perform glucose monitoring around exercise sessions, as summarized in Table 4. These individuals are best supported through personalized, team-based preventive and treatment approaches that include input from their diabetes team, a diabetes and sports RDN, other physicians, and coaches/athletic trainers. Having a basic understanding of glucose metabolism during exercise, nutritional requirements, blood glucose management, medications, and sport-related considerations is important for health care professionals who care for people with type 2 diabetes.

No potential conflicts of interest relevant to this article were reported.

All of the authors contributed to the literature search and writing of this manuscript and approved the final draft for submission. S.N.S. is the guarantor of this work and, as such, takes full responsibility for the integrity of the content.