Diet, Nutrition and Hydration
Why Nutrition Matters in Sport
Food is the body’s fuel. The amount, type and timing of what an athlete eats directly affects how much energy they have during training, how quickly they recover afterwards, whether their body composition supports their sport and how strongly their immune and skeletal systems resist injury and illness. A thorough understanding of nutrition is therefore not a separate subject from PE — it is integral to physical performance.
Macronutrients
Macronutrients are the energy-providing nutrients. The three macronutrients are carbohydrates, fats and proteins. A balanced diet for an active individual should provide approximately 55–60% of energy from carbohydrates, 25–30% from fats and 15–20% from proteins.
Carbohydrates are the body’s preferred and most readily available energy source. They are broken down to glucose, which is used directly for energy production or stored as glycogen in the liver and skeletal muscles. Glycogen is the key fuel for moderate-to-high-intensity exercise. When glycogen stores are depleted, performance deteriorates rapidly — athletes call this “hitting the wall” in endurance events. Carbohydrate loading is a strategy used by endurance athletes such as marathon runners, who increase carbohydrate intake in the days before a race to maximise glycogen stores. A runner preparing for a marathon might shift from a standard 55% carbohydrate diet to 70% in the final 48 hours before the race.
Proteins are made up of amino acids and are essential for the growth, repair and maintenance of body tissues, particularly muscle. During resistance training, microscopic tears occur in muscle fibres. Protein consumed after training provides the amino acids needed to repair these tears and build new, stronger muscle tissue — a process called hypertrophy. The timing of protein intake matters: consuming protein within 30–60 minutes after a resistance session maximises the muscle-repair response. For power athletes such as weightlifters or sprinters, protein intake is especially important because muscle mass is central to their performance.
Fats are a concentrated energy source providing more than twice the energy per gram as carbohydrates or proteins. They are the primary fuel for low-intensity, prolonged exercise — long, easy-paced cycling or walking. The body stores fat in virtually unlimited quantities and draws on it during aerobic activity when glycogen stores begin to deplete. Fats are also essential for the absorption of fat-soluble vitamins (A, D, E, K), the production of hormones and the maintenance of cell membranes. Unsaturated fats from sources such as olive oil, nuts and oily fish are health-promoting, whereas excessive saturated fat (from red meat and full-fat dairy) raises LDL cholesterol and increases cardiovascular disease risk.
Balanced diet macronutrient split: carbohydrates 55–60% (primary fuel, stored as glycogen), fats 25–30% (secondary fuel, hormone production), proteins 15–20% (tissue repair and hypertrophy). Endurance athletes increase carbohydrate proportion before major events; power athletes prioritise protein timing after resistance training.
Micronutrients
Micronutrients — vitamins and minerals — do not provide energy but are essential for many critical processes. Iron is needed to form haemoglobin, the oxygen-carrying protein in red blood cells; a deficiency causes anaemia, reducing aerobic capacity. Calcium is the primary mineral in bone structure; adequate calcium intake throughout adolescence maximises bone density, reducing lifelong fracture risk. Vitamin D enables calcium absorption from the gut and is therefore equally important for bone health. Athletes who train indoors or in low-sunlight climates may develop vitamin D deficiency, increasing their risk of stress fractures. Water and fibre, while not traditional micronutrients, are also essential: fibre supports healthy digestion and water enables every cellular process in the body.
Many students believe that eating large amounts of protein will automatically cause muscle bulk. In reality, dietary protein alone does not cause hypertrophy — it must be combined with progressive resistance training. Protein provides the raw material (amino acids) for muscle repair; the training stimulus provides the reason to repair bigger. An endurance athlete who increases protein intake without changing their training will simply excrete the excess as urea rather than building new muscle. This is an important nuance for 4-mark explain questions about nutrition and performance.
Energy Balance
Energy balance is the relationship between energy consumed (through food and drink) and energy expended (through basal metabolic rate and physical activity). When intake equals expenditure, weight is stable. When intake exceeds expenditure, the surplus is stored as fat and weight increases. When expenditure exceeds intake, the body draws on fat stores and weight decreases. Maintaining an appropriate energy balance is essential for athletes: too little energy causes fatigue, muscle loss and hormonal disruption; too much causes unwanted fat gain and reduced power-to-weight ratio. The correct balance varies between sports: an ultra-endurance athlete needs a very high caloric intake, while a gymnast needs carefully controlled intake to maintain the strength-to-weight ratio that their sport demands.
Hydration and Dehydration
Water makes up around 60% of adult body mass and is involved in every physiological process relevant to exercise: transporting nutrients and oxygen in the blood, regulating body temperature through sweating, lubricating joints and removing waste products via urine. Even mild dehydration — a loss of just 2% of body water — measurably impairs aerobic performance and decision-making ability. As dehydration worsens, blood volume decreases, which makes the blood thicker. This raises blood viscosity, forcing the heart to work harder to deliver the same amount of oxygen to the muscles. Heart rate rises, perceived effort increases and endurance performance deteriorates.
At higher levels of dehydration (above 5% body water loss), the risk of heat exhaustion and heat stroke increases significantly. Cramping, dizziness and collapse become possible during prolonged exercise in warm conditions. Athletes should begin exercise well hydrated (pale yellow urine is a common guide) and drink regularly throughout prolonged activity rather than waiting until they feel thirsty — thirst is already a late signal of dehydration. Sports drinks that contain electrolytes (sodium and potassium) as well as carbohydrates are useful in events lasting more than 60 minutes, as they replace both fluid and the electrolytes lost in sweat.
Dehydration chain: blood volume decreases → blood viscosity increases → heart works harder to maintain cardiac output → heart rate rises → fatigue increases and endurance performance deteriorates. Even 2% dehydration measurably impairs aerobic performance.
Carbohydrate loading works by maximising glycogen storage in the muscles and liver before a prolonged endurance event. Athletes typically taper their training (reducing volume while maintaining intensity) and simultaneously increase carbohydrate intake in the 48–72 hours before competition. The strategy is effective for events lasting more than 90 minutes (such as a marathon or triathlon) because it delays the point at which glycogen depletion causes fatigue. It is not appropriate for short, high-intensity events (such as the 100m sprint) where glycogen stores are not the limiting factor. A sprinter does not benefit from carbohydrate loading in the same way because their event is over in under 15 seconds and relies primarily on the ATP-PC (phosphocreatine) energy system.