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close this bookActivity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)
close this folderThe energy requirements of growth and catch-up growth
close this folder8. Extent to which colonic fermentation of carbohydrates contributes to energy requirements in childhood
View the document8.1. Colonic fermentation
View the document8.2. Energy from SCFA
View the document8.3. Factors influencing SCFA production
View the document8.4. Gross versus metabolizable energy
View the document8.5. Faecal energy and non-starch polysaccharide
View the document8.6. Faecal energy in cystic fibrosis

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.