Undigested carbohydrate reaching the colon can be absorbed to a certain extent, but its fate is generally to be converted to short chain fatty acids (SCFA) by the colonic microflora. This undigested carbohydrate will include some simple sugars and polysaccharides, as well as the non-digestible fibre components of plants. Roughly half of the SCFA produced in the colon are volatile, and include acetate, propionate and butyrate. Short chain fatty acids can contribute to energy needs in humans, but their contribution is based on the amount of dietary fibre ingested. An intake of around 10 grams of dietary fibre will provide around 100 mmol of acetate which is equal to around 54 kcals of energy. High fibre diets will therefore derive a higher percentage of energy from SCFA, and this is illustrated in herbivores that can derive around 22 % of their basal energy needs from the production of SCFA.
As well as providing energy, SCFA can stimulate a number of secretions that are necessary for proper colonic physiology. Short chain fatty acids are able to diffuse into the enterocytes of the gut and can be exchanged in a counter-transport system for chlorine, which is co-transported with sodium. In this way SCFA are intimately involved in the absorption of sodium, with water following passively via osmotic forces. Detrimental effects on the microflora of the gut by use of antibiotics may decrease SCFA production, with a result of decreasing sodium and water absorption leading to diarrhoea. Short chain fatty acids are also thought to stimulate enterocyte cell division, with butyrate being more potent that either propionate or acetate. This may occur via a lowering of pH which stimulates mucosal growth, via the stimulation of growth factors or via the ability of SCFA to increase blood flow, thus facilitating optimal growth conditions.
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