Dietary Fibre For Food Labels: What Is It And How Is It Measured?
To the layman, fibre is an edible material that is hard to digest, which has health benefits for the individual. Writing a completely satisfactory scientific definition for fibre is almost impossible because of the wide variety of substances it may contain.
Similarly, it is quite hard to measure the amount of fibre in a food product. Laboratory methods try to mimic the digestive system, but that’s a very difficult task. In general, the methods used involve dissolving, digesting or otherwise removing all the other material in order to weigh the fibre at the end. Some material may not be removed, but it is corrected for in a calculation. Because of all the manipulation, fibre analysis generally lacks precision.
There are various analytical methods for determining fibre and these different methods reflect differences in how fibre might be defined.
In the UK, the Englyst method used to be the most popular. The well-known ‘McCance and Widdowson’s The Composition of Foods’, 6th Edition (2002) used the Englyst method for fibre values and Englyst is still used by dieticians in the UK.
When UK food products first began to be labelled with nutrition information, Englyst values were almost always used. However, the UK was out of sync with most other countries, particularly in the European Union, so, when a unified approach to nutrition labelling was adopted by the EU, the UK began to use the AOAC method which has been generally favoured abroad.
As a result, the 7th Edition of McCance and Widdowson was published with both AOAC and Englyst values, although there were a lot of gaps.
AOAC values are higher than Englyst. McCance gives values for wheat bran of 41% using the AOAC method, compared with 33% for the Englyst method.
So the standard AOAC method no. 985.29 may not be appropriate in all cases and, if your product contains soluble fibre, it’s important to tell the laboratory about that when requesting a fibre analysis.
Like most food analytical methods, these fibre methods work well for a majority of food types. However, where unusual ingredients, such as soluble fibre appear, the results can be hopelessly low if an incorrect method is used which dissolves it [It might sound as though a fibre couldn’t be soluble, because soluble substances should digest easily, however the soluble fibre forms a gel in digestion, which is very difficult to digest.]
One consequence of having incorrect dietary fibre values is that the energy values will be wrong. In the UK and EU, 1g dietary fibre is assumed to provide 2kcal, whereas for carbohydrate it is 4kcal.
If using NutriCalc to obtain your nutrition information, these complications with fibre and energy values will not arise. Sometimes though we have had to provide support to a user when they have found a fibre value obtained in the laboratory is too low compared with the NutriCalc value, for the reasons given above.