Energy and calories
Learn what a calorie measures, where the energy in food comes from, and why your needs differ from someone else's.
Kilocalorie (kcal)
The energy needed to warm 1 litre of water by 1 °C. The "calories" on a food label are really kilocalories: "Calories", "Cal" and "kcal" all mean the same thing there. Many labels also give kilojoules (kJ), the metric unit: 1 kcal is about 4.2 kJ (4.184, to be exact).
A label that says 250 kcal might also say about 1,050 kJ. Same energy, two units.
WHERE ENERGY COMES FROM
Four things in food carry energy
Per gram, using the standard figures behind food labels:
- Carbohydrate: about 4 kcal
- Protein: about 4 kcal
- Alcohol: about 7 kcal
- Fat: about 9 kcal
Fibre gives only a little (about 2 kcal per gram). Water, vitamins and minerals give none, however important they are.
Fat carries more than twice the energy of the same weight of carbohydrate or protein. That's why oily and fried foods add up so quickly.
Check yourself
Gram for gram, which of these gives your body the most energy?
- Protein
- Carbohydrate
- Fat
- Water
Show the answer
Fat
Right. Fat gives about 9 kcal per gram, more than twice the roughly 4 kcal of protein or carbohydrate.
A snack bar's label says:
carbohydrate 10 g
protein 5 g
fat 10 g
carbohydrate 10 g x 4 = 40 kcal
protein 5 g x 4 = 20 kcal
fat 10 g x 9 = 90 kcal40 + 20 + 90 = about 150 kcal.
Fat is a third of the weight but more than half the energy.Labels round their numbers, so your sum and the printed total may differ by a few kcal. That's normal.
Check yourself
Maryam's box of dates lists 20 g of carbohydrate per serving and no protein or fat. About how much energy is in one serving?
- About 20 kcal
- About 80 kcal
- About 180 kcal
- It can't be worked out from the label
Show the answer
About 80 kcal
Yes. Carbohydrate gives about 4 kcal per gram, so 20 × 4 = about 80 kcal.
WHERE ENERGY GOES
Most of it just keeps you alive
For many people, roughly 60 to 70 % of the day's energy goes on basal metabolism: breathing, a beating heart, a working brain, a steady body temperature, all while you rest. About 10 % goes on digesting food. The rest goes on movement, from walking to the bus to climbing stairs or sport.
Even on a day spent mostly sitting down, your body uses a lot of energy. That's why nobody's needs drop to zero on a lazy day.
Check yourself
For a typical adult who isn't an athlete, where does most of the day's energy go?
- Digesting food
- Exercise and sport
- Keeping the body running at rest
- Growing new muscle
Show the answer
Keeping the body running at rest
Right. Roughly 60 to 70 % goes on basal metabolism: breathing, heartbeat, brain and body temperature, even while you rest.
What sets your energy needs
- Age: needs are highest in teenagers and young adults and tend to fall slowly with age.
- Sex and body size: a bigger body, and more muscle, uses more energy even at rest.
- Activity: a builder or a nurse on her feet all day uses more than someone at a desk.
- UK reference figures are about 2,000 kcal a day for women and about 2,500 for men. They are averages across many people, not targets.
- The 2,000 kcal (8,400 kJ) "reference intake" printed on labels in the UK and EU is a yardstick for comparing foods, not your personal number.
Check yourself
The 2,000 kcal printed on a food label is the amount every adult should eat each day.
Show the answer
False
False. It's a reference figure, an average used so that labels can be compared. Your own needs depend on your age, sex, body size and how active you are, and can be well above or below it.
Energy balance is a bank account, read monthly
Food is money coming in; keeping alive and moving is money going out. Take in more than you spend over weeks and the body saves the extra, mostly as fat. Take in less and it draws on its savings. One big wedding dinner or one skipped lunch is a single transaction: what shapes the balance is the pattern across weeks. Where the analogy breaks: your body isn't a calculator. Appetite, sleep and health all nudge how much you eat and use.
Same calories, different food
200 kcal of soft drink
About half a litre. Almost all sugar, no protein, no fibre. It goes down in a minute and does little to fill you up.
200 kcal of cooked lentils
A generous bowl. Brings protein, fibre, iron and folate along with the energy, and keeps you full for hours. The number is the same; what comes with it is not.
Check yourself
Sort these by whether they give your body energy.
- A glass of sweet tea
- A glass of plain water
- A spoon of olive oil
- The vitamin C in an orange
- A glass of orange juice
- The iron in a bowl of lentils
Show the answer
Gives energy (kcal): A glass of sweet tea, A spoon of olive oil, A glass of orange juice
Gives no energy: A glass of plain water, The vitamin C in an orange, The iron in a bowl of lentils
Lesson recap
- The "calories" on a label are kilocalories (kcal); 1 kcal is about 4.2 kJ.
- Per gram: carbohydrate and protein about 4 kcal, alcohol about 7, fat about 9. Water, vitamins and minerals give none.
- Roughly 60 to 70 % of your energy keeps you alive at rest; about 10 % goes on digestion; the rest on movement.
- About 2,000 kcal for women and 2,500 for men are reference averages, not personal targets.
- Energy balance is about the pattern over weeks, and calories are only part of the story: what comes with them matters too.