Kilocalories and Kilojoules
At recess you eat a quesadilla and your friend Diego eats a granola bar. Which one gives you more energy? And how much energy do you need in total in one day? To answer, you need to know what unit food energy is measured in and how to calculate it from what the label says.
The energy in food: kcal and kJ
Your body runs on chemical energy stored in nutrients: carbohydrates, proteins, and lipids. When you digest and use them, that energy is released and you use it to move, think, maintain your temperature, and grow.
That energy is measured in two units:
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The kilocalorie (kcal), the one most used for food. On labels it appears as "Calories" (with a capital C) or "Energy."
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The kilojoule (kJ), the International System of Units (SI) unit that appears on many labels next to the kcal.
Why do you multiply when going to kJ? Because the kJ is a smaller unit: it takes 4.184 kJ to equal a single kcal, so the same food gives a larger number in kJ.
The label on a can of tuna says that one serving provides 120 kcal. Express that energy in kilojoules using the equivalence $1 \text{ kcal} = 4.184 \text{ kJ}$ and round…
The label on a can of tuna says that one serving provides 120 kcal. Express that energy in kilojoules using the equivalence $1 \text{ kcal} = 4.184 \text{ kJ}$ and round to the nearest unit.
How many kcal are in a dish
Each nutrient releases a different amount of energy per gram. These values are obtained by measuring how much energy foods release when they are used, and they are taken as averages:
| Nutrient | Energy per gram |
|---|---|
| Carbohydrates | 4 kcal/g |
| Proteins | 4 kcal/g |
| Lipids (fats) | 9 kcal/g |
Lipids give more than double the energy per gram. Their carbon chains are less "oxidized" and store more chemical energy, which is why the body uses them as a reserve.
To calculate the energy in a food: multiply the grams of each nutrient by its value and add.
$$E = (g_{\text{carb}} \times 4) + (g_{\text{prot}} \times 4) + (g_{\text{lip}} \times 9)$$A quesadilla made with a corn tortilla and cheese contains approximately 30 g of carbohydrates, 12 g of protein, and 10 g of lipids. Calculate the total energy content…
A quesadilla made with a corn tortilla and cheese contains approximately 30 g of carbohydrates, 12 g of protein, and 10 g of lipids. Calculate the total energy content in kilocalories using $4 \text{ kcal/g}$ for carbohydrates and proteins and $9 \text{ kcal/g}$ for lipids.
Energy density
Not all foods "go the same distance." A serving of potato chips and a serving of apple can weigh the same, but the energy is very different. Energy density is the energy that each gram of food provides.
A food with a lot of water and fiber (fruits, vegetables) has low density. One with a lot of lipids and little water (fried snacks, chocolate, nuts) has high density.
A 40 g granola bar provides 190 kcal and a 150 g apple provides 80 kcal. Calculate the energy per gram of each food in $\text{kcal/g}$ and explain which one is more…
A 40 g granola bar provides 190 kcal and a 150 g apple provides 80 kcal. Calculate the energy per gram of each food in $\text{kcal/g}$ and explain which one is more "energy-dense". Round to two decimal places.
Your daily energy and physical activity
Your body uses energy all day long, even when you're asleep: your heart beats, you breathe, you maintain your 37 °C. If you study and play sports, you use more. A reference value for a teenager is around 2,200 kcal a day, although it varies with age, size, and activity: someone who trains in soccer every day needs more than someone who hardly moves.
To see how much of the day's total a meal covers, we use a percent:
$$\% = \frac{\text{energy of the meal}}{\text{daily energy}} \times 100$$A reasonable split over the day is usually breakfast, lunch, and dinner that each provide between 20 % and 35 %, with small snacks in between. If a single meal covered more than half of your energy, the rest of the day would be unbalanced.
A school meal provides 750 kcal and a 13-year-old teenager needs about 2,200 kcal per day. Calculate what percentage of the daily energy that meal covers, and express…
A school meal provides 750 kcal and a 13-year-old teenager needs about 2,200 kcal per day. Calculate what percentage of the daily energy that meal covers, and express both amounts in kilojoules as well. Is it reasonable for a single meal to cover that fraction of the daily total? Comment on your answer.
Go back to the beginning: to find out which of the two foods gives you more energy, add up the kcal from each nutrient (4, 4, and 9 kcal/g) or read the label; and to find out how much you need in a day, the reference for a teenager is around 2,200 kcal.