8.1. Colonic fermentation

During the past decade there has been an increased interest in the effects of dietary fibre on human metabolism and physiology. It is now widely appreciated that substantial fermentation of dietary carbohydrate takes place in the large intestine, but the contribution made by fermentation to the energy economy of the body is less clear. Fermentation is an important component of normal large-bowel activity (CUMMINGS and ENGLYST, 1987). It is the process whereby anaerobic bacteria (and yeasts) break down dietary and other substrates, principally carbohydrate, to obtain energy for growth and the maintenance of cellular function. A variety of potentially fermentable substrates may enter the large intestine. These include dietary fibre (non-starch polysaccharide, NSP), unabsorbed dietary residue (starches and sugars) and endogenous secretions and cellular debris (intestinal glycoproteins and mucopolysaccharides). The relative proportions of each of the substrates will vary with dietary intake, the extent of maldigestion or malabsorption and mucus production within the gut. The end-products of anaerobic carbohydrate breakdown are short-chain fatty acids (SCFA: acetic, proprionic and butyric acids), gases (hydrogen, carbon dioxide and methane) and energy. The energy is used by the intestinal microflora for growth, whereas most of the SCFA are absorbed through the colonic mucosa and may contribute to the host's energy supply. The output of SCFA in faeces is minimal (RUBENSTEIN, HOWARD and WRONG, 1969). The absorption of SCFA stimulates sodium and water absorption, thereby being an important contributor to salt and water homeostasis in the colon. The production of SCFA may be of importance in maintaining the health of the epithelium of the large bowel. The epithelial cells of the colon metabolize SCFA, especially butyrate, which is their preferred fuel (ROEDIGER, 1980). Following absorption, SCFA pass into the portal vein and thence to the liver where proprionate and some acetate are taken up. The remaining acetate may pass on to peripheral tissues, being metabolized by muscle.

8.2. Energy from SCFA

The total production of SCFA in the human intestine is unknown. The factors which influence both production and absorption have yet to be established. Attempts have been made to estimate SCFA production from a stoichiometric appraisal based on the formula for fermentation derived by MILLER and WOLIN (1979):

34.4 C₆H₁₂O₆ ® 64 SCFA + 34.23 CO₂ + 10.5 H₂O

It can be calculated, using molar ratios, that approximately 10 mmol of SCFA will be produced for each gram of carbohydrates broken down in the human colon, yielding approximately 12 kJ (3 kcal) as available energy. Consequently, even modest amounts of fermentation from dietary fibre alone would contribute to metabolizable energy intake (e.g., 10 g/d carbohydrate fermentation would yield 120 kJ (30 kcal)/d in the form of SCFA).

McNEIL (1984) has calculated the amount of carbohydrate fermented and SCFA produced on the basis of the growth requirements of intestinal bacteria (approximately 0.1 mol ATP is needed to generate 1 g dry weight of bacteria). As faeces from adults on a British diet contain an average of 15 to 20 g bacteria (dry) (STEPHEN and CUMMINGS, 1980), approximately 1.5 to 2.0 mol ATP would be needed to replace the daily faecal output of bacteria. As 1 mol hexose yields 5 mol ATP when metabolized anaerobically, 50 to 65 g of hexose are required to produce 1.5 to 2.0 mol ATP. Of this, 10 to 15 g may be derived from dietary NSP, the remaining 35 to 50 g must be derived from unabsorbed starches and sugars or endogenous secretions. The yield would be approximately 500 to 600 mmol of SCFA, with a total energy value of 600 to 750 kJ (144 to 180 kcal)/d. This represents approximately 75% of the original energy content of the carbohydrate; the remaining 25% may be used by the colonic microflora for growth. This kind of calculation has led to the view that between 6 to 10% of daily energy needs could be met by fermentation of the fibre typically consumed in the Western World.

8.3. Factors influencing SCFA production

The extent of the production of SCFA and the contribution to metabolizable energy intake appears to depend on several factors.

Firstly, the amount of dietary fibre and the degree to which it is digested. There has been considerable interest in the digestibility of different forms of dietary fibre within the human large intestine (CUMMINGS, 1984). However, the extent to which microbial fermentation takes place and the extent to which the end-products of fibre fermentation are absorbed and contribute to metabolizable energy intake remain an area of controversy. Increases in microbial cell excretion were observed in association with an increased intake of vegetable fibre from cabbage whereas the increased consumption of the much less digestible wheat fibre resulted in only a small change in faecal microbial excretion (STEPHEN and CUMMINGS, 1980). Thus, the higher the fibre intake, particularly that derived from beans, vegetables and fruits, the greater the potential contribution from colonic fermentation.

Secondly, the amount of unabsorbed carbohydrate delivered to the large intestine. Clearly, this may be increased as a result of maldigestion and malabsorption associated with disease (e.g., cystic fibrosis or lactose intolerance). However, it is now clear that some foodstuffs contain relatively large amounts of starch in a form that is resistant to digestion - both in vitro and by human digestive enzymes. A number of studies have demonstrated that not only does the resistant starch largely escape digestion in the human small intestine, but that other types of starch may also pass into the large intestine to undergo fermentation (CUMMINGS and ENGLYST, 1987).

Thirdly, the quantity and nature of endogenous material delivered to the large intestine is not clear. Mucus degradation by colonic microflora has been well documented, and the presence of bacterial sub-populations that produce extracellular glycosidases with the specific role of degrading complex oligosaccharides of mucin in the gut lumen have been identified (HOSKINS and BOULDING, 1981). It has not been possible to directly quantify mucus production and epithelial cell losses, hence the extent of the contribution made by fermentation of mucopolysaccharides and glycoproteins to metabolizable energy remains unknown. However, in circumstances where mucus production is substantially elevated (e.g., in cystic fibrosis), the potential capacity for energy to be salvaged might be substantial (see later).

Finally, what is the overall magnitude of the gross energy intake and to what extent can the intake satisfy the energy needs of the individual? The relative contribution made by colonic fermentation may be relatively small when the gross energy intake is high. In contrast, for individuals on a marginal intake of energy, or in whom requirements are elevated, the relative contribution will be greater and may be critical.

8.4. Gross versus metabolizable energy

Further support for the role that colonic fermentation may play in meeting the energy needs of the individual comes from balance studies in which the true metabolizable energy intake (gross intake minus faecal and urinary losses) has been compared with the metabolizable energy intake estimated from food composition tables. There are several approaches in use for calculating the metabolizable energy content of mixed diets from the foods and the nutrients they contain. In 1899, ATWATER and BRYANT published factors that could be applied to the protein (4 kcal/g), fat (9 kcal/g) and carbohydrate (4 kcal/g) content of different foods based upon balance experiments in human subjects. The factors were subsequently modified by MERRILL and WATT (1973) who stated that if the revised factors were used, the deviation between the true and calculated metabolizable energy intake would not exceed 5% of the true value for most diets. In the United Kingdom, a slightly different approach has been used (PAUL and SOUTHGATE, 1978). The methods differ in the approach adopted for the calculation of metabolizable energy from carbohydrate. Paul and Southgate gave the metabolizable energy as 'available carbohydrate' (effectively excluding NSP) x 3.75 kcal/g. In contrast, MERRILL and WATT (1973) used a range of values, 3.87 to 4.12 kcal/g for carbohydrate. Using the first approach, SOUTHGATE and DURNIN (1970) found that the contribution of dietary NSP to metabolizable energy could be disregarded on diets containing low levels of NSP, up to 32 g/d.

GÖRANZON and FORSUM (1986) showed that when the potential contribution made by dietary fibre (NSP) was excluded there was a consistent underestimation of the metabolizable energy for diets high in dietary fibre. They calculated that dietary fibre, derived mainly from cereals, would contribute 10 kJ (2.5 kcal)/g to metabolizable energy. The dietary fibre from beans, vegetables and fruits would provide 13 kJ (3.1 kcal)/g. These values are in general agreement with the findings from studies in ruminants where approximately 70-75% of the heat of combustion of NSP may be available for metabolism (WHISKER, MALTZ and FELDHEIM, 1988). If these figures are applicable in humans, the energy available from dietary fibre would be a maximum of 13 kJ (3.1 kcal)/g (CUMMINGS, 1981).

8.5. Faecal energy and non-starch polysaccharide

Whilst there is evidence that NSP may contribute to the metabolizable energy of an individual, there is also evidence that increasing the amount of NSP in the diet may also lead to an increased excretion of fat, nitrogen and energy; the result being a decrease in the apparent digestibility of fat and protein and a reduction in available energy (SOUTHGATE and DURNIN, 1970; WISKER, MALTZ and FELDHEIM, 1988). In these studies, the total increase in energy losses associated with the increased intake of NSP exceeded the gross energy contained in the NSP itself. On the basis of these observations, FAO/WHO/UNU (1985) have proposed that no extra correction needs to be made to the metabolizable energy derived using Atwater factors, when the diet contains small amounts of dietary fibre. With increasing amounts of dietary fibre the calculated metabolizable energy should be reduced by about 5%. It is proposed that metabolizable energy may need to be further reduced when the consumption of dietary fibre is high: of the order likely to be ingested in many developing countries. This recommendation should not be applied uncritically, without an improved understanding of the potential contribution made by colonic fermentation to metabolizable energy and the nature and origin of the energy within the stool.

8.6. Faecal energy in cystic fibrosis

One example where the colonic fermentation of carbohydrate may be of importance both in the recovery of dietary energy and in reducing the energy losses associated with endogenous secretions, is cystic fibrosis (CF). We have studied a series of 16 children with CF on pancreatic replacement therapy and 20 healthy control children between 6 and 20 years of age. Gross and metabolizable energy intakes were estimated from 7-day records of weighed food intake using food composition tables and either heat of combustion values (MERRILL and WATT, 1973) or the modified Atwater factors (PAUL and SOUTHGATE, 1978). The faecal energy excretion was measured over 3 days. The microbial mass within the stool was determined according to the method of STEPHEN and CUMMINGS (1980). The results are summarised in Table 4.

Table 4. Studies were carried out in 16 children with cystic fibrosis, and 20 healthy controls between the ages of 6 and 20 years. Gross and metabolizable energy intakes were estimated from 7-day records of weighed food intake, using food composition tables and either heat of combustion values or the modified Atwater factors. The faecal energy excretion was measured over 3 days, and the microbial content of the stool was determined

1 Assumes heat of combustion of dietary fibre = 17.5 kJ/g.

² Energy content of pooled CF microbial mass = 23.8 kJ/g. Energy content of pooled control microbial mass = 20.4 kJ/g.

³ Assumes 0.1 mol ATP to synthesise 1 g of microbial mass and that 33 g hexose fermentation will generate 1 mol ATP.

⁴ Assumes 17.5 kJ/g hexose.

⁵ Total energy yield minus faecal microbial energy.

Gross energy intakes were comparable between both groups with the potential energy from dietary NSP contributing approximately 3 to 4% of the gross energy intake. However, in CF children the faecal energy losses were considerable, substantially reducing the available energy. Faecal mass was nearly 3 times greater in the CF group, with a corresponding increase in the microbial mass in the stool. The microbial mass accounted for approximately 35% and 25% of the dry mass of the stool in CF and controls respectively. This percentage is lower than that reported for adults by STEPHEN and CUMMINGS (1980) and may reflect the higher intake of non-digestible polysaccharides in their subjects.

In order to estimate the amount of energy within the microbial mass, a portion of the dried microbial mass from each sample was pooled for both CF and control subjects and the energy content was determined by bomb calorimetry. Using these values (approximately 20 to 25 kJ (4.8 to 6.0 kcal)/g dry microbial mass), the daily excretion of energy could be estimated. It is clear that both the total faecal energy and the energy within the microbial mass of the stool were substantially greater than the energy within the dietary NSP consumed by the CF subjects. This would imply that a substantial amount of substrate other than dietary NSP had been fermented. Using the same approach as McNEIL (1984), it was possible to estimate the carbohydrate fermented and the total energy yielded by that fermentation. Subtraction of the energy within the faecal microbial mass from this total energy yield provides a value for the amount of energy that may be made available through SCFA absorption. From these calculations it would appear that colonic fermentation may provide an additional 7.3% energy (range 3.0 to 12.5%) in the CF group compared to 2.4% in the control group (range 1.0 to 3.6%).

This interpretation indicates that the colonic fermentation of carbohydrate affords the opportunity to recover, at least in part, some of the energy from either unabsorbed starches and sugars or endogenous secretions that would otherwise be lost in the stool in children with cystic fibrosis.