Low Blood Sugar Workout: Causes and Safety Tips

Low blood sugar during a workout is a genuine safety concern for people with diabetes, particularly those using insulin or sulphonylureas. Hypoglycaemia, defined in the UK as a blood glucose reading below 4.0 mmol/L, can develop quickly during physical activity as working muscles draw glucose from the bloodstream at an accelerated rate. Knowing how to prevent, recognise and treat exercise-related hypoglycaemia is essential for exercising safely. This article covers the causes, warning signs, prevention strategies and nutritional guidance to help you stay safe and active.

Why does exercise cause low blood sugar?

Exercise causes low blood sugar because working muscles absorb glucose rapidly, increasing insulin sensitivity and potentially overwhelming the liver's compensatory glucose release, particularly in people using insulin or sulphonylureas.

Exercise causes low blood sugar (hypoglycaemia) because working muscles absorb glucose from the bloodstream at a much faster rate than usual, which can outpace the body's ability to maintain stable blood glucose levels. In the UK, hypoglycaemia is generally defined as a blood glucose reading below 4.0 mmol/L. Under normal circumstances, the liver releases stored glucose (glycogen) to compensate, but this mechanism can be overwhelmed during prolonged or intense activity, particularly in people who take insulin or certain glucose-lowering medicines such as sulphonylureas.

During aerobic exercise, insulin sensitivity increases significantly, meaning that any insulin already circulating in the body becomes more effective at driving glucose into cells.^[7][8] This is beneficial for long-term metabolic health, but in the short term it raises the risk of blood glucose falling too low. People with type 1 diabetes are at the greatest risk, though those with type 2 diabetes managed with insulin or sulphonylureas are also vulnerable.

Other medicines used in diabetes management, including metformin, DPP-4 inhibitors (such as sitagliptin), SGLT2 inhibitors (such as dapagliflozin), thiazolidinediones and GLP-1 receptor agonists, carry a much lower intrinsic risk of causing hypoglycaemia on their own.^[2][12] However, the risk increases if any of these are used alongside insulin or a sulphonylurea. Understanding your individual risk profile, ideally with guidance from your diabetes care team, is an important first step before increasing your activity levels.

Which types of workout are most likely to drop your blood sugar?

Prolonged aerobic exercise lasting more than 30 minutes poses the highest risk, but HIIT and resistance training can cause delayed hypoglycaemia several hours after the session ends.

Prolonged aerobic exercise, such as running, cycling, swimming or brisk walking lasting more than 30 minutes, is most likely to cause a significant drop in blood sugar. These activities rely heavily on glucose as a fuel source and sustain elevated insulin sensitivity throughout and for several hours afterwards, creating a window of increased hypoglycaemia risk that can extend well into the evening or overnight.

High-intensity interval training (HIIT) and resistance exercise present a more complex picture. Short bursts of intense effort can temporarily raise blood glucose by triggering the release of stress hormones such as adrenaline and cortisol, which stimulate the liver to release glucose.^[6][9] However, once the session ends and hormone levels fall, blood glucose can drop sharply, sometimes several hours later. This delayed hypoglycaemia is a particular concern after evening workouts.

Team sports and mixed-intensity activities, such as football or tennis, combine aerobic and anaerobic phases, making blood glucose responses less predictable. Aquatic exercise can also make it harder to notice early warning signs of hypoglycaemia, as the physical sensations of being in water, such as shivering or fatigue, may overlap with hypo symptoms.

Yoga and gentle stretching carry a lower risk overall, though they are not entirely without risk for those on insulin. Regardless of the activity type, it is advisable to keep fast-acting carbohydrate accessible during all sessions and to avoid exercising alone if you are at higher risk of hypoglycaemia. Monitoring blood glucose before, during (for sessions over 45 minutes) and after exercise is the most reliable way to understand your individual response.

How can you prevent hypoglycaemia before and during exercise?

Check blood glucose before exercise, aim for a pre-activity level of 6 to 10 mmol/L, consider insulin dose adjustments with your care team, and consume additional carbohydrates during sessions lasting over 45 minutes.

Preventing hypoglycaemia during a workout requires planning ahead, and the most effective strategies depend on the type of activity, its duration and your current diabetes management regimen. People using insulin should discuss with their diabetes care team whether a dose reduction before exercise is appropriate, as this is a well-established approach recommended in NICE guidance for people with type 1 diabetes (NG17). Pump users may benefit from a temporary basal rate reduction; your care team can advise on this. Never adjust insulin doses without professional advice.

Checking blood glucose before starting exercise is essential. A general target range before activity is often cited as 6 to 10 mmol/L, though your care team may advise a different target based on your individual circumstances and regimen.^[19][5] If your reading is below 5 mmol/L, it is advisable to consume a carbohydrate snack and recheck before beginning.^[19][1] If it is above 14 mmol/L and you use insulin, check for ketones; if ketones are present, postpone exercise, follow your sick-day rules and seek clinical advice before resuming activity.^[5][3]

For sessions lasting longer than 45 to 60 minutes, consuming additional carbohydrates during the activity can help maintain blood glucose. The amount needed varies between individuals and should be established through careful monitoring over time.

Wearing a continuous glucose monitor (CGM) or flash glucose monitor can provide real-time data and early alerts, significantly improving safety during exercise. However, CGM readings reflect interstitial glucose, which can lag behind blood glucose by several minutes, particularly during rapid changes.^[9][14] If you feel symptomatic or your CGM shows an unexpected trend, confirm with a finger-prick blood glucose test before acting.

Carrying fast-acting carbohydrate at all times during exercise is important. Where appropriate, your diabetes team may also recommend keeping a glucagon rescue kit available and ensuring that any companions or fellow gym-goers know how to use it.

For those exploring structured weight-management support alongside their diabetes care, Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online consultations with prescriber review and pharmacist support, where clinically appropriate treatment options may be considered as part of a broader plan.

What are the warning signs of low blood sugar during a workout?

Warning signs include shakiness, sweating, rapid heartbeat, dizziness and sudden disproportionate fatigue; stop activity immediately and treat with 15 to 20 g of fast-acting carbohydrate.

The warning signs of low blood sugar during a workout include shakiness, sweating, a rapid heartbeat, dizziness, difficulty concentrating and a sudden feeling of weakness or fatigue that seems disproportionate to the effort being made. These symptoms arise because the brain and nervous system are highly dependent on glucose, and when levels fall, the body releases adrenaline as an emergency response, producing many of the classic hypoglycaemia symptoms.

During vigorous exercise, some of these signs can be difficult to distinguish from normal exertion. Sweating, an elevated heart rate and breathlessness are expected during a hard workout, which means hypoglycaemia can go unrecognised. This is a practical challenge that is separate from true hypoglycaemia unawareness. True hypoglycaemia unawareness is a clinical condition in which the body's counter-regulatory responses are impaired, reducing or eliminating the usual warning symptoms even at rest. It is more common in people who have had diabetes for many years or who experience frequent low blood sugar episodes, and it warrants specialist review. If you think you may have reduced awareness of your hypo symptoms, discuss this with your diabetes care team.

If you experience any warning signs during exercise, stop activity immediately and treat as follows: take 15 to 20 g of fast-acting carbohydrate (for example, 4 to 5 glucose tablets, approximately 150 to 200 ml of fruit juice or non-diet cola, or one tube of glucose gel), wait 10 to 15 minutes and recheck your blood glucose. If it remains below 4.0 mmol/L, repeat the fast-acting carbohydrate. Once your reading has recovered, follow up with a longer-acting carbohydrate snack, such as a small sandwich or a few plain biscuits, to help stabilise levels.

More severe symptoms, which indicate that blood glucose has fallen significantly, include confusion, slurred speech, blurred vision, difficulty coordinating movements and, in rare cases, loss of consciousness. A companion or fellow gym-goer should be aware of your condition so they can assist if needed, including using a glucagon rescue kit if one is available and they have been trained to do so.

If you experience severe hypoglycaemia with loss of consciousness, seizures or inability to swallow safely, call 999 immediately. For urgent advice about a hypoglycaemic episode that has not resolved with treatment, contact NHS 111. Routine concerns about recurring low blood sugar during exercise, or if you notice reduced warning symptoms, should be discussed with your GP or diabetes care team, who can review your management plan.

If you suspect that a medicine you are taking has caused or contributed to a hypoglycaemic episode, you can report this to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

What should you eat and drink around exercise to stay safe?

Eat a small carbohydrate snack before aerobic exercise, consume 15 to 20 g of fast-acting carbohydrate during prolonged sessions, and follow up with a balanced carbohydrate and protein meal within an hour of finishing.

Eating the right foods at the right time around exercise is one of the most practical ways to reduce the risk of low blood sugar during a workout. Before aerobic exercise lasting more than 30 minutes, a small carbohydrate-containing snack, such as a banana, a slice of wholegrain toast or a small pot of porridge, can help maintain blood glucose levels throughout the session. The timing and quantity depend on your current blood glucose reading and the intensity of the planned activity.

During prolonged exercise, fast-acting carbohydrates are the most effective option for preventing or treating a drop in blood glucose. Aim for approximately 15 to 20 g of fast-acting carbohydrate. Suitable choices that provide roughly this amount include:

• 4 to 5 glucose tablets

• Approximately 150 to 200 ml of fruit juice or non-diet cola

• One tube of glucose gel

• Jelly sweets providing around 15 to 20 g of glucose (check the packet for gram amounts, as portion sizes vary)

• A sports drink containing glucose (check the label for carbohydrate content)

After exercise, blood glucose can continue to fall for up to 12 hours, particularly after aerobic or mixed-intensity sessions.^[13][11] Eating a balanced meal or snack containing both carbohydrate and protein within an hour of finishing can help stabilise levels. If you exercise in the evening, a small carbohydrate snack before bed may reduce the risk of nocturnal hypoglycaemia. After evening sessions, additional overnight glucose checks may be advisable if you are at higher risk; your diabetes team can advise on whether this is appropriate for you.

Hydration also matters. Dehydration can affect blood glucose readings and impair the body's ability to regulate glucose, so drinking water consistently before, during and after exercise is important. Alcohol should be avoided around exercise, as it suppresses the liver's ability to release glucose and significantly increases hypoglycaemia risk.^[1][19]

Scientific References

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3. Type 1 diabetes | Treatment summaries - BNF - NICE.
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7. Heat Shock Proteins and Exercise in Skeletal Muscle Insulin Resistance: Protective Mechanisms and Therapeutic Potential.
8. Molecular basis of insulin resistance and its impact on the brain: the role of physical exercise.
9. A Comparison of Glucose and Additional Signals for Three Different Exercise Types in Adolescents with Type 1 Diabetes Using a Hybrid Closed-Loop System.
10. High intensity interval training as a novel treatment for impaired awareness of hypoglycaemia in people with type 1 diabetes (HIT4HYPOS).
11. A Randomized Crossover Trial Comparing Glucose Control During Moderate-Intensity, High-Intensity, and Resistance Exercise With Hybrid Closed-Loop Insulin Delivery.
12. Dipeptidyl Peptidase 4 Inhibitors: Novel Therapeutic Agents in the Management of Type II Diabetes Mellitus.
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14. Dual-hormone artificial pancreas: benefits and limitations compared with single-hormone systems.
15. Long-term intensive therapy of IDDM patients with clinically overt autonomic neuropathy: effects on hypoglycemia awareness and counterregulation.
16. Anticipated Basal Insulin Reduction to Prevent Exercise-Induced Hypoglycemia in Adults and Adolescents Living with Type 1 Diabetes.
17. Improved Open-Loop Glucose Control With Basal Insulin Reduction 90 Minutes Before Aerobic Exercise in Patients With Type 1 Diabetes on Continuous Subcutaneous Insulin Infusion.
18. Timing of insulin basal rate reduction to reduce hypoglycemia during late post-prandial exercise in adults with type 1 diabetes using insulin pump therapy.
19. Diabetes and Exercise.

Frequently Asked Questions