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

When energy expenditure consistently exceeds energy intake:

• The body draws on fat stores for energy
• Over time, this leads to weight loss
• A deficit of approximately 3,500 kcal leads to a loss of about 0.45 kg (1 lb) of body fat

Weight Maintenance

When energy in equals energy out over time:

• Body weight remains stable
• This is the goal for most healthy individuals

Exam Tip: Edexcel questions often ask you to explain how an athlete can adjust their energy balance. Always refer to both sides of the equation: they can either increase energy out (more exercise) or decrease energy in (adjusted diet), or both.

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Energy Balance and Different Performers

Different athletes have different energy needs:

Performer	Energy Needs	Reason
Marathon runner	Very high energy in; high carbohydrate intake	Endurance activity burns thousands of calories; needs glycogen replenishment
Sprinter	Moderate-high energy in; high protein intake	Needs energy for explosive training but must maintain optimal body composition
Jockey	Low energy in; careful weight management	Must maintain very low body weight to meet race weight limits
Sumo wrestler	Very high energy in; deliberate positive energy balance	Needs high body mass for competitive advantage
Gymnast	Moderate energy in; careful balance	Needs strength but must maintain low body weight for skills and aesthetics

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The Link Between Energy Balance and Health

Energy Balance State	Health Consequence
Chronic positive balance	Obesity, CHD, type 2 diabetes, hypertension
Chronic negative balance	Malnutrition, weakened immune system, fatigue, loss of muscle mass, eating disorders
Balanced	Healthy weight, optimal performance, reduced disease risk

Both extremes are harmful. The goal is to maintain a healthy energy balance appropriate for the individual's activity level, age and goals.

graph TD
    A["Chronic Positive Balance<br/>(Too Much Energy In)"] --> B["Obesity"]
    B --> C["CHD, Diabetes,<br/>Hypertension"]
    D["Chronic Negative Balance<br/>(Too Little Energy In)"] --> E["Malnutrition"]
    E --> F["Fatigue, Weakness,<br/>Eating Disorders"]
    G["Balanced Energy"] --> H["Healthy Weight"]
    H --> I["Optimal Health<br/>and Performance"]
    style A fill:#e53935,color:#fff
    style D fill:#e53935,color:#fff
    style G fill:#27ae60,color:#fff
    style I fill:#c8e6c9,stroke:#2e7d32

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Common Exam Mistakes

1. Forgetting BMR. Energy out is not just exercise — BMR accounts for 60–75% of total energy expenditure. Always mention it.
2. Confusing positive and negative energy balance. Positive = weight gain (energy in exceeds out). Negative = weight loss (energy out exceeds in).
3. Not linking energy balance to specific sports. The exam may ask how a particular athlete manages energy balance — always relate your answer to the demands of their sport.
4. Ignoring the dangers of chronic negative energy balance. Students often focus only on obesity. Chronic undereating is also dangerous and leads to malnutrition and eating disorders.

Exam Tip: When a question asks about energy balance, always define it first ("the relationship between energy consumed and energy expended") and then apply it to the specific scenario. Use the terms "positive energy balance" and "negative energy balance" precisely — they are key command phrases.

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Summary