Chemistry Reference
In-Depth Information
cholesterolgenesis. Levrat et al.
33
showed that high levels of propionic acid fermenta-
tions were present in the cecum of rats fed moderate amounts of inulin. Similarly,
Eberhard et al.
51
showed inulin supplementation in piglets decreased cecal acetate.
This suggests that one role of prebiotics or probiotics is to alter the proportion of
these breakdown products produced during fermentation. While intriguing, this fact
is controversial and does not seem to play a major role in the cholesterol-lowering
effects of prebiotics.
9
10.6.3 Increase in Cholesterol Excretion
Studies suggest that an interruption of the enterohepatic circulation of bile acids
and enhanced fecal excretion may have a major impact on the hypocholesterolemic
effect of prebiotics. In a study by Vanhoof and Schrijver,
52
normocholesterolemic
rats were fed a bread diet with cornstarch or 6 percent inulin in either cholesterol-
free diets or diets with 1 percent cholesterol and 0.1 percent cholic acid. There was
significant reduction in plasma cholesterol in those rats fed inulin and a cholesterol-
free diet. Also seen was a tendency toward greater fecal excretion of neutral ste-
roids. The authors speculated that the greater cholesterol excretion could be due
to a decrease in cholesterol absorption as a result of a higher viscosity in the upper
intestinal tract. Fecal loss results in higher hepatic cholesterol catabolism. This is
supported by an inverse relationship between liver cholesterol concentrations and
daily fecal bile acid excretion. Greater excretion is facilitated by a lower cecal pH as
seen in those rats consuming inulin. At a lower pH, the amount of soluble bile acids
decreases, resulting in less lipid absorption. A similar experiment with hypercholes-
terolemic rats, however, showed a tendency toward greater bile acid excretion, but no
effect on serum or hepatic cholesterol.
52
In humans, a study of 12 healthy volunteers fed short-chain fructo-oligosac-
charides for 4 weeks revealed an increase in fecal cholesterol concentration during
ingestion, and a return to baseline 4 weeks after completion of the study. This was
correlated with a rise in the number of fecal bifidobacteria and a decrease in fecal pH
during the study period, with a subsequent return to baseline.
53
10.6.4 Effect on bacterial Flora
Most prebiotics promote lactic acid-producing bacteria. As previously dis-
cussed, in animals the use of fermented dairy products to lower cholesterol has been
demonstrated. The combination of different types of bacteria, such as
Lactobacillus
acidophilus, L. casei,
and
Bifidobacterium bifidum,
may be responsible for the
cholesterol-lowering action of dairy products.
54
Although animal studies appeared
promising regarding the ability of probiotics to lower cholesterol, their effect in
humans is unclear. The mechanisms by which probiotics exert an effect were previ-
ously discussed.
