Definition of Energy Terms56

Gross energy or heat of combustion Hc

Net energy NE

Heat energy released by complete combustion of the nondigestible carbohydrate, i.e., heat of combustion (17.2 kJ/g)

Gross energy corrected for fractional availability of energy from the portion of the nondigestible carbohydrate that is fermented (f)

1. What is the percentage of the ingested dose (and thus the energy intake) that is absorbed or hydrolyzed in the gastrointestinal tract?

2. What percentage of the ingested dose is fermented by the intestinal microflora?*

3. What is the efficiency of microbial biomass production, and how much of the C atoms and energy in inulin-type fructans are lost during the fermentation in the intestine?

4. What is the ATP yield (number of mols of ATP produced) of the metabolism of the fermentation end products by the host?

Answering the last two questions is, by far, the most difficult part. The fermentation process is complex and likely to be dependent on the composition of the intestinal microflora which varies among individuals. One approach is the "theoretical" approach.8 It is based on the hypothesis that inulin-type fructans are specifically metabolized by bifidobacteria (see Chapter 9 for more discussion of that hypothesis) that are known to have a unique carbohydrate metabolic pathway. Bifidobacteria lack both aldolase (EC 4.1.2.13) and glucose-6-phosphate NADP+ oxidoreductase (EC 1.1.1) but, instead, use phosphoketolases (EC 4.1.2.22 and 4.1.2.9) in the so-called "bifid shunt" (see Chapter 5, Section 5.3.3.2 and Figure 5.4).9 The theoretical approach, then, uses the following steps and subsequent calculations (Figure 7.1):

1. Calculate the value, Equation (7.3), for fructose (and thus inulin-type fructans) oxidation by bifidobacteria according to the stoichiometry of the bifidus pathway.10

2. Calculate the value, Equation (7.4), for the fermentation of fructose (and thus inulin-type fructans) by the mixed colonic microflora, by reference to the metabolic pathways that further oxidize the end products of the metabolism of this carbohydrate by bifidobacteria (especially lactate and pyruvate)1112 (see Chapter 5, Section 5.3.3 and Section 5.3.4).

3. Based on Equation (7.4), calculate the percentage of the C atoms from the fermented fructose that are recovered in SCFAs, lactate, and CO2.

4. Calculate the percentage of C atoms produced by fructose fermentation and metabolism of end products that are used for bacterial growth.

5. By combining steps 3 and 4, calculate the overall balance of fructose fermentation by the colonic microflora in terms of SCFAs, lactate, CO2, and bacterial biomass.

6. Based on published information, estimate the percentage of SCFAs and lactate that are absorbed and reach the host's metabolic pools.

* In the present discussion, the expression "fermentation by intestinal microflora" is used to take into account the fact that the small intestine, even in humans but certainly in most pets and domestic animals, is not germ-free and thus may, along with the large bowel, contribute to fermentation of nondigestible food components.

7. Based on metabolic charts including both catabolic and anabolic pathways for the major SCFAs (i.e., acetate, propionate, and butyrate), calculate the yield of ATP.

8. Summing up of all the information, calculate the energy value of fermented fructose (and thus inulin-type fructans).

A similar approach has been taken by an expert group of FASEB/LSRO13 for evaluating the energy content of polyol. These experts have applied the factorial method which uses the following formula:

NE = (A □ B) □ (1 □ C) □ (1 □ D) □ E (7.3)

In this equation:

• A = the energy (kcal or kJ) present in the intestine as fermentable substrate

• C = the proportion of C atoms going into bacterial biomass

• D = the loss of C atoms and energy due to fermentation

• E = the efficiency of utilization of the fermentation end products by the host compared to glucose

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