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Unlock This CourseEvery single thing your body does requires energy - from breathing and keeping your heart beating, to running for the bus or thinking about your homework. Just like a car needs petrol to run, your body needs energy from food to function properly.
Energy requirements refer to the amount of energy your body needs each day to carry out all its functions. This energy comes from the food we eat and is measured in units called kilojoules (kJ) or kilocalories (kcal).
Key Definitions:
Your body uses energy for three main purposes: keeping basic body functions running (like breathing and circulation), digesting food and physical activity. The biggest chunk - about 60-70% - goes to basic functions, even when you're sleeping!
Not everyone needs the same amount of energy. Your personal energy requirements depend on several important factors that make each person unique.
Your age plays a huge role in determining how much energy you need. Babies and children need lots of energy for growth, whilst teenagers need even more during growth spurts. Adults generally need less energy as they get older because their metabolism slows down.
Need about 7,500-8,500 kJ per day. Growing bodies require extra energy for development and they're usually very active.
Need about 9,000-12,000 kJ per day. This is the highest energy requirement due to rapid growth and high activity levels.
Need about 8,500-10,500 kJ per day. Requirements stabilise but vary based on activity level and body size.
Males typically need more energy than females of the same age. This is mainly because males usually have more muscle mass and muscle tissue burns more energy than fat tissue, even when resting.
A 16-year-old male footballer might need 12,000 kJ per day, whilst a 16-year-old female of similar height who doesn't play sport might need only 9,000 kJ per day. The difference comes from muscle mass, activity level and natural metabolic differences.
The amount of physical activity you do has a massive impact on your energy requirements. The more active you are, the more energy your body needs to fuel those activities.
Physical activities can be grouped into different categories based on how much energy they use:
Walking slowly, light housework, desk work. Uses about 150-300 kJ per hour above resting level.
Brisk walking, cycling, swimming leisurely. Uses about 300-600 kJ per hour above resting level.
Running, competitive sports, heavy manual work. Uses 600+ kJ per hour above resting level.
Bigger bodies need more energy to function, just like bigger cars need more petrol. But it's not just about total size - the type of tissue matters too.
Muscle tissue is much more active than fat tissue. Even when you're sitting still, your muscles are using energy to maintain themselves. This is why people with more muscle mass need more energy, even at rest.
Where you live and the conditions around you can affect your energy needs too. Your body works harder in extreme temperatures to maintain its normal temperature.
In very cold conditions, your body burns extra energy to keep warm through shivering and increased metabolism. In very hot conditions, your body uses energy for cooling mechanisms like sweating, though this increase is usually smaller than the cold response.
Arctic explorers can need up to 20,000 kJ per day - nearly double a normal adult's requirement. This is due to the extreme cold, heavy physical work of travelling across ice and the extra weight of equipment they must carry.
To meet our energy requirements, we need to understand how much energy different foods provide. Food energy is measured using the same units as our energy requirements.
Different nutrients provide different amounts of energy per gram:
Provide 17 kJ per gram. These are the body's preferred energy source for quick energy.
Provide 17 kJ per gram. Mainly used for growth and repair, but can be used for energy if needed.
Provide 38 kJ per gram. The most concentrated energy source, used for long-term energy storage.
Energy balance is the relationship between the energy you consume (from food) and the energy you use (for body functions and activity). This balance affects your body weight and health.
When energy in equals energy out, your weight stays stable. This is the ideal situation for most people who have finished growing.
Problems occur when there's a mismatch between energy intake and energy use:
Positive Energy Balance: Taking in more energy than you use leads to weight gain, as excess energy is stored as fat. Over time, this can lead to obesity and related health problems.
Negative Energy Balance: Using more energy than you consume leads to weight loss, as the body breaks down stored fat and eventually muscle for energy. Extreme negative balance can lead to malnutrition.
A 17-year-old student who trains for swimming 2 hours daily needs about 13,000 kJ per day. During exam periods when training stops but stress increases, they might only need 9,500 kJ. If they don't adjust their food intake, they could gain weight during exams despite eating the same amount.
Understanding energy requirements helps us make better decisions about food and activity. It explains why athletes need to eat more, why growing teenagers are always hungry and why older adults might need to eat less to maintain their weight.
To maintain good health, it's important to match your energy intake to your needs. This means eating more on days when you're very active and being mindful of portion sizes when you're less active.
On days with sports practice, PE lessons, or lots of walking, your body needs extra energy. Choose energy-rich foods like pasta, bananas, or nuts to fuel your activities.