Nutrition and digestion · Quantitative
Energy in food and what you need
A label says 1050 kJ. Your body needs about 9500 kJ a day. Neither number means anything until you know whose day it is.
Start here
Two people eat exactly the same food for a month.
Same meals, same portions, same times — about 11 000 kJ a day each. One is a 68-year-old who spends the day reading. The other is a 25-year-old cyclist covering 120 km a day. After a month, one has gained mass and one has lost it.
Same food in. Why the different outcome?
There is no such thing as the right amount of food. There is only the amount that matches what a particular body transfers in a day — and that number differs by a factor of two between the two people in this paragraph.
At the bench · feed a person for a day
Pick who you are feeding, then add food
0 portions on the plate
Who is eating
A 4-year-oldA 13-year-oldAn adult with a desk jobAn adult doing heavy manual workAn Olympic rower in training
Needs 5,800 kJ / dayNeeds 9,500 kJ / dayNeeds 9,000 kJ / dayNeeds 13,500 kJ / dayNeeds 25,000 kJ / day
Small body, so little tissue to keep warm and fuel — but growing fast, and a surprising share of the total goes into building new tissue rather than moving about.Almost an adult’s requirement, in a body that is still smaller than one. Adolescent growth is the most energy-expensive period of a human life after infancy.Full adult size, no growth to pay for, and very little movement. Roughly three quarters of this goes on simply staying alive and at 37 °C.The same body as the office worker, doing eight hours of lifting. The difference between these two rows is activity and nothing else.Large muscular body plus six hours of hard training. Athletes at this level struggle to eat enough, which is a problem most people never have.
0 kJ of 9,500 kJ
empty plate
Tap to add a portion · tap again to remove one
Within 5% of the target
That day matches a 4-year-old.
That day matches a 13-year-old.
That day matches an adult with a desk job.
That day matches an adult doing heavy manual work.
That day matches an Olympic rower in training.
Now switch person without changing the food. The same plate is a surplus for one body and a shortfall for another, and nothing about the food has changed — which is the whole idea.
Doing it on paper
Three sums, and the units that make them behave.
Sum 1
Total the day up
Nothing clever — multiply each portion by how many, then add. Marks are lost far more often on arithmetic and on forgetting a portion than on method.
4 slices toast and butter
4 × 780 = 3120 kJ
1 plate of pasta
= 2400 kJ
1 banana
= 450 kJ
1 chocolate bar
= 1050 kJ
total
= 7020 kJ
Sum 2
Compare with the requirement
A shortfall is not automatically a fault in the day — it is a statement that stores will be drawn on. Over one day that is normal. Over a month it becomes mass loss.
requirement (13-year-old)
= 9500 kJ
intake
= 7020 kJ
shortfall
9500 − 7020 = 2480 kJ
Sum 3
Turn a label into a share
The same bar is 18% of a 4-year-old’s day and 4% of a rower’s. A percentage on a packet is only meaningful once you say whose requirement it is a percentage of — which is exactly what packets do not say.
bar
= 1050 kJ
requirement
= 9500 kJ
share
1050 ÷ 9500 × 100 = 11%
Key fact
Energy requirement is not a property of food. It is a property of the person eating it — set by their size, their age, their growth and how much they move.
Forward pointer · Physics
You have used kJ here as a bookkeeping unit — energy in versus energy needed. The full treatment belongs to Physics: what a joule actually is, how energy is transferred rather than used up, and how to calculate it from power and time. Nothing here contradicts it; it is the same joule, counted less formally.
Think again
“Everyone needs about 2000 calories a day.”
That figure is a labelling convention, not a fact about a person. It exists so packets can print a percentage, and it is the rounded average for an adult woman doing moderate activity. Run the bench above through all five people and the honest range is roughly 6000 to 25 000 kJ a day — more than a fourfold spread, and the top of it is a professional athlete rather than an extreme case. A 4-year-old fed an adult's day would be badly overfed; a rower fed a 4-year-old's day would collapse in a week. When a question gives you a number for a requirement, that number belongs to somebody, and part of the answer is saying who.
“The energy in food gets used up and disappears.”
You met this in Energy stores and it comes back wearing a chef's hat. Nothing about eating destroys energy. The chemical store in your food is transferred: some to movement when you walk, some to the chemical store of new tissue while you grow, and — this is the part people miss — most of it to the thermal store of your surroundings. You are a 70-watt heater. Sit in a small room with the door shut and the temperature rises measurably, and that rise is the energy from your breakfast, still entirely present, just no longer anywhere useful. “Where did the energy go” always has an answer.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Do the sum
A 13-year-old needing 9500 kJ eats: cereal (1100 kJ), two chapatis with dal (1750 kJ), an apple (320 kJ), pasta (2400 kJ) and two chocolate bars (1050 kJ each). What is their intake, and how does it compare?
Rung 2 · The one that catches people
A chocolate bar label says “1050 kJ — 11% of your daily needs”. A 4-year-old eats one. What share of their day is it?
Rung 3 · Explain the pair
The office worker and the bricklayer in the bench above are the same age and size, but their requirements differ by 4500 kJ a day. Explain where that difference goes, and why the 4-year-old needs less than either despite growing fastest.
Rung 4 · Take it somewhere new
A school wants one standard lunch, the same portion for every pupil aged 11 to 16. Using what you now know about requirements, say what is wrong with the plan, what you would do instead, and one thing you would need to find out first.
Key note
Energy in food is measured in kilojoules. A person's daily requirement depends on their age, size, sex, rate of growth and level of activity, so there is no single correct figure. Take in more than you transfer and the surplus is stored; take in less and stores are broken down.
Going further
The numbers on a food packet were originally obtained by burning the food. A weighed sample goes into a sealed steel vessel packed with oxygen, the vessel sits in a known mass of water, and the sample is ignited electrically; the rise in water temperature gives the energy released. The device is called a bomb calorimeter, and the name is literal enough that early ones occasionally lived up to it. It measures the total chemical energy in the sample, which is why the figure for dietary fibre had to be corrected downwards later — a calorimeter happily burns cellulose, and your gut cannot digest a gram of it. The instrument was answering a chemistry question when what was wanted was a biology one.
Before this lesson
Continues in Physics
- Energy in food (Physics)
Where comparing energy values from labels in kJ is taught as physics — the arithmetic and the units.
- Calculating energy transferred
Where a joule is derived from power and time rather than counted off a packet.
At GCSE this becomes
- Energy budgets, metabolic rate, and the effect of exercise and body composition on requirement.
Where to next
Ask Mr Badmus AI
Want to check a requirement calculation of your own?
Requirement figures are rounded typical values used to teach the principle. They are not a guide for anyone's own eating, and this lesson does not set targets for any individual. Questions about your own diet belong with a doctor or a registered dietitian.
Lesson content © MrBadmusAI.