Introduction to Human Nutrition

Digestion and Metabolism of Carbohydrates 83

host, and return to the host the by-products of their
metabolism. The major substrate that the bacteria
receive from the host is carbohydrate, mostly in the
form of polysaccharides. They obtain nitrogen from
urea (which diffuses into the colon from the blood)
and undigested amino acids and proteins. Fermentation
is the process by which microorganisms break down
monosaccharides and amino acids to derive energy for
their own metabolism. Fermentation reactions do not
involve respiratory chains that use molecular oxygen
or nitrate as terminal electron acceptors. Most of the
fermentation in the human colon is anaerobic, i.e., it
proceeds in the absence of a source of oxygen. Different
bacteria use different substrates via different types
of chemical reaction. However, as a summary of the
overall process, fermentation converts carbohydrates
to energy, plus various end-products, which include
the gases carbon dioxide, hydrogen, and methane, and
the SCFAs acetic (C2), propionic (C3), and butyric
(C4) acids. Acetate, propionate, and butyrate appear
in colonic contents in approximate molar ratios of
60:20:20, respectively. Most of the SCFAs produced are
absorbed and provide energy for the body (Figure
5.3).
The roles of SCFAs in metabolism are discussed
later in this chapter. Formic acid (C1) and minor
amounts of longer chain SCFAs and branched-chain
SCFAs may also be produced. In addition, lactic
and succinic acids and ethanol or methanol may be
intermediate or end-products depending on the
conditions of the fermentation. For example, rapid
fermentation in an environment with a low pH results
in the accumulation of lactic and succinic acids.

The fi rst step in fermentation is the breakdown of

polysaccharides, oligosaccharides, and disaccharides

to their monosaccharide subunits. This is achieved

either by the secretion of hydrolytic enzymes by bac-

teria into the colonic lumen or, more commonly, by

expression of such enzymes on the bacterial surface

so that the products of hydrolysis are taken up directly

by the organism producing the enzyme. To degrade

the NSP of dietary fi ber, the bacteria may need to

attach themselves to the surface of the remnants of

the plant cell walls or other particulate material.

Once the monosaccharide is internalized, the

majority of carbohydrate-fermenting species in the

colon use the glycolytic pathway to metabolize carbo-

hydrate to pyruvate. This pathway results in the

reduction of NAD+ to NADH. Fermentation reactions

are controlled by the need to maintain redox balance

between reduced and oxidized forms of pyridine

nucleotides. The regeneration of NAD+ may be

achieved in a number of different ways (Figure 5.4).

Electron sink products such as ethanol, lactate,

hydrogen, and succinate are produced by some

bacteria to regenerate oxidized pyridine nucleotides.

These fermentation intermediates are subsequently

fermented to SCFAs by other gut bacteria, and are

important factors in maintaining species diversity in

the ecosystem.

Fate of short-chain fatty acids

Colonic fermentation can be viewed as a way in which

the human host can recover part of the energy of

malabsorbed carbohydrates. The amount of energy

recovered from fermentation depends on the fer-

mentability of the carbohydrate (which can range

from 0% to 100%) and the nature of the products of

fermentation. On a typical Western diet, about 40–

50% of the energy in carbohydrate that enters the

colon is available to the human host as SCFAs. The

rest of the energy is unavailable to the host, being lost

as heat or unfermented carbohydrate or used to

produce gases or for bacterial growth (Figure 5.5).

SCFAs are almost completely absorbed and, while

some butyrate is oxidized by colonocytes (the epithe-

lial cells lining the colon), most arrives at the liver via

the portal vein. Propionate and butyrate are removed

in fi rst pass through the liver, but increased concen-

trations of acetate can be observed in peripheral

blood several hours after consumption of indigestible

but fermentable carbohydrates. These absorbed

Carbohydrate

Pyruvate

60% Acetate

20% Propionate

20% Butyrate

CO 2

H 2

CH 4

Absorbed

and utilized Feces

Breath

Flatus

Figure 5.3 Overview of carbohydrate fermentation in the human
colon.