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What's the difference between a calorie and a kilojoule?

Tuesday 21 February 2017

A man stands in a supermarket reading the nutrition label on a box of food.
Understanding how your body makes energy from food can help you achieve a healthy diet.

Sometimes, a unit of measurement lingers long after it’s been replaced. For example, we still often talk about a person’s height in feet and inches. In the field of nutrition there is the calorie, which was superseded by the kilojoule in Australia over 40 years ago.

If you want to know why we made the switch, the clue is in the name: like the kilogram and the kilometre, the ‘kilo’ part of kilojoule represents the measurement of one thousand of a certain metric, in this case joules. And joules? They’re the unit used to measure energy.

Where did calories come from, anyway?

Calories are a way of measuring the potential energy contained in food, and a single calorie is the amount of energy, or heat, required to heat a gram of water by 1° Celsius.

The calorie system for measuring food was put together by a man named Edward Atwater, who began experiments into the science of food and diet back in the early 1900s. He had a particular fascination with the chemical composition of foods, the effects of food processing and cooking on nutritional quality, and on the effects of nutrients on the human body.

He also calculated the calorie content of specific foods through the use of a device called a bomb calorimeter.

The name is impressive, but the process is obvious once you know what calories are measuring. Essentially, Atwater put food in a sealed container full of water, ran electricity through the food until it was fully burned, then measured the difference in the water temperature after he was done.

And so, the calorie content of food was first measured.

Why are we still talking about them?

Essentially, the difference between calories and kilojoules is terminology - they're two different ways of measuring the energy contained in food and the energy we expend. You can convert calories to kilojoules by multiplying the calories by 4.2.

Unlike the inch or the mile, which have fallen out of popular usage in Australia, the calorie remains pretty current when it comes to the way people think about food. The science community may have swapped over to kilojoules, but the calorie is everywhere when consumers start looking at weight loss, dieting, and apps for measuring their daily energy consumption.

The resistance to update is understandable: unlike kilometres and kilograms, which we all use throughout our day-to-day life, people who aren't involved in science don't usually start engaging with kilojoules and calories until they start looking into healthy eating or weight loss. Old terminology tends to linger, particularly when one of the countries that stuck with the older measures is the United States of America.

That said, if you are interested in tracking the energy levels in your food, it might be worth switching your thinking over to kilojoules for the same reason it's easier to think in kilometres when travel long-distances in Australia - all the official signs and food packaging use those measurements, and you don't have to rely on someone translating them for you.

Regardless of the term you use, here's what really matters

Whether you use calories or kilojoules, what really matters is how they're related to achieving and maintaining a healthy weight.

You naturally take in energy from the food that you eat. You naturally burn off energy through the process of living your life, with everything from sleeping to lifting weights taking something from your energy reserves.

Your exact daily energy requirements will vary based on your age, height, gender, and general level of physically activity, but weight gain tends to happen when you consume more energy than you expend. If you're curious about what your daily energy requirements are, you can use this calculator to find out.

Learning your energy requirements is your first step, but your second is learning to recognise that not all kilojoules are created equal. A single chocolate bar can contain as many kilojoules as a serve of healthy mixed-veggie mash, but the latter will see you consuming approximately 250 grams of nutrient-rich vegetables that will help leave you feeling full, compared to just 40 grams of sugar-rich chocolate.

Focusing on healthy eating means achieving parity between the energy you bring in and the energy you expend, regardless of how it's being measured, and learning to identify the most beneficial foods to consume.

If you'd like to start getting a better understanding of what you're eating, and how to eat healthier, check out our Healthier. Happier. website for information and healthy recipes.

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Last updated: 23 June 2020