Belgium Study: How Your Blood Sugar Really Behaves During Exercise with Type 1 Diabetes

A study from Belgium offers some of the most detailed real-world data yet on what happens to blood sugar, lactate, and ketone levels when people with type 1 diabetes (T1D) exercise — and the findings could change how you prepare for your next workout.

Why an International Perspective Matters for US Patients

Exercise management in T1D remains one of the most challenging aspects of daily diabetes care worldwide. This Belgian research from the University of Antwerp adds to a growing body of global evidence that helps US clinicians and patients move beyond generic exercise warnings toward specific, data-driven strategies. Because the study used standardized protocols and continuous glucose monitoring (CGM) — technology widely used in the US — its findings translate directly to American patients using devices like the Dexcom G7 or FreeStyle Libre.

What the Study Did

Researchers at the University of Antwerp enrolled 21 men with T1D (average age 29, HbA1c of 7.2%) and had them complete two separate exercise sessions within six weeks:

• Anaerobic exercise: A cardiopulmonary exercise test (CPET) — essentially a maximum-effort cycling test
• Aerobic exercise: 60 minutes of moderate cycling at 60% of peak oxygen capacity (VO₂ peak)

Before each session, participants ate a standardized breakfast but skipped their pre-meal insulin. Insulin pump users also suspended their basal insulin.

Key Findings — In US Units

Anaerobic (High-Intensity) Exercise

• Blood sugar spiked to a peak of 331 mg/dL within 1–3 hours after exercise
• Blood sugar dropped to a low of 176 mg/dL six hours post-exercise
• Lactate peaked at 108 mg/dL (6.0 mmol/L), with a maximum of 243 mg/dL (13.5 mmol/L)

Aerobic (Moderate-Intensity) Exercise

• Blood sugar peaked at 305 mg/dL about 80 minutes after exercise ended
• Blood sugar reached a low of 211 mg/dL six hours later
• Lactate rose quickly, reaching 77 mg/dL (4.3 mmol/L) within just 10 minutes

Ketones — Good News

Ketone levels stayed very low (≤3.6 mg/dL / 0.2 mmol/L) during both types of exercise. No ketosis was recorded at any point — an important safety reassurance for T1D patients who worry about diabetic ketoacidosis (DKA) during exercise.

The Tradeoff: No Hypoglycemia, But Higher Blood Sugar

Skipping pre-meal insulin successfully prevented hypoglycemia (low blood sugar) during exercise — a major fear for many T1D patients. However, combined with the pre-exercise carbohydrate meal, it caused post-exercise blood sugars that exceeded the American Diabetes Association (ADA) target range of 70–180 mg/dL. This suggests the carbohydrate amount in the pre-exercise meal may have been too high when insulin is omitted entirely.

Why This Matters for US Patients

Roughly 1.6 million Americans live with type 1 diabetes, and fear of exercise-induced hypoglycemia is a documented barrier to physical activity in this population. This Belgian study provides concrete evidence that:

• High-intensity exercise raises blood sugar — not just lowers it — which may actually protect against lows during the session itself
• A six-hour window after exercise is when blood sugar drops most significantly, meaning post-workout CGM monitoring is critical
• Ketosis risk during exercise is low, which should reassure patients and providers
• Insulin and carbohydrate adjustments need to be fine-tuned together — omitting insulin alone without adjusting carb intake may overshoot blood sugar targets

US patients should discuss these findings with their endocrinologist or certified diabetes care and education specialist (CDCES) before changing any insulin protocols around exercise.

Original Study Citation

De Ridder, Ledeganck, De Winter. Trends of glucose, lactate and ketones during anaerobic and aerobic exercise in subjects with type 1 diabetes: The ACTION-1 study. Diabetes/Metabolism Research and Reviews, 2022. PMID: 35533265. DOI: 10.1002/dmrr.3537. ClinicalTrials.gov: NCT05097339.