Intermediate Summary

People with T1D face a distinctive challenge in that they cannot mimic the complex neurohormonal regulatory responses required for various types and durations of exercise. The combination of changes in fuel selection and oxidation leads to a mismatch between peripheral glucose disposal (i.e., increased muscle glucose uptake) and glucose provision by the liver (i.e., decreased output/supply from the liver). This imbalance often leads to rapid declines in glucose levels during exercise, contributing to a substantial risk of hypoglycaemia both during and after activity in people with T1D.

How Exercise Affects Blood Glucose Levels in People with T1D

Exercise significantly impacts glucose levels in people with T1D [4, 25, 44-46], but the responses vary greatly from person to person. While many factors (such as age, sex, fitness level, time of day, stress level, menstrual cycle, previous exercise or hypoglycaemia) influence how blood glucose responds to a bout of exercise [4, 47, 48], perhaps the most important determinants include:

• The duration, intensity and, type of exercise [4, 49]. In other words, different forms of exercise tend to elicit different glycaemic effects (Figure 2).
• The time elapsed since the last meal: exercise in a fasted versus a fed state elicits different effects on blood glucose levels [50-52]. In particular, the impact of the prandial state (whether and how long ago you ate a meal) is a key factor for the blood glucose response to exercise.
• In line with the previous point, the level of circulating insulin before, during, and after the exercise session [53], also referred to as ‘insulin on board’. Indeed, it has been stated that a key reason for the reported heterogeneity in blood glucose outcomes in response to exercise is likely the varying insulin concentrations at the time of exercise [46].
• Pre-exercise blood glucose concentration [53, 54].
• Whether carbohydrates (and protein [55]) are consumed before and during exercise. If so, how much?

In addition to exercise type and duration, several individual-related and event-related factors can influence the blood glucose response to exercise. Recently, the T1DEXI study explored these factors and found that higher pre-exercise glucose concentration, declining blood glucose levels preceding exercise, lower baseline HbA1c levels, and higher insulin levels onboard were the four significant predictors of a more considerable glucose decline during exercise [56].

Still, as stated above, exercise intensity is a significant determinant of blood glucose levels in people with T1D [4]. Understanding the hormonal responses during different types and durations of exercise helps determine the appropriate insulin dose and nutritional adjustments for managing blood glucose, which will be discussed in Chapter 4.

Low- to Moderate-Intensity Aerobic Exercise in T1D

Moderate-intensity aerobic exercise is typically defined as prolonged exercise lasting over 30 minutes, performed at an intensity of around 50-80% 𝑉̇O2max. Generally, for people with T1D, moderate-intensity aerobic exercise leads to a decrease in blood glucose concentrations [44, 46], with an increased risk for exercise-induced hypoglycaemia [4, 57].