Several metabolic conditions first become apparent during exercise, because muscular energy demand exposes a defect in the pathway that supplies it. This page covers the two groups a sport and exercise medicine (SEM) clinician meets most often in young athletes: diabetes, where the challenge is managing glucose around training and competition, and the inherited metabolic myopathies, in which a child or adolescent presents with exercise intolerance, cramps, or recurrent dark urine after exertion and is repeatedly told they are simply unfit or dehydrated. Obesity-related metabolic disease, endocrine disorders, and relative energy deficiency in sport and disordered eating are important in this age group but are covered elsewhere on StudySEM. Here the focus is recognition from the pattern and timing of symptoms, and keeping young athletes participating safely.
Type 1 diabetes should not limit participation, and young people with it compete successfully at every level, but glucose management around exercise takes planning. The central difficulty is that different types of exercise move glucose in opposite directions. Sustained aerobic exercise tends to lower blood glucose and carries a clear risk of hypoglycaemia, whereas short high-intensity and resistance work, through the catecholamine response, can transiently raise it. Mixed team sports do both within a single session, which is why a young athlete can finish a match with a high reading and then drop overnight.
Management must be individualised rather than generic. The appropriate insulin and carbohydrate adjustments depend on the insulin delivery method, the type, timing and intensity of the exercise, the current glucose trend, any recent hypoglycaemia and the athlete's established diabetes plan, which is agreed with their diabetes team. Continuous glucose monitoring (CGM) is increasingly used and helps considerably by showing the direction and rate of change rather than a single value, but interstitial readings lag behind blood glucose during rapid change, symptoms that do not match the sensor should prompt capillary confirmation where possible, and alarms and trend arrows support rather than replace clinical judgement. Automated insulin delivery systems require device-specific exercise settings and planning rather than a generic dose reduction.
Common strategies include reducing rapid-acting insulin for the meal before planned exercise and taking additional carbohydrate during prolonged sessions, always within the athlete's own plan. Delayed and nocturnal hypoglycaemia in the hours after hard training is the most dangerous and most commonly overlooked risk and should be planned for explicitly. Fast-acting glucose must be immediately available at the pitchside rather than in a bag in the changing room, and coaches and teammates should know how to recognise and treat hypoglycaemia. Exercise should not be undertaken with moderate or large ketonaemia or ketonuria, because it can worsen metabolic decompensation; the athlete's individual sick-day and correction plan should be followed and the situation reassessed before restarting. Type 2 diabetes is now seen in adolescents and is managed with the same emphasis on activity, with fewer hypoglycaemia concerns unless the young person is on insulin or a sulfonylurea. Anti-doping deserves a mention: insulin is prohibited at all times, so an athlete subject to testing should check the applicable therapeutic use exemption requirements before competing.
The inherited metabolic myopathies impair the supply of energy to working muscle, and the single most useful diagnostic clue is when in exercise the symptoms appear.
Disorders of glycogen metabolism produce symptoms within seconds to minutes of starting brief, high-intensity or isometric activity, because muscle depends on glycogen early in exercise. The best known is McArdle disease, a glycogen storage disease (GSD) type V caused by myophosphorylase deficiency, in which the young person describes premature fatigue, cramping and muscle pain in the first few minutes of effort. Its hallmark is the second wind phenomenon: if they persist at low intensity, symptoms and an inappropriately high heart rate improve markedly after roughly ten minutes as blood flow and alternative fuels take over. A clearly described second wind is highly characteristic and should prompt specialist confirmation. Resting creatine kinase (CK) is often persistently raised, and lactate fails to rise normally on specialist non-ischaemic forearm exercise testing; older ischaemic protocols are avoided because they can provoke painful contracture and muscle injury. Tarui disease, a phosphofructokinase deficiency, resembles McArdle disease but has no second wind, and characteristically symptoms are made worse rather than better by taking carbohydrate before exercise, sometimes with a mild haemolytic picture.
Disorders of fatty acid oxidation behave in the opposite way. Because fat becomes the dominant fuel during prolonged, lower-intensity activity, conditions such as carnitine palmitoyltransferase II deficiency and very long-chain acyl-CoA dehydrogenase deficiency cause symptoms after extended endurance exercise, and are typically provoked by fasting, intercurrent fever or infection, and cold. They classically present in adolescence with recurrent exercise-induced rhabdomyolysis. Mitochondrial disorders commonly cause disproportionate endurance intolerance, although the phenotype, any associated neurological features and the risk of rhabdomyolysis vary considerably. A separate pattern worth knowing is exertional rhabdomyolysis occurring in hot, humid conditions, which should raise the possibility of a variant in the ryanodine receptor 1 gene. That is not a classic metabolic myopathy at all but a disorder of calcium handling associated with malignant hyperthermia susceptibility, and it belongs in the differential for a different reason: it has direct implications for any future anaesthetic for the young person and their relatives.
Most young athletes with muscle soreness after unaccustomed training have delayed onset muscle soreness and need nothing more than reassurance and sensible progression. The features that should change that judgement are recurrent episodes of severe exertional muscle pain or cramping consistently out of proportion to the workload, any episode of dark or cola-coloured urine after exercise, a creatine kinase that remains clearly raised between episodes rather than settling, a clearly described second wind, symptoms reliably triggered by fasting or intercurrent illness, and a family history of similar problems, unexplained muscle disease or an adverse anaesthetic reaction.
Initial investigation includes creatine kinase measured both acutely and after a period of rest, renal function and electrolytes, and thyroid function. Urine testing is most usefully described as a dipstick positive for blood with few or no red cells on microscopy, which supports myoglobinuria, since direct urinary myoglobin assays are not always available and myoglobin clears rapidly. A persistently raised resting creatine kinase supports an underlying muscle disorder and is particularly recognised in McArdle disease and the muscular dystrophies, but it is not present in every fatty acid oxidation or mitochondrial disorder, so a normal value between episodes does not exclude them.
Second-line testing is arranged through a specialist metabolic or neuromuscular service and may include a plasma acylcarnitine profile and urine organic acids, which are most informative when taken during or shortly after an episode and may be normal between them, structured specialist exercise testing, and genetic panel testing, which has reduced the need for muscle biopsy although biopsy remains useful in selected mitochondrial, structural or unresolved cases. Referral should not wait for a diagnosis to be certain, because recurrent exertional rhabdomyolysis carries a real risk of acute kidney injury.
The aim is almost always to keep the young person active while removing the triggers, rather than to withdraw them from sport. In McArdle disease, regular moderate aerobic conditioning is safe and beneficial because it improves the muscle's capacity to use fat, and taking carbohydrate shortly before planned exercise together with a deliberate low-intensity warm-up helps the athlete reach their second wind and train more comfortably.
In fatty acid oxidation disorders the general principles are to avoid prolonged fasting, maintain carbohydrate intake around exercise and during illness, and avoid extremes of duration, heat and cold, with the clear instruction not to train through a febrile illness. The specific dietary strategy, and the limits on duration and temperature, differ between individual disorders and must be set with a specialist metabolic team and a specialist dietitian rather than applied as one generic plan.
Across all of these conditions the athlete, their family and the coaching staff need a written emergency plan. It should set out the criteria for stopping activity, the carbohydrate and hydration plan, precautions around illness and fasting, when to attend emergency care, and how the plan is communicated to school and coaching staff, along with the instruction to seek urgent medical assessment if urine turns dark after exercise. Where a ryanodine receptor variant or malignant hyperthermia susceptibility is suspected, specialist assessment should be arranged because of the implications for future anaesthesia. Care is best shared with paediatric specialist services, with the SEM clinician's role being to recognise the pattern, protect the athlete from repeated episodes, and keep them participating safely.
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