Share

Kilojoules, calories: which is it?

If you're confused about the difference between kilocalories and kilojoules, the amount of foods you need to cut out of your diet to lose weight and the whole concept of "negative-energy foods", this article is for you.

But before we even discuss calories and kilojoules, it's important to clear up the confusion that exists about one of the most commonly used concepts in dieting: the general use of the word “calories”.

Calories = kilocalories
Strictly speaking, the non-metric unit of energy is the kilocalorie or kcal, which equals 1000 “small” calories. South Africans are used to the idea of kilojoules (kJ) and use the term when we speak of the amount of energy in foods or when describing how much energy we've used during exercise.

However, most of us don’t think of kilocalories when we ponder the energy contents of foods or energy expenditure. We think of “calories”, even if we're strictly speaking about kcals.

To try to make this concept understandable, let’s take a look at the scientific definition of a calorie and a kilocalorie:

  • Calorie: The amount of energy required to raise the temperature of 1 millilitre of water by 1 degree Celsius at 15 degrees Celsius.
  • Kilocalorie: The amount of energy required to raise the temperature of 1000 millilitres (or 1 litre) of water by 1 degree Celsius at 15 degrees Celsius.

So, a kcal = 1000 calories as the “kilo” part of the name indicates (kilo = one thousand).

It's important to keep in mind that when we talk about food energy in non-metric units we are actually speaking about kilocalories, not “little” calories. You may have noticed that the labels of food products imported from other countries list the energy content of the foods as kcals (not cals). So although it's common practise to speak of calories, what we actually mean is “kilocalories”.

What's a kilocalorie or a kilojoule?
A kilocalorie or a kilojoule is a measure or unit of energy. Kilocalories and kilojoules are units used in similar fashion to kilograms or metres, but instead of measuring weight and height, they measure energy.

If you say that a food contains 100 kilocalories or 420 kilojoules, this means that if the food is completely metabolised or ‘‘burnt’’ in the human body, 100 non-metric units or 420 metric units of energy will be released for use by the body. The body can use energy for physical activity and for keeping the basic metabolic processes (digestion, breathing and so forth) running. Your body may use the 100 kilocalories in the food to do daily work or just to provide energy for your body processes when you're sleeping.

If you take in more energy in the form of kilocalories or kilojoules than your body requires for everyday processes and physical activity, that energy will be stored in the form of fat. If you take in less energy than your body requires, it will have to get some energy out of its storage depots and break down some fat to obtain the required energy, thus causing weight loss.

What's the difference between kilocalories and kilojoules?
In the previous section we defined kilocalories as the non-metric, and kilojoules as the metric, unit of energy. So, although they're both units of energy, they differ because they're used in different measuring systems. We all know that pounds are the non-metric measure of weight and kilograms the metric measure of weight. In a similar fashion, kilocalories and kilojoules both indicate how much energy is contained in a food or how much energy we use up when we're active.

The difference between kilocalories and kilojoules is that one kilocalorie equals 4.2 kilojoules, and vice versa. So, if you read on a food label that a portion of the food contains 100 kilocalories, you can multiply that value with 4.2 to work out how many kilojoules the food contains.

Example: 100 kilocalories x 4.2 = 420 kJ

Conversely, you can calculate that a portion of food that contains 420 kJ (metric system) will contain 420 divided by 4.2 = 100 kilocalories The same applies when you want to work out how many calories or kilojoules a diet contains. Multiply kilocalories by 4.2 to obtain kilojoules and divide kilojoules by 4.2 to obtain kilocalories.

Although we use larger numbers when working with metric kilojoules this does NOT mean that kilojoules contain more energy – just that we're using a different system to express energy content.

How many kcal/kJ must I eat if I want to lose weight?
Generally speaking, we need to reduce our energy intake by 500 kcal or 2100 kJ (500 x 4,2 = 2100) to lose between 0.5 and 1kg per week – a rate that will help to keep the weight you lose from being regained.

An average, moderately active woman between the ages of 18 and 50 needs 2200 kcal or 9250 kJ (rounded off to the nearest 50) a day to maintain her weight. To lose weight, she needs to reduce her energy intake by 500 kcal or 2100 kJ down to 1700 kcal or 7150 kJ a day.

An average, moderately active man between the ages of 18 and 50 needs 2900 kcal or 12200 kJ (rounded off to the nearest 100) to maintain his body weight. To reduce his weight, our standard man would have to reduce his energy intake by 500 kcal or 2100 kJ down to 2400 kilocalories or 10 100 kJ a day.

Is food X fattening?
Many people wonder about specific foods or beverages and whether they're fattening or not. The answer to this question is that all foods and drinks are potentially fattening if consumed in excess – the only exceptions being artificially sweetened drinks and the foods that we'll discuss below under ‘‘negative-energy foods’’.

This means that even if you eat healthy foods (e.g. yoghurt, fruit, vegetables, vegetarian dishes etc.) to such an extent that you push up your energy intake beyond your daily requirement, these good and healthy foods will still make you gain weight. The fact that a food or beverage has excellent nutritional properties doesn't mean that it doesn't have the potential to make you fat if you overdo your intake.

Conversely, many foods that have a very high energy content, such as olive oil, are so nutritious that it's a good idea to include them in your diet, but in moderate quantities.

What are negative-energy foods?
The concept of negative-energy foods and drinks is also cause for much confusion. The theory of negative-energy foods and drinks is based on the fact that we use up some energy to digest foods and drinks.

Some foods have such a low energy content that we theoretically use up more energy than they contain to digest them, thus one can regard these foods as negative-energy foods. Examples include lettuce, celery, tomatoes, cucumber, gherkins, lemon juice, grapefruit, strawberries and other berries, cabbage, sugarless chewing gum and artificially sweetened beverages.

The problem is that people imagine that they can eat only these foods when they want to lose weight. No one can exist on lettuce and tomatoes for days on end. This is a totally unbalanced diet and will result in a variety of negative effects, including deficiencies, constipation, lack of energy, faintness and shakiness.

The ironic thing is that if you try and lose weight by ingesting negative-energy foods and artificially sweetened drinks on their own, your body may react to this starvation regimen by shutting off its weight-loss systems and you may stop losing weight altogether.

Negative-energy foods are excellent fillers when you're on a slimming diet. For example, having a large salad made up of these foods with fat-free cottage cheese and wholewheat bread will make a filling, low-fat, low-energy meal that will keep you feeling satisfied for longer. But forget about just living on salads – you won’t survive and ultimately you’ll stop losing weight.

- (Dr Ingrid van Heerden, DietDoc, November 2008)

We live in a world where facts and fiction get blurred
Who we choose to trust can have a profound impact on our lives. Join thousands of devoted South Africans who look to News24 to bring them news they can trust every day. As we celebrate 25 years, become a News24 subscriber as we strive to keep you informed, inspired and empowered.
Join News24 today
heading
description
username
Show Comments ()
Editorial feedback and complaints

Contact the public editor with feedback for our journalists, complaints, queries or suggestions about articles on News24.

LEARN MORE