Herbivores obtain a considerable proportion of energy requirements from carbohydrate by the chain of anaerobic carbohydrate fermentation producing short-chain fatty acids that are absorbed then metabolized. The evidence for this sequence occurring in the large intestine of man is reviewed and estimated to produce 5 to 10% of human energy requirements. Further small amounts of energy may come from large intestinal absorption of fat and the bacterial breakdown products of protein.

SUMMARY Short chain fatty acid absorption from the human rectum has been studied in 46 subjects attending an obesity clinic, using a dialysis bag technique. From a mixed electrolyte solution, acetate concentrations fell from 97 0 to 64-2 mmol/l, and sodium from 97-8 to 85 1 mmol/l with respective net absorption rates of 8-1 and 5-2 [Lmol/cm2/h. From a solution with mixed short chain fatty acids acetate concentratio4 fell from 62-3 to 37-6 mmol/l, propionate from 20'2 to 11 '5mmol/l, and butyrate from 25*7 to 17-3 mmol/l with absorption rates of 5'2, 1 8, and 1 9 ttmol/cm2/h. Lowering pH from 7-2 to 5'5, to test the possibility that absorption occurred by passive non-ionic diffusion, had no effect on absorption rates, although pH rose rapidly in the dialysis fluid. These results are comparable with rates of acetate absorption from the animal large intestine. The hypothesis that short chain fatty acids are not absorbed from the large gut and therefore contribute to faecal bulk by retaining water in the bowel lumen may need revision.One of the best documented physiological actions of dietary fibre is its capacity to increase faecal bulk (Cummings et al., 1976;Mitchell and Eastwood, 1976). The greater part of this increase in faecal weight is water (Cummings et al., 1976). In vitro, fibre has been shown to have the property of holding water (McConnell et al., 1974) and it is this which is thought to account for faecal bulking. An alternative suggested mechanism is the retention in the colon of water in osmotic association with unabsorbed short chain fatty acids derived from the breakdown of fibre (Williams and Olmsted, 1936). This was thought to be the mechanism which led to diarrhoea in children with glucosidase deficiency described by Weijers et al., (1961).Short chain fatty acids (acetic, propionic, and butyric) are the major anions in adult human faeces (Rubinstein et al., 1969) and are thought not to be absorbed by the human colon (Bustos-Fernandez et al., 1971). However, many animal studies have shown that they can absorb short chain fatty acids rapidly from the colon (Myers et al., 1967;Henning and Hird, 1972;Argenzio and Southworth, 1975) and that both ruminant and monogastric species rely on short chain fatty acids as an important source of energy.'Present address, and address for correspondence: Department of Medicine, University College Hospital Medical School, University Street, London WC1E 6JJ. Received for publication 29 March 1978 819In the rabbit these contribute 30-40% of basal energy requirements (Parker, 1976). The fate of short chain fatty acids in the human colon is important. They may be involved in the control of faecal bulk, are a potential source of energy and, despite the major anion in the faeces, their effect on colonic water and electrolyte transport in man is unknown. We have, therefore, studied short chain fatty acid absorption from the human rectum using the dialysis technique of Edmonds (Edmonds, 1971) and found that they are rapidly absorbed. MethodsForty-six subjects (10 men, 36 ...

~_ _ _ _An in vitro faecal incubation system was used to study the metabolism of complex carbohydrates by intestinal bacteria. Homogenates of human faeces were incubated anaerobically with added lactulose, pectin, the hemicellulose arabinogalactan, and cellulose, both before and after subjects had been pre-fed each carbohydrate. Fermentation of added substrate was assessed by the production of short-chain fatty acids (SCFA) and suppression of net ammonia generation over 48 h of incubation. Control faecal homogenates to which carbohydrate was not added yielded an average increment of SCFA of 43 mmol/l, equivalent to 172 mmol/kg in the original stool. The addition of lactulose, pectin and arabinogalactan each increased the yield of SCFA by a similar amount, averaging 6.5 mmol/g carbohydrate or 1.05 mol/mol hexose equivalent; organic acid yield was not increased by pre-feeding these substances for up to 2 weeks. Acetate was the major SCFA in all samples at all times and, after pre-feeding with extra carbohydrate, butyrate concentrations exceeded propionate in all samples. Faecal homogenates incubated with cellulose showed no greater SCFA production than controls over the first 48 h, but there was a slight increase when samples from two subjects pre-fed cellulose were incubated for 14 d. Net ammonia generation was markedly suppressed by addition of lactulose to faecal incubates with an initial period of net bacterial uptake of ammonia. Pectin and arabinogalactan also decreased ammonia generation, but the reductions were not significant unless subjects were pre-fed these materials ; cellulose had no effect on ammonia generation.

SUMMARY To investigate factors which predispose to relapse in patients with ulcerative colitis, we conducted a survey to compare the events occurring in the four weeks preceding the clinic attendance of 62 outpatients in remission with those taking place in the same period before the onset of relapse in 21 patients attending with active disease. The only event which occurred significantly more often in patients who subsequently relapsed was ingestion of paracetamol and other inhibitors of prostaglandin synthesis (76% (16/21) relapse vs 39% (24/62) remission, p

1. The dietary intakes of sixty-three adults, randomly-selected from the electoral role of a large village near Cambridge, have been measured using the weighed-intake technique for 7 d. 2. Mean (+/- SD) daily intakes (g) for men and women respectively were: energy (MJ) 10.0 +/0 2.4, 8.2 +/- 2.1; fat 104 +/- 27, 90 +/- 27; protein 77 +/- 20, 67 +/- 16; carbohydrate 285 +/- 81, 229 +/- 74; sucrose 91 +/- 47, 57 +/- 33. 3. When interviewed at the end of the study 40% of subjects said they were watching their weight. 4. Women ate less food over all than men, and proportionately less potato and bread, and used only one-third as much sugar in drinks, probably in an attempt to control their weight. Men took considerably more alcohol than the women. In the age-group 20-39 years alcohol provided 9% (1.0 MJ/d) of the total energy intake in the men. 5. Wide variation in the intake of nutrients was observed amongst the individuals. For vitamin C and fibre intake this was partly partly explained by seasonal variation but for most nutrients total energy intake and food choice were the main determinants. The range of intakes of nutrients such as fat was similar in these individuals to that seen amongst countries internationally. It is suggested that if differences in nutrient intake amongst the various populations of the world can be associated with disease risk, then the same interpretation should be possible in individuals.