Short-chain fatty acid formation at fermentation of indigestible carbohydrates

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 cmphazised. 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 SCFAformed 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 apotential that in the future it will be possible to control SCFAproduction in the colon regarding pattern and place for release.


Introduction
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).A considerable microbial flora, which amounts to 10" bacteria per gram content, is present in this organ.The colonic microflora contains about 400 to 500 bacterial species, but most are in small numbers (2).Most of the microorganisms are saccharolytic.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.However, it is now known that about 90% of these SCFAs are rapidly absorbed by the colon, stimulating water and sodium absorption (3).Fermentation of indigestible carbohydrate to SCFAs thus reduces the osmotic load and there is also increasing evidence that the individual SCFA may have specific roles, including beneficial health implications.

Nutritional implications of short-chain fatty acids
The SCFAs absorbed from the colon can be utilized as an energy source by the host, but they contribute only to a small part (5-10%) 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 SCFAs that escape metabolism in the colonocytes enter the hepatic portal blood.Acetic acid is utilized by the liver where it is transferred into Acetyl-CoA, which can act as a precursor for lipogenesis (6), but also stimulates gluconeogenesis (7).Low concentrations of acetic acid can also be detected in venous blood in peripheral tissues (8).
SCFAs formed at fermentation of carbohydrates in the colon have been reported to affect carbohydrate metabolism (9).Thus, barley containing high amounts of fermentable carbohydrates improved glucose tolerance in healthy subjects compared to rice with a lower amount of indigestible carbohydrates.The effect may be ascribed to propionic acid.Propionic acid is mainly metabolized in the liver and has been shown to inhibit gluconeogenis and increase glycolysis in rat hepatocytes (10).It has also been proposed that propionic acid may lower plasma cholesterol concentrations by inhibiting hepatic cholesterogenesis (1 1).
However, the results from studies examining the effects of propionic acid on cholesterol metabolism are not consistent.Both lack of cholesterol lowering (12) and increased plasma cholesterol levels (13) have been observed in pigs caecally infused with propionate.In humans, however, the synthesis of cholesterol from acetic acid decreased when propionate was infused rectally (14), suggesting that the ratio of SCFAs formed upon colonic fermentation may also be of physiological importance.Butyric acid is the main energy substrate for the colonocytes (15) and is metabolized by the cells in preference to glucose or glutamine, accounting for 70% of the total energy demand of the colonic mucosa (1 6).Butyric acid has been reported to be important in the prevention and treatment of diseases of the colonic mucosa, such as distal ulcerative colitis (17) and cancer (18).A diminished oxidation of butyric acid in the colonocytes has been suggested to contribute to the genesis of ulcerative colitis (19).Several clinical studies have been performed, where enemas with an SCFA mixture (20,21) or butyrate alone (22) were associated with reduced symptoms, e.g., bowel frequency, rectal bleeding, and reduced histological scores, in ulcerative colitis patients.The effect appeared to be concentration-dependent, and lower concentrations of butyric acid had no effect (23).A recent intervention study in ulcerative colitis patients has demonstrated reduced symptoms regarding abdominal pain and gastroesophageal reflux when a diet rich in P-glucans from oats was consumed (Hallert C, Bjorck I, Nyman M, Granno C, Svensson H, unpublished results).The P-glucan rich diet was also associated with an increased faecal concentration of butyric acid; in fact, consumption of 10 g P-glucans in the form of oat bran specifically increased the faecal concentration of butyric acid with no effect on the concentration of other SCFAs.
Although butyric acid serves as the primary energy source for the normal colonic epithelium and stimulates growth of colonic mucosa, the growth of colonic tumour cell lines has been reported to be slowed down by butyrate (24).Butyrate also appears to reduce cell differentiation (25) and stimulate apoptosis (26) in tumour cell lines.In rat models, high butyric acid concentration in the colon from fermentation of wheat bran, resistant starch and fructooligosaccharides has been shown to be protective against carcinogenesis in the colon of chemically induced cancer models (27,28).However, Zoran and co-workers found that wheat bran reduced tumour incidence in a rat model independently of butyric acid concentration (29).
In addition to serving as a nutrient to the mucosa cells, SCFAs have other specific colonic effects such as increasing mucosal blood flow (30).Further, the decreased luminal pH induced by SCFA production may stimulate mineral absorption through increased mineral solubility (3 1,32).A lower luminal pH may also reduce secondary bile acid formation (33).Further, SCFAs have been shown to influence the gastrointestinal motility by mechanisms that are not yet fully understood (34).

Impact of substrate
The content and distribution of SCFAs is dependent on the colonic microflora and/or the type and amount of indigestible carbohydrate available.The bacterial species present in the colon use different fermentation pathways (35,36), leading to differences in the SCFA pattern generated.Indigestible carbohydrates that reach the colon are mainly non-starch polysaccharides (NSP), resistant starches (RS) and certain oligosaccharides.The fermentability of NSPis highly dependent on its physico-chemical properties.Soluble fibres, e.g.pectin and guar gum, are fermented more readily than insoluble fibres, e.g.cellulose and wheat bran.These types of NSP instead, create faecal bulk rather than contribute to SCFA production (37).RS can be classified into three major types (38): starch entrapped in the cell wall of plants and thereby physically inaccessible to aamylase (RS 1); starch stored in granules in the native crystalline form, which can be made accessible to enzymes by gelatinization (RS 2), and finally starch that has retrograded as a result of cooling of gelatinized starch (RS 3).Like NSP, different starches are fermented to various extents (39,40), and in rats retrograded starch has been found to be less fermented than native starch (RS2) (40).
Most mono-and disaccharides are rapidly absorbed in the 166 Scand J NutdNaringsforskning 4/01 upper intestinal tract and provide a readily available source of energy.An exception is lactose, a P-linked disaccharide, which is poorly absorbed by most of the adult population in the world due to low levels of lactase in the small intestine (41).When lactose is malabsorbed in the small intestine it reaches the colon for fermentation.Resistant peptides may also enter the colon and amino acid fermentation yields branched SCFAs (i.e.iso-butyric and iso-valeric acid) (42).
It is difficult to study fermentation in man due to the inaccessibility of the proximal colon where the main fermentation takes place.Human studies are thus generally based on indirect measures of fermentation, such as analysis of faecal SCFAs or breath hydrogen.Thus, in vitro and animal models are extensively used to study the formation of SCFAs from indigestible carbohydrates.In vitro studies, where the substrates are incubated with human faecal inocula, are easy to perform, but lack of standardization makes comparisons difficult (43).Further, the food needs to be subjected to in vitro digestion to mimic the action of the small intestine prior to the fermentation tests.
Rats and pigs are the most frequently used animals for fermentation studies.Pigs are colon fermenters, like man, and they also have a similar microflora to man (44).However, in pigs there is also a considerable fermentation in the ileum ( 4 3 , which may affect the results, and dietary fibre fermentation has been reported to be considerably higher in pigs than in man.In contrast to man, the rat is a caecum fermenter.However, the degree of fermentation of a variety of dietary fibres has been shown to correlate well between rat and man (46).Further, there are similarities in SCFApattern when comparing in vitro fermentation data for different dietary fibres (i.e.pectin and soybean fibre), using rat and human faecal inocula (47).
The molar distribution of acetic, propionic and butyric acid produced at fermentation of indigestible carbohydrates varies between substrates, as demonstrated in Table 1.Although the relative proportion of the organic end products from fermentation may vary, acetate is the major anion.Pectin is aparticularly good source for acetic acid (48,49), whilst arabinogalactan and guar gum are the poorest sources (48,50,51).By contrast, arabinogalactan and guar gum have been shown to give high amounts of propionic acid.Butyric acid production varies over a wide range.Starches have been shown to give high proportions of butyric acid in vitro (49,50,52).In humans it has been demonstrated that administration of an a-amylase inhibitor, acarbose, to subjects resulted in enhanced butyric acid in faeces, measured either as concentration or as percentage of total SCFAs (53).However, studies in rats (54)(55)(56) and pigs (45,57,58) have shown important variations in fermentation profiles for RS.An explanation could be that the production of butyric acid may vary for different sources of RS.In addition, the time allowed for adaptation of the microflora to the RS may have an impact on the SCFA pattern.We have found in rats fed raw potato starch, that an adaptation period of 4 w gave significantly higher proportions of butyric acid in caecum (19%) compared to an adaptation period of 2 w (6%) (Henningsson A, Nyman M, Bjorck I, unpublished results).A prolongation of the adaptation (6 w) did not have any further effect on butyric acid proportion.P-glucans and raffinose are other substrates that have been shown to give high amounts of butyric acid upon fermentation (48,49,52).
In contrast, fermentation of lactose with human faecal inocula in vitro was reported to give mainly acetic acid, and when the amount of lactose increased, the proportion of acetic acid increased further (59).Lactulose is an indigestible disaccharide and is often used to mimic lactose malabsorption.It is completely indigestible (60), and the fermentation pattern is identical to that of lactose (42).
Most studies have been performed on substrates tested as single sources of indigestible carbohydrates, which is not representative of a human diet that contains a complex mixture of carbohydrates.The combination of different indigestible carbohydrates may influence the fermentation pattern, and a mixture of pectin and guar gum gave a higher proportion of butyric acid in the caecum of rats (15%) compared to the individual substrates (10 and 6%, respectively) (Henningsson A, Bjorck I, Nyman M, unpublished results).Further, the site for SCFAgeneration in the colon is likely to be dependent on how rapidly fermented the carbohydrate is.Interestingly, it has been demonstrated that the fermentation of easily fermentable high-amylose maize starch could be shifted to the distal colon of rats, when fed in mixture with a slowly fermentable dietary fibre, e.g.psyllium (61).This is an interesting observation since it provides a dietary tool to increase butyric acid formation in the distal colon where most colonic cancers appear in humans (62).

Conclusions and future perspectives
There are indications that SCFAs may have health-promoting effects, both locally in the colon and systemically, e.g. on glucose and cholesterol metabolism.The design of carbohydrate foods that generate specific SCFApatterns at controlled sites in the large bowel is a challenge for the future with possible health consequences, e.g. in the prevention and treatment of colonic diseases.

Figure 1 .
Figure 1.Carbohydrate fermentation in the human colon.