Calories, Kilojoules, and how they work in the body?

I’m sure you’ve all heard of kilojoules and calories. What are they exactly, and how do they work in the body? There is often confusion behind the terms and if there is any difference between them. We are going to clear that up in this blog.


What is a calorie or a kilojoule?

A calorie (abbreviated kcal) or a kilojoule (abbreviated Kj) is a measure or unit of energy found in food sources. Calories and kilojoules are units used in similar fashion to kilograms or metres, but instead of measuring weight and height, they measure energy.


100 calories x 4,2 = 420 kJ

If we say that a food contains 100 calories 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, brain function, etc.) operating optimally. Your body may use the 100 calories in the food to do daily activities or just to provide energy for your body processes while the body as rest (while you sleep).


If you take in more energy in the form of calories or kilojoules than your body requires for everyday processes and physical activity, that energy will be stored in the form of fat in the adipose cells. If you take in less energy than your body requires, it will have to utilize some energy out of its storage depots and break down some fat to obtain the required energy, thus causing weight loss. However, in cases where calorie intake is extremely low for extended periods; the body will start to get require nutrients from lean muscle and fat. This causes catabolism, which can result in severe negative effects.


What is the difference between calories and kilojoules?


Calories are the non-metric units, and kilojoules as the metric, unit of energy. So, although they are both units of energy, they differ because they are used in different measuring systems. The difference between calories and kilojoules is that one calorie equals 4,2 kilojoules and vice versa. So, when you read on a food label that a portion of the food contains 100 calories, you can multiply that value with 4,2 to work out how many kilojoules the food contains:

Conversely, you can calculate that a portion of food that contains 420 kJ (metric system) will contain 420 divided by 4,2 = 100 calories. The same applies when you want to work out how many calories or kilojoules a diet contains. Multiply calories by 4,2 to obtain kilojoules and divide kilojoules by 4,2 to obtain calories. Although we use larger numbers when working with metric kilojoules this does NOT mean that kilojoules contain more energy than calories, just that we are using a different metric system to express energy content.


How many calories/kilojoules must I eat if I want to slim?

Everyone’s body differs, and what works for one person may not necessarily work for another. So, when it comes to reducing calories to lose weight; there are a lot of variables to take into consideration (such as activity, levels, age, body type, medication, etc) Generally speaking, we need to reduce our energy intake BY 500 cal or 2100 kJ (500 x 4,2 = 2100) to lose between 0,5 and 1 kg per week – a rate that will help to keep the weight you lose from being regained.

This means you need to eat 500 calories (2100kj) less than what you have currently been eating. When a plateau is reached you reduce by 500calories again; until your daily calorie intake is at a healthy recommended intake.

An average, moderately active woman between the ages of 18 and 50 needs 2 200 kcal or 9250kj (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 (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 12200kj (rounded off to the nearest 100) to maintain his body weight. To reduce his weight, your standard man would have to reduce his energy intake by 500 kcal (2100 kJ down to 2400 calories or 10 100 kJ a day).

Is food X fattening?

Many people are misled about specific foods or beverages and whether they are fattening or not. The answer, is that all foods and drinks are potentially fattening if consumed in excess. The only exceptions being artificially-sweetened drinks and the foods which we will discuss below under ‘negative energy foods’.

This means that even if you eat what many regard as “good or healthy” foods (e.g. yoghurt, fruit, vegetables, nuts, etc) and you eat too much that you push up your energy intake beyond your daily requirement ( kj’s in, is more than kj’s expended), these good and healthy foods will still make you gain weight if you consume them in excess. The fact that a food or beverage has excellent nutritional properties does not mean that it does not have the potential to make you fat if you overdo your intake. However, many foods that have a very high energy content, such as olive oil, are so nutritious that it is 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, how we use up some energy to digest foods and beverages. Some foods have such a low energy content that we theoretically use up more energy than they contain to digest them, thus they are regarded 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 live on lettuce and tomatoes for days on end. This is an incredibly unbalanced diet and will result in a variety of negative effects, including deficiencies, constipation, lack of energy, faintness and other symptoms.

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 for when you are on a slimming diet. For example, having a large salad made up of these foods with cottage cheese and whole-wheat bread will make a nutritious, low-fat, low-energy meal that will keep you feeling satisfied for longer, but forget about just living on salads, you can’t maintain health and ultimately, you’ll stop losing weight.


Michaela xx

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