Exercise in type 1 diabetes: hypoglycemia before and after

A 23-year-old student has had type 1 diabetes since age 12 and uses a continuous glucose sensor with an insulin pen. He plays soccer two nights a week. On game nights, glucose rises in the middle of the match, he corrects with insulin and, at three in the morning, wakes up to the sensor alarm showing hypoglycemia. He has thought about quitting the game.

The story brings together what makes exercise difficult in type 1 diabetes: glucose can fall or rise depending on the type of effort, and the risk continues for hours after the training ends. Even so, exercise brings cardiovascular, metabolic and well-being benefits, and the goal of care is for the person to stay active safely.

This text explains the physiology behind these fluctuations, compares aerobic, interval and strength exercise, and organizes strategies for before, during and after training, including the use of the sensor and automated insulin systems.

Key points

• The risk of hypoglycemia runs from the start of exercise to 24 hours later, because of increased insulin sensitivity (Adolfsson, 2022).
• In a cohort of 497 adults, glucose fell most with aerobic exercise, less with interval exercise and even less with strength exercise (Riddell, 2023).
• Late afternoon exercise is associated with nocturnal hypoglycemia, often 7 to 11 hours later (Adolfsson, 2022).
• Doing strength exercise before aerobic exercise in the same session left glucose more stable in a crossover trial (Yardley, 2012).
• During exercise, the European position suggests keeping sensor glucose between 90 and 180 mg/dL in most adults (Moser, 2020).
• Insulin and carbohydrate adjustments are individual and are calibrated with measurements, without universal numbers.

Summary

• Why type 1 changes the physiology of exercise
• Aerobic, interval and strength: how each one affects glucose
• Before exercise: starting glucose, ketones and insulin
• During: sensor, carbohydrate and when to stop
• After: late hypoglycemia and the early hours
• Pumps and automated systems
• How to organize it in practice
• How NEXO helps train this reasoning
• Frequently asked questions
• Take-home messages

Why type 1 changes the physiology of exercise

In people without diabetes, the pancreas reduces insulin secretion when exercise begins and increases glucagon. This exchange releases glucose from the liver at the same rate the muscle consumes it, and blood glucose barely changes.

In type 1 diabetes, insulin comes from outside. The amount circulating during training is the amount applied beforehand, and it does not fall because the muscle started working. With high insulin in the blood, the liver releases less glucose, and the muscle, which already takes up glucose through contraction, takes up even more. The ISPAD guideline notes that counterregulatory hormones, such as glucagon, catecholamines, cortisol and growth hormone, and starting glucose also influence the fall during aerobic exercise (Adolfsson, 2022).

The result is that hypoglycemia is the only common adverse event of exercise in type 1 diabetes, at any age (Colberg, 2016). Fear of it is one of the most cited reasons for avoiding physical activity, and the continuous glucose sensor can reduce that fear by showing trends that allow earlier prevention and treatment of the fall (Colberg, 2016).

What exercise offers

The effort of adjusting is worth it. In type 1 diabetes, aerobic training increases cardiorespiratory capacity, reduces insulin resistance and improves lipids and endothelial function (Colberg, 2016). Strength training adds gains in muscle mass, strength, bone density, blood pressure and mental health, described for all adults (Colberg, 2016).

In young people, a meta-analysis of 10 trials with those under 18 found improved glycated hemoglobin in those who exercised, and more than three sessions per week of more than one hour each, combining aerobic and strength exercise, seemed to bring greater benefit (Colberg, 2016). ISPAD recommends that children and adolescents with diabetes be encouraged to do 60 minutes of moderate to vigorous activity every day, the same goal as the general population (Adolfsson, 2022).

The gains from regular training, such as higher capillary density and greater oxidative capacity of the muscle, are lost when the person stops training (Colberg, 2016). That is why the team's goal is to find a routine the person can sustain, and not just survive an isolated workout. Those who give up sport for fear of hypoglycemia lose these benefits, and the role of care is to restore confidence with concrete strategies.

This conversation tends to be more productive when the professional starts with what the person already experiences in training, as the text on how to listen for change talk in motivational interviewing suggests. For the context of type 1 diagnosis before symptoms, see the stages of type 1 diabetes.

Aerobic, interval and strength: how each one affects glucose

Continuous aerobic

Running, cycling, swimming and long walks mainly use oxidative metabolism, and glucose tends to fall continuously. In a crossover trial, plasma glucose fell from 9.2 to 5.8 mmol/L during 45 minutes of aerobic exercise (Yardley, 2013). ISPAD states that insulin adjustments are needed above all for aerobic exercise (Adolfsson, 2022).

Anaerobic and interval

In short, intense efforts such as sprints and weightlifting, the muscle generates energy through anaerobic glycolysis, phosphocreatine and free ATP (Adolfsson, 2022). These efforts raise catecholamines and cortisol, which release glucose from the liver, and that is why very intense exercise is usually associated with high glucose, not low (Adolfsson, 2022).

Mixed sports such as soccer, basketball and interval training are associated with more stable glucose than predominantly aerobic activities, although results vary between studies (Colberg, 2016). A systematic review of 15 small crossover studies concluded that high-intensity intermittent exercise may be safer than continuous exercise because it causes a smaller glucose drop, but data on the number of hypoglycemia events are lacking (Hasan, 2018).

Strength

In the same crossover trial, glucose fell from 8.4 to 6.8 mmol/L during strength exercise, a much smaller drop than with aerobic exercise (Yardley, 2013). The sensor detected nine nocturnal hypoglycemia events after strength and four after aerobic exercise and rest, a difference without statistical significance (Yardley, 2013).

What real-life data show

The T1DEXI study followed 497 adults with type 1 diabetes randomized to structured sessions of aerobic, interval or strength exercise (Riddell, 2023). Glucose fell on average 18 mg/dL with aerobic, 14 mg/dL with interval and 9 mg/dL with strength, with similar results among users of an automated system, a standard pump and multiple injections (Riddell, 2023). Days with exercise increased time in range, but may have slightly increased time below range (Riddell, 2023).

The average hides a lot of variation: the standard deviations were larger than the averages themselves, which means the same person can have different drops on different days (Riddell, 2023). That is why the sensor and the training log matter more than any table.

The order within the session

Twelve physically active people with type 1 diabetes did, on different days, 45 minutes of running followed by 45 minutes of strength, or the reverse order (Yardley, 2012). Starting with strength kept glucose more stable during exercise and reduced the duration and severity of hypoglycemia afterward (Yardley, 2012). The ADA position statement lists short sprints and strength before aerobic exercise among the strategies to prevent hypoglycemia (Colberg, 2016).

Before exercise: starting glucose, ketones and insulin

When to postpone

Some situations contraindicate exercise for a while. ISPAD cites severe hypoglycemia or repeated hypoglycemia in the previous 24 hours and glucose of 270 mg/dL or more with ketones due to lack of insulin (Adolfsson, 2022). Blood ketones of 1.5 mmol/L or more contraindicate exercise, and values between 0.6 and 1.4 mmol/L call for postponing until the cause is assessed and corrected (Adolfsson, 2022).

Alcohol should be avoided before and during exercise, because it increases the risk of hypoglycemia, including nocturnal hypoglycemia (Adolfsson, 2022). Acute injury and infection are also reasons to postpone (Adolfsson, 2022).

Starting glucose

The EASD and ISPAD position on sensors advises postponing exercise until glucose reaches at least 90 mg/dL in most adults, with slightly different ranges for those at high risk of hypoglycemia (Moser, 2020). Once the value needed to start is reached, the person only eats carbohydrate again when the trend arrow starts to fall (Moser, 2020).

Insulin before training

The two tools are reducing the insulin for the meal before exercise and, in some cases, basal insulin. How much to reduce depends on the time of day, the duration, the intensity, the type of insulin and the person's own response in previous training sessions. The guidelines offer starting ranges, but the fine adjustment comes from sensor records, and these numbers should not become a rule for everyone.

Correcting high glucose before or during exercise deserves caution. ISPAD suggests treating acute hyperglycemia with conservative corrections or with components of light aerobic activity and cool-down, which lower lactate and catecholamines (Adolfsson, 2022). In the student's case, the full correction in the middle of the game added insulin to the sensitivity increased by exercise and helped produce the hypoglycemia in the early hours of the morning.

During: sensor, carbohydrate and when to stop

The target during training

Regardless of the type of exercise, the European position suggests keeping sensor glucose between 90 and 180 mg/dL and, in prolonged aerobic exercise, ideally between 126 and 180 mg/dL in most adults (Moser, 2020). For adults at low risk of hypoglycemia, the guidance is to eat 10 to 15 g of carbohydrate when the sensor reads 126 mg/dL with a horizontal arrow (Moser, 2020).

For safety, exercise should be interrupted, at least temporarily, if sensor glucose falls below 70 mg/dL, with oral carbohydrate intake (Moser, 2020). For those who prefer a practical example of fast-absorbing carbohydrate, gel sachets, glucose tablets or a sports drink fit in a pocket or at the edge of the court.

The sensor lag

The sensor measures glucose in the interstitial fluid, which follows blood glucose with some delay. At rest, this delay is about 5 minutes in healthy people, and it can reach 12 to 24 minutes or more during exercise in people with type 1 diabetes (Moser, 2020). ISPAD also notes that the sensor lags during prolonged aerobic exercise (Adolfsson, 2022).

In practice, the trend arrow matters as much as the number. A glucose of 130 mg/dL with a rapidly falling arrow calls for action before one of 100 mg/dL that is stable, and symptoms of hypoglycemia call for confirmation with a capillary blood glucose reading when the sensor and the body disagree.

In children and adolescents

ISPAD strongly recommends the sensor during exercise in children and adolescents with type 1 diabetes, because symptoms of hypoglycemia and hyperglycemia can be hard to notice (Adolfsson, 2022). For those who do not use a sensor, the guideline recommendations start from a capillary blood glucose reading every 30 minutes during the activity (Adolfsson, 2022).

After: late hypoglycemia and the early hours of the morning

Why glucose falls hours later

After training, the muscle replenishes the glycogen used and remains more sensitive to insulin. ISPAD describes that, after 45 minutes of exercise, the risk of hypoglycemia lasts 7 to 11 hours, which makes the early hours of the morning dangerous for activities done after 4 pm (Adolfsson, 2022). In adolescents, the need for glucose after moderate exercise in the afternoon increases in two phases, one right after training and another during the night (Adolfsson, 2022).

What the trials tested

In a trial with children on an insulin pump, reducing basal insulin by 20 percent for six hours at bedtime raised the lowest glucose value of the night to 172 mg/dL, against 127 mg/dL on the night without the intervention (Taplin, 2010). The reduction decreased readings below 70 mg/dL but increased readings above 250 mg/dL (Taplin, 2010).

In ten men on multiple injections who ran early in the evening, the group without adjustment had a glucose drop after six hours, and nine of them had nocturnal hypoglycemia (Campbell, 2015). With reduced basal insulin, meal insulin, and a low-glycemic-index snack before bed, all were protected from hypoglycemia for 24 hours (Campbell, 2015).

These numbers describe protocols from small studies, with few people and controlled conditions. They show that the combination of less insulin and well-chosen carbohydrate works, and they serve as a starting point for individual adjustment, not as a prescription.

What to do on the night of training

The position statement on sensors provides glucose targets to guide carbohydrate intake in the period after exercise, including the night after a late-afternoon training session (Moser, 2020). In the first 90 minutes after exercise, the suggested range for most people at low risk is 80 to 180 mg/dL (Moser, 2020). Setting the sensor's hypoglycemia alarm slightly higher on the night of training gives room to act before the drop.

Pumps and automated systems

Automated insulin systems adjust basal according to the sensor, but still require the user to announce meals and prepare for physical activity (Moser, 2025). The function that raises the glucose target has different names in each system, such as exercise mode, activity mode, or temporary target (Moser, 2025).

The EASD and ISPAD position statement recommends setting the higher target 1 to 2 hours before activity when a glucose drop is expected (Moser, 2025). Activating the target only at the start of exercise is less effective at reducing hypoglycemia, but is still encouraged, especially for long activities (Moser, 2025). For planned exercise up to 2 hours after a meal, the guidance is to raise the target first and then reduce the meal insulin (Moser, 2025).

Suspending the system or disconnecting the pump may be necessary in some sports, which may require another way of administering insulin (Moser, 2025). The text on the insulin pump on the way up and on the way down shows how the same equipment behaves in another risk setting.

How to organize in practice

A summary by moment

Moment	What to do	Basis
Before	Postpone with severe hypoglycemia in the previous 24 hours or blood ketones of 1.5 mmol/L or more (Adolfsson, 2022)	ISPAD guideline
Before	Start with glucose of at least 90 mg/dL in most adults	Position statement on sensors (Moser, 2020)
Before, with automated system	Set a higher target 1 to 2 hours before	EASD and ISPAD position statement (Moser, 2025)
During	Keep the sensor between 90 and 180 mg/dL and stop below 70 mg/dL	Position statement on sensors (Moser, 2020)
In the session	Do strength work before aerobic exercise or include intermittent efforts	Crossover trial (Yardley, 2012)
After, at night	Less insulin, low-glycemic-index carbohydrate, and adjusted sensor alarm	Randomized trial (Campbell, 2015)

Applying to the case

For the student, the first step is to review the sensor records from the last nights of games. The rise in the middle of the match is expected in a sport with sprints, and a full correction at that moment explains much of the overnight drop. The team can agree on smaller corrections during the game, or none if glucose is only moderately high and there are no ketones (Adolfsson, 2022).

On the night of the game, he can test with the team a reduction in basal insulin or the last meal and a snack before bed, as in the study protocols (Campbell, 2015). A slightly higher sensor hypoglycemia alarm and recording each game allow the strategy to be adjusted within a few weeks.

The goal is for him to keep playing. Before proposing tests or major changes, it is worth remembering the lesson of quaternary prevention on cascades of care and of negotiating the agenda with the patient. To review the classification of hypoglycemia, see how to define and classify hypoglycemia, and for the place of the sensor in treatment, continuous glucose monitoring from diagnosis.

The limits of the evidence

Many trials on exercise strategies in type 1 diabetes have few participants, often young and trained men, under laboratory conditions (Hasan, 2018). Real-world data such as those from T1DEXI help, but they came from adults with good control, and the response varies greatly between sessions (Riddell, 2023). That is why guidelines classify many recommendations as expert consensus (Moser, 2020).

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by looking for the ICD-10 code.

The limits of using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information on curation and on creators, and institutions can learn about the proposal on the partnerships page. For the most frequent reasons for consultation, see the most common codes in primary care.

Frequently asked questions

Why does glucose rise during some workouts?

Short, intense efforts raise catecholamines and cortisol, which release glucose from the liver (Adolfsson, 2022). This is common in sprints, combat sports and weightlifting, and the correction should be cautious.

Which exercise causes less hypoglycemia?

In T1DEXI, glucose fell less with strength training and interval training than with aerobic exercise (Riddell, 2023). Doing strength training before aerobic exercise also kept glucose more stable (Yardley, 2012).

How long after training does the risk last?

Up to 24 hours (Adolfsson, 2022). After late-afternoon workouts, nocturnal hypoglycemia usually appears 7 to 11 hours later (Adolfsson, 2022).

How much should I reduce insulin before exercise?

It depends on the time of day, the duration, the intensity and your response in previous workouts. The studies used specific reductions in controlled protocols (Campbell, 2015), and personal adjustment should be agreed with the team and calibrated by the sensor.

Is the sensor reliable during exercise?

It is very useful, but it lags. The lag can reach 12 to 24 minutes or more during exercise (Moser, 2020), and the trend arrow helps you act earlier.

Can I train with ketones?

Blood ketones of 1.5 mmol/L or more contraindicate exercise, and values between 0.6 and 1.4 mmol/L call for postponing and investigating (Adolfsson, 2022).

Do people who use an automated system need to do anything?

Yes. The guidance is to set the higher target 1 to 2 hours before the activity (Moser, 2025), in addition to adjusting insulin for nearby meals.

Take-home messages

• In type 1 diabetes, injected insulin keeps acting during the workout, and hypoglycemia is the most common adverse event.
• Glucose falls more with aerobic exercise, less with interval training and even less with strength training.
• Short, intense efforts can raise glucose, and the correction should be cautious.
• The risk lasts up to 24 hours, and late-afternoon workouts threaten the early hours of the night.
• Doing strength training before aerobic exercise and including intermittent efforts keeps glucose more stable.
• The sensor, with attention to the arrow and the lag, is the main adjustment tool.
• The adjustment numbers come from study protocols and serve as a starting point, not as a rule.

Play the case of the day on NEXO.

References

1. Adolfsson P, Taplin CE, Zaharieva DP, et al. ISPAD Clinical Practice Consensus Guidelines 2022: Exercise in children and adolescents with diabetes. Pediatr Diabetes. 2022. PMID 36537529. DOI 10.1111/pedi.13452.
2. Colberg SR, Sigal RJ, Yardley JE, et al. Physical Activity/Exercise and Diabetes: A Position Statement of the American Diabetes Association. Diabetes Care. 2016. PMID 27926890. DOI 10.2337/dc16-1728.
3. Riddell MC, Li Z, Gal RL, et al. Examining the Acute Glycemic Effects of Different Types of Structured Exercise Sessions in Type 1 Diabetes in a Real-World Setting: The Type 1 Diabetes and Exercise Initiative (T1DEXI). Diabetes Care. 2023. PMID 36795053. DOI 10.2337/dc22-1721.
4. Moser O, Riddell MC, Eckstein ML, et al. Glucose management for exercise using continuous glucose monitoring (CGM) and intermittently scanned CGM (isCGM) systems in type 1 diabetes: position statement of the European Association for the Study of Diabetes (EASD) and of the International Society for Pediatric and Adolescent Diabetes (ISPAD) endorsed by JDRF and supported by the American Diabetes Association (ADA). Pediatr Diabetes. 2020. PMID 33047481. DOI 10.1111/pedi.13105.
5. Moser O, Zaharieva DP, Adolfsson P, et al. The use of automated insulin delivery around physical activity and exercise in type 1 diabetes: a position statement of the European Association for the Study of Diabetes (EASD) and the International Society for Pediatric and Adolescent Diabetes (ISPAD). Diabetologia. 2025. PMID 39653802. DOI 10.1007/s00125-024-06308-z.
6. Hasan S, Shaw SM, Gelling LH, et al. Exercise modes and their association with hypoglycemia episodes in adults with type 1 diabetes mellitus: a systematic review. BMJ Open Diabetes Res Care. 2018. PMID 30397494. DOI 10.1136/bmjdrc-2018-000578.
7. Yardley JE, Kenny GP, Perkins BA, et al. Resistance versus aerobic exercise: acute effects on glycemia in type 1 diabetes. Diabetes Care. 2013. PMID 23172972. DOI 10.2337/dc12-0963.
8. Yardley JE, Kenny GP, Perkins BA, et al. Effects of performing resistance exercise before versus after aerobic exercise on glycemia in type 1 diabetes. Diabetes Care. 2012. PMID 22374639. DOI 10.2337/dc11-1844.
9. Taplin CE, Cobry E, Messer L, et al. Preventing post-exercise nocturnal hypoglycemia in children with type 1 diabetes. J Pediatr. 2010. PMID 20650471. DOI 10.1016/j.jpeds.2010.06.004.
10. Campbell MD, Walker M, Bracken RM, et al. Insulin therapy and dietary adjustments to normalize glycemia and prevent nocturnal hypoglycemia after evening exercise in type 1 diabetes: a randomized controlled trial. BMJ Open Diabetes Res Care. 2015. PMID 26019878. DOI 10.1136/bmjdrc-2015-000085.