Energy Use and Calorie Requirements

Understanding how the body uses energy is essential for AQA GCSE PE. This lesson covers how energy is measured, what daily calorie requirements are, and the factors that affect how much energy a person needs. This knowledge underpins your understanding of diet, nutrition and the relationship between exercise and weight management.

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How Is Energy Measured?

Energy from food and drink is measured in kilocalories (Kcal), commonly referred to as "calories" in everyday language. One kilocalorie is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius.

Unit	Symbol	Used For
Kilocalorie	Kcal	Measuring the energy content of food and drink
Kilojoule	kJ	The SI unit of energy; 1 Kcal ≈ 4.18 kJ (less commonly used in PE)

Exam Tip: AQA uses Kcal as the standard unit for energy in GCSE PE. You do not need to convert between Kcal and kJ, but you should know that Kcal is how food energy is measured.

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Daily Calorie Requirements

The amount of energy a person needs each day depends on several factors, but the general guidelines are:

Group	Recommended Daily Intake
Average adult male	Approximately 2,500 Kcal per day
Average adult female	Approximately 2,000 Kcal per day

These are averages — actual requirements vary significantly from person to person depending on the factors discussed below.

Children and Teenagers

Children and teenagers generally require fewer calories than adults, but their needs increase as they grow:

Age Group	Approximate Daily Requirement
Children (5–10)	1,400 – 2,000 Kcal (depending on age and activity)
Teenagers (11–18)	1,800 – 3,000 Kcal (depending on age, sex and activity)

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Factors Affecting Calorie Requirements

Not everyone needs the same number of calories. Four key factors determine how much energy a person requires:

1. Age

Effect of Age on Calorie Needs	Detail
Children and teenagers	Need calories for growth, development and high activity levels
Adults	Calorie needs stabilise, reflecting work and activity levels
Older adults	Calorie needs decrease because metabolic rate slows and activity levels typically reduce

2. Gender (Sex)

Male vs Female	Detail
Males generally need more calories	Males tend to have more muscle mass and a higher metabolic rate
Females generally need fewer calories	Females tend to have a higher body fat percentage and lower metabolic rate

The difference in recommended intake (2,500 vs 2,000 Kcal) reflects these average physiological differences, though individual variation is significant.

3. Height and Body Size

Effect of Body Size	Detail
Taller / larger people need more energy	More body mass to maintain and move requires more calories
Shorter / smaller people need less energy	Less body mass means lower energy requirements for basic functions

A person who is 6'4" (193cm) and weighs 90kg will need significantly more calories than a person who is 5'2" (157cm) and weighs 55kg, simply because they have more tissue to fuel.

4. Energy Expenditure (Physical Activity Level)

Effect of Activity Level	Detail
Highly active people need more calories	Athletes, manual labourers and very active individuals may need 3,000–5,000+ Kcal per day
Sedentary people need fewer calories	Office workers and inactive individuals may need fewer than the average recommendation
Type of exercise matters	Endurance activities (marathon running, cycling) burn far more calories than low-intensity activities