Diet for Physical Activity

This lesson builds on the components of a balanced diet covered in Lesson 6 and focuses on how diet should be adapted for physical activity. The OCR GCSE PE (J587) specification requires you to understand how energy is used during exercise, what performers should eat before, during and after exercise, and how hydration affects performance. This is a frequently examined topic, so precise knowledge is essential.

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How Diet Affects Energy for Exercise

The body uses different energy sources depending on the intensity and duration of exercise:

Exercise Type	Primary Energy Source	Why
High-intensity, short-duration (e.g. sprinting, weightlifting)	Carbohydrates (glucose/glycogen)	Glucose can be broken down quickly (anaerobically and aerobically) to provide rapid energy
Low-intensity, long-duration (e.g. walking, gentle cycling)	Fats	Fat provides more energy per gram and is the preferred fuel when oxygen supply is plentiful and intensity is low
Moderate-intensity, sustained (e.g. jogging, swimming)	Mixture of carbohydrates and fats	Both contribute to energy production; the balance shifts towards carbohydrates as intensity increases

graph LR
    A["Low Intensity<br/>(e.g. walking)"] -->|"Primary fuel: FAT"| B["Moderate Intensity<br/>(e.g. jogging)"]
    B -->|"Mixed fuel:<br/>FAT + CARBS"| C["High Intensity<br/>(e.g. sprinting)"]
    C -->|"Primary fuel:<br/>CARBOHYDRATE"| D["Maximum Effort"]
    style A fill:#27ae60,color:#fff
    style B fill:#f39c12,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e74c3c,color:#fff

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Pre-Exercise Nutrition

What a performer eats before exercise significantly affects their performance.

General Guidelines for Pre-Exercise Eating

Guideline	Reason
Eat 2–4 hours before exercise	This allows time for digestion and absorption; exercising on a full stomach causes discomfort and diverts blood to the digestive system
Focus on complex carbohydrates	Pasta, rice, bread, oats — these provide a sustained release of energy throughout the activity
Include moderate protein	Supports muscle function during exercise
Limit fat and fibre	Both are slow to digest and can cause stomach discomfort during exercise
Stay hydrated	Drink water in the hours leading up to exercise to ensure the body is well-hydrated

Carbohydrate Loading

Carbohydrate loading is a dietary strategy used by endurance athletes in the days before a competition:

Aspect	Detail
What it is	Eating a high-carbohydrate diet in the 2–3 days before a long-duration event
Purpose	To maximise glycogen stores in the muscles and liver
Who uses it	Endurance athletes — marathon runners, long-distance cyclists, triathletes
How it works	Increased carbohydrate intake fills glycogen stores to maximum capacity, providing more energy during prolonged exercise
Not suitable for	Short-duration events (e.g. 100m sprint) or sports requiring repeated short bursts — glycogen stores are already sufficient for these

Exam Tip: Carbohydrate loading is a popular OCR exam topic. Remember it is only relevant for endurance events lasting longer than 60–90 minutes. It would not benefit a sprinter or a weightlifter because their glycogen stores are not the limiting factor in their performance.

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During-Exercise Nutrition

During exercise, the primary needs are energy maintenance and hydration:

Need	Strategy	Example
Hydration	Drink water regularly in small amounts throughout the activity	150–200ml every 15–20 minutes
Electrolyte replacement	During prolonged exercise (60+ minutes), sports drinks containing sodium and potassium help replace minerals lost through sweat	Isotonic sports drinks
Energy top-up	During very long events (90+ minutes), small amounts of easily digestible carbohydrates help maintain blood glucose	Energy gels, bananas, jelly sweets

Types of Sports Drinks

Drink Type	Composition	When to Use	Purpose
Hypotonic	Low concentration of carbohydrate; lower osmolality than blood	During exercise when rapid hydration is the priority	Replaces fluid quickly without providing much energy
Isotonic	Similar concentration of carbohydrate to blood	During moderate-to-long exercise	Replaces fluid AND provides energy; balanced approach
Hypertonic	High concentration of carbohydrate; higher osmolality than blood	After exercise, or during ultra-endurance events	Provides a large amount of energy but absorbed slowly

graph TD
    A["Sports Drinks"] --> B["Hypotonic"]
    A --> C["Isotonic"]
    A --> D["Hypertonic"]
    B --> B1["Low sugar<br/>Quick hydration"]
    C --> C1["Balanced sugar<br/>Fluid + Energy"]
    D --> D1["High sugar<br/>Energy recovery"]
    B --> E["Best DURING<br/>high-intensity exercise"]
    C --> F["Best DURING<br/>prolonged exercise"]
    D --> G["Best AFTER<br/>exercise"]
    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff

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Post-Exercise Nutrition

What a performer eats after exercise is crucial for recovery:

Nutrient Need	Reason	Example Foods
Carbohydrates	To replenish glycogen stores that were depleted during exercise	Rice, pasta, bread, potatoes, fruit
Protein	To repair micro-tears in muscle fibres caused by exercise and stimulate muscle growth	Chicken, fish, eggs, beans, protein shake
Water	To replace fluid lost through sweating and breathing	Water, milk, diluted fruit juice
Electrolytes	To replace sodium, potassium and other minerals lost through sweat	Sports drinks, bananas, nuts

The Recovery Window

Timing	What to Eat	Why
Within 30 minutes of exercise	A snack combining carbohydrates and protein (e.g. a banana with peanut butter, chocolate milk, a recovery shake)	The body absorbs nutrients most efficiently during this period — glycogen resynthesis is fastest in the first 30 minutes
Within 2 hours of exercise	A full balanced meal with carbohydrates, protein and vegetables	Continues the recovery process and provides the full range of nutrients needed for repair

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Hydration for Exercise

Hydration is one of the most important and frequently tested topics in OCR GCSE PE.

Why Hydration Matters During Exercise

Factor	Explanation
Sweating increases	During exercise, the body produces sweat to cool down. This loses water and electrolytes
Blood volume decreases	Dehydration reduces blood volume, meaning less oxygen can be transported to working muscles
Heart rate increases	The heart must beat faster to circulate the reduced blood volume, increasing cardiovascular strain
Performance declines	Even mild dehydration (1–2% body mass loss) reduces endurance, strength, concentration and reaction time
Risk of heat illness	Severe dehydration impairs the body's ability to cool itself, risking heat exhaustion or heat stroke

Hydration Guidelines

When	How Much	What
Before exercise	500ml in the 2 hours before activity	Water
During exercise	150–200ml every 15–20 minutes	Water (for <60 minutes) or isotonic sports drink (for 60+ minutes)
After exercise	1.5 litres for every kg of body mass lost through sweating	Water or sports drink with electrolytes

Signs of Dehydration

Sign	What It Means
Dark-coloured urine	The body is conserving water; urine is concentrated
Thirst	By the time you feel thirsty, you are already mildly dehydrated
Headache	Reduced blood volume and brain dehydration cause pain
Dizziness and fatigue	Reduced blood flow to the brain and muscles
Muscle cramps	Electrolyte imbalance from fluid and mineral loss

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Diet Considerations Across Age Groups

The OCR specification requires you to consider how dietary needs vary by age: