Abstract
More than 29 million people in the United States have diabetes mellitus, including both type 1 and type 2 diabetes. The CDC also estimates that upward of 86 million people can be classified as prediabetic, with as many as 30% of these people transitioning into diabetes within the next 5 years. Individuals with type 1 diabetes account for roughly 5% of those patients. Dating back to 2008 and 2009, roughly 18 000 youth were diagnosed with type 1 diabetes each year. The prevalence of diabetes is well known; most of the studies that are completed today relate to the progression and/or treatment of those with type 2 diabetes. Yet most physicians will have to take care of a type 1 diabetic patient who will want to be active. Having a fundamental knowledge of how exercise affects insulin and blood glucose and how to manage these patients is important. Time must be taken to modify each treatment regimen for each individual. One cannot stress enough the importance of providing patient education, ensuring adequate hydration, recognizing signs and symptoms of hypoglycemia/hyperglycemia, and how to treat and prevent these serious complications. All patients must have a care plan and access to supplies during exercise. It is known that poorly controlled blood glucose can have detrimental consequences in the long term. The question is if type I diabetic athletes who are allowed to have higher blood glucose during exercise are at the same risk for these potential complications.
“These athletes should be allowed to take ownership of their condition and encouraged to take an active role in their treatment.”
Background
To understand the role exercise can play and how it can affect the diabetic athlete, the basic differences between the nondiabetic and the diabetic athlete must be understood. It is also important to understand the difference between type 1 and type 2 diabetes mellitus. In type 1 diabetes, the body does not make a sufficient amount of insulin, secondary to autoimmune destruction of the B-cells of the pancreas. This can develop at any age but is usually diagnosed during childhood. There is no known way to prevent type 1 diabetes. In type 2 diabetes, the body is unable to utilize insulin appropriately.1-4 This is seen typically in obese, older patients. However, with the rise in childhood obesity, more patients are being diagnosed at a younger age. Type 2 diabetes mellitus can be prevented with the initiation and maintenance of healthy dietary choices and regular exercise. Obtaining an insulin level and a C-peptide level can differentiate between patients with type 1 and type 2 diabetes. 4 In type 1 diabetes mellitus, both these values are low. Other diagnostic tests can be done, including antibody testing, but that is beyond the scope of this review article.
Common signs and symptoms of poor glucose control include, but are not limited to, polyuria, polydipsia, polyphagia, weight loss, visual disturbances, fatigue, and/or ketosis.2-4 If any of these symptoms are present in the patient, definitive workup should be started, including testing for diabetes.
Diabetes and Exercise
Many primary care physicians will eventually encounter a patient who has type 1 diabetes, and this patient may want to participate in some form of physical activity. Exercise should never be discouraged in these patients because the benefits can far outweigh the risks. At times, when dealing with a younger athlete struggling with the diagnosis, it should be kept in mind that they should not be discouraged. Real-life examples of professional level athletes who were still able to function at the highest level despite their diagnosis should be provided—for example, Jay Cutler, quarterback for the Chicago Bears, and Gary Hall Jr, former Olympic Gold Medal swimmer. The most difficult aspect in dealing with these patients is providing education and insight into their disease process. These athletes should be allowed to take ownership of their condition and encouraged to take an active role in their treatment.
Exercise can be divided into 2 forms: aerobic and anaerobic. Aerobic exercise pertains to those activities that involve continuous, rhythmic, repeated movements of the same larger muscle groups for at least 10 minutes. It utilizes oxidative metabolism to provide adequate energy.5,6 Examples include walking, jogging, cycling, or basically any endurance-type activity. Anaerobic exercise is primarily resistance-type exercise, using the muscle strength to move a force with a short burst of activity. This pathway utilizes nonoxidative metabolism. Examples include sprinting and weight lifting.
The main fuel sources for any athlete during exercise are lipids and carbohydrates. The exact mix for their use depends on the type, intensity, and duration of exercise. Initially, muscle contraction uses stores of ATP and glycogen in the muscle itself. With increase in duration, a shift occurs to use outside sources for energy, including free fatty acids and blood glucose. During prolonged exercise, the main source of energy comes from blood glucose, which in turn comes from the breakdown of liver glycogen and ingested carbohydrates. With higher-intensity exercises, the need for the carbohydrates is even greater. In those with type 1 diabetes, ability to oxidize the ingested carbohydrates is impaired. Thus, the athlete relies more on muscle glycogen and free fatty acids/lipids. This makes them more prone to ketosis with vigorous exercise.5-7
During aerobic exercise, there is a lowering of blood glucose because it is being utilized for energy to create work. In comparison, during anaerobic exercise, there is usually a resultant increase in blood glucose level secondary to the rise of catecholamines (epinephrine, norepinephrine, growth hormone, and cortisol). Most sporting activities are a combination of both types of exercise, and one can see a moderating effect on glucose control during exercise.5,6
In nondiabetic patients at rest, as blood glucose concentration decreases, counterregulatory hormones are triggered. As blood glucose levels fall, insulin secretion is reduced, glucagon and epinephrine secretion increases, followed by norepinephrine, growth hormone, and cortisol release. With the increase in these hormones, glucose is released. However, if the counterregulatory hormones fail to increase the blood glucose adequately, impairment can be seen. Similar effects occur during exercise. These same counterregulatory hormones function similarly in a person with type 1 diabetes. However, most studies conclude that the response is impaired. The main difference is that the patient with type 1 diabetes cannot alter his/her insulin levels prior to exercise. If insulin is not properly dosed during exercise, it can result in a hyperinsulin state, leading to more complications.5,6
Poor Glycemic Control During Exercise
Despite the attempts at adequate control, complications can occur. Because the type 1 athlete is in a relatively hyperinsulinemic state at the onset of exercise and because exercise increases insulin sensitivity, if the insulin dose is not adequately reduced prior to initiating exercise, hypoglycemia can ensue.3,5,8 This is a main concern for the exercising diabetic patient. Symptoms of hypoglycemia can include autonomic dysfunction presenting as tachycardia, diaphoresis, palpitations, headache, dizziness, or neurogenic dysfunction causing vision abnormalities, fatigue, cognitive impairment, or even loss of consciousness. Hypoglycemic episodes are usually seen during or following evening exercising, rather than in the morning, secondary to the higher levels of cortisol in the morning hours. Hypoglycemia can occur immediately during exercise or it can be delayed, occurring 12 to 24 hours after exercise.2,3,5 In 1 study, 80% of patients who had a blood glucose of <120 mg/dL prior to start of exercise became hypoglycemic during exercise. 5 Despite this high percentage, an even more detrimental problem can occur in those who become hypoglycemic later. The delayed reaction is thought to be secondary to the duration of the insulin used combined with the impaired counterregulatory hormonal response in those with type 1 diabetes. This can result in low blood sugars around 2:00 or 3:00 in the morning, when the athlete is sleeping, which can be fatal.
Although hypoglycemia may be the most feared complication in diabetic patients who exercise, the potential for hyperglycemia should not be forgotten. Hyperglycemia is usually seen in those athletes who are poorly controlled, but it can also be seen in those performing high-intensity, vigorous exercise.2,3,5,8 The physical strain of the exercise, combined with the psychological stress of exercise, can cause an increase in the glucose-altering catecholamines, resulting in a higher blood glucose level.5,8 This is why it is of utmost importance to monitor type 1 athletes before, during, and after exercise. Symptoms of hyperglycemia can include nausea, dehydration, decreased cognitive function, fatigue, rapid breathing, increased thirst, and increased urination. Risk factors for both hypoglycemia and hyperglycemia are given in Table 1.
Risk Factors.
Of note, prior episodes of either hypoglycemia or hyperglycemia can result in an increased risk of future occurrence. Also, exercising in a warm and humid environment can cause either reaction. Hypoglycemia is secondary to the increase in insulin sensitivity seen with increased warmth. In regard to hyperglycemia, the warmer, more humid environment exacerbates the dehydration that can be seen in those with poor control, increasing the risk for ketoacidosis. 5
Physical Exam
The key to proper management is taking a team approach. This includes involving the physician, the athlete, the parents, coaches, athletic trainers, and any other necessary school personnel. All diabetic athletes should have testing supplies, blood sugar goals, and treatment and prevention plans as well as emergency contact information readily available at all practices and games. This can be referred to as the diabetic action care plan.2,3,8,9
Poor control can first be screened during the preparticipation physical. Both the Pennsylvania Interscholastic Athletic Association for high school sports and the National Collegiate Athletic Association for the collegiate athletes require this exam prior to start of competition.2,3 The goal of this physical is to identify those with inherent risks of exercising, including cardiac problems. But this can be used to also screen for those diabetic patients who are poorly controlled. Questioning the athlete’s knowledge or concept of his/her disease and treatment regimen can indicate how much in control of their diabetes they are. Providers can also use this time to provide education, ensuring that the athlete is obtaining appropriate screening tests, which can be seen in Table 2.3,4,8,9
Recommended Screening.
Nutrition
Nutritional recommendations are set forth by the American Academy of Sports Medicine and the American Diabetic Association. The current recommendations are as follows9,10:
Carbohydrates: 6 to 10 g/kg body weight per day
Proteins: 1.2 to 1.4 g/kg body weight for endurance athletes and 1.6 to 1.7 g/kg body weight in strength training athletes
Fats: 20% to 25% of total daily calories
With these recommendations, all diets, as well as insulin regimens, must be individualized for each and every athlete. In general, 15 to 30 g of carbohydrates should be ingested for every 30 minutes of exercise, with a small portion being consumed 15 to 30 minutes prior to start of exercise. Some advocate that carbohydrate intake should be increased 12 to 24 hours prior to exercise to help avoid potential complications of exercise.3,5,9 The patient must have a good understanding of his/her disease and insulin treatment regimen. Given the fact that the energy requirements vary greatly, close monitoring is needed.
Insulin Treatment
When at adequate levels, insulin can promote an increase in protein synthesis, glycogen synthesis, and muscle/adipose tissue uptake. Proper management can result in a higher functional exercise capacity, similar to that of a nondiabetic. The preferred site of injection is subcutaneously in the abdomen. Other injection sites can have variable absorption rates.2,3 Again, individualization is the key. All regimens need to be tailored for each individual athlete. Modifications of the insulin regimen with dose reductions or increases may be needed depending on the type of exercise and the intensity of the activity. Long-acting basal insulins, such as glargine (Lantus) and detemir (Levemir), are often utilized. The long-acting insulins are then used primarily in combination with short-acting insulin (Humalog, Novolog, and Apidra; Table 3). The very-rapid-acting insulin (Humulin and Novolin) is generally avoided because the absorption rates can vary from day to day, even in the nonathlete.3,5 Insulin treatment regimens can vary not only in terms of what type of insulin is used but also with regard to the method of treatment, such as multiple daily injections (MDIs) versus continuous subcutaneous insulin infusion (insulin pumps). Both options have their benefits and pitfalls. Using MDIs is typically more cost-effective; however, compliance is a concern. Also worrisome is that the basal dose of insulin cannot be altered during exercise because the MDI regimen usually utilizes long-acting insulin combined with short-acting insulin at mealtime. This becomes a problem in those who are not being followed closely and who do not alter their regimen with exercise.3,5,8 Insulin pumps are more patient friendly because of ease of dose altering, no multiple injections, and option of suspending/disconnecting the pump. However, pump failure is a huge risk because if the pump becomes dysfunctional, ketoacidosis can ensue within 4 hours. This method is also very costly.3,5,8
Types of Insulin.
Blood Glucose Monitoring
Most experts recommend monitoring blood glucose every 30 minutes during exercise. At least 2 measurements need to be taken in the hour prior to starting exercise as well, at 30-minute intervals.2,3,5 A diabetic athlete may require checking blood glucose upward of 6 times daily. Ideally, checking every 30 minutes during exercise may not be feasible, depending on the sporting event. In football, one can check the blood glucose twice prior to the start of the game—at the start of warm-ups, then 30 minutes later before the start of the actual game—then again at halftime, and later at the end of the game. Each testing regimen should be individualized. Certain recommendations should be followed based on the blood glucose findings prior to exercise, specifically if the blood sugar is <100 mg/dL or >250 mg/dL; see Table 4.3,5,8,9 If blood glucose is below 100, one may try to supplement with carbohydrate loading and recheck as below. If the supplementation fails to increase the blood glucose after 2 attempts, the athlete may not participate. If the blood glucose is >250 mg/dL, urine should be checked for ketones; if no ketones are present, they may participate in mild exercise (walking). If ketones are present, they may not participate, and if the blood glucose level is >350 mg/dL, there should be no exercise regardless of the ketone finding. Generally, most physicians should err on the side of caution. If the blood glucose is ever >250 mg/dL, exercise should be withheld until better controlled to avoid potential complications. Based on the blood glucose monitoring, the athlete may be restricted from activity.
Prior-to-Exercise Recommendations.
Evidence-Based Table.
Modification With Exercise
This is by far the hardest task a physician will face when caring for a diabetic athlete. Most adjustments are made based on the history of how the individual is affected by exercise. This is important to achieve control in a newly diagnosed diabetic athlete. Attempting to mimic the diet and exercise routine that he/she will be using helps tailor their individual regimen. Athletes, to ensure adequate treatment prior to competition, must consume food as if they would be competing and exercise with similar intensity and duration to that of actual competition. Most individuals with type 1 diabetes may require 0.5 units of insulin/kg body weight even when they are not being active. Therefore, most athletes may require a decrease in this usual dose. General recommendations have been made in regard to altering the athlete’s insulin regimen based on duration and timing of exercise. If the athlete is exercising less than 1 hour, rapid-acting insulin should be decreased by 30%. If exercise lasts 1 to 2 hours, it should be decreased by 40%. And if exercise lasts longer than 2 hours, it should be decreased by 50%. If exercise is occurring in the evening, mealtime insulin should be decreased by 50% as well to avoid postexercise hypoglycemia.3,5,9 Athletes may be asked to eat more 12 to 24 hours after competition, again to avoid the delayed hypoglycemia. During days of less activity, or off-season, the athlete may need more insulin, less food, or a combination of the two to achieve adequate control. Not only should the bolus insulin be reduced, but the athlete’s basal dose should be reduced as well with days of exercise, usually 25% to 40% depending on the demand of the activity.3,5 When dealing with an athlete who has an insulin pump, the pump may be disconnected with certain activities such as football, wrestling, showering, swimming, and so on. It is important to make sure that 50% of the usual basal rate of insulin is infused prior to pump removal.3,5,9 Experts recommend limiting the time that the pump will be disconnected to less than 2 hours. The risk of ketoacidosis/hyperglycemia can occur within 3 to 4 hours of pump removal.3,6 If the pump remains on during exercise, the 50% reduction in basal dosing should be started 4 hours prior to exercise and continued at that rate until 1 hour postexercise.3,5,9
Along with modifying the athlete’s insulin regimen, some may advocate for increasing their carbohydrate intake prior to, during, and after exercise. In a study of 67 athletes, Grimm et al 12 found that adequate carbohydrate replacement was superior to modifying insulin alone. That being said, they still agree that modifying the insulin regimen was also a necessity specifically in longer duration of exercise.
Benefits of Exercise
Most studies on the benefits of exercise in those with diabetes are done in the type 2 patient. However, these benefits can also be seen in type 1 diabetic patients. If the athlete has adequate control, it facilitates the development of muscle mass because insulin has an effect on the muscle in relation to proper use of amino acids. Reduction in microvascular complications and cardiovascular risks can be seen. Good control results in being able to participate and can lead to improved self-esteem and self-confidence of the athlete.3,6 Thus, the benefits of exercise in the diabetic patient far outweigh the risks, as long as the athlete remains under good control throughout the exercise process.
Good Control?
Many type 1 athletes allow their sugars to run higher because they fear that hypoglycemic episodes will affect their performance. Compliance is an issue with most diabetic patients. When combining denial and the overall attitude the athlete may have in relation to his/her disease, it can lead to great difficulty in achieving adequate control. For example, when caring for a wrestler with type 1 diabetes, the constant flux of weight gain or loss can lead to devastating complications. Athletes may withhold their insulin, resulting in elevation of sugars. This causes an increase in urination and loss of water weight, resulting in worsening dehydration and increasing the potential of ketoacidosis. Education is of utmost importance here. However, a recent study in 2011 showed in a survey of endurance athletes with type 1 diabetes that most do not follow the guidelines set forth by their physician and the American Diabetes Association. 11 Thus, despite our best efforts with education, these athletes still do what they want. Not many studies have been done on the long-term outcomes of athletes with type 1 diabetes in relation to their glucose control. More longitudinal studies need to be completed to see if an increase in morbidity and mortality in this specific patient population occurs.