Abstract
Short chain fatty acids (SCFAs; acetic, propionic and butyric acid) are formed during bacterial fermentation of carbohydrates in the colon. The interest in SCFA production is related to an increasing body of knowledge of the physiological effects of these acids. SCFAs are important anions in the colonic lumen and serve locally as nutrients for the mucosa cells, stimulating mucosal proliferation and blood flow. Especially butyric acid has been emphasized. It is the main energy substrate for the colonocytes and has been suggested to play a role in the prevention and treatment of diseases of the colonic mucosa, such as distal ulcerative colitis and cancer. SCFA production decreases the luminal pH, and may thereby stimulate mineral absorption and reduce secondary bile acid formation in the colon. Colonic generation of SCFAs has also been related to systematic and metabolic effects, e.g. SCFAs may influence the motility along the gastrointestinal tract and propionic acid has been suggested to inhibit the cholesterol synthesis from acetic acid in the liver. The SCFA formed at fermentation is quantitatively and qualitatively influenced by the type and amount of carbohydrate substrate. Further, certain combinations of carbohydrates may have synergistic effects on the SCFA pattern and may also shift the site of fermentation. This opens possibilities to design foods with tailored features regarding SCFA release in the human colon with potential health implications. There is a potential that in the future it will be possible to control SCFA production in the colon regarding pattern and place for release. Keywords: Carbohydrates, dietary fibre, fermentation, SCFA, resistant starch
Highlights
In Western societies, about 20-60 g of the daily carbohydrate intake escapes digestion and absorption in the small intestine and reaches the colon as a potential source for fermentation [1]
It is known that about 90% of these short-chain fatty acids (SCFAs) are rapidly absorbed by the colon, stimulating water and sodium absorption [3]
The content and distribution of SCFAs is dependent on the colonic microflora and/or the type and amount of indigestible carbohydrate available
Summary
In Western societies, about 20-60 g of the daily carbohydrate intake escapes digestion and absorption in the small intestine and reaches the colon as a potential source for fermentation [1]. The fermentation products, with carbohydrates as fermentation substrate, are mainly short-chain fatty acids (SCFAs; acetic, propionic and butyric acid) and gases (CO,, CH, and H,). These end products are either excreted in the stool or absorbed from the colon (Figure 1).In the 1960s,SCFAs were believed to be poorly absorbed, causing diarrhoea through osmotic fluid retention in the stool. The SCFAs absorbed from the colon can be utilized as an energy source by the host, but they contribute only to a small part (510%) of total energy in healthy individuals on Western diets [4]. The colonic mucosa obtains its energy by oxidizing mainly SCFAs in the order of butyric>propionic>acetic acid [5]. The results from studies examining the effects of propionic acid on cholesterol metabolism are not consistent
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