Factor 1. Pre-Exercise Glucose Levels
Blood glucose concentration before exercise can affect how glucose levels change during exercise [1-3]. The ideal starting glucose level depends on the duration and intensity of the exercise and the amount of insulin on board. Pre-exercise glucose levels also determine when carbohydrates should be consumed before and during exercise and whether you need to reduce your pre-exercise bolus insulin, especially if you exercise after a meal (see below for more information on postprandial exercise).
General Guidelines:
Ideal Starting Glucose Level: Aim to start the exercise with stable blood glucose levels between 7.0 and 14.0 mmol/L (126-250 mg/dL), with low blood ketone levels (i.e., <0.6 mmol/L) or trace amounts in urine [4].
Low Glucose Levels: If glucose levels are below 5.0 mmol/L (90 mg/dL) before exercising, consider having 10–20 grams of glucose before starting.
Moderately Low Glucose Levels: If levels are between 5.0 and 6.9 mmol/L (90-125 mg/dL), 10 grams of carbohydrate is recommended before beginning exercise.
High Glucose Levels: If blood glucose levels are high (>14.0 mmol/L or 250 mg/dL) before exercise, consider administering a small correction bolus of insulin and waiting for them to decrease before starting the workout. Alternatively, consider going for a walk or starting some low-activity exercise to help bring your glucose levels down.
Additional Considerations:
- Adjust pre-exercise bolus insulin as needed, especially if exercising after a meal.
- Monitor blood glucose levels closely during exercise to prevent hypoglycaemia or hyperglycaemia.
Table 1: Recommended glycaemic values before starting exercise in individuals with T1D. Note that the direction of glucose trend arrows on CGM will alter these suggestions.
<90 mg/dL (<5.0 mmol/L)
→ Do NOT start exercising and measure glycaemia with finger prick. → Carbohydrate intake needed: ~10-20 grams of fast-acting carbohydrates. → Re-check after 15 minutes and repeat carbohydrate intake if still not >90 mg/dL.
90-125 mg/dL (5.0-6.9 mmol/L)
→ Starting anaerobic exercise and HIIT is safe. → Delay aerobic exercise; first ensure intake of ~10 grams of fast-acting carbs. Take into account trend arrows on CGM.
126-180 mg/dL (7.0-10.0 mmol/L)
→ All types of exercise can be started. → Carbohydrate intake can be initiated depending on the type of exercise and glucose trend arrows on your CGM.
181-270 mg/dL (10.1-15.0 mmol/L)
→ Starting to exercise is safe but watch out with anaerobic exercise as glucose might rise substantially. → Wait with carbohydrate intake for at least ~20-30 minutes depending on glucose trend arrows on your CGM.
>270 mg/dL (>15.0 mmol/L)
→ Do NOT start exercising and measure ketone bodies if hyperglycaemia is unexplained (i.e., not due to recent food intake).
- When not or only in mild ketosis (<0.6 mmol/L) and/or hyperglycaemia is due to recent food intake: safe to perform exercise but regularly re-check blood glucose.
- Significant ketosis? (≥0.6 mmol/L) → Do NOT start exercise; first:
o Ketones 0.6-1.4 mmol/L: corrective insulin dose must be administered, drink water, wait with exercise until ketone levels decrease to <0.6 mmol/L, and regularly check blood glucose.
o Ketones ≥1.5 mmol/L: exercise is contra-indicated, and glucose management should be initiated as soon as possible by a medical healthcare professional.
Factor 2. Type, Duration, and Intensity of Exercise
As discussed in Chapter 3, the type of exercise, how long you do it, and how intense it is can significantly affect the direction and extent of blood glucose changes during exercise. These factors influence the amount of carbohydrates needed and the insulin bolus adjustments required. Understanding the specific demands of the exercise type, duration, and intensity is key to tailoring carbohydrate intake and insulin adjustments to maintain stable blood glucose levels. In general, the following may apply:
Long, Moderate-Intensity Aerobic Exercise: For more extended sessions of moderate-intensity aerobic exercise, you’ll need more carbohydrates and lower insulin levels. This contrasts with a short, low-intensity walk, which typically does not require extra carbohydrates and can generally be done safely with a lower starting glucose concentration.
Short, High-Intensity Exercise: For shorter, high-intensity exercise bouts such as a 30-minute HIIT session, repeated track sprints, or an intense weight training session, you might not need as many carbs or any. Sometimes, you might even need a correction insulin bolus to maintain stable blood glucose levels and avoid highs.
Competitive Exercise: Regarding the “type” of exercise, mentioning competitive exercise (races, events, etc.) is of special interest. This will likely need even more trial and error. Competition will be associated with a higher stress level and carbohydrate intake (i.e., fuelling more frequently and/or intensively). It is of utmost importance to avoid hypoglycaemia at all costs during competition. We have a dedicated section on competition-related stress and glucose management in Chapter 9.
Factor 3. Insulin Adjustments and Exercise
Understanding the impact of insulin on board (IOB) is very important for people with T1D who want to exercise [5]. Having a firm grasp on how IOB affects glucose levels during exercise can empower individuals with T1D to make better-informed decisions about when and how to exercise and adjust insulin and carbohydrate intake accordingly.
Insulin Management Before and During Exercise
Generally, to minimise the risk of hypoglycaemia during exercise, people with T1D are recommended to have low insulin levels on board. This is to mimic the body’s natural physiological situation as closely as possible. Adjustments can be made to the quick-acting (bolus) or the background (basal) insulin to achieve this. Specific changes to insulin will depend on the starting blood glucose, type, intensity, and duration of the intended exercise bout, as well as the time of day the exercise is performed. We’ll talk more about strategies for managing insulin after exercise later in the Recovery section of this chapter.
Insulin Adjustments for People Using Multiple Daily Injections (MDI)
MDI users generally have less flexibility with insulin adaptations around exercise than pump users, as they cannot alter insulin infusion rates. For mealtime bolus insulin, Table 2 offers some general guidelines on how much to reduce. Getting this right will require some trial and error, and glucose changes around exercise can sometimes be unpredictable.
Generally, if you exercise within two hours after a carb-rich meal, you might need to reduce your mealtime insulin by 25-75%. The bolus reduction depends on exercise duration and intensity 4 (Table 2), insulin levels already on board, and other factors (as you’ve probably gathered by now, “other factors” are a general theme here!).
Adjusting basal insulin before exercise isn’t always recommended for MDI users. However, experienced individuals might lower their evening basal insulin dose by 10% if an aerobic exercise session of substantial duration (i.e., more than 1 hour) is planned for the following morning. More substantial basal insulin reductions are recommended if multi-day activities or training camps are scheduled. If basal insulin has a duration of action over 24 hours (for example, insulin degludec), the insulin reduction may have to take place 2-3 days beforehand.
Insulin Adjustments for People Using Insulin Pumps
Modulating circulating insulin levels before and during exercise is arguably more manageable with an insulin pump, as the user can adapt the infusion rate [6]. For bolus insulin reductions at meals before exercise, the same guidelines as for patients on MDI (Table 2) can be followed. Later, as part of Factor 7 (Diabetes Technology), we’ll dive deeper into recommendations on insulin infusion rate adaptations, pump suspension, and using an ‘exercise mode’ in the newer hybrid closed-loop systems.
Table 2: Suggested adjustments of exogenous bolus insulin doses before exercise (according to exercise intensity and duration) in individuals with T1D planning to exercise within ~two hours after a meal (From: Riddell et al. 2017 [4]).
Low-intensity aerobic exercise (approximately 25% VO2max)
- 30 minutes: 25% reduction
- 60 minutes: 50% reduction
Moderate-intensity aerobic exercise (approximately 50% VO2max)
- 30 minutes: 50% reduction
- 60 minutes: 75% reduction
High-intensity aerobic exercise (70-75% VO2max)
- 30 minutes: 75% reduction
- 60 minutes: Not applicable unless very highly trained
Intense aerobic or anaerobic exercise (>80% VO2max)
- 30 minutes: No reduction recommended
- 60 minutes: Not applicable
Factor 4. Nutritional Considerations: Carbohydrate Intake Around Exercise
Eating the appropriate amount of carbohydrates before and during exercise is another important consideration for managing stable blood glucose levels and avoiding lows. Carbohydrates also provide the fuel (energy) for the workout. The amount and type of carbohydrates required will depend on the intensity and duration of the exercise session. Other considerations may include competition, training goals, and weather conditions (see Chapters 5, 6 and 9 for more information).
Determining the Amount of Carbohydrates You Need
Balancing blood glucose levels while providing enough fuel for the activity can be challenging. Because of this ‘dual task’ of managing blood glucose levels and providing energy for the workout, it is very difficult to provide one-size-fits-all advice for carbohydrate intake during exercise. The ideal amount and type of carbohydrates will also depend on several factors [4]:
Starting Glucose Levels [7] and Trend Arrows [8]: The initial blood glucose level and whether it is rising or falling. Trend arrows on your CGM can help here.
Type, Duration and Intensity of Exercise: The specific demands of the exercise session.
Pre-exercise Meal: Whether a meal was consumed before exercise, and if so, the meal composition [9] and the impact of insulin taken.
Circulating Insulin Levels (Insulin on Board): This is especially important if exercising after a meal.
General Guidelines for Carbohydrate Intake
Short, Low- to Moderate-Intensity Exercise (<30 minutes): Little to no carbohydrates may be required for performance, but having 10-20 grams of fast-acting carbohydrates can help prevent hypoglycaemia, depending on glucose levels.
Moderate-Duration Exercise (30-60 minutes): Around 30 grams of carbohydrates may be needed to avoid lows, especially with higher insulin levels [7].
High-Intensity Exercise (>70% VO2max): For activities such as running or cycling, around 45 grams of carbohydrates per hour may be needed to prevent hypoglycaemia [7].
Long-Duration Exercise (>60 minutes): To prevent hypoglycaemia and maintain endurance performance, carbohydrate intake generally ranges between 45 and 70 grams per hour. If carbohydrates are consumed in large amounts during exercise, additional insulin may be needed to avoid hyperglycaemia and facilitate glucose uptake into the muscles. Chapter 5 has a separate section on carbohydrate “fuelling” during exercise.
The Impact of Carbohydrate Type on Blood Glucose Levels During Exercise
Not all carbohydrates are the same, and the type of carbohydrate can significantly impact glucose levels.
High-Glycaemic Index Carbohydrates: Some carbohydrates, such as those found in sugary drinks and processed foods, are quickly digested and absorbed, leading to a rapid spike in blood glucose levels.
Low-Glycaemic Index Carbohydrates: These are found in whole grains, fruits, and vegetables, which are digested and absorbed more slowly. This results in a more gradual and sustained release of glucose into the bloodstream.
The Role of Fibre and Other Macronutrients
Fibre: Found in foods like whole grains, legumes, and vegetables, fibre slows down the digestion and absorption of carbohydrates. This helps prevent rapid fluctuations in blood glucose levels.
Protein and Fats: Consuming carbohydrates alongside protein and fats can further slow digestion and promote stable blood glucose levels during exercise.
Macronutrient Balance:
Pay attention to the overall macronutrient content of your meals.
Protein: Including protein in post-exercise nutrition can aid muscle repair and glycogen resynthesis.
Healthy Fats: Consuming healthy fats can provide sustained energy during prolonged exercise.
Factor 5. Timing of Exercise: When You Exercise Matters
The time of day you exercise can significantly impact your blood glucose response. For example, morning workouts might require different insulin and nutritional strategies than afternoon or evening sessions. The timing of the exercise in relation to meals can also affect your blood glucose levels.
Exercise Timing in Relation to Meals: Fasted vs. Post-Meal Workouts – Should You Exercise Before or After Eating?
Fasted Exercise: A Safer Bet?
Studies have shown that afternoon resistance exercise in people with T1D decreases blood glucose during activity [10, 11]. However, doing a similar resistance exercise workout in the morning while fasting resulted in either no change [12] or a slight increase [13] in blood glucose. Similar findings have been reported regarding fasting and aerobic exercise. Ruegemer and colleagues [14] found that 30 minutes of aerobic exercise in the afternoon decreased blood glucose, while the same exercise performed in the morning caused an increase. Another study found that people with T1D performing both moderate aerobic and high-intensity intermittent exercise in the fasted state did not experience declines in blood glucose during either exercise protocol [13]. These findings contrast with the declines in blood glucose observed during later-day (fed state) aerobic exercise [15-20] and high-intensity interval exercise [15-19].
Considerations Regarding Fed vs. Fasted Exercise:
While more research is needed to confirm these theories, it can be proposed that individuals struggling with hypoglycaemia during exercise or aiming to avoid additional carbohydrates to manage their weight might benefit from early morning/fasted exercise compared to exercising later in the day.
On the other hand, if your goal is to reduce post-meal highs, pre-meal exercise might also be advantageous. Turner et al. reported that pre-dinner walks are slightly better than post-dinner walks for improving glycaemia following the meal in T1D individuals using a hybrid closed-loop system [21].
Potential Explanations:
Lower Circulating Insulin Levels: During fasted exercise, lower circulating insulin decreases the suppression of hepatic glycogenolysis, increasing blood glucose during exercise.
The “Dawn Phenomenon” [22]: People with T1D may experience an early morning rise in blood glucose, possibly due to increased circulating growth hormone and cortisol [23-25]. Previous studies suggest that higher growth hormone levels could preserve blood glucose 11, 26 by stimulating greater breakdown of fat stores – a process known as lipolysis [27].
Post-Meal Exercise and Glucose Response
The term "early" or "true" post-meal exercise refers to physical activity that occurs within 2 hours after a meal [28], as illustrated in Figures 2 and 3. During this period, the body is still digesting and absorbing nutrients, which can influence how exercise affects blood glucose levels. People with T1D must pay close attention to their insulin and glucose management strategies during this time.