Physical activity and sport in type 1 diabetes

Absolutely. Many Olympic and professional athletes have type 1 diabetes, proving that the condition does not limit athletic potential when it is well managed [1]. Among them are Olympic swimmers, Tour de France cyclists, tennis players, professional footballers and elite marathon runners. The key to success is meticulous planning and intensive monitoring with the glucose sensor (CGM) during training. Precise adjustment of insulin and carbohydrate doses for the specific type of effort matters just as much, alongside a medical team specialized in diabetes and sport [2]. Modern technology (pumps with an exercise mode, CGM with predictive alarms) greatly reduces the technical barriers, but does not remove them completely: cost, availability and sensor errors during the effort remain.

Specific challenges include the different management of aerobic sports (which lower glucose) versus anaerobic sports (which raise it temporarily) [3]. Other issues to address include travel to competitions across time zones, competitive stress (which raises glucose) and sports regulations on the use of medical devices during competition. The adjustment period can last up to a year, during which you learn your individual response to the different types and intensities of effort. Nutritional discipline and careful glucose monitoring, needed in diabetes anyway, can also be of help in training [1].

Preparation starts 2-3 hours beforehand. Aim for a starting glucose of 126–180 mg/dL (7-10 mmol/L) for aerobic exercise and 120 mg/dL (6.7 mmol/L) for strength training [3]. Don't forget to check the trend on your CGM (a stable or slightly rising arrow) [4]. Many protocols reduce the bolus for the preceding meal by 50% and/or the basal rate by 50% starting 90 minutes before the effort.

For unplanned exercise, take 15 g of carbohydrates without insulin if your glucose is below 150 mg/dL (8.3 mmol/L) [3]. Check for ketones if your glucose is above 250 mg/dL (13.9 mmol/L) [3]. Their presence temporarily contraindicates exercise until it is corrected. The sports kit includes a glucometer/CGM, fast-acting carbohydrates (gels, glucose tablets), complex carbohydrates (a cereal bar), nasal glucagon for emergencies and a visible medical identification bracelet. Adequate hydration and electrolytes help you achieve optimal performance safely.

Aerobic exercise increases insulin sensitivity [5]. Muscle glucose uptake rises up to 50-fold compared with rest, through insulin-independent mechanisms [5]. Muscle contraction activates glucose transporters without requiring insulin. At the same time, circulating insulin from earlier injections continues to act [6]. The effect is amplified during efforts lasting more than 30 minutes, when muscle glycogen stores are depleted and the muscles draw even more glucose from the blood.

The drop in glucose is accelerated by raised body temperature (vasodilation and rapid absorption), the time of day (greater sensitivity in the afternoon), exercise on an empty stomach and the amount of alcohol consumed recently [3]. Paradoxically, very intense or competitive exercise can initially raise glucose through the release of stress hormones (adrenaline, cortisol), followed by a significant delayed drop [7]. The glucose-lowering effect persists for 24 hours after exercise [5]. During this time muscle glycogen stores are replenished, generally requiring a reduction in overnight basal insulin, as the figure below shows.

The paradoxical rise appears after intense anaerobic exercise (sprinting, weight lifting), stressful competitions or HIIT sessions, when the counter-regulatory hormones (adrenaline, cortisol, growth hormone) outweigh the glucose-lowering effect of the effort [7]. Glucose can climb temporarily by 100 mg/dL (5.6 mmol/L) in the first 60 minutes, but don't rush to correct it [3]. Within 2-3 hours a significant drop usually follows, as the hormones normalize and insulin sensitivity increases. If a correction is needed, it is done conservatively, with half the calculated dose, under close monitoring.

For prevention, you can give a small bolus 15 minutes before an intense exercise that you know raises your glucose [1]. The dose is not the same for everyone, so you set it in advance with your medical team: too large a bolus before the effort can lead to hypoglycemia. As an alternative, you can pair the anaerobic exercise with 15 minutes of light aerobic activity to balance things out [3]. Don't confuse a temporary physiological rise in glucose with a significant insulin deficiency. Closed-loop systems manage these fluctuations automatically.

If a planned exercise is coming up and you use a pump, you can reduce the basal rate by 50% starting 90 minutes beforehand [3]. The bolus for the meal before the exercise is also reduced by 50%, with a pump and with a pen alike [3]. With a pen, the basal insulin has already been injected and can no longer be reduced starting 90 minutes beforehand, so the preparation relies on the smaller bolus and on the extra carbohydrates. You can cut the dose even further for a longer aerobic exercise, or less for a shorter session, especially if it also involves strength work. For team sports with variable intensity, consider temporarily suspending the pump during the effort and reconnecting during breaks, giving micro-boluses as needed [1].

Basal reductions for exercise are decided together with your medical team, starting from your training journal. After exercise, the bolus for the next meal is usually reduced by 20% and the overnight basal rate by 20% to prevent delayed hypoglycemia [8]. If you start an unplanned exercise, take 15 g of carbohydrates without a compensatory bolus, repeating every hour or more often depending on how your glucose behaves [3]. Keep a detailed journal (type of exercise, duration, intensity, adjustments made, results) to identify your personal patterns. The response varies enormously between individuals and requires continuous adjustments over a month for each new type of activity [1].

For moderate exercise under 45 minutes, use glucose tablets or sports gels. You need 3-4 tablets for 15 g, absorbed in 10 minutes, or one gel sachet of 20-25 g, which acts within 15 minutes [9]. Both are enough for rapid glucose corrections. For prolonged effort lasting more than an hour, combine fast-acting carbohydrates with complex ones, such as:

• a banana (25 g) — also provides potassium, a mineral lost through sweat;
• cereal bars (20-30 g) — easy to carry;
• dried fruit with nuts (30 g) — with fats for a sustained effect;
• a jam sandwich (40 g) — balanced.

Isotonic sports drinks (8% carbohydrate) provide both hydration and carbohydrates.

As a general estimate, you need 30-60 g of carbohydrates per hour for moderate exercise and 75-90 g for very high-intensity effort [9]. Take these carbohydrates in small amounts, every 15 minutes [3]. Avoid foods high in fiber, fat or protein before and during short-duration effort. They slow absorption when you need quick energy. Test all products during training, before competitions, to avoid gastrointestinal surprises.

No. Blood ketones above 1.5 mmol/L or urine ketones (++ or more, that is at least a moderate amount on the test strip) contraindicate exercise [3]. Values between 0.6 and 1.5 mmol/L call for postponing the effort until the correction and the recheck described below. Without enough insulin, exercise accelerates ketone production and can trigger diabetic ketoacidosis within just a few hours [10]. If you have a low ketone level (urine + or blood 0.6-1.5 mmol/L), correction with insulin and rehydration follow the protocol agreed with the medical team. Recheck after two hours to see whether it has resolved, so that you can do light exercise. You need to distinguish between "starvation" ketones and truly pathological ones. The first appear after prolonged fasting, with normal or low glucose. The pathological ones are associated with a severe insulin deficiency, with high glucose [3].

If your glucose is above 250 mg/dL (13.9 mmol/L), always check for ketones before sport [3]. If they are present, you hydrate intensively and make the correction following the sick-day protocol agreed in advance with your medical team; it can reach around 150% of the usual dose. Between 1.5 and 2.9 mmol/L, contact your medical team immediately. At 3.0 mmol/L or above, or if vomiting, difficulty breathing or drowsiness appear, go to the emergency room straight away. Postpone exercise until ketones are negative (under 0.6 mmol/L) and glucose falls under 250 mg/dL (13.9 mmol/L). With an insulin pump, the presence of ketones together with high glucose suggests an insulin delivery problem. In that case, change the infusion set and give a correction with a pen or with the pump once you have fitted the new set.

Post-exercise nocturnal hypoglycemia occurs on up to 25% of the nights following a day of intense sport [11]. It is due to increased insulin sensitivity and the replenishment of muscle glycogen stores. Preventive strategies include reducing the overnight basal rate by 20% and raising the bedtime glucose target to 180 mg/dL (10 mmol/L) [8]. Monitoring with a glucose sensor is essential [4]. Set the alarm threshold higher than usual on the nights following more intense physical effort, especially effort performed toward the evening.

Alcohol consumed after exercise amplifies the risk of hypoglycemia [3]. The individual pattern varies, with the highest risk usually in the first 6 hours, although some people may experience it up to 12 hours later [11]. Keep fast-acting carbohydrates and glucagon by the bed and instruct your partner about the symptoms and treatment of nocturnal hypoglycemia. Hypoglycemia means a blood glucose below 70 mg/dL (3.9 mmol/L): you take 15 g of fast-acting carbohydrate and recheck after 15 minutes. The complete steps are on the pages about what hypoglycemia is and about how to treat it.

All sports are possible, but some offer added advantages for glycemic management [4]. Moderate aerobic exercise (brisk walking, low-intensity swimming, long-distance cycling) has a predictable glucose-lowering effect that is easy to manage. Strength training with moderate weights maintains muscle mass (a major glucose reservoir) and improves insulin sensitivity in the long term [5]. Yoga, Pilates and tai chi combine metabolic benefits with stress reduction and improved balance.

Team sports (basketball, football, tennis) provide social motivation but require complex strategies to adapt to the variable intensity of the effort [1]. Extreme sports (mountaineering, diving, skydiving) require special safety protocols and long experience in diabetes management [3]. Swimming presents unique challenges because the glucose sensor does not transmit data underwater, access to carbohydrates is limited, and the symptoms of hypoglycemia can be masked [2]. Choose sports you enjoy and will practice consistently. Regularity matters, because over time you learn how you respond to a variety of situations.

Most glucose sensors are resistant to sweat and moderate impact, but they need extra protection for contact or water sports [2]. For swimming, most sensor models temporarily lose their Bluetooth transmission underwater but keep recording the data, which syncs once you get out. Check the trend before starting the effort and check the sensor every 20 minutes during it, especially for endurance sports [4].

Calibration on a day when you have already done intense exercise can be affected by dehydration or variations in tissue perfusion [12]. Compression of the sensor area (from equipment or specific positions) can sometimes give falsely low readings [13]. In that case, carefully relieve the pressure on the sensor and check your glucose with a glucometer. Set the alarms somewhat higher to give yourself time to react during the effort [4]. After sport, the sensor may show a longer lag until peripheral circulation has fully normalized [12].