Energy Balance

Energy balance is a central concept in the Edexcel GCSE PE specification. It describes the relationship between energy in (the energy consumed through food and drink) and energy out (the energy expended through physical activity and bodily functions). Understanding energy balance is essential for explaining weight gain, weight loss, weight maintenance, and the link between diet and physical activity.

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What Is Energy Balance?

Energy balance is the equation:

Energy In = Energy Out

When energy in equals energy out, body weight remains stable. When they are unequal, weight changes.

State	Energy Relationship	Outcome
Balanced	Energy in = Energy out	Weight maintained
Positive energy balance	Energy in > Energy out	Weight gain (excess stored as fat)
Negative energy balance	Energy in < Energy out	Weight loss (body uses fat stores for energy)

graph LR
    A["Energy In<br/>(Food & Drink)"] --> B{"Energy Balance"}
    C["Energy Out<br/>(Activity + BMR)"] --> B
    B -->|"In = Out"| D["Weight Maintained"]
    B -->|"In > Out"| E["Weight Gain"]
    B -->|"In < Out"| F["Weight Loss"]
    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#ffcdd2,stroke:#c62828
    style F fill:#fff9c4,stroke:#f9a825

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Energy In

Energy in comes from the food and drink we consume. Energy is measured in kilocalories (kcal) or kilojoules (kJ).

Macronutrient	Energy per gram
Carbohydrates	4 kcal per gram
Proteins	4 kcal per gram
Fats	9 kcal per gram
Alcohol	7 kcal per gram (not a nutrient but does provide energy)

The total energy intake depends on:

• The type of food eaten (fats are more energy-dense than carbohydrates or proteins)
• The amount of food eaten (portion size)
• The frequency of meals and snacks

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Energy Out

Energy out has two main components:

1. Basal Metabolic Rate (BMR)

BMR is the amount of energy the body uses at rest to maintain basic functions — breathing, circulating blood, maintaining body temperature, cell repair.

Factor Affecting BMR	Effect
Age	BMR decreases with age as muscle mass declines
Sex	Males typically have a higher BMR due to greater muscle mass
Body size	Larger bodies require more energy to maintain
Muscle mass	More muscle = higher BMR (muscle tissue is more metabolically active than fat)
Genetics	Some people naturally have a higher or lower BMR

BMR accounts for approximately 60–75% of total daily energy expenditure in most people.

2. Physical Activity

Physical activity accounts for the remaining energy expenditure:

• Exercise and sport — training, matches, gym sessions
• Non-exercise activity — walking, housework, fidgeting, standing
• NEAT (Non-Exercise Activity Thermogenesis) — the energy used for everything that is not sleeping, eating or formal exercise

Activity	Approximate Energy Expenditure (kcal/hour for 70 kg person)
Sleeping	~60
Sitting (desk work)	~100
Walking (moderate)	~250
Cycling (moderate)	~400
Running (8 km/h)	~500
Swimming (moderate)	~450
Football	~550

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Applying Energy Balance to Weight Management

Weight Gain

When energy intake consistently exceeds energy expenditure:

• The surplus energy is stored as adipose tissue (body fat)
• Over time, this leads to weight gain and potentially obesity
• A surplus of approximately 3,500 kcal leads to a gain of about 0.45 kg (1 lb) of body fat

Weight Loss