Understanding food labels: portions, energy


Most people don't have a clear understanding of what the information displayed on food labels really mean.

This is worrying, as food labels can be a source of a great deal of knowledge about the foods we buy and eat. Learn more about what you should be looking for on food labels.

Portions vs. 100g

One source of great confusion when reading food labels is that, at present, manufacturers may list the nutritive value of the food per portion, per 100g or per 100ml (liquids), or both per portion and per 100g (100ml).

So, if you pick up an energy bar, which weighs 50g and you look at the Nutrition Information Table on the label, then you may need to check if the values in the Table are listed per portion, which in this case is per 50g, or if the Table refers to nutrient value per 100g.

The former method of transmitting information means that you know exactly how much of each listed nutrient you will be getting when you eat the energy bar, which weighs 50g. On the other hand, listing the nutrient content per 100g when the bar you are going to eat only weighs 50g, means that you will be getting 50/100 or half the amount of each nutrient when you eat the bar.

Hopefully this source of confusion will be eliminated when the new SA Labelling Regulations come into effect some time soon. Manufacturers will be obliged to list nutrition information both per portion (in this case per 50g bar) and per 100g (or 100ml).

Once this regulation is law, consumers will just need to ensure that they look at the correct column in the Table when trying to figure out what they are eating. In the case of the energy bar, you would look at the column under 'portion' and you would then know how much of each nutrient you are ingesting when you eat an energy bar weighing 50g.

Energy value

a) Kilojoules

As South Africa is a metric country, the energy value of foods must be expressed in kilojoules (kJ). So, if a food's energy value is listed as 500kJ per portion, then this is how much energy you will ingest when you eat a portion of this food.

b) Calories or kilo-calories

However, many manufacturers still list the energy content of foods in calories (cal) or use both kJ and calories. Confusion arises when the label expresses the energy content in kcal (or kilo-calories).

Energy contained in food is always expressed in kcal, but most manufacturers forget to use the 'k' in front of the 'cal' in the abbreviation. So, although the abbreviations 'cal' and 'kcal' actually have different scientific meanings, manufacturers use them interchangeably.

When a food label states that the product contains 50cal and another product contains 50kcal, it actually means the same thing.

Low-energy foods

Since so many people are trying to lose weight, it is important for them to understand which foods are low in energy.

Generally speaking, a food that contains less than 170kJ per 100g or less than 80kJ per 100ml in the case of liquids can be regarded as 'low in energy'.

Translated into daily energy requirements, this means that an entire 100g of this food or 100ml of this liquid will only supply you with 2,7% of the energy you require when you are on a slimming diet with a total energy content of 6300kJ.

This is a very small amount of energy. You will probably know that most commercially prepared foods will not meet this stringent requirement.

When next you look at a food label that says 'low in energy', first check the label to see how much energy it really contains. That energy bar we were discussing earlier may contain as much as 500kJ per 50g portion (which means it contains 1000kJ per 100g). Therefore, it is anything but 'low in energy'.

Energy content of slimming products

The same is often true for a variety of foods and drinks that are sold as 'diet products', e.g. diet shakes or slimming bars and snacks.

Look at the labels. Do these foods contain 170kJ per 100g, or per 100ml, or less? If the answer is 'No', then these products are NOT low-energy foods/drinks. Eating these foods will not help you to lose weight, and may actually hamper your weight loss. Worst of all, these foods could cause weight gain.

Keep this in mind when you are using diet shakes and slimming bars. These products most certainly don't only contain 170kJ per 100g.

However, if we use a more lenient criterion, namely that a slimming shake that you use to replace one of your three daily meals should provide 1/3 of your daily energy intake of 6300/3 = 2100 kJ, then you can check how much energy one glass of the slimming shake contains. If it contains more than 2100 kJ per glass, using the product will probably not cause weight loss!

(Bronstein, P (2006). A Simple Solution. Time. Oct 16, 2006, pp:36-43)

Read more:

The parent's guide to food labels

Spot the kilojoule bombs

The kilojoule conundrum

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