Resistance Exercise and T1D
The hormonal and metabolic responses to resistance exercise in people with T1D likely vary depending on the specific workout routine. In people without T1D, high-resistance, low-repetition workouts trigger similar responses to anaerobic activities like high-intensity running, potentially increasing blood glucose levels [58]. On the other hand, high-repetition, low-resistance programs with short rests between sets are more aerobic and may decrease blood glucose levels.
There haven’t been many studies on how blood glucose responds to resistance exercise in people with T1D. Of the limited research in this area, most studies have used a moderate-intensity protocol involving three sets of eight repetitions [59-62]. Yardley and colleagues [59] found that a resistance training protocol performed in the afternoon was associated with a decline in blood glucose, although to a lesser extent than a comparable period of moderate-intensity aerobic exercise (60% 𝑉̇O2peak). Resistance exercise was also associated with more stable blood glucose levels than aerobic exercise for several hours post-exercise. However, an almost identical protocol performed while fasting in the morning caused an increase in blood glucose and post-exercise hyperglycaemia [61, 62]. This phenomenon of fed vs. fasted exercise will be discussed in more detail in Chapters 4, 6 and 7 during the sections on fasted exercise in T1D.
When combined with aerobic exercise, resistance exercise might offer some protection against hypoglycaemia. One study showed that performing 45 minutes of resistance exercise before 45 minutes of treadmill running (60% 𝑉̇O2peak) delayed the decline in blood glucose during aerobic exercise [60]. Conversely, blood glucose levels decreased immediately and rapidly when aerobic exercise was performed first, but this decline stopped when switching to resistance exercise. The authors of these studies suggested that, based on these findings, individuals experiencing high blood glucose levels during exercise should perform aerobic activities first. In contrast, those struggling with hypoglycaemia during exercise should start their sessions with resistance exercise. This advice on the ordering of exercise types is now fairly common. This information can be used to plan an exercise session based on starting glucose and highlights the importance of understanding the physiology of glucose responses based on the type of exercise being undertaken.
As mentioned earlier, research on resistance training and T1D is limited, and studies generally have included small sample sizes. Further research is needed to fully understand how factors like the speed of movement, amount of weight lifted, number of repetitions and sets, and duration of rest intervals influence hormonal responses in people with T1D [63]. It is also unclear whether physiological factors such as age, sex, and physical fitness can modulate these responses [48].
High-Intensity Interval Training (HIIT)
HIIT is a popular workout that involves repeated bouts of high-intensity exercise interspersed with low-intensity recovery periods. Certain forms of HIIT are a time-efficient alternative to moderate-intensity exercise and offer similar cardio-metabolic health benefits in individuals without T1D [64-66]. Studies investigating how blood glucose responds to HIIT in people with T1D have shown mixed results. HIIT has been shown to increase, decrease, or stabilise blood glucose levels 50, 67. The variety in these findings likely results from differences in exercise protocols (intensity and duration of intervals), exercise modality (cycling vs. whole-body calisthenics), and the time of day the exercises were performed. When HIIT is undertaken in a fasted state or when insulin levels are significantly lowered, a variable rise in glucose levels is often seen [68, 69].
Adding Short Sprints to Moderate-Intensity Workouts
Seeing how blood glucose levels rise during high-intensity exercise, researchers hypothesised that adding a short sprint to a lower-intensity workout could balance the glucose-lowering effects and stabilise blood glucose levels in individuals with T1D. A series of studies tested this idea and found that incorporating a short (10-second) sprint before [70] or after [71] a 20-minute bout of moderate-intensity (40% 𝑉̇O2max) exercise temporarily reduced the post-exercise decline in blood glucose [70-73].
In these studies, moderate-intensity exercise alone caused a significant drop in blood glucose. However, including a 10-second sprint lessened these declines. Stabilising glucose levels following the sprint was associated with elevated catecholamines (stress hormones such as adrenaline), growth hormone, and cortisol. The underlying mechanism was later revealed to involve a reduction in glucose disposal rate rather than its appearance rate [73].
By How Much Does Blood Glucose Concentration Change During Exercise?
The most “predictable” type of exercise to estimate blood glucose changes (although this remains an estimation due to the impact of several factors) is moderate-intensity aerobic exercise since there is a relatively constant and substantial glucose uptake into the muscle. A meta-analysis of five studies on moderate-intensity continuous exercise reported that a decline in blood glucose concentration between 4-5 mmol/L (70-90 mg/dL) per hour can be expected [46]. On the other hand, resistance exercise appears to have a more modest or stabilising effect on glycaemia [46, 59, 62], whilst very short, intense exercise can often cause hyperglycaemia [15]. HIIT yields varying results, with studies reporting both increases and decreases in glycaemia [4, 44, 46, 74-76]. The recent TDEXI study showed that the glucose decline during structured at-home exercise was greatest with aerobic exercise, followed by HIIT and then resistance exercise, causing the smallest decline 56. For more information on the magnitude of blood glucose changes, specifically during postprandial exercise [77, 78] – which will generally be greater due to higher levels of insulin on board – we refer to Chapter 4.
It’s Not Over Yet After Exercise: Late Effects of Exercise on Blood Glucose Levels in T1D
Exercise has lasting effects on glucose metabolism in the hours following a workout. Both muscle glucose uptake and insulin sensitivity remain elevated compared to baseline [79], with variations based on the duration and intensity of exercise. While glucose uptake rates drop quickly after exercise (because muscle contractions have stopped), insulin sensitivity remains high for several hours [80]. This heightened insulin sensitivity and continued glucose absorption may be due to increased activity of the enzyme glycogen synthase, which acts to replenish glycogen stores [81, 82]. Additionally, increased GLUT4 translocation and muscle microvascular perfusion play key roles in the changes in glucose uptake [83, 84].
Nguyen and colleagues found a way to differentiate and quantify insulin-independent and insulin-dependent glucose uptake after aerobic exercise in individuals with T1D [80]. They found that non-insulin-mediated glucose uptake peaks during exercise but drops quickly afterwards. In contrast, insulin-mediated glucose uptake has a prolonged response, lasting several hours post-exercise before returning to baseline [80]. This highlights the importance of post-exercise insulin reduction strategies, which will be discussed in the next chapter.
For people with T1D, the glucose requirements to maintain glucose levels within the target range after exercise are increased in two phases [85]. There’s an early risk of hypoglycaemia (within the first 2 hours) and a late risk (starting approximately 7-11 hours after exercise and lasting for up to 6 hours) [85]. The increased insulin sensitivity and prolonged insulin-mediated glucose uptake likely contribute to this long-term impact. Therefore, exercise can affect blood glucose levels for several hours (6-15 hours and sometimes up to 24 hours) post-exercise [4, 25, 57, 85, 86], increasing the risk of late-onset post-exercise hypoglycaemia [85-87]. Extra precautions may be necessary following afternoon or evening exercise due to the heightened risk of nocturnal hypoglycaemia 47. Notably, the risk of nocturnal hypoglycaemia following 45 minutes of moderate-intensity exercise has been reported to be as high as 30-40% [87-89]. However, this risk may be significantly lowered if a hybrid-closed loop system is used overnight.
Therefore, the "classic 3-phase glucose response to aerobic exercise" in individuals with T1D consists of:
A significant blood glucose decline during exercise, mainly depending on exercise duration and intensity, insulin on board, and several other factors;
Often, an almost immediate and transient post-exercise rise in glycaemia due to a relative decrease (compared to during exercise) in glucose uptake in the muscle after cessation of exercise (as muscle contractions are not taking place anymore) without an equally fast decrease in hepatic glucose production (as counterregulatory hormonal stimulation is still active);
There is an increased risk for late-onset post-exercise hypoglycaemia in the subsequent hours.
Understanding these phenomena helps provide insights into mitigation strategies to improve glucose management during and after exercise.
The Final Word
Managing glucose levels during and after exercise is a considerable challenge for people living with T1D. As we’ve discussed, besides the effect of the type, duration, and intensity of the exercise, there is a wide range of other factors to consider, including the location and method of insulin delivery, amount of insulin in the circulation, pre-exercise blood glucose concentration, and the composition of the last meal. The changes in fuel selection and oxidation often lead to a mismatch between muscle glucose uptake and liver glucose output. This imbalance can cause a rapid decline in glucose levels during exercise, increasing the risk of hypoglycaemia both during and after activity. A solid understanding of the hormonal mechanisms behind why glucose responds as it does during exercise can empower people living with T1D to exercise safely and regularly.