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Energy in, energy out
A science lesson on how the body uses food as energy: energy per gram of each macronutrient, what "energy balance" means, and simple arithmetic worked on an example scenario. Framed strictly as the science of energy, with a deliberate steer away from dieting and weight for this age group.
Learning objectives
By the end of the activity, students can:
- State the approximate energy in one gram of carbohydrate (4 kcal), protein (4 kcal) and fat (9 kcal).
- Explain energy balance: energy taken in from food versus energy used by the body.
- Calculate the energy in a simple food from its macronutrients.
- Describe how the body uses energy even at rest, and how activity adds to it.
The activity
- Starter (10 min): where does the energy in food go? Draw out that the body uses energy constantly — even asleep — to run the heart, brain and cells, and uses more during activity.
- Input (15 min): give the energy values (carbohydrate and protein ≈ 4 kcal/g, fat ≈ 9 kcal/g) and model the calculation for a simple snack.
- Task (20 min): students complete the worksheet calculations and the energy-balance questions.
- Plenary (10 min): discuss that balance is about the whole day and week, that needs differ between people, and that growing bodies need plenty of energy.
The worksheet
This prints as a clean sheet — press print and the site header, footer and this instruction row drop away. Photocopy for a class as needed.
Energy in food: 1 g of carbohydrate ≈ 4 kcal, 1 g of protein ≈ 4 kcal, 1 g of fat ≈ 9 kcal. Use these to answer the questions.
Worked example
A snack has 20 g carbohydrate, 5 g protein and 10 g fat. Energy = (20 × 4) + (5 × 4) + (10 × 9) = 80 + 20 + 90 = 190 kcal.
1. A cereal bar has 30 g carbohydrate, 3 g protein and 7 g fat. Work out its energy in kcal.
2. Gram for gram, which macronutrient carries the most energy? By roughly how many times more than carbohydrate?
3. Explain, in your own words, what "energy balance" means.
4. Name three things the body spends energy on even when you are completely still.
5. Two people eat the same meal but one plays sport all afternoon. How does their energy "out" differ, and why?
Answer key
- 1. (30 × 4) + (3 × 4) + (7 × 9) = 120 + 12 + 63 = 195 kcal.
- 2. Fat, at about 9 kcal/g — a little over twice as much as carbohydrate (4 kcal/g).
- 3. Energy balance is the comparison between energy taken in from food and drink and energy used by the body over time.
- 4. Any of: heartbeat, breathing, brain activity, keeping warm, running the cells/organs, digestion.
- 5. The sportsperson uses more energy, because physical activity adds to the energy the body uses at rest.
Teacher notes
- Teach this as physics and biology of energy, NOT as diet advice. Do not set tasks that ask students to count their own calories, judge their meals, or link food to their own body weight — that framing is inappropriate for the age group and can be harmful.
- Emphasise that growing young people need plenty of energy and that "less" is not "better". Keep examples about generic snacks, not students' own intake.
- If any pupil shows anxiety around food or eating, follow your school's safeguarding and pastoral procedures. Cross-links to the physics of energy and the biology of respiration.
A teaching resource, not medical or dietary advice. Handle food, sleep, sugar and energy topics sensitively with young people, and follow your school's pastoral and safeguarding procedures if a pupil raises a concern.
Frequently asked questions
What level is this for?
Suits lower-secondary science (energy, the human body).
Can I print and photocopy it?
Yes — press print (Ctrl+P / Cmd+P) and the site chrome disappears, leaving the worksheet. It is free for classroom use.
Is there an answer key?
Yes, at the foot of this page. Decide whether to project, hide or hand it out.
Source: Standard public guidance (EFSA reference values, EU labelling rules, Atwater energy factors) and simple arithmetic · checked 2026-09-02
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