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'C-atoms of hexose monomers recovered (%) as SCFAs. 2Numbers in () is the relative percent.

'C-atoms of hexose monomers recovered (%) as SCFAs. 2Numbers in () is the relative percent.

ways necessary for growth and proliferation.10 172021 According to De Vries and Stouthamer,10 Baldwin,22 Soergel,17 and Cummings,23 the fermentation of 1 mol of hexose supports the production of 37 g, 25-31 g, 15-60 g, or 40-57.5 g of bacterial biomass (dry weight), respectively (Figure 7.2).

In vitro experiments have shown that the fermentation of oligofructose by human fecal slurries yields 40% C atoms as SCFAs, 15% as lactate and 5% as CO2, thus leaving some 40% for bacterial (mainly bifidobacteria) growth.24 A similar figure has been reported by Baldwin,22 Payne,25 Isaacson et al.,26 and Cummings et al.,27 for the growth of mixed populations of bacteria fermenting a hexose substrate.

In vivo experiments with rats fed a diet containing 10-20% oligofructose have shown that the increased fecal excretion (dry weight) during a 24-h period corresponded to 41-47% of the C atoms ingested as oligofructose being incorporated into new bacteria.8 By applying similar calculations on data reported by Nakae et al.28 and Nyman et al.29 who fed rats mixtures of dietary fibers, the C atoms recovered in fecal bacterial biomass account for 45-55% and 39-54%, respectively.

Taken together, this information indicates that, in terms of C atoms, the intestinal fermentation of 1 mol fructosyl equivalent from inulin-type fructans produces:

40% SCFAs 15% lactate 5% CO2

40% bacterial biomass (mainly bifidobacteria)

FIGURE 7.2 Energy and C-atoms balance chart of fermentation of fructose by intestinal microflora.

The 40% figure for the yield of SCFAs is similar to that reported by Cummings et al.27 who have quantitatively calculated that the fermentation of 10 g nonstarch polysaccharides in the human hind gut is likely to produce some 75 mmol of SCFAs which, based on the recovery of C atoms, corresponds to a yield of approximately 45%. Such a figure falls within the extreme values proposed by Baldwin,22 and Bernier and Pascal,30 and it is close to the values proposed by Bar.19 Moreover, assuming that 40-45% of the C atoms of 1 mol of fermented fructose unit in inulin-type fructans are used to build up bacterial biomass is compatible with the ATP cost of the process (0.1-0.2 mol ATP/2-6 g bacterial biomass).31 In molar terms, this means that the fermentation of 1 mol fructose unit in inulin-type fructans gives about 1 mol SCFAs and about 0.3 mol of lactate. By reference to Table 6.3, the relative distribution of the SCFAs is 58% or 0.58 mol acetate, 20% or 0.2 mol propionate, and 22% or 0.22 mol butyrate.

7.3.4 ATP Yield of the Metabolism of the Fermentation End Products by the Host

Both CO2 and bacterial biomass are excreted. They do not participate in the cellular metabolism of the host, and they are calorie free. This is certainly not the case for SCFAs and lactate that are absorbed and used by host's cells to produce energy and serve as anabolic substrates. The questions that remain to be solved are thus:

1. What proportion of these molecules is absorbed and how much is excreted?

2. What is the efficiency in terms of ATP production and utilization of their cellular metabolism as compared to fructose?

7.3.4.1 Absorption and Excretion of SCFAs and Lactate

The intestinal absorption of SCFAs is a very efficient process that takes place mostly in the cecum and the colon.30-33 Only 5-10% are excreted in feces.3133 Of 90-95% of SCFAs absorbed, majority of butyrate is used by colonocytes for maintenance energy;3435 the totality of propionate is metabolized in the liver whereas acetate serves as metabolic substrate both for the liver (50-70%) and peripheral tissues (mainly muscles).3637

Lactic acid is also likely to be absorbed. Moreover, many different intestinal bacteria metabolize it. It has been estimated that some 50% of L-lactate (the only isomer produced by bifidobacteria) will reach the systemic circulation.8

7.3.4.2 Cellular Metabolism of SCFAs and Lactate and ATP Yield

SCFAs are metabolized at three major sites in the host body.

1. Cells of the ceco-colonic epithelium that use butyrate as a major substrate for maintenance-energy producing pathways

2. Liver cells that fully metabolize residual butyrate and propionate via gluconeogenesis and 50-70% of acetate

3. Muscle cells that generate energy from the oxidation of the residual acetate

Even though little information is available on the metabolism of L-lactate originating from intestinal carbohydrate fermentation, it is likely that it serves as substrate both for gluconeogenesis and lipogenesis; part is also excreted in urine.

The host cells' metabolic pathways that use SCFAs and lactate originating from intestinal carbohydrate fermentation are yet not fully understood. Nevertheless, the relative efficiency of utilization of their energy content compared to glucose or fructose can be estimated, on a theoretical basis, by using literature information.38 A theoretical approach has been published and is summarized by Roberfroid et al.8

Figure 7.3 shows an average ATP yield of 14 mol from the cellular metabolism of a typical mixture of 0.9-0.95 mol SCFAs and 0.075 mol L-lactate resulting from the fermentation of 1 mol of inulin-type fructan followed by its absorption through the intestinal wall. As the ATP yield from the complete metabolic oxidation of glucose or fructose is 38 mol ATP,38 the net energy content (see Equation 7.1) of inulin-type fructan is 14/38 □ 100 = 36.8% that of glucose or fructose, i.e.,

Similar figures have been reported for the ATP yield of SCFAs resulting from the intestinal fermentation of polyols.19

1 mol INULIN-type FRUCTANS

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