Exercise and hypoglycemia

Muscle uses glucose during exercise and then stays more sensitive to insulin. In a person without diabetes, the automatic response is to reduce insulin secretion. In type 1 diabetes, however, insulin given subcutaneously keeps being absorbed and keeps working, and exercise can speed up its absorption [1].

The fall in blood glucose is greater if you start with a lot of active insulin, for example after a recent dose of rapid-acting insulin (bolus) [2]. Muscle also takes up glucose from the blood in the hours after training. Its aim is to rebuild its glycogen stores, the form in which it keeps glucose for later. This is where the second part of the risk comes from, delayed hypoglycemia [3].

For many hours, including the following night. The risk of delayed hypoglycemia persists for up to 24 hours after the end of significant physical exertion [4]. A workout in the afternoon or evening can cause hypoglycemia long after midnight [3].

The reason is the rebuilding of glycogen stores in the muscles and liver, a process that takes time. The risk increases with the duration and intensity of exercise and is higher in untrained people or in those who exercise irregularly [4] [5]. That is why treatment adjustments do not stop when the workout ends. Reducing basal insulin by about 20% after prolonged or intense exercise can prevent delayed hypoglycemia, especially overnight [6].

A level of 126–180 mg/dL (7.0–10.0 mmol/L) at the start of exercise, as the figure below shows. The value is chosen individually, however, according to the type, intensity and duration of exercise and to your insulin treatment [5]. Below this range, you add carbohydrates before you start. Above it, you check whether the high blood glucose might come from a significant lack of insulin [5]. Above 270 mg/dL (15 mmol/L), you measure ketone bodies before you start [3].

Postpone intense exercise if your blood glucose is 350 mg/dL (19.4 mmol/L) or higher [5]. Do not exercise at all if you also have blood ketone bodies above 1.5 mmol/L or at least a moderate amount in your urine [4]. Ketone bodies appear when the body does not have enough insulin and starts burning fat for energy. Be careful with lower blood ketone levels too, from 0.6 mmol/L upward [5]. If your blood glucose is high but you have no ketone bodies, the cause is usually not a lack of insulin but rather a recent meal. Discuss with your doctor the thresholds that are right for you, depending on your treatment and type of exercise.

Usually two doses, often reduced together. You reduce the insulin dose for the meal before exercise and, if needed, also lower your basal insulin [4]. With pumps that have no automated system, the basal rate is usually reduced by about 10–50% or even more [5]. The reduction starts about 90 minutes before planned exercise [3]. Another option is to stop the pump during exercise, but for no more than 1–2 hours [5]. A lack of insulin leads to ketone bodies within a few hours, so do not keep the pump stopped beyond this interval [3].

Your doctor sets the exact figures, because they depend on the type of exercise, the time of day you do it and your treatment plan. Automated systems have a special physical activity mode, started 1–2 hours before you begin [2]. The timing of an unannounced snack before exercise also matters, because a rise in blood glucose can make the automated system deliver more insulin [4].

Before exercise, if your blood glucose is below the target range, you add fast-acting carbohydrates. If you have reduced your insulin dose before exercise in advance, 15–20 g of carbohydrates per hour may be enough [7].

During exercise that follows an insulin bolus, you need more carbohydrates. In this case, 0.5–1.0 g of carbohydrates per kilogram of body weight per hour is recommended, about 30–60 g per hour, for most types of exercise [3] [5]. These amounts are close to those used by athletes without diabetes [5]. The total is spread over the hour, not taken all at once. After a workout, a meal with carbohydrates and protein helps rebuild glycogen stores and lowers the risk of hypoglycemia during the night [3] [8]. The exact figures are set with your doctor, because they depend on your weight and on the active insulin available at any given time.

Yes. It is a common surprise. Very intense, short exercise, such as sprinting, weightlifting or a competition, increases the release of adrenaline and other stress hormones, which release glucose from the liver [3]. In this case, blood glucose rises instead of falling [9].

The trap is what comes next. Once the effect of the hormones wears off, blood glucose can fall a lot, sometimes only after a few hours or during the following night [3]. If an insulin correction is needed, it is done cautiously, following the plan agreed with your doctor.

Not always. You use it for direction and speed, not for the exact number. During physical exertion, blood glucose changes quickly. In this situation, the sensor delay increases from about 5 minutes to 12–24 minutes [3]. When blood glucose is falling, the sensor usually shows a higher value than the real one [10]. That is why the arrow showing that blood glucose is falling matters more than the number displayed.

Set your low glucose alarm to a higher value on training days and check any surprising reading with your glucometer [3]. Sensor accuracy varies with the type of exercise [5]. Sweat can weaken the adhesive, and the sensor may then come off more easily. Always keep your glucometer in your gym bag.

You stop. No performance is worth a half-treated low. You take 15 g of fast-acting carbohydrates, wait 15 minutes and check again before going back into play [11]. During exercise, the 15 g rule also applies to automated systems, where 5–10 g is usually enough in other situations [11].

Preparing in advance makes the difference. Your coach should know that you have diabetes, what hypoglycemia looks like and where the glucose and, if needed, the glucagon are kept [3]. Keep your glucose supply at the edge of the field, not in the locker room. After a competition, the risk of hypoglycemia continues for hours, so checking your blood glucose in the evening and then during the night is important [4].

Yes, more than people usually think. A half-hour walk after a meal noticeably lowers blood glucose, especially if your mealtime insulin is still active [12]. Shopping, gardening or cleaning can have a similar effect.

This is both the advantage and the trap. Light activity is good for blood glucose, but it is often unplanned, without any prior adjustment of your mealtime insulin dose [8]. If you know a day with a lot of movement is coming, discuss with your doctor how to reduce your insulin doses, and always carry glucose with you.

Fast-acting glucose within reach, not in the locker room. Also take your glucometer, your phone with the sensor alarms turned on and glucagon, if possible [2]. Add a card or bracelet that shows you have diabetes.

The remedy for hypoglycemia should be a few steps away from you. Hypoglycemia in the middle of the field, with your glucose locked in a locker, is exactly the situation that can quickly turn into a severe episode. Tell a teammate where you keep your glucose and glucagon. If you cannot swallow, do not respond or have seizures, the people around you give you nothing by mouth, give you glucagon and call emergency services immediately [11].