This study evaluated, in the context of a combined resistance and aerobic exercise session, the effects of exercise order on blood glucose levels in individuals with type 1 diabetes. As we had anticipated, performing resistance exercise before aerobic exercise rather than the reverse resulted in attenuated declines in glucose concentration during exercise, fewer exercise-induced hypoglycemic events, and less need for carbohydrate supplementation. Furthermore, we observed beneficial effects from this sequence on subsequent 12-h glycemic trends where the duration and severity of hypoglycemia was reduced. The benefits of performing resistance exercise before aerobic exercise instead of the reverse were observed despite overall energy expenditure being equal between experimental sessions.
Resistance exercise is a primarily anaerobic activity. Other types of high-intensity exercise combining aerobic and anaerobic metabolism (e.g., high intensity cycling) can increase the rate of glucose appearance to a greater extent than the rate of glucose utilization (seven and four times, respectively) during exercise in type 1 diabetes (18
). This may cause glucose levels to increase during exercise, producing postexercise hyperglycemia if intense exercise is sustained for 12 or more minutes (19
). Shorter anaerobic exercise bouts (intermittent 4-s sprints or 10-s sprints before or after low-intensity aerobic exercise) attenuated declines in blood glucose both during and after exercise when combined with low-intensity (40% o2peak
) cycling (6
). Elevated glucose production from very high-intensity exercise is generally attributed to increased levels of circulating epinephrine [known to triple with short sprints (6
) and increase up to 14 times its resting value (18
) after 12 min of exhaustive cycling] and norepinephrine, which augment glycogenolysis throughout exercise and early recovery (18
Although we did not measure catecholamines during the sessions, responses to high-intensity exercise are known to be comparable (18
) or slightly attenuated (19
) in individuals with type 1 diabetes compared with nondiabetic counterparts. Catecholamines can increase to three or four times resting values during moderate-intensity resistance exercise in individuals without diabetes (22
), with responses increasing in proportion to exercise intensity (23
). If our participants experienced similar responses to resistance exercise as individuals without diabetes, then increases in epinephrine may have contributed to the attenuated rate of decline in blood glucose during the first 15 min of aerobic exercise in RA, and to the increase in glucose during resistance exercise in AR (). The latter should be interpreted with caution, because most participants needed glucose supplements to prevent hypoglycemia during aerobic exercise in this session.
It is also possible that exercise-related growth hormone (GH) secretion differed between treatments, potentially affecting fuel selection during exercise. Goto et al. (24
) found that in nondiabetic individuals, endurance exercise performed before resistance exercise produced lower GH secretion than resistance exercise alone. They also found that resistance exercise performed 20 min or less before endurance exercise produced elevated levels of GH and greater rates of lipolysis during the subsequent aerobic activity compared with endurance exercise alone (16
). Because higher GH levels are known to decrease muscle glucose uptake and increase lipolysis in nondiabetic individuals (25
), this may have been a factor in the attenuated declines in blood glucose during aerobic activity in RA.
High-intensity cycling increases blood lactate levels during and up to 40 min after exercise in individuals with type 1 diabetes (6
). We are unaware of published data describing lactate responses to resistance exercise in this population. Resistance exercise protocols similar to the one we used have produced lactate concentrations up to four times those measured at rest, with levels remaining significantly higher than baseline until 30 min postexercise in trained nondiabetic individuals (26
). Because elevated lactate could serve to increase gluconeogenesis (7
), it could be a contributing factor in the attenuated decline in glucose during the first 15 min of aerobic exercise in RA as well as in the increases in postexercise glucose levels in AR.
Studies suggest that high-intensity exercise may be associated with a greater frequency of nocturnal hypoglycemia in type 1 diabetic individuals (10
). Our participants experienced nocturnal hypoglycemia as frequently postexercise as on nonexercise nights. Nocturnal hypoglycemia has been identified as a risk inherent with intensive insulin therapy (27
), and it is possible that overnight hypoglycemia in our study was more related to insulin therapy than to exercise. It is noteworthy that hypoglycemic events occurring after AR tended to be longer and more severe than those experienced in RA, as demonstrated by a greater AUC. Studies using glucose clamp found that counter-regulatory responses to subsequent hypoglycemia were blunted after exercise, even in the absence of significant changes in glucose levels during exercise (28
). In addition, even mild hypoglycemia (3.9 mmol/L) in nondiabetic individuals is sufficient to elicit counter-regulatory reactions that can blunt neuroendocrine responses to subsequent hypoglycemia within 24 h (29
). Because decreases in blood glucose were greater during AR (reaching a mean of 5.5 ± 2.4 vs. 6.9 ± 3.1 mmol/L in RA), it is plausible that subsequent responses to declining blood glucose could have been subject to impairment after exercise.
Although there are advantages to admitting study subjects the night before testing to control participant activity and food intake, we chose a study design more reflective of real-life conditions. Participants controlled their meals and insulin but were asked to eat the same breakfast, lunch, and dinner at the same time for every day of sensor wear and to match their insulin intake as closely as possible. Exercise took place at 1700 h when many individuals who work during the day opt to exercise, unlike several other studies where midmorning exercise was performed (6
Several aspects of resistance training in type 1 diabetic individuals require further scrutiny. Glucose responses may be different if exercise is performed at another time of day because hormone and exogenous insulin concentrations are both likely to be different. Our participants were fit, habitual exercisers, and the effects of exercise may be less pronounced in unfit individuals exercising at the same relative intensity because the activity would be at a lower absolute intensity. In nondiabetic subjects running at very high relative intensity, glucose production and catecholamine concentrations increase more in athletes than in physically untrained individuals, resulting in hyperglycemia after exercise in the former group because glucose production falls more slowly than glucose utilization when exercise ends (30
). Further research on different subpopulations of type 1 diabetic individuals, including those with lower fitness levels and poorer glycemic control, is warranted.
This study is limited by its small sample size (n = 12), which may have prevented us from finding all of the significant differences in plasma glucose levels during exercise. To examine our participants in a real-life scenario, we compromised a certain amount of experimental control such as having complete control over all food and insulin intake. The ability to interpret the data would have been improved by having catecholamine, lactate, and GH measurements. Finally, having a relatively fit sample with moderate to good control of their diabetes makes the applicability of the outcomes to individuals who are inactive or have poor glycemic control uncertain.
In summary, our findings suggest that trained individuals with type 1 diabetes who perform both resistance and moderate aerobic exercise should consider performing their resistance exercise first if they tend to develop exercise-associated hypoglycemia because doing so may attenuate declines in glucose levels during subsequent aerobic exercise. This order of exercise could lead to a lower reliance on glucose supplementation during exercise and might also decrease the severity of potential nocturnal hypoglycemia. Conversely, individuals having exercise-associated hyperglycemia may wish to perform aerobic exercise before resistance training. Both approaches should still be accompanied by careful monitoring of blood glucose levels, both during and after exercise.