Introduction to Human Nutrition

84 Introduction to Human Nutrition

Hexose

NAD+

NAD+ NAD+

2 ATP

ATP

ATP ATP

CO 2

NADH 2

NADH 2

NADH 2

CH 4

H 2

H+

Lactate Pyruvate Succinate

Ethanol Acetyl CoA Biotin CO^2

Biotin

Acetate Butyrate

Propionate

Figure 5.4 Summary of biochemical

pathways used by the anaerobic

bacteria in the colon. Acetyl CoA, acetyl

coenzyme A.

Carbohydrate

entering the colon

(17.2kJ/g)

SCFA

(7.2kJ/g)

H 2 and CH 4
(0.6kJ/g)

Heat

(0.6kJ/g)

Unfermented

carbohydrate

(5.2kJ/g)

Bacterial

matter

(3.6kJ/g)

Feces

(8.8kJ/g)

Colon

Figure 5.5 Quantitative fate of carbohydrate in the colon. SCFA,
short-chain fatty acid.

SCFAs are readily oxidized and contribute modestly
(up to 10%) to the body’s energy supply.
There is considerable interest in the possible effects
of individual SCFAs on the health of the colon and
the whole body. The strongest evidence to date is for
an anticancer effect of butyrate, which may be due to

the ability of butyrate to induce differentiation and

apoptosis (programmed cell death) of colon cancer

cells. There is some support for the hypothesis that

propionate may help to reduce the risk of cardiovas-

cular disease by lowering blood cholesterol concen-

tration and/or by an effect on hemostasis, but the

evidence so far is not conclusive.

5.5 Carbohydrates and dental caries

The resident bacteria in the mouth ferment carbohy-

drates to yield acidic end-products (mainly lactic acid

but also some formic, acetic, and propionic acids),

which result in a drop in dental plaque pH. When the

pH falls below 5.5, the dental enamel dissolves in the

plaque fl uid and repeated exposure to periods of very

low pH can lead to caries. Not all carbohydrates are

equally cariogenic. The sugars found commonly in

human foods, e.g., sucrose, fructose, glucose, and

maltose, are all readily fermented by bacteria in the

mouth. Lactose, galactose, and starches are less cario-

genic, while sugar alcohols such as xylitol (used as a

sweetener in some confectionery and chewing gums)

are noncariogenic. Eating sugars with meals reduces

the risk of caries, as does the consumption of cheese,

which provides phosphates to prevent demineraliza-

tion and to encourage demineralization of the enamel.

Fluoride ingestion in foods and drinking water or