Diabetes in Sport

Insulin is the hormone that lowers blood glucose, driving it into muscle and other tissues and switching off glucose release from the liver. In type 1 diabetes there is no insulin of the body's own, so it is given by injection or pump, and the amount on board at any moment strongly influences what happens during exercise.

Exercise changes glucose in opposite directions depending on its nature. Sustained aerobic or endurance exercise increases the uptake of glucose by working muscle and raises insulin sensitivity, which tends to lower blood glucose, and when there is injected insulin still acting, this can drive glucose down into hypoglycaemia. This effect outlasts the session, because sensitivity stays high and the muscles restock their glycogen, so glucose can fall for many hours afterwards, including overnight.

By contrast, short bursts of high-intensity or anaerobic effort, and the adrenaline of competition, release stress hormones that push glucose up, so a sprint session or a big race can raise glucose rather than lower it.

The practical consequence is a balancing act. Too much insulin or too little carbohydrate tips an athlete into hypoglycaemia, while too little insulin, particularly in type 1 diabetes, allows glucose to climb and ketones to form, and exercising in that state is unsafe. The response is individual and shaped by exercise type, intensity, duration, timing and the starting glucose.

The danger to watch for is hypoglycaemia. Its early, autonomic features are sweating, shakiness, palpitations, hunger and anxiety, and if glucose keeps falling the brain is affected, giving confusion, poor coordination, altered behaviour, drowsiness and, at worst, seizures or loss of consciousness. In an athlete this is genuinely tricky, because sweating, a racing heart and fatigue are also the normal signs of hard exercise, so a hypo can hide in plain sight. Repeated hypoglycaemia can also blunt the warning symptoms over time, a state of impaired awareness that raises the risk further.

Hyperglycaemia tends to be less dramatic in the moment, causing thirst, needing to pass urine, tiredness and flat performance. The situation that must not be missed is high glucose together with ketones in someone short of insulin, which can progress to diabetic ketoacidosis (DKA), with nausea, vomiting, abdominal pain, deep laboured breathing and drowsiness. That is a medical emergency, and exercising with high glucose and ketones makes it worse rather than better.

Glucose is the measurement that matters around exercise. Traditional capillary finger-prick testing still has a place, but continuous glucose monitoring (CGM), a small sensor giving real-time readings and, crucially, the direction and rate of change, has transformed how athletes manage sport, letting them see a fall coming and act before symptoms arrive. Checking glucose before exercise is a habit worth building, since it guides whether to take carbohydrate or adjust insulin.

Ketones should be checked, in blood or urine, whenever glucose is high or the athlete is unwell, because they signal insulin deficiency and the risk of ketoacidosis. Glycated haemoglobin (HbA1c) reflects overall control over the preceding months and is useful for the bigger picture, but it is the real-time glucose, not the HbA1c, that governs a given training session. Monitoring extends beyond the session itself, given the risk of delayed and overnight lows.

Managing diabetes around exercise rests on anticipating the glucose swing and heading it off. Before exercise the athlete checks glucose and acts on it, taking extra carbohydrate when glucose is low or in the lower part of the range, and being cautious when it is high, avoiding exercise altogether when it is high with ketones until that is corrected. During and after longer or harder sessions, carbohydrate top-ups and monitoring help hold glucose steady.

Insulin is adjusted rather than fixed. Reducing the relevant insulin dose around planned exercise, and timing sessions to avoid the peak action of a recent dose, both reduce the chance of a hypo, and these adjustments are worked out individually with the diabetes team. Injection site matters too, since insulin injected into a muscle that is then worked hard is absorbed faster and can trigger an acute hypo, so a site away from the muscles about to be exercised is preferred.

A hypoglycaemic episode is treated promptly with fast-acting carbohydrate, such as glucose tablets, gel or a sugary drink, then rechecked and followed with longer-acting carbohydrate, and rapid carbohydrate should always be to hand. Anyone likely to be around a severe hypo should know how to help, including the use of glucagon.

Two wider points matter. In type 2 diabetes, exercise is treatment in its own right, improving insulin sensitivity, glucose control, weight and cardiovascular risk, with the main hypoglycaemia risk arising in those on insulin or certain tablets rather than on metformin alone. And there is an anti-doping dimension: insulin is a prohibited substance, so an athlete with diabetes who needs it requires a Therapeutic Use Exemption (TUE), which their genuine clinical need supports, and any other glucose-lowering medicine should be checked against the current list.

Diabetes is fully compatible with sport, including at elite level, and the work is in the ongoing fine-tuning rather than any restriction. Each athlete needs an individualised glucose management plan, built with their diabetes team, that they revisit as training load, sport and circumstances change, since what keeps glucose steady in one phase may need adjusting in another.

Practical preparation makes a real difference, including carrying glucose at all times, wearing or carrying identification, planning for heat, altitude, travel and illness, and having clear sick-day rules. Particular vigilance is needed for impaired hypoglycaemia awareness, which should prompt review with the diabetes team. With good preparation and monitoring, athletes with diabetes train and compete fully and safely.

• •International consensus guidance on exercise management in type 1 diabetes sets out glucose targets around exercise, carbohydrate and insulin strategies, and the prevention of hypoglycaemia.
• •Diabetes UK and United Kingdom diabetes guidance cover glucose monitoring, continuous glucose monitoring, and the safe use of insulin and other treatments.
• •National guidance on type 2 diabetes places structured physical activity at the centre of management alongside other measures.
• •UK Anti-Doping and World Anti-Doping Agency guidance confirm that insulin is prohibited and requires a Therapeutic Use Exemption for the athlete who needs it.

• •Type 1 diabetes needs lifelong insulin, while in type 2 diabetes exercise is a cornerstone of treatment.
• •Aerobic exercise tends to lower blood glucose, whereas high-intensity effort and competition stress can raise it.
• •Hypoglycaemia is the key danger, is easily masked by exercise, and can be delayed for hours including overnight.
• •Avoid injecting insulin into a muscle about to be exercised hard, as faster absorption can cause a hypo.
• •Do not exercise with high glucose and raised ketones, because of the risk of diabetic ketoacidosis, and use continuous glucose monitoring and pre-exercise checks to guide carbohydrate and insulin.
• •Insulin is prohibited in sport and requires a Therapeutic Use Exemption for the athlete with diabetes who needs it.