The two-track approach to dietary fibre

By Lorraine Heller

- Last updated on GMT

Related tags: Nutrition, Carbohydrate

Dietary fibre is increasingly being linked to health benefits, but progress in the application of emerging science is being stunted by the lack of a clear definition of the term, according to the British Nutrition Foundation.

Fibre intake has been shown to benefit gastrointestinal health, glucose handling, heart health, cancer risk and satiety, but these benefits are dependent on the types of fibre present in foods, said the foundation’s director general Professor Judith Buttris.

“There are many different types of undigested carbohydrates, and they don’t all exert the same physiological effects,”​ said Professor Buttris last week at a meeting of the Food & Health Forum at the Royal Society of Medicine (RSM) in London.

This has led to a potentially confusing situation, which has hampered progress in the fibre research field, she said. It could also be preventing the implementation of research findings in food development, as well as their communication to consumers.

Two-track approach

The problem is that there is no single definition for fibre that is accepted worldwide.

Professor Butriss told delegates at RSM’s forum on Nutritional Approaches to Digestive Health that there is currently a “two-track” ​approach to dietary fibre.

Some believe that the inclusion of material other than plant cell walls as dietary fibre (e.g. oligosaccharides, resistant starch) is inappropriate and potentially misleading to consumers. Others accept these types of undigested carbohydrate can contribute to some of the health benefits associated with high fibre intakes, and should therefore also be considered as fibre.

Definitions of ‘fibre’

Although all definitions classify fibre as carbohydrate that is undigested in the small intestine, there remain some inconsistencies.

The Codex ​Alimentarius Commission has for the past 15 years debated a definition that it hopes can be used internationally. This is based on the method developed by the Association of Official Analytical Chemists (AOAC) and describes fibre as carbohydrate polymers with a degree of polymerization not lower than three (to exclude mono- and disaccharides).

It includes: Naturally occurring edible carbohydrate polymers; carbohydrate polymers obtained from food raw material by physical, enzymatic or chemical means; and synthetic carbohydrate polymers.

The WHO/FAO​ definition of fibre, established in December 2007 (European Journal of Clinical Nutrition – ​Nishida et al 2007) describes fibre as intrinsic plant cell wall polysaccharides.

It says the term should be “reserved for cell wall polysaccharides of vegetables, fruit and wholegrains, the health benefits of which have been clearly established, rather than synthetic, isolated or purified oligosaccharides with diverse, and in some cases unique, physiological effects.”

The UK’s Department of Health​ in 1991 defined fibre as non-starch polysaccharides (NSP), which comprise cellulose, hemicelluloses, pectin, arabinoxylans, betaglucan, glucomannans, plant gums and mucilages and hydrocolloids, all of which are principally found in the plant cell wall.

The American Association of Cereal Chemists (AACC​) in 2001 defined fibre as including oligosaccharides. The group said: “Dietary fibre is the remnants of the edible part of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine. Dietary fibre includes polysaccharides, oligosaccharides, lignin and associated plant substances.”

The Institute of Medicine’s (IOM​) Food and Nutrition Board in the US in 2001 defined total fibre as the sum of dietary fibre (NSB fibre) and functional fibre (isolated, non-digestible carbohydrates that have beneficial physiological effects in humans).

In July 2007, the European Food Safety Authority (EFSA) ​provided an opinion to the European Commission (EC) on the issue, which stated that the problem in differentiating between NSP fibre and ‘functional fibre’ is that no analytical method differentiates between them once they are mixed in food. It also said that NSP from plant cell walls cannot be distinguished from added NSP with a similar chemical structure.

EFSA’s panel recommended that dietary fibre should include all non-digestible carbohydrates plus lignan, which comproses NSP, resistant oligosaccharides (including fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS)), resistant starch, and lignin.

In January 2008, the EC ​published a working draft amendment to the Directive 90/496/EEC, which defines dietary fibre. This has passed through the standing committee with one abstention, and is now due to be presented to Parliament.

The Commission defines fibre as carbohydrate polymers with three or more monomeric units that are neither digested nor absorbed in the small intestine. These include: Edible carbohydrate polymers naturally occurring in the food as consumed; carbohydrate polymers that have been obtained from food raw material by physical, enzymatic or chemical means; or synthetic carbohydrate polymers. With the exception of naturally present carbohydrate polymers, evidence of beneficial physiological effects is also required for claims.

Progress constrained

“Progress in the application of emerging evidence regarding dietary fibre and health is constrained by a need to resolve the definition and methodology issues,”​ wrote Professor Buttriss and colleague C.S. Strokes in a British Nutrition Foundation review earlier this year.

“Most consumers are likely to be unaware of the more recent developments an dthe overlapping properties of different types of carbohydrate that resist digestion in the small intestine, and the particular sources of these carbohydrates.”

The review – Dietary fibre and health: An overview​ (Nutrition Bulletin​, 33, 186-200) – concluded that the translation of the growing knowledge about the physiological properties of carbohydrates, particularly non-digestible carbohydrates, into clear and unambiguous public health messages is long overdue.

To access the full review, click here​.

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