Energy, Diet, Nutrition and Hydration

Energy: The Fuel for Sport and Life

Everything the body does — breathing, thinking, sprinting, lifting — requires energy. That energy is measured in kilocalories (Kcal) and comes entirely from the food we eat. The average adult male requires approximately 2,500 Kcal per day, while the average adult female requires around 2,000 Kcal per day. However, these are only averages — how much energy any individual actually needs depends on four key factors: age, gender, height, and energy expenditure (how much exercise they do). A 16-year-old county-level swimmer training twice a day will have far greater energy needs than a sedentary adult of the same height and gender.

The Balanced Diet: Proportions and Purposes

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. No single food provides every nutrient — variety is essential. Unused energy, particularly from saturated fats, is stored as body fat, which can contribute to obesity over time. The specification requires you to know the approximate percentage breakdown of a balanced diet: 55–60% carbohydrate, 25–30% fat, and 15–20% protein. These proportions reflect the primary roles each macronutrient plays in the body.

Macronutrients: Carbohydrate, Fat and Protein

Carbohydrate is the primary and preferred fuel for all types of exercise, regardless of intensity. Whether a performer is jogging at a comfortable pace or sprinting at maximum effort, carbohydrates are the first fuel the body calls upon. 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 marathon runners prepare for competition partly by ensuring glycogen stores are fully loaded through carbohydrate-rich meals in the days before an event.

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 demand. Fat also has important non-energy roles: it insulates the body, protects organs, and is a carrier for fat-soluble vitamins.

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, for example, follows a protein-rich recovery plan after intense pool sessions to support muscle repair and adaptation. Consuming insufficient protein slows recovery and limits the adaptations (such as hypertrophy) that training is designed to produce.

Hydration and the Consequences of Dehydration

Water is not just a drink — it is the medium in which almost every chemical reaction in the body takes place. Maintaining water balance (hydration) 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 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 severe cases, heart rhythm may become irregular. Meanwhile, the body’s ability to shed heat through the blood vessels near the skin is compromised, causing core body temperature to rise — increasing the risk of overheating and heat stroke.

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. For a road cyclist like Geraint Thomas, any cramping or slowing of reaction time in the final kilometre of a race could be decisive.