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Exercise and Diabetes Control

A Winning Combination

Sheri R. Colberg, PhD; David P. Swain, PhD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 4 - APRIL 2000


In Brief: It is becoming evident that in many individuals, diabetes of all types can be precipitated or exacerbated by inactivity. Exercise is a cornerstone in diabetes management and conveys many health benefits. Different forms of exercise can have varying effects on the blood sugar response, especially in individuals using insulin. Appropriate exercise for effective management of blood sugar levels and published clinical exercise recommendations for individuals with type 1 and type 2 diabetes include additional blood glucose monitoring, modified insulin doses, and supplemental carbohydrate intake. Physicians who treat exercising patients with diabetes should tailor programs to meet individual requirements.

The prevalence of diabetes mellitus is increasing worldwide. In the United States alone, an estimated 16 million individuals have diabetes mellitus, and in close to a third of them the disease is undiagnosed. A vast majority (90% to 95%) have type 2 diabetes (formerly called non-insulin-dependent, or NIDDM), but many of these individuals also use exogenous insulin (1). More than 1 million individuals have type 1 diabetes (formerly called insulin-dependent, or IDDM). Although all types of diabetes mellitus result in hyperglycemia, the etiology depends on the type. In most individuals with type 1 diabetes, an environmentally triggered autoimmune process destroys pancreatic beta-cells, rendering the body unable to manufacture insulin. Type 2 diabetes results more often from an insulin resistance syndrome that is usually exacerbated by excess body fat (2). A third form, gestational diabetes, occurs in women mainly during their third trimester of pregnancy. Although the condition usually resolves in the postpartum period, it is associated with a higher risk of type 2 diabetes later in life (3).

Benefits of Exercise

Exercise plays a role in the prevention of type 2 diabetes as well as other serious health problems, including cardiovascular disease and certain cancers. It is becoming evident that many cases of type 2 diabetes may be precipitated by inactivity. Manson et al (4) examined the potential role of physical activity in the primary prevention of type 2 diabetes by studying a cohort of 87,253 middle-aged US women (ages 34 to 59) for 8 years. The investigators found that women who engaged in vigorous exercise at least once a week had a lower risk of developing diabetes (age-adjusted relative risk of 0.67; P < 0.0001) compared with women who did not exercise, irrespective of obesity (4). In another study (5), participation in leisure-time physical activity was inversely related to the development of type 2 diabetes, especially for men at highest risk. Each 500-kcal increment in weekly energy expenditure reduced the age-adjusted risk of diabetes by 6%. More recently, Hu et al (6) published the results from a longitudinal study of 70,102 female nurses who were studied from 1986 to 1992. The researchers showed that exercise need not be vigorous to have the same preventive effect. In these women, a greater physical activity level, even in those who did only moderately paced activities such as walking, was associated with a substantial risk reduction for the development of type 2 diabetes.

Being physically active not only can reduce the risk for developing type 2 diabetes, but exercising regularly can also bring many health benefits to all, especially for those with diabetes. Recent research has shown that exercise lowers the risk of macrovascular disease (7), obesity, hypertension (8), certain cancers such as breast (9) and colorectal (10), and all-cause mortality in the general population. Kampert et al (11) studied 7,080 women and 25,341 men (mean age, 43 years; age range, 20 to 88) for about 8 years and assessed baseline physical fitness with a maximal treadmill test and physical activity via questionnaire. They found a strong inverse relationship between all-cause mortality and level of physical fitness in both men and women (P for trend < 0.01). Physically active men, but not women, were also at lower risk of death from cancer and all causes.

Exercise and Diabetes Therapy

Exercise is a cornerstone in the treatment of diabetes, along with proper diet and medication, but it may present greater obstacles for the type 1 patient (see below). Physical exercise confers the well-established benefit of improved insulin sensitivity for all patients (12). Increased sensitivity may improve glucose homeostasis and necessitate lower doses of medications. Physically conditioned individuals with diabetes also have a heightened insulin sensitivity, which allows glucose to enter muscle cells more efficiently, both acutely and chronically with exercise (13). Acute changes probably stem from an increased rate of muscle glycogen repletion, while chronic changes likely indicate increases in the total amount of metabolically active muscle. These adaptations may result in lower basal and postprandial insulin needs for diabetic patients (12). Increased insulin sensitivity begins to decline, however, in as little as 1 to 2 days without exercise (14).

While exercise is considered a cornerstone in type 2 management, glycemic control in individuals with type 1 diabetes can be challenging for medical care-givers because of the complexities of regulating blood sugar with the added, and often unpredictable, variable of exercise. Furthermore, unlike most exercisers with type 2 diabetes, some patients with type 1 diabetes may not have improved glycemic control with regular exercise training if changes in their diet and insulin dosage do not appropriately match exercise requirements (15).

Choosing Appropriate Exercise

Different forms of exercise have varying effects on the blood sugar response, especially in patients using insulin. Variables that must be considered include exercise type, duration, and intensity and the patient's fitness level.

Exercise type. For individuals with diabetes, the type of exercise done (aerobic vs anaerobic) exerts a significant effect on blood sugar responses during the activity.

Anaerobic. Activities lasting less than 2 minutes (such as sprinting or power lifting) are primarily anaerobic in nature and are fueled by phosphagens (adenosine triphosphate [ATP] and creatine phosphate) and carbohydrates (glycogen) stored in skeletal muscle. The contributions of phosphagenic and glycolytic metabolism depend on the duration and intensity of the activity (figure 1).

[Figure 1]

Feed-forward glycemic control (that due to immediate activation of the sympathetic nervous system at the onset of exercise) causes an immediate rise in blood glucose levels, and only a minimal amount of blood glucose is used to fuel anaerobic activities. Blood glucose levels are easier to maintain during such exercise but can also rise as a result. Kjaer et al (16) demonstrated that individuals with type 2 diabetes experienced an exaggerated rise in blood glucose levels during, and for 60 minutes after, maximal exercise on a dynamic cycle ergometer. But patients then experienced increased insulin sensitivity that lasted for at least 24 hours postexercise.

Repeated bouts of an intense activity (eg, interval or circuit-type weight training) can result in significant muscle glycogen depletion that greatly increases postactivity insulin sensitivity. Eriksson et al (17), who studied individuals who had impaired glucose tolerance, found that several months of resistance training led to a much greater increase in insulin sensitivity than seen in patients who engaged in aerobic or no training. This was attributed primarily to an increase in glycogen storage.

Aerobic. For any exercise lasting 2 minutes or longer, the body uses all three of the different energy systems (see figure 1): phosphagens, anaerobic glycolysis (use of glycogen stores), and ATP derived from aerobic metabolism of fats and carbohydrates. In addition, many hormonal changes occur in normal individuals to maintain blood sugar levels around 100 mg/dL (5.5 mM). Sustained aerobic activities such as running, cycling, swimming, and aerobic dance rely on a mix of aerobically processed fuels, but the major sources are fats and carbohydrates (both muscle glycogen and blood glucose).

For exercise done at higher intensities (70% to 75% of maximal aerobic capacity or higher), carbohydrates are the body's fuel of choice regardless of training status. Blood glucose use can become quite significant during these activities, especially as muscle glycogen stores become more depleted during prolonged exercise. As more glycogen is depleted, the risk of hypoglycemia occurring during muscle glycogen replenishment increases, especially in those who do not adjust their insulin doses. For shorter duration and more intense activities, carbohydrate supplementation alone is effective in maintaining euglycemia. For prolonged exercise such as in marathons, triathlons, and long practices, most diabetic athletes must reduce their insulin dose because an increased carbohydrate intake by itself is not adequate to compensate for the accelerated glucose uptake during the activity (18).

An individual's training status will additionally affect the fuels used during an activity. As exercise intensity increases, the body switches from using a mix of fats and carbohydrates to primarily carbohydrates for fueling prolonged activity. Chronic training increases the proportion of fat used during low- or moderate-intensity activity, delaying the "crossover" point to greater carbohydrate use with increasing exercise intensity (19). Metabolism of more fats (both intramuscular triglycerides and circulating plasma free fatty acids) spares muscle glycogen and blood glucose and allows trained individuals with diabetes to maintain blood sugar levels more effectively during endurance activities.

Exercise Prescription for Individuals With Diabetes

Before beginning an exercise program, individuals with diabetes should have a medical evaluation to screen for macrovascular and microvascular complications that may be exacerbated by exercise. The exam should screen for cardiovascular health, peripheral arterial disease, retinopathy, nephropathy, and peripheral and autonomic neuropathy. A graded exercise test is recommended to screen for cardiovascular disease in individuals who meet any of the criteria in table 1 and who wish to participate in moderate- to high-intensity exercise programs (20).


TABLE 1. Criteria* for Assessing the Need for Graded Exercise Testing Among Patients Who Have Type 1 or Type 2 Diabetes


Age > 35 years

Type 2 diabetes > 10 years' duration

Type 1 diabetes > 15 years' duration

Presence of any additional risk factor for coronary artery disease

Presence of microvascular disease (proliferative retinopathy or nephropathy, including microalbuminuria)

Peripheral vascular disease

Autonomic neuropathy


*If an individual meets any one of these criteria, exercise testing is recommended prior to participation in moderate- to high-intensity exercise programs.


ACSM recommendations. The American College of Sports Medicine (ACSM) recommendations (table 2) for all individuals (including those with diabetes) is that aerobic physical activity be done a minimum of 3 to 5 days a week, for 20 to 60 minutes at 40% to 85% of maximum oxygen uptake reserve (VO2R or heart rate reserve [HRR]), or at 55% to 90% of maximal heart rate (21). For less conditioned individuals, exercise can be done at the lower intensity level for a longer duration, at least until a higher level of fitness can be achieved. Individuals with type 2 diabetes should especially be encouraged to progress to a higher total duration of exercise (eg, 1 hour daily) to facilitate fat loss.


TABLE 2. American College of Sports Medicine Guidelines for Aerobic Exercise Programs

Exercise Characteristic Recommendation*

Mode Continuous, rhythmic, prolonged activities using the large muscle groups of the arms and/or legs
Intensity Range of 55%-90% of maximal heart rate, 40%-85% of VO2R or HRR, or RPE of 12-16 (somewhat hard to hard)
Duration Minimum of 20-60 min of continuous aerobic activity to improve fitness and endurance capacity
Frequency Minimum of 3-5 dy/wk, with frequency determined by exercise duration and intensity
Rate of progression Initial conditioning of 4-6 wk, improvement phase lasting 4-5 mo, and maintenance thereafter

*Resistance-type and flexibility training are recommended 2-3 days per week. VO2R = VO2 reserve; HRR = heart rate reserve; RPE = rating of perceived exertion


Monitoring intensity. Exercise intensity can be prescribed and monitored in several ways. Oxygen uptake reserve is a percentage of the difference between maximal and resting oxygen uptake (VO2) (22). If metabolic data from an exercise stress test are available, a target VO2 can be determined at a desired percentage of VO2R and then translated into a workload for a given exercise. More typically, exercise intensity is established using heart rate. Calculation of a training heart rate or range using the HRR method is done with the following formula:

Training HR = intensity fraction 3 (HRmax - HRrest) + HRrest, where the intensity fraction is chosen between 0.40 and 0.85 (the lower end of the range is used for patients with low initial fitness). A known value of HRmax, obtained from a maximal stress test, should be used if available; otherwise, HRmax may be estimated by subtracting the patient's age from 220. For example, calculation of a training HR at 50% of HRR for a patient with a maximal HR of 170 beats per minute (bpm) and a resting HR of 80 bpm would result in a training HR of 125 bpm:

Training HR = 0.50 3 (170 - 80) + 80 = 45 + 80 = 125 bpm.

Resistance training. Resistance or weight training that includes at least one set of each of 8 to 10 different exercises using the major muscle groups is also recommended 2 to 3 days a week. Each set should consist of 8 to 12 repetitions, with the amount of weight increased when the individual can complete 12 or more repetitions. For those age 50 or older who have diabetes or preexisting health concerns such as hypertension, more repetitions (12 to 15) done at a lower weight may be more suitable. Flexibility training should be incorporated into the overall fitness routine a minimum of 2 to 3 days per week to develop and maintain joint range of motion and to minimize the potential loss of flexibility resulting from glycosylation of various joint structures. Stretching for 5 to 10 minutes should be done either after an aerobic warm-up or following the completion of an exercise session.

Warm-up and cooldown. No matter what type of activity is done, the standard recommendation for all individuals (with or without diabetes) is to include proper warm-up and cooldown periods. A warm-up consists of 5 to 10 minutes of a lower-intensity aerobic activity that uses the same muscles that will be exercised at a higher intensity (eg, walking before starting to jog). The cooldown consists of 5 to 10 minutes of less intense activity. Such activities may ease the cardiovascular transition between rest and exercise and help prevent muscle and joint injuries.

Exercise Recommendations for Blood Glucose Management

Exercise recommendations published collaboratively by the American Diabetes Association and the ACSM give general guidelines for maintaining metabolic control and avoiding hypoglycemia (20). The recommendations address management of blood sugar levels in type 1 diabetes during exercise and explain metabolic control before exercise, blood glucose monitoring before and after exercise, and food intake (table 3). The following sections serve to assist the physician in fully understanding all aspects of the recommendations.


TABLE 3. American College of Sports Medicine and American Diabetes Association General Guidelines for Exercise and Type 1 Diabetes


Metabolic Control Before Exercise
Avoid exercising if fasting glucose levels are > 250 mg/dL and ketosis is present; use caution if glucose levels are > 300 mg/dL and no ketosis is present

Ingest added carbohydrates if glucose levels are < 100 mg/dL

Blood Glucose Monitoring Before and After Exercise
Identify when changes in insulin or food intake are necessary

Learn the glycemic response to different exercise conditions

Food Intake
Consume added carbohydrate as needed to avoid hypoglycemia

Carbohydrate-based foods should be readily available during and after exercise


Maintaining glycemic control. Individuals should identify when changes in insulin or food intake are necessary and learn their own glycemic response to different exercise conditions. Glycemic balance is essential to optimal exercise performance. The key to sustaining normal blood glucose levels during physical activity is frequent blood sugar monitoring coupled with appropriate preventive and corrective changes to insulin dosage and/or food intake.

In a recent study (23), one of us (SRC) reported on the extent of usage of current recommendations by 238 exercisers with type 1 diabetes. Participants stated that the general nature of available guidelines requires a trial-and-error period when they participate in new or unusual physical activities. To establish their usual glycemic patterns, exercisers had to engage in more blood sugar monitoring before, during, and after exercise. Once a pattern was found, then each one's subsequent response was easier to predict.

A recommendation addressing metabolic control states that type 1 diabetic individuals should avoid exercise if blood glucose levels are greater than 250 mg/dL (13.9 mM) and ketosis is present, and use caution if glucose levels are greater than 300 mg/dL (16.8 mM) and no ketosis is present (table 4) (23). While this recommendation is well advised in most cases, the physician should allow for individual variations if an exerciser has an established pattern such as short excursions in blood glucose levels postprandially without ketosis. Trained athletes with type 1 diabetes report optimal performance when starting exercise at lower levels of blood glucose (70 to 200 mg/dL) (23).


TABLE 4. Exercise Recommendations for Type 1 Diabetes Based on Preexercise Blood Glucose Levels

Glucose
Level
Ketones Exercise
Advised?

< 100 mg/dL - Yes, but carbohydrate snack may be needed first (allows for individual variation in response)
100-250 mg/dL - Yes
> 250 mg/dL No Yes*
> 250 mg/dL Yes No
> 300 mg/dL No Use caution*
> 300 mg/dL Yes No

*At these levels, some athletes may choose to inject a small dose of short-acting insulin prior to exercise.

Reprinted with permission from Colberg S: Use of clinical practice recommendations for exercise by individuals with type 1 diabetes. Diabetes Educator 2000;26(2);265-271.


Modifying insulin dosage. The current ubiquity of blood glucose monitors affords many individualized modifications of insulin and diet to avoid hypoglycemia during exercise. Although the actual reduction will be affected by the individual's insulin regimen, fitness level, exercise choice, and other factors, a general guideline for insulin users is to reduce by 30% to 50% their dose of short-acting insulin (usually regular or lispro, a modified insulin) within 2 to 3 hours of beginning exercise. Alternately, some insulin pump users simply choose to reduce or eliminate their basal infusion of insulin during exercise. These recommendations are, however, merely starting points, as individual responses will vary greatly. The onset and peak of lispro insulin action are much more rapid than those of regular insulin, and greater reductions may be needed if lispro is given before exercise.

Because "tighter" (closer to recommended) metabolic control is achievable with various insulin regimens and frequent blood glucose monitoring, most diabetic exercisers can experience a decrease in their blood sugar levels while doing aerobic exercise, regardless of their starting blood sugar levels, as long as they have some circulating insulin "on board." Many exercisers report injecting 1 to 3 units of regular or lispro insulin before exercising when they have preexercise blood sugars of 250 to 300 mg/dL or greater (23). They find that the combination of exercise-induced and insulin-mediated glucose uptake lowers blood sugar levels more rapidly than exercise or insulin alone.

Carbohydrates for exercise. Other exercise recommendations for individuals with type 1 diabetes advise having extra carbohydrates handy and consuming them as needed to avoid hypoglycemia during and after exercise. When insulin doses can be reduced before exercise, additional carbohydrates may not be needed. For more spontaneous exercise, or if an individual prefers not to modify insulin dosages, glucose monitoring should be used to determine the actual amount of carbohydrate needed; a typical intake for endurance activities is 15 to 30 g of simple carbohydrates (such as sports drinks, juice, regular soda, glucose tablets, hard candy, bagels, fruit, or dried fruit) every 30 to 60 minutes during prolonged exercise. Access to simple carbohydrates is essential for the rapid treatment of hypoglycemia, should it occur. In addition, a glucagon emergency kit should be available, especially during longer endurance sporting activities.

Another recommendation is that exercisers ingest additional carbohydrates if glucose levels are below 100 mg/dL (5.5 mM) before exercise. At this level, an exerciser may need to ingest 10 to 15 g of simple carbohydrates and wait 5 to 10 minutes before beginning to exercise. As noted for other recommendations, this serves as a starting point until an individual's glycemic response can be determined. Additional carbohydrates may not be needed for exercise of short duration (< 30 minutes) if the exerciser has sufficiently reduced the preexercise insulin dosage. Intense, shorter exercise usually requires a lower carbohydrate intake as well.

Although a snack may not be needed immediately to maintain blood glucose levels after exercise, research has shown that consuming carbohydrates within 30 minutes after exhaustive, glycogen-depleting exercise allows for more efficient restoration of muscle glycogen (24). This step may also help prevent postexercise, late-onset hypoglycemia, which can occur up to 24 hours following such exercise (25). During the time of heightened insulin sensitivity, muscle uptake of glucose to restore glycogen can be accomplished with minimal circulating insulin. However, some individuals will require supplemental insulin—albeit less than usual—along with these carbohydrates to prevent hyperglycemia.

Type 2 diabetes. For individuals with type 2 diabetes who are not using supplemental insulin, such stringent recommendations are not necessary to maintain proper blood glucose levels during exercise. Blood glucose monitoring should be done before and after an activity to assess its effect on glycemia. Supplemental carbohydrates are generally not needed in these patients; however, blood glucose monitoring will reveal which individuals may need additional carbohydrates to prevent hypoglycemia during and following exercise. Use of certain oral hypoglycemic agents such as sulfonylureas carries a higher risk of exercise-induced hypoglycemia due to their longer half-lives. Other recommendations concern safe participation for type 2 exercisers with existing or developing diabetes-related complications such as cardiovascular disease, hypertension, neuropathy, or microvascular changes.

Exercise Risks and Precautions

Exercise under some hyperglycemic conditions—especially if insulin deficiency and ketosis are present—can actually worsen metabolic control. This disruption stems from excessive secretion of counterregulatory hormones that may increase already high levels of glucose and ketones. Hyperglycemia alone can stimulate urine production and increase fluid losses; this phenomenon combined with sweat and other fluid losses from exercise can cause dehydration in diabetic individuals.

Avoiding hypoglycemia. The most immediate and serious potential risk is that exercise can result in hypoglycemia (blood glucose level of 65 mg/dL [3.6 mM] or lower). Supranormal levels of circulating insulin resulting from the mobilization of injected insulin during exercise can attenuate or prevent the normal mobilization of glucose and other substrates and increase muscle uptake of glucose (26). In fact, the risk of hypoglycemia during or following exercise is substantial, especially in insulin users who take a preset insulin dosage, unless appropriate modifications in food or insulin are made.

A preventive strategy is to exercise when circulating insulin levels are lower (at least 3 to 4 hours after the last injection of short-acting insulin, and not during a dose peak). This makes insulin levels during exercise more similar to those in a nondiabetic individual. Morning exercise, especially if done before any insulin injection, usually exerts less hypoglycemic effect than the same exercise done later in the day. This results from the effects of higher circulating cortisol and other glucose-raising hormone levels early in the day (ie, insulin resistance is generally greater in the morning), along with lower circulating levels of insulin (27). Conversely, evening exercise conveys the greatest risk for nocturnal hypoglycemia unless the patient makes preventive changes in food intake or insulin doses.

Mitigating diabetic complications. People with diabetes may experience a variety of macrovascular and microvascular complications that can complicate exercise. Any individual with a high risk for underlying cardiovascular disease should be considered for a graded exercise test before beginning a moderate- to high-intensity exercise program (22). For exercise involving the feet, precautionary measures are recommended, especially if peripheral neuropathy is present. Use of silica gel or cushioned midsoles, polyester or synthetic-blend socks, and proper footwear is essential to prevent blisters, keep the feet dry, and minimize or prevent trauma. Non-weight-bearing exercise such as aqua aerobics can be substituted for weight-bearing activities.

Hypotension and hypertension following vigorous exercise are more likely to develop in individuals with autonomic neuropathy. Their thermoregulatory capacity may be inadequate, and special care is needed to maintain adequate hydration. Active, strenuous exercise is contraindicated for individuals with active vitreous hemorrhages due to unstable proliferative retinopathy. Intense exercise involving straining, jarring, or Valsalva-like maneuvers should be avoided as well. When nephropathy is present, individuals may have a reduced exercise capacity, but low- to moderate-intensity activities can be done.

Parting Advice

Individuals with diabetes can learn how to improve their blood glucose management during exercise. Making the appropriate regimen adjustments before, during, and after exercise is a trial-and-error process for each new activity. Physicians should encourage diabetic exercisers to test their blood glucose levels before, during (for new activities), and after exercise. They should also assist these individuals in predicting their exercise responses to new or unusual activities based on the type, duration, and intensity of exercise, insulin regimens, starting blood sugar levels, carbohydrate intake, and timing of exercise; all can affect blood sugar responses. All exercisers should be advised to consume extra rapidly absorbed carbohydrates as necessary to prevent or treat hypoglycemia during and following exercise.

Although the general exercise recommendations can be helpful, physicians may have to aid patients in modifying diet and insulin regimens because the recommendations require tailoring to meet individual needs (23). Furthermore, if a patient's preexercise metabolic control is poor or diabetes-related complications are present, clinical recommendations should be followed to prevent worsening or onset of complications. Exercise can be done safely by individuals with diabetes, and the health benefits are undeniable. Diabetes control and regular exercise truly form a winning combination.

References

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Dr Colberg is an assistant professor and Dr Swain is an associate professor in the department of exercise science, physical education, and recreation at Old Dominion University in Norfolk, Virginia. Address correspondence to Sheri R. Colberg, PhD, Dept of Exercise Science, Physical Education and Recreation, Old Dominion University, HPE Bldg, Rm 140, Norfolk, VA 23529-0196; e-mail to [email protected].


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