Type 1 diabetes mellitus (T1DM) is an autoimmune disease that causes the body to attack insulin-producing cells and accounts for 5% of all cases of diabetes (4,6,27). Formerly known as juvenile diabetes, T1DM can occur at any time in a person’s lifetime, although the diagnosis typically is made before the age of 30 years. Although significant advancements have been made to better understand the underlying factors, such as genetic risk and immunologic triggers, the causes of T1DM are still largely unknown (4). Although serious in nature, the advances in treatment options make living with T1DM very manageable giving individuals the opportunity to live full, active lives (2,12). Unfortunately, physical activity carries a high risk of potentially dangerous glucose variability, particularly hypoglycemia (23,28). Treatment recommendations regarding the inclusion of physical activity, exercise and/or competitive sports remains unclear because of the lack of research within this population (12). The research that is available for individuals living with T1DM indicates that, similar to individuals without diabetes and those living with type 2 diabetes mellitus (T2DM), training should result in health improvements and long-term adaptations. In addition, through these adaptations, someone with T1DM could naturally reduce insulin requirements of the body by incorporating physical activity or exercise into his or her daily routine. The available data with exercise and T1DM have produced weak, but meaningful, meta-analyses and recommendations for incorporating physical activity and exercise into the treatment regimen (8,12,25). This column will address the current state of research and provide evidence-based recommendations for individuals with T1DM.
Predicting the glucose response during and after exercise is dependent on intensity as well as the type and duration (7,10,11,13–15,19,21,23–26,29). Increased intensity and/or longer aerobic exercise produce different glucose responses when compared with moderate intensity of shorter duration. Formal sporting events that include periods of inactivity interrupted by sporadic exercise of variable intensity further complicate the glucose response. Both individual and team sports require the athlete to switch from moderate to vigorous intensities resulting in glycogen conversion to glucose, not all of which is used for energy, resulting in significant changes in blood glucose concentration. The degree of this glucose response is unique for the individual with T1DM.
Although available studies for review are limited in size (Table 1), there are two common themes. All studies suggest that exercise can be safe and beneficial for individuals living with T1DM (17,19,21,29). Investigators also present the need for stronger research studies (7,8,20,25) to determine a more accurate, predictable response to various intensity levels and durations of activity, as well as develop ideal insulin doses and carbohydrate supplementation guidelines.
Exercise Testing and Physical Activity Recommendations for Individuals Living with T1DM: Should they Mimic T2DM?
Because there is limited research to properly determine ideal exercise testing procedures that are both safe and effective for persons with T1DM, the American College of Sports Medicine (ACSM) currently recommends similar procedures for exercise testing that are used for the general healthy population as long as the individual’s blood glucose level is greater than 100 mg/dL and less than 250 mg/dL (1). If blood glucose level is outside this range, then exercise could be detrimental because of the increased risk of hypoglycemia during and/or hours after the exercise bout as well as hyperglycemia, which ultimately could further damage blood vessels and potentially result in a diabetic coma.
Aside from a few special considerations, the exercise recommendations for individuals living with T1DM and T2DM are the same (1). Furthermore, some considerations made, such as ingesting carbs and reducing insulin pump activity, do not provide numbers or ranges. This lack of specificity may increase the risk of hyper- or hypoglycemia. Because of research limitations regarding physical activity and exercise for T1DM, ACSM’s current recommendations include the same FITT (Frequency, Intensity, Time, Type) principles for aerobic exercise as the general population (1). This includes 150 minutes per week of moderate- to vigorous-intensity aerobic exercise. However, no recommendations are given for resistance exercise or competitive sports because of a lack of available research data. There is some evidence showing that combination aerobic and resistance training improves the blood glucose response compared with a single modality. Interestingly, recent evidence also would suggest that including high-intensity interval training could help reduce the risk of nocturnal hypoglycemia (14,24).
With competitive sports, the particular sport involved will dictate the glucose response. Random sporadic bouts of moderate to vigorous intensity, followed by rest, are different across sports and will affect glucose levels in different ways. For example, golf typically involves frequent periods of activity interspersed with periods of rest. When walking the course, muscle and liver glycogen may break down and result in a transient increase in blood glucose. Yet, when riding in a golf cart, the glucose may subsequently drop. Another individual may have a completely opposite response. This unpredictable nature of glucose variability during sporting activities also may lead to under- or overuse of insulin, increasing the risk for hyper- or hypoglycemia later in the day or while sleeping.
One of the concerns with the current recommendations is the lack of large clinical trials demonstrating the heterogeneous characteristics of the different forms of diabetes in response to varying exercise intensities and forms. In particular, there is a paucity of specific information on proper insulin and carbohydrate supplementation. Because of the lack of available data, it is currently recommended that individuals with T1DM follow similar guidelines put forth for those with T2DM using exogenous insulin therapy.
When developing an exercise prescription for individuals with T1DM, attention to intensity and duration is of the utmost importance. Because of the complex nature of the internal response to maintaining homeostatic glucose concentrations, the bout of exercise could have a very different effect whether it is a moderate-intensity workout day or a more prolonged high-intensity competition. Regardless of intensity, time, or type of exercise, frequency of glucose monitoring, carbohydrate intake, and insulin usage must be taken into careful consideration. Using evidence-based data and recommendations from the American Diabetes Association, we have put together a suggested list of special considerations to enhance patient safety (Table 2).
Two special considerations that warrant further discussion for individuals with T1DM are hypoglycemia and hyperglycemia. Intensity level ultimately will affect the level of circulating blood glucose. Vigorous-intensity levels of activity have been shown to impact the risk of hyperglycemia because of the natural response of the sympathetic nervous system (16). Similar to chronic stress, vigorous exercise intensity will trigger a hormonal response resulting in increased levels of circulating catecholamines, cortisol, and growth hormone. Formal sporting events can further increase risk of hyperglycemia due to the heightened cognitive stress of competitions releasing catecholamines and cortisol beyond that of the typical exercise response. The cycle of events induces internal glucose production known as gluconeogenesis and inhibits peripheral glucose use. With training, this overproduction of glucose from glycogen stores should lessen. In addition, adaptations to exercise training in general populations will reduce the hormonal response but still effectively maintain needed glucose production. Similar responses should be observed in an individual with T1DM.
Intensity level ultimately will affect the level of circulating blood glucose. Vigorous-intensity levels of activity have been shown to impact the risk of hyperglycemia because of the natural response of the sympathetic nervous system (16). Similar to chronic stress, vigorous exercise intensity will trigger a hormonal response resulting in increased levels of circulating catecholamines, cortisol, and growth hormone.
Studies have consistently shown that those with T1DM experience a greater decline in their glucose levels (hypoglycemia) both during and after exercise if only moderate-intensity exercise is performed, as well as the hours after recovery (23,26). Similar declines in blood glucose also have been shown after a maximal exercise test (13). As a result, nocturnal hypoglycemia after a large bout of aerobic exercise is a common concern and oftentimes a barrier to physical activity or sport participation. In contrast to data on hypoglycemia risk from prolonged moderate-intensity exercise, evidence suggests that the inclusion of intermittent high-intensity exercise actually reduces the decline in blood glucose among individuals with T1DM when compared with moderate-intensity exercise alone (10,12,14). Furthermore, the addition of simple wind sprints into a moderate-intensity workout was shown to lessen the drop in blood glucose and even increase glucose levels (14,15). This, in turn, could possibly lead to postexercise hyperglycemia, so caution should be taken with this approach. In contrast, Maran et al. (15) reported higher rates of delayed nocturnal hypoglycemia after high-intensity intermittent exercise. These findings conflict with other study results, demonstrating that more research is warranted. For a safer approach, and if vigorous-intensity levels should be avoided, risk of hypoglycemia can be minimized in one of two ways: 1) appropriate reductions in insulin dosage and/or 2) ingesting additional carbohydrates (3,9,12).
For individuals with T1DM, the general goal is to improve self-management of blood glucose to maintain levels within a normal physiologic range as frequently and as safely as possible during exercise and physical activity. The primary cautionary measures that need to be taken before, during, and after exercise are adequate carbohydrate supplementation, frequent glucose monitoring, and adjustments to insulin therapy (Table 2). Nocturnal hypoglycemia after exercise has been well demonstrated and remains of particular concern. Some current evidence suggests reducing insulin dosage by up to 50% after exercise (23). To improve safety, we advocate that individuals participating in moderate to intense exercise during the day consider additional nocturnal glucose monitoring, particularly if glucose response to certain activities has not been well established. Exercise professionals working with individuals who have T1DM also should maintain an ongoing relationship with the client’s health care provider to closely monitor the short- and long-term effects on the glucose response and changes in insulin sensitivity. Sharing of information can help the exercise professional determine whether any changes need to be made to the intensity or duration of exercise as well as whether the physician should adjust medications or daily insulin dosage.
The primary cautionary measures that need to be taken before, during, and after exercise are adequate carbohydrate supplementation, frequent glucose monitoring, and adjustments to insulin therapy (Table 2).
This column presented exercise guidelines for the safety of T1DM individuals. The next Clinical Applications column will present a primer to health advocacy for the personal trainer and clinician. This information will help the exercise professional develop skills to defend his or her health and fitness outcomes to his or her supervisors and their legislators.
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