The Legal Definition
Standard 1.2.8 of the Food Standards Code defines dietary fibre by two linked criteria. First, behaviour: the substance must resist digestion and absorption in the small intestine, typically undergoing complete or partial fermentation in the large intestine. Second, structure: it must be a polysaccharide or oligosaccharide with a degree of polymerisation greater than two (meaning more than two sugar units joined together), or a lignin.
The Code also lists recognised physiological effects fibre is associated with, a list carried over from international definitions rather than specific to any one ingredient, used as part of how a new ingredient gets assessed for dietary fibre status rather than as a claim about any particular product.
Why It's Defined This Way
The "resists digestion" criterion is what separates fibre from other carbohydrates, like starch or sugar, which are broken down and absorbed in the small intestine for direct energy. The structural test (chain length, polymerisation) is what lets regulators assess new, often manufactured ingredients against a consistent standard, rather than relying only on their plant origin or marketing description.
Natural vs Manufactured Fibre
Some fibres, like psyllium husk or wheat bran, are naturally occurring plant material that clearly meets the definition without needing a specific regulatory application. Others, like resistant dextrin, are manufactured: starch treated with heat, acid or enzymes until the result meets the same structural and digestive-resistance tests. Resistant dextrin needed a specific FSANZ application to establish that a manufactured ingredient met the definition; naturally occurring fibres generally didn't.
How This Differs From "Fibre" in Everyday Use
In everyday use, "fibre" often just means "the roughage in plant food." The regulatory definition is narrower and more specific: it has to meet the digestion-resistance and structural tests above, regardless of whether it's commonly thought of as "fibrous." This is also why an ingredient can be a dietary fibre under the Code without being obviously plant-like in appearance, such as a manufactured ingredient derived from starch.
How It's Measured
Schedule 11 of the Code prescribes specific AOAC laboratory methods for measuring dietary fibre. Total dietary fibre in general is measured using AOAC sections 985.29, 991.43 or 2017.16. Resistant maltodextrins specifically require section 2001.03, because the general methods only capture part of what a resistant maltodextrin actually contains. Inulin and fructooligosaccharide have their own dedicated methods (997.08 and 999.03), as do polydextrose (2000.11) and resistant starch (2002.02). If a food's fibre content has been tested by more than one of these methods, the Code sets out how to combine the results without double-counting. This is why the gram figure on a label reflects a specific, defined laboratory test matched to the ingredient, not a general estimate applied uniformly across every fibre type.
Why the Definition Keeps Expanding
The list of approved analytical methods has grown over time as new fibre ingredients have come onto the market, particularly manufactured ones that didn't exist when the original definition was drafted. Each addition follows the same pattern: an ingredient is proposed, assessed against the structural and digestion-resistance criteria, and if it qualifies, a method for measuring it is added to Schedule 11. Inulin, fructooligosaccharide and polydextrose were added this way, and resistant maltodextrin followed later through FSANZ Application A491. The underlying definition hasn't changed; what's expanded is the list of ingredients and methods recognised as meeting it.
Sources
- Australia New Zealand Food Standards Code, Standard 1.2.8, Nutrition Information Requirements
- Australia New Zealand Food Standards Code, Schedule 11, Calculation of Values for Nutrition Information Panel
- FSANZ, Final Assessment Report, Application A491, Resistant Maltodextrin as Dietary Fibre