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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Energy Requirements and Activity
Our bodies need energy for everything we do - from keeping our hearts beating to running a marathon. This energy comes from the food we eat, which is why understanding our energy requirements is essential for maintaining good health.
Key Definitions:
The energy in our food comes from three main nutrients:
When we eat these nutrients, our digestive system breaks them down and through cellular respiration, they release energy that our cells can use.
Our bodies use energy for:
Your BMR is the amount of energy your body needs to function at rest. It accounts for about 60-70% of your total daily energy expenditure for most people who aren't very active.
There are several formulas to estimate BMR. One common method is the Harris-Benedict equation:
For males:
BMR = 66.5 + (13.75 × weight in kg) + (5.003 × height in cm) - (6.755 × age in years)
For females:
BMR = 655.1 + (9.563 × weight in kg) + (1.850 × height in cm) - (4.676 × age in years)
For example, a 16-year-old female who weighs 60 kg and is 165 cm tall would have a BMR of approximately:
655.1 + (9.563 × 60) + (1.850 × 165) - (4.676 × 16) = 1,425 kcal per day
Children and teenagers need more energy per kg of body weight than adults because they're growing. BMR decreases with age as muscle mass typically decreases and metabolism slows.
Males typically have higher energy requirements than females of the same age and weight because they generally have more muscle mass and less body fat. Muscle tissue uses more energy than fat tissue.
People with more muscle mass have higher BMRs because muscle tissue is metabolically active and requires more energy to maintain than fat tissue.
People living in very cold environments need more energy to maintain body temperature. Similarly, very hot environments can increase energy needs as the body works to cool down.
Physical and emotional stress can increase energy requirements as stress hormones like adrenaline and cortisol raise metabolic rate.
Illness, infection and recovery from surgery or injury can significantly increase energy needs as the body works to heal and fight infection.
Physical activity is the most variable component of energy expenditure and can significantly increase your daily energy requirements above your BMR.
To calculate total daily energy expenditure, we multiply BMR by an activity factor:
Different activities require different amounts of energy:
| Activity | Energy (kcal/hour for 70kg person) |
|---|---|
| Sleeping | 60 |
| Sitting quietly | 100 |
| Walking (moderate pace) | 280 |
| Cycling | 450 |
| Swimming | 500 |
| Running (8 km/h) | 700 |
Energy balance is the relationship between energy intake (from food and drink) and energy expenditure (through BMR and physical activity).
When energy intake equals energy expenditure, body weight remains stable. This is called energy balance.
When energy intake exceeds energy expenditure, the excess energy is stored as fat, leading to weight gain.
When energy expenditure exceeds energy intake, the body uses stored energy (primarily fat), leading to weight loss.
Elite athletes have significantly higher energy requirements than the average person. For example:
These extreme examples show how physical activity dramatically increases energy requirements and how the body adapts to different energy demands.
Understanding energy requirements has practical applications in everyday life:
Food labels provide energy values in kcal and kJ. Being able to interpret these values helps with making informed food choices.
For example, if a chocolate bar contains 250 kcal, that represents about:
Knowing your energy requirements helps you plan a diet that provides the right amount of energy. A balanced diet should:
Energy requirements vary significantly between individuals based on factors like age, sex, body composition and activity level. Understanding your own energy needs is essential for maintaining a healthy weight and supporting physical activity.
Remember these key points:
By understanding these principles, you can make informed choices about your diet and activity levels to support your health and wellbeing.
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