The digestion and absorption of food is an interesting and not fully understood topic. The lack of understanding stems from the difficulty is accessing the absorptive surfaces of the gut to be able to measure the transport of nutrients, and the interactions between foods makes the study of digestion difficult. That being said, animals and cell culture models have given insight into the digestive and absorptive processes in man. Traditionally the small intestine was seen as the location of maximum nutrient absorption, particularly for the main macronutrients. In contrast, the colon was seen as a location of water absorption, with minimal contribution to the overall digestive and absorptive process. However, increasingly the colon is being seen as a vital element in digestion and absorption due to the presence of the high numbers of gut bacteria that inhabit the colon. These commensal bacteria have been shown to play a major role in the digestive process due to their production of a wide variety of metabolic and digestive enzymes.
Fermentation is a process by which bacteria metabolise carbohydrates to alcohols, gases and acids for their own use. This process is highly important in ruminants such as cattle, and it has been shown that the fermentation of carbohydrates is a significant route of starch disappearance in such animals1. Increasingly it is being shown that the metabolism of nutrients in the colon, including the fermentation of nondigestible carbohydrates, is an important route for the digestion of energy in humans. Many products of bacterial fermentation are absorbed from the colon and this can not only be an important route for the absorption of energy, but also an important route for the absorption of other non-energy nutrients such as phytochemical metabolites. Many such metabolites have not been fully categorised and their fate is often unreported. The polyphenol group of metabolites are extensively metabolised in the colon, and there is evidence that they are then absorbed in humans, where they may be then further metabolised in the liver.
Originally dietary fibre was considered to provide roughage, to allow the more efficient transport of food through the intestine. However, increasingly it has been shown that fibre is fermented in the colon, in a similar way to the fermentation of carbohydrates in ruminants. The fermentation process results in the production of volatile short chain fatty acids such as butyrate, propionate and acetate, that are then absorbed. These volatile fatty acids may contribute significantly towards the energy needs of humans, showing that dietary fibre may be an important source of energy. This contradicts the current paradigm that suggests that fibre is a non-energy nutrient and contributes nothing to the energy needs of humans. Some estimates claim that the contribution of fibre to the energy requirements of humans, following high fibre diets, may be a significant source of energy. The short chain fatty acids are also pivotal to gut health, providing an acid environment to prevent the growth of deleterious bacterial and yeast strains.
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