Possible metabolic consequences of fermentation in the colon for humans

03CG9877/89/0029-0161/$10.00

Medul Hypotheses (19Sd) 29. 161-166 0 Lonaman Group UK Ltd 1989

Possible Metabolic Consequences in the Colon for Humans

of Fermentation

C. S. VENTER and H. H. VORSTER Departments

of Dietetics and Physiology,

Potchefstroom

University,

Potchefstroom,

South Africa

Abstract - We postulate that the short chain fatty acids, produced in the large gut by the microbial fermentation of dietary fiber, improve glucose tolerance and inhibit hepatic cholesterol and fibrinogen synthesis, probably by preventing an increase in serum levels of free fatty acids, and by improving insulin sensitivity. Since hypercholesterolemia, hyperfibrinogenemia and glucose intolerance are important risk factors for coronary heart disease, this could serve as a basis for recommendations that Western populations at risk should increase their dietary intake of substrates for short chain fatty acids.

Introduction

Increased intake of dietary fiber is associated with several beneficial effects on carbohydrate and lipid metabolism (1) and possibly also on haemostasis (2, 3, 4). Various mechanisms have been proposed to explain these effects (5). Dietary fiber influences the secretion (6), diges(8) and fermentation tion (7), absorption functions (9). as well as the morphologic structure of the gastrointestinal tract (10) and the transit time of food through the gut (11). This may lead to changes in blood nutrient dynamics and endocrine responses, which may influence cellular substrate availability and hormonal regulation of enzyme activities, with resultant changes in metabolism. Recently, questions have arisen about the physiological importance of short chain fatty acid metabolites of certain plant fibers (9). Some investigators have speculated that the short chain fatty acids (SCFA) have

metabolic effects that may explain some of the physiologic responses attributed to dietary fiber (5). The SCFA, acetic, propionic and butyric acid, are produced in large quantities by anaerobic bacteria in the human colon, mainly as a result of the breakdown of carbohydrate, a process usually referred to as fermentation. The main substrates are plant cell wall polysaccharides such as cellulose, pectins and hemicelluloses,, currently referred to as dietary fibers, and some starch that may escape digestion. Other substrates include protein (dietary as well as endogenous) and mucopolysaccharides (12). It is estimated that the daily production of SCFA is 200 to 300 mmol, in the molar ratios of acetate: propionate: butyrate of approximately 60:25: 15 (13). These SCFA are rapidly absorbed into the portal vein and transported directly to the liver, except for some butyrate which is used by

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MEDICAL HYPOTHESES

DemignC et al. (24) reported complete suppression of lactate utilization in isolated rat hepatocytes by propionate at concentrations close to those found in portal veins in rats fed a high fiber diet (0.5 mM). When propionate and butyrate were simultaneously present at higher concentrations (2.0 mM), the stimulating effect of butyrate on lactate utilization prevailed over the inhibitory effect of propionate. At lower concentrations, however, there was an apparent mutual neutralization of the effects of propionate and butyrate on lactate utilization. Since propionate uptake by liver cells in rats adapted to a high fiber diet was found to be 1.8-fold higher than butyrate uptake (24); it could be inferred that the effects of propionate might prevail in vivo. Acetate and butyrate appears to stimulate hepatic gluconeogenic rates largely by activating pyruvate carboxylase via acetyl CoA generation Effects of propionate on hepatic carbohydrate (27, 28, 29). Propionate, on the other hand, metabolism appears to lower hepatic acetyl CoA concentration (25), and inhibits pyruvate carboxylase The effects of propionate on glucose metabolism CoA and have been extensively studied in a number of activity further via methylmalonyl succinyl CoA, metabolites of propionate, which ruminant species. The majority of studies on monogastric animals have been carried out in are specific inhibitors of pyruvate carboxylase (26). Glycolytic rates are regulated by the rats. The rat is regarded as a useful experimental activity of phosphofructokinase. Citrate, which model for the prediction of fermentative breakdown and bulking capacity of dietary fiber in appears to be an inhibitor of this enzyme, is metabolites (29, 30). man (22). However, primates should be a more ’ reduced by propionate suitable model to investigate the association of Anderson and Bridges (23) conclude that the increase in glycolysis that they observed with fiber-depleted diets and Western life-style and valerate may be related to diseases, such as ischaemic heart disease. We are propionate decreased hepatic citrate concentrations. presently examining the influence of propionate In vivo studies by DemignC et al. (24) revealed supplementation of a Western diet in baboons. that rats adapted to a high fiber diet used mainly There are some data suggesting that propionate as gluconeogenic substrate during is a potent inhibitor of lactate conversion to propionate periods of starvation, as opposed to rats on a glucose in the liver (23-26). Anderson and diet who predominantly utilized Bridges (23) studied the effects of SCFA at fiber-free lactate. If the use of propionate as gluconeogenic concentrations ranging from 1 to 10 mM on rates substrate could be of such an extent as to postof glucose production from lactate and on glycolpone the rise in free fatty acid (FFA) ysis in rat hepatocytes. Physiological concentration in the serum, this could have a concentrations (0.15-0.3 mM) of propionate are beneficial effect on insulin sensitivity. High difficult to investigate with isolated hepatocytes, since this substrate could be exhausted within a serum concentrations of FFA are associated with few minutes. SCFA with even numbers of insensitivity to insulin and inhibition of glucose uptake by muscle cells (31). Increased intake of carbon atoms, acetate and butyrate. significantly dietary fiber, the main substrate for SCFA increased glucose production from lactate, increases insulin sensitivity in whereas propionate and valerate significantly production, humans (32, 33). Increased insulin responsivedecreased this process. The effect of these odd ness, as measured by percent insulin stimulation numbered SCFA on glycolysis also differed from of adipose tissue pyruvate dehydrogenase that of acetate and butyrate. Propionate and valerate significantly increased glycolysis, but activity, has also been reported in rats with increased dietary fiber intake, (34). Various acetate and butyrate had the opposite effect (23). colonic epithelial cells as respiratory fuel (14, 15). Butyrate also affect nucleic acid metabolism in colonic cells through its capacity to stabilize chromatin structure during cell division (16, 17). Absorbed butyrate and propionate are cleared by the liver and little appears in peripheral blood (18, 13). Acetate, however, passes into the circulation to peripheral tissues where it is metabolized by muscle (19, 20). Thus, the SCFA may influence intermediary metabolism in the colonic epithelium, liver and peripheral tissues. In this contribution we wish to summarize recent literature on the possible effects of propionate on carbohydrate and lipid metabolism in the liver. Also, we would like to examine the possibility that observed effects of dietary fiber on haemostasis (3, 21) may be mediated through the production and metabolic effects of propionic acid.

POSSIBLE METABOLIC

CONSEQUENCES

OF FERMENTATION

effects of guar gum, a water soluble fiber concentrate, on the activities of some key enzymes of carbohydrate and lipid metabolism have been reported in mouse liver (35). The authors speculated that these effects could be ascribed to changes in the rate of secretion of gastrointestinal hormones or the rate at which lipogenic precursers are supplied to the liver due to changes in t.he rate of their absorption from the intestine. We would like to propose a third possibility, namely the effects of propionate on liver metabolism. Effects of propionate on lipid metabolism

The hypocholesterolemic effect of soluble dietary fiber components is well documented (1). It has been suggested that this may be due, in part, to propionate derived from gut fermentation. Confirmatory results from various studies on animals have recently been summarized (36). Bush and Milligan (37) reported that the addition of propionic acid at concentrations of 15 and 30 mM to bovine liver homogenates inhibited 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase activity by 30% and 58% respectively, while Ide et al. (38) showed that short chain fatty acids lowered the activity of HMG-CoA reductase, the rate limiting enzyme of cholesterol synthesis in animal tissues. cholesterol synthetic rate in Consequently, isolated rat hepatocytes decreased by 45%. Inhibition of cholesterogenesis in isolated hepatocytes was also found by other investigators (39). In pigs, diets supplemented with 6.9 and 10.4% propionic acid reduced serum cholesterol by 14% (40). The decrease in serum cholesterol in this study occurred at the expense of HDLcholesterol. However, in a similar study with rats, 0.5% sodium propionate lowered serum and liver cholesterol concentrations without affecting HDL-C (41). In another study with pigs. the inclusion of 5% propionic acid in diets containing tallow reduced the rate of cholesterol synthesis (42). The metabolic effects of rapid intestinal absorption of dietary propionate and gradual colonic absorption of propionate derived from microbial fermentation of plant fiber may possibly not be the same. In rats fed oat bran, which is extensively fermented in the large bowel to SCFA, decreased serum and liver cholesterol was found by some authors (43), whilst raised hepatic hpogenesis and cholesterogenesis was found by others (44). It is important to note that

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163

with respect to cholesterol metabolism, propionate has a dual role. In addition to decreasing cholesterol concentration in serum, propionate also seems to be a substrate for cholesterogenesis, as has been demonstrated in the rat, mouse, chicken, cow and pig (45). All tissues tested used both acetate and propionate as cholesterol precursers, but the rate of propionate conversion was about 20% of the rate of acetate conversion to cholesterol. The net effect of propionate on cholesterol homeostasis will depend on the balance between its conversion to cholesterol and its regulatory effects on cholesterol synthesis, probably through inhibition of HMG-CoA synthase and HMG-CoA reductase. The physiological importance of the effects of propionate on cholesterol homeostasis in humans have not been investigated to the best of our knowledge. In addition to effects on cholesterol homeostasis, propionate also inhibits the synthesis of fatty acids, as shown in rat hepatocytes (41). Possible effect of SCFA on haemostasis

Abnormal haemostasis, characterised by hypercoagulability as a result of raised plasma fibrinogen levels and factor VII coagulant activity, appears to be an important risk factor, not only for thrombosis, but also for the development of atherosclerosis and coronary heart disease (46, 47). Evidence is accumulating that diet may influence haemostasis. Associations have been found between dietary fat and in vitro clotting time (48), bleeding time (49), and the activity of factor VII in plasma (50). Very little information is available regarding the influence of other dietary components on coagulation factors and clotting times. Studies done in hospitals in the UK showed that the incidence of post-operative venous thrombosis could be reduced by feeding patients high fiber diets (2, 51). Simpson and his colleagues (52) showed that an increased intake of dietary fiber, mainly as leguminuous vegetables, improved some coagulation factors of diabetic subjects. They failed to demonstrate an effect on fibrinogen levels. However, Koepp and Hegewisch (3) reported decreased plasma viscosity and fibrinogen levels in diabetic children after supplementation of their diet with guar gum. Supplementing a Westernised diet of baboons with konjacglucomannan lowered fibrinogen and factor X levels (21). Guar gum and konjac-glucomannan are both water soluble polysaccharides which are

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readily fermented by bacteria in the hind gut to SCFA. The possibility that SCFA may have an inhibitory effect on blood coagulation was first mentioned by Malhotra (53), an Indian physician, who found longer mean clotting time in men in Udaipur, North India, and soft, jelly-like clots in comparison to men in Madras in South India. Ruling out other possibilities, such as smoking, physical exercise and serum lipids, he concluded that these differences could be ascribed to differences in diet, with a preponderance of SCFA (milk, ghee and fermented milk products) and the presence of cellulose and vegetable fibers in the diets of the North Indian group. The mechanism whereby SCFA may influence the coagulation system is not clear. It is possible that by stimulating glycolysis (23) and inhibiting the release of fatty acids (41), propionic acid may inhibit the synthesis of fibrinogen and other coagulation factors in the, liver. Pilgeram and Pickart (54) have shown that raised circulating FFA levels stimulate fibrinogen synthesis. Interestingly, they demonstrated that shorter chain fatty acids (hexanoate) and unsaturated longer chain fatty acids (oleic and linoleic acid) induced a lesser, but still significant increase in fibrinogen synthesis than stearate and palmitate. Furthermore, the association between raised serum concentrations of FFA and insulin insensitivity (31) offers the possibility that fibrinogen synthesis may be influenced through a second mechanism. According to Regoeczi (55) fibrinogen synthesis may be controlled on an inhibitory level. We have recently proposed that’ a relative insulin deficiency (insulin resistance) with resultant increased circulating FFA may result in raised fibrinogen levels (4). Circumstantial evidence that supports this supposition, comes from the work of Antoniades and Westmoreland (56) who demonstrated that in vivo intravascular coagulation and thrombosis in the rat were more pronounced in fasted, diabetic and obese animals, all characterized by a relative insulin deficiency and raised FFA circulating concentrations. A possible inhibitory effect of the SCFA, by decreasing circulating FFA and increasing insulin sensitivity, on blood coagulation merits further investigation. Conclusion

It

seems

that

hypercoagulability,

especially

HYPOTHESES

raised fibrinogen levels and factor VII activity may play an important role, not only in thrombosis, but also in the development of atherosclerosis. Hypercholesterolemia and glucose intolerance are both known risk factors for atherosclerosis. Increased intakes of dietary fiber may protect against all three these risk factors. Amongst other, one of the possible mechanisms may involve the effect of SCFA, particularly propionic acid, which are produced in the large gut by microbial fermentation of dietary fiber, absorbed into the portal blood, and transported to the liver. In the liver, propionate may inhibit the synthesis of cholesterol, and may influence fibrinogen synthesis through effects on insulin sensitivity and FFA, thereby reducing the risk for coronary heart disease. References 1. Anderson J W. Fiber and health: an overview. The American journal of gastroenterology 81(10): 892-897, 1986. 2. Latto C. Postoperative deep-vein thrombosis, pulmonary embolism, and high-fibre diet (letter). Lancet 11: 1197, 1976. 3. Koepp P, Hegewisch S. Effects of guar on plasma viscosity and related parameters in diabetic children. European journal of pediatrics 137: 31-33, 1981. 4. Vorster H H. Venter C S, Silvis N, van Eeden T S. Huisman H W, Walker A R P. Dietary influences on haemostasis may affect risk for coronary heart disease. South African journal of science (in press), 1988. 5. Anderson J W, Chen W L. Plant fiber: carbohydrate and lipid metabolism. American journal of clinical nutrition 32, 346-363, 1979. 6. Eastwood M, Brydon G. Physiological effects of dietary fibre on the alimentary tract..p 105-131 in Dietary fibre, fibre-depleted foods and disease. (H Trowell, D Burkitt. K Heat&r, eds) Academic Press, ‘London, 1985. 7. Southgate DAT. The relation between composition and properties of dietary fiber and physiological effects. p 35-48 in Dietary fiber: basic and clinical aspects. (G V Vahounv. D Kritchevskv, eds) Plenum, London. 1986. 8. Jenkins. D J A. Wolever T M S. Leeds A R, Gassul M A. Haisman P. Diliwari J, Goff D V, Metz G L, Alberti K G M M. Dietary fibres, fibre analogues, and glucose tolerance: importance of viscosity: British medical iournal 1: 1392-1394. 1978. 9. Cummings J H. Fermentation in the human large intestine: evidence and implications for health. Lancet 1: 1206-1209, 1983. 10. Vahouny G V. Cassidy M M. Dietary fiber and intestinal adaption. v 181-209 in Dietarv fiber: basic and clinical aspects. (G V Vahouny,. D Kritchevsky. eds) Plenum, London. 1986. 11. Read N W. Dietary fiber and bowel transit. p 81-100 in Dietary fibre: basic and clinical aspects. (G V Vahouny. D Kritchevsky. eds) Plenum, London, 1986. in the 12. Cummings J H. Englyst H N. Fermentation human large intestine and the available substrates.

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METABOLIC

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OF FERMENTATION

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MEDICALHYPOTHESES

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