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🍎 Calculating Dietary Energy

Discover how the energy in food is measured and learn to work it out from a nutrition label.

lessons

~15 min

to learn

🔬 Science

subject

Teens

level

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What you’ll learn

Energy in FoodKnow that food energy is measured in kilocalories (kcal) or kilojoules (kJ) and where to find it on a label.Food gives the body the energy it needs, and that energy is measured in kilocalories, often written as 'Calories', or in kilojoules, with 1 kcal about equal to 4.18 kJ. Nutrition labels show the energy along with the grams of fat, carbohydrate, and protein, which are the numbers used to calculate energy.
Energy per GramState how much energy fat, carbohydrate, and protein each release per gram.The three energy-giving nutrients release different amounts per gram: fat gives about 9 kcal, while carbohydrate and protein each give about 4 kcal. Fat is stored energy, carbohydrate is the body's quick energy, and protein is mainly used for growth and repair.
Doing the MathsCalculate a food's total energy by multiplying each nutrient's grams by its energy per gram and adding the results.To find a food's total energy, multiply the grams of fat by 9 and the grams of carbohydrate and protein each by 4, then add the three amounts. For example, a snack with 10 g fat, 20 g carbohydrate, and 5 g protein gives 90 + 80 + 20 = 190 kcal.

Questions this course answers

Which units are used to measure the energy in food?

Food energy is measured in kilocalories (kcal, also written 'Calories') or in kilojoules (kJ).

About how many kilojoules are in one kilocalorie?

The two units are linked, with 1 kcal equal to about 4.18 kJ.

Which nutrient gives the most energy per gram?

Fat releases about 9 kcal per gram, more than twice the energy of carbohydrate or protein.

About how much energy does one gram of carbohydrate or protein give?

Carbohydrate and protein each release about 4 kcal per gram, less than fat.

A snack has 10 g fat, 20 g carbohydrate and 5 g protein. What is its total energy?

Fat gives 10 × 9 = 90, carbohydrate 20 × 4 = 80, and protein 5 × 4 = 20, so 90 + 80 + 20 = 190 kcal.

How do you find a food's total energy from its nutrients?

You multiply the grams of each nutrient by its energy per gram and then add the results for the total.

Grounded in trusted sources

BBC Bitesize
KidsHealth
Britannica
Britannica Kids

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