Structurally, dietary fibres are sugar molecules joined through non-digestible glycosidic bonds. The sugar energy within the molecule is not absorbed dirtectly as the fibre passess through the upper gastrointestinal tract undigested. However, as with ruminant animals, humans can ferment fibre in the colon, and this results in the production of short chain fatty acids that can significantly contribute to the daily energy requirements. Any molecule with indigestible glycosidic bonds could be classified as dietary fibre, and a number of such molecules have been developed by the food industry for their superior sensory properties. Two such fibres, polydextrose and soluble corn fibre, are commercially added to foods for their taste properties, but these fibres may provide health benefits as with natural plant fibres. Polydextrose is formed by the random polymerisation of glucose resulting in a polysaccharide with α-1,6 linkages, whereas soluble corn starch consists of mixtures of α-1,6, α-1,2 and α-1,3 glycosidic bonds.
Some of the physiological effects of polydextrose and soluble corn fibre have been tested by researchers. For example in a study published in the Journal of Nutrition<\sup>1<\sup>, researchers assessed the laxative effects of the fibres in a double blind, placebo controlled study. Subjects were supplements with 20 grams of fibre per day (as a muffin and cereal) or a low fibre control for 10 days, followed by a 2 week washout period before they commenced one of the other treatments. The polydextrose and soluble corn starch both increase faecal weight, but only the polydextrose treatment increased daily faecal output, the number of stools per day and softened the stool. Neither polydextrose or soluble corn starch had any effect on total gut transit time compared to the control. Production of short chain fatty acids is an important component of the health benefits of fibre, and only the polydextrose lowered faecal pH significantly compared to the control, indicating the production of short chain fatty acids.
The manufactured fibres polydextrose and soluble corn starch therefore show some of the same properties that are seen in natural dietary plant fibres. That the properties of the fibres differ is not surprising because natural fibre too show wide variations in their ability to alter physiology in humans. That the fibres had an effect on stool weight and defecation frequency suggest that they affect laxation in humans. The fibres were well tolerated at the 20 gram dose, which would be large compared to the amount that would normally be ingested in foods during ad libitum feeding. Interestingly, the effects of the fibre were seen on top of the subjects normal diets which contained 13 to 14 grams of natural based fibres. This suggests that the effects of the manufactured fibres would be even greater had the fibre intake of the subject’s diets been lower. Adding manufactured fibres with improved sensory properties to Western foods may therefore be beneficial to those consuming such low quality diets.
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