Athlete Nutrition and Hydration Principles

The single most useful concept is energy availability, the amount of energy left to support the body's basic functions once the energy burned in exercise is taken away, judged against the athlete's lean body mass. When an athlete does not eat enough to cover both training and normal physiology, energy availability falls, and the body downregulates functions it treats as non-essential, including hormonal, bone and immune processes. Low energy availability, whether from deliberate restriction, disordered eating, or simply failing to keep up with a heavy training load, is the root cause of many of the problems seen later in this topic.

The practical principle that follows is to fuel for the work required. Intake is not a fixed daily target but something that should rise and fall with training load, higher around hard or long sessions and competition, and lower on rest days. Athletes who under-fuel around their most demanding sessions train in a chronically depleted state, recover poorly and adapt less well, even when their weight looks stable.

Carbohydrate is the main fuel for moderate and high-intensity exercise, and the body's stores of it are limited, so intake is scaled to training load rather than fixed. General guidance runs from a few grams per kilogram (g/kg) of body mass a day for light training up to much higher intakes for heavy endurance loads, with more taken around demanding sessions and less when training is light. For efforts lasting beyond about an hour, taking carbohydrate during exercise helps maintain performance.

Protein supports the repair and adaptation of muscle and other tissues. What matters is not only the total but its distribution, with moderate amounts spread across the day and a serving after training generally advised, rather than one large intake. Typical guidance sits well above the requirement for a sedentary person, and is higher during heavy training, injury or energy restriction, when protecting muscle matters most.

Fat is essential, provides energy and carries fat-soluble vitamins, and should not be over-restricted, since very low fat intake can compromise health and hormones. Around the day the useful ideas are simple: fuel before hard sessions, take on carbohydrate during long ones, and combine carbohydrate and protein afterwards to refill stores and support repair. Whole foods cover most needs, with supplements playing a limited and considered role.

Body water is lost through sweat during exercise, at a rate that varies widely between individuals and with the heat, and even modest dehydration can impair endurance, strength and concentration. The practical aim is to begin exercise well hydrated, to limit large fluid deficits during prolonged exercise by drinking to a sensible plan, and to rehydrate afterwards, replacing both the fluid and the sodium lost in sweat. Because sweat rates differ so much, generic volumes are less useful than an individual sense of how much a given athlete loses, which can be gauged simply from the change in body mass across a session.

Sodium and other electrolytes are lost in sweat and matter in prolonged or very sweaty exercise, where drinking large volumes of plain water alone carries a specific danger. Drinking beyond thirst over long events can dilute the blood sodium and cause exercise-associated hyponatraemia (EAH), which can be serious. The message for athletes is therefore not to drink as much as possible, but to drink appropriately, guided by thirst and a plan rather than forcing large volumes, and to include sodium when sessions are long or sweat losses are heavy.

The clinical face of chronic low energy availability is Relative Energy Deficiency in Sport (RED-S), a syndrome in which inadequate fuelling impairs many body systems at once. Its consequences are exactly the ones an MSK clinician meets: reduced bone density and recurrent bone stress injuries, disturbance or loss of menstrual periods in women and lowered testosterone in men, frequent illness, poor recovery, mood change, and a plateau or decline in performance. It can affect any athlete, is not confined to those who look underweight, and often coexists with disordered eating, which must be handled sensitively.

At the other extreme of the hydration story, exercise-associated hyponatraemia presents after prolonged exercise with headache, nausea and confusion and, when severe, seizures or collapse, and is a medical emergency.

Assessment is mostly clinical and starts with a careful history: the training load, a picture of typical eating around it, weight history and any restriction, and, in women, a menstrual history, since menstrual disturbance is a sensitive marker of low energy availability. Examination and simple investigations are guided by the picture, and may include blood tests for deficiencies such as low iron, hormonal assessment, and bone density measurement by dual-energy x-ray absorptiometry (DEXA) where bone health is a concern or bone stress injuries recur. The mainstay of management is restoring adequate energy and nutrient intake, best done with a registered sports dietitian and, in established Relative Energy Deficiency in Sport or where disordered eating is present, a multidisciplinary team. The SEM clinician's job is to recognise the pattern, exclude other causes, begin sensible advice on fuelling and hydration, and refer for specialist input rather than manage complex cases alone.

• •International Olympic Committee (IOC) consensus statements on Relative Energy Deficiency in Sport and on dietary supplements inform current understanding of energy availability and its consequences.
• •Sports nutrition position statements from bodies such as the American College of Sports Medicine and international dietetic associations set out the macronutrient and hydration principles summarised here.
• •Guidance on exercise-associated hyponatraemia advises against overdrinking and informs the recognition and emergency management of low blood sodium.
• •National Health Service and British Dietetic Association resources provide accessible, UK-facing advice, and a registered sports dietitian is the appropriate specialist for individualised plans.

• •Energy availability is intake minus exercise energy expenditure relative to lean mass, and low energy availability is the root of many athlete nutrition problems.
• •Fuel for the work required: carbohydrate in particular should be scaled up around demanding sessions and down when training is light.
• •Protein supports repair and adaptation, and is best spread through the day and taken after training rather than in one large amount.
• •Relative Energy Deficiency in Sport links under-fuelling to reduced bone density, menstrual disturbance, recurrent bone stress injuries, illness and underperformance.
• •Recurrent bone stress injuries or menstrual disturbance should prompt assessment of energy availability, not just local treatment.
• •Athletes should drink to a sensible plan and to thirst, since overdrinking in long events risks exercise-associated hyponatraemia.