02 May 2012

Dieting – cut the confusion

If you're confused about calories, kilojoules, good foods vs. fatty foods, and the concept of "negative energy foods", this article is for you. DietDoc answers crucial questions.


If you're confused about the difference between calories 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. DietDoc answers some crucial questions.

What is a calorie or a kilojoule?

A calorie or a kilojoule is a measure or unit of energy. Calories 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 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 etc.) running. Your body may use the 100 calories in the food to do daily work or just to provide energy for your body processes when you are sleeping.

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. 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 is the difference between calories and kilojoules?

In the previous section we defined calories as the non-metric, 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. We all know that pounds are the non-metric measure of weight and kilograms the metric measure of weight. In a similar fashion, calories and kilojoules both indicate how much energy is contained in a food or how much energy we use up when we are active.

The difference between calories and kilojoules is that one calorie equals 4,2 kilojoules and vice versa. So, if 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:

Example: 100 calories 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 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, just that we are using a different system to express energy content.

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

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.

An average, moderately active woman between the ages of 18 and 50 needs 2 200 cal or 9250 (rounded off to the nearest 50) kJ a day to maintain her weight. To lose weight, she needs to reduce her energy intake by 500 cal or 2100 kJ down to 1700 cal or 7150 kJ a day.

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

Is food X fattening?

Many people wonder about specific foods or beverages and whether they are 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 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, vegetarian dishes etc.) and you eat so much 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 does not mean that it does not 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 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 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 are on a slimming diet. For example having a large salad made up of these foods with 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)

Any questions? Ask DietDoc

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