When ingested, carbohydrate (starch) is passed to the small intestine, where enzymes hydrolyse the glycosidic bonds to produce glucose unit, and these are then absorbed and appear in the circulation as blood sugar. However, some carbohydrate in the diet is not available for digestion and absorption in the small intestine in humans because the carbohydrate molecules contain chemical bonds for which the specific digestive enzymes are absent. These unavailable carbohydrates fall into the general category of dietary fibre, and can include oligosaccharides, inulin, pectin, lignin, cellulose, alcohol sugars and resistant starches. Dietary fibres tends to add bulk to the food, and pass to the colon where they undergo fermentation by colonic microflora producing a range of products including short chain fatty acids, gases and lactate. These products are then absorbed and can be used in metabolic pathways and can contribute to the energy needs of the body.
Carbohydrates digested by colonic microflora contribute to the energy needs of the body but result in a considerable delay in the delivery of that energy to circulation because of the increased digestive time. Research published in the American Journal of Clinical Nutrition in 20001 investigated the effects of varying quantities of different types of carbohydrates on substrate oxidation and hunger. The researchers found that diets with higher proportions of carbohydrate in the form of dietary fibre, produced lower feelings of hunger than diets that were high in freely digestible starch. In addition, although the subjects on the diets did not differ in their total energy expenditure or substrate oxidation, the diet high in indigestible dietary fibres did produce a significant lower and delayed rise in post-prandial carbohydrate oxidation.
The researchers used breath hydrogen as a test for the fermentation of dietary fibre in the colon, and found that breath hydrogen increased in the subjects on the high fibre diet. In these subjects, hydrogen in the breath increased 4 hours after the meal and was still higher the next morning, suggesting that the fibre was slowly being fermented in the colon by microflora. Dietary fibre in not usually though of as contributing energy to the body, but the total nutrient oxidation was not significantly different between the two diets. Higher fat oxidation and lower carbohydrate oxidation was seen in the high fibre diets after the meal, presumably because lower levels of glucose were provided to the circulation. In contrast, during the night the high fibre diets had reduced oxidation of fat such that the total macronutrient oxidation was similar.
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