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Several methods exist to determine the energy concentration of dietary fibers. These were reviewed by Fahey and Grieshop (2000) and include gross energy determination, energy balance method, factorial calculation of the metabolizable energy value, calculation of the net energy of maintenance, indirect calorimetry, breath hydrogen determination, and radiolabel technique.
Because the process of fermentation is anaerobic, less energy is recovered from dietary fiber than the 4 kcal/g obtained from aerobic glycolysis. Fermentation balance equations and molar ratios of short chain fatty acids in human stool can be used to estimate that anaerobic metabolism yields 2 to 3 kcal/g of hexose fermented (Hungate, 1966; Miller and Wolin, 1979), a calculation that assumed that short chain fatty acids were actively absorbed from the large intestine and that those generated by one strain of bacteria were not utilized by another microbial species.
Available data suggest that neither of these assumptions apply. It appears that the short-chain fatty acid, propionate, is utilized by some bacteria and is, therefore, unavailable for absorption. Also, data suggest that in monogastric species, short-chain fatty acids are passively absorbed from the large intestine, meaning that only when the concentration is greater in the colonic lumen than in the adjacent tissue does absorption occur (Fleming and Yeo, 1990).
Absorption of short-chain fatty acids is closely linked to the movement of water and electrolytes from the lumen, and participation in the normal secretory and absorption activities in the colon is one of the important physiological functions of short-chain fatty acids (Argenzio et al., 1975). Short-chain fatty acids are the main anions in human feces (Høverstad et al., 1984). Although this has not been well documented, humans, in contrast to most animals that consume highly defined diets, consume excess electrolytes and protein, the latter requiring ample buffering capacity. The role of short-chain fatty acids in electrolyte and acid-base balance undoubtedly dominates over their absorption and subsequent use as an energy source. Therefore, it is not possible for anaerobic fermentation to generate 4 kcal/g, and it is unlikely that the theoretical yield of 3 kcal/g is absorbed from the large intestine. Indeed, data indicate that the average energy yield from dietary fiber fermentation in monogastric species is in the range of 1.5 to 2.5 kcal/g (Livesey, 1990; Smith et al., 1998).