Calories, kilojoules: which is it?

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

Before we even discuss calories and kilojoules, let's 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 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, the word “calories” is widely and loosely used, even if we're referring to kcals.

The scientific definitions are:

  • 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).

Kcal vs kJ
Both a kilocalorie and a kilojoule are a measure or unit of energy. If you say that a food contains 100 kcal or 420 kilojoules, this means that if the food is completely metabolised, 100 non-metric units or 420 metric units of energy will be released for use by the body. Apart from needing energy for physical activity, the body uses energy to keep the basic metabolic processes (digestion, breathing and so forth) running.

If you take in more energy 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, 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.

To convert kcal to kJ, which is the preferred unit of measure in the metric measuring system in use in SA, the formula is 1 kcal = 4.2 kilojoules.

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 a day to lose between 0.5 and 1kg a 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 a day to maintain her weight. To lose weight, therefore, she needs to reduce her energy intake 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 to maintain his body weight, so to lose weight he'd have to reduce his intake to 2400 kilocalories or 10 100 kJ a day.

Is food X fattening?
Most foods and drinks are potentially fattening if consumed in excess. This means that even if you only eat healthy foods (yoghurt, fruit, vegetables, vegetarian etc), you'll still put on weight if you take in more energy than you need. 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.

The concept of negative-energy foods and drinks is based on the principle 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 making them "negative-energy". Examples include lettuce, celery, tomatoes, cucumber, gherkins, lemon juice, grapefruit, strawberries and other berries, cabbage, and artificially sweetened beverages.

So if you want to lose weight you eat only those foods?

Absolutely not. An unbalanced diet is, in the end, counterproductive and will result in a variety of negative effects, including deficiencies, constipation, lack of energy, faintness and shakiness. Ultimately, your body may react to what is effectively a starvation regimen by shutting off its weight-loss systems: you stop losing weight altogether.

Negative-energy foods are excellent if you use them as 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.

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

Any questions? Ask DietDoc

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