Diet, Nutrition and Hydration

Energy Use in Sport and Exercise

Everything the body does — breathing, thinking, sprinting, lifting — requires energy. That energy comes entirely from the food we eat and is derived from a combination of macronutrients. The amount of energy an individual needs depends on several factors: age, gender, body size and, critically for athletes, the amount and intensity of exercise performed. A 16-year-old county-level swimmer training twice a day will have far greater energy needs than a sedentary adult of the same age and size.

OCR focuses on energy use — understanding how the body uses different nutrients as fuel — rather than precise calorie calculations. The key principle is matching food intake to energy demand: an athlete needs to consume enough of the right foods to fuel training, support recovery and maintain a healthy body composition.

The Balanced Diet: Seven Components

A balanced diet does not mean eating equal amounts of everything. It means eating the right proportions of different food types to supply the body with all the nutrients it needs. OCR requires knowledge of seven components: carbohydrates, proteins, fats, minerals, vitamins, fibre and water. No single food provides every nutrient — variety is essential. The approximate proportions for macronutrients in a balanced diet are: 55–60% carbohydrate, 25–30% fat, and 15–20% protein.

Macronutrients: Carbohydrate, Fat and Protein

Carbohydrate is the body’s main fuel for exercise, and it becomes the dominant fuel as intensity rises. At an easy jog the body burns a mix of fat and carbohydrate; at sprint pace it relies almost entirely on carbohydrate. They are broken down into glucose, which is either used immediately or stored as glycogen in the muscles and liver for use during exercise. Elite distance runners and cyclists often practise ‘glycogen loading’ — consuming carbohydrate-rich meals in the days before an event to maximise glycogen stores. A marathon runner, for example, eats carbohydrate-rich meals for two or three days before a race so that muscle and liver glycogen stores are full at the start.

Fat provides more energy per gram than carbohydrate, but it can only be used efficiently as a fuel source at low intensity — for example, during a slow, steady-state walk or easy cycle ride. When exercise intensity increases, the body switches back to carbohydrate because fat metabolism is too slow to meet the energy demand. Fat also has important non-energy roles: it insulates the body, protects vital organs, and acts as a carrier for fat-soluble vitamins (A, D, E and K).

Protein is not a primary energy source. Its main role is the growth and repair of muscle tissue — essential after heavy training when muscle fibres have been broken down by the work they have done. Adam Peaty follows a protein-rich recovery plan after intense pool sessions to support muscle repair and adaptation. Consuming insufficient protein slows recovery and limits the muscle adaptations that training is designed to produce.

Micronutrients: Vitamins, Minerals and Fibre

Vitamins and minerals do not supply energy directly, but they are essential for the efficient working of all body systems. Vitamins support processes from immune function (vitamin C) to bone metabolism (vitamin D); minerals such as calcium are vital for bone density and muscle contraction, while iron is essential for oxygen transport in the blood via haemoglobin. A diet lacking variety will typically be deficient in vitamins and minerals even if its total energy looks adequate.

Fibre is the indigestible component of plant-based foods — found in fruit, vegetables, wholegrain cereals and legumes. Its primary role is to support healthy digestion by adding bulk to stools and maintaining regular bowel function. Fibre also helps regulate blood glucose levels after meals by slowing the absorption of sugar, which is particularly important for athletes trying to maintain stable energy levels across a training day.

Hydration and the Consequences of Dehydration

Water is the medium in which almost every chemical reaction in the body takes place. Maintaining water balance is critical to performance. Even a 1–2% reduction in body mass through fluid loss can begin to impair sporting performance. Dehydration occurs when the body loses more fluid than it takes in. During intense exercise in warm conditions — such as a cross-country race or a summer football tournament — sweat rates can be high, meaning fluid must be replaced regularly to maintain both performance and safety.

Effects of Dehydration on the Body and Performance

Dehydration triggers a series of interconnected physiological problems. As fluid is lost, blood plasma volume falls and the blood becomes thicker. This increase in blood viscosity means the heart must work harder to push blood around the circulatory system, driving up heart rate. Delivery of oxygen and nutrients to muscles slows, accelerating fatigue. In warm conditions, the body’s ability to shed heat through blood vessels near the skin is also compromised, raising core body temperature and increasing the risk of overheating.

Beyond the cardiovascular effects, dehydration also affects the nervous system. Reaction time slows and decision-making deteriorates — a basketball player who is even mildly dehydrated may make slower, poorer decisions on court. Muscle function degrades too: dehydrated muscles fatigue more rapidly and are more prone to cramping, because the electrolyte balance (sodium, potassium) that enables muscle contraction is disrupted. Staying hydrated before, during and after exercise is therefore a non-negotiable aspect of sports performance for athletes at any level.