Page 17 - Petelin, Ana. 2020. Ed. Zdravje delovno aktivne populacije / Health of the Working-Age Population. Proceedings. Koper: University of Primorska Press.
P. 17
hed strongly affect their bioavailability. Bioavailability is a crucial factor in dietary polyphenols and their effect on the gut microbiota and human health 15
determining their biological activity in vivo (Manach et al., 2005).
The bioavailability of dietary polyphenols is, in general, low. Small
amounts of their intake (about 5-10 %) may be absorbed in the small intestine,
mainly those with monomeric and dimeric structures. The released aglycones
enter the enterocytes by passive diffusion. Once absorbed, polyphenols reach
the liver through the portal circulation. Here, they undergo biotransformation
via phase I (oxidation, reduction and hydrolysis) and phase II (conjugation) re-
actions. These transformations produce water-soluble conjugated metabolites
(glucuronide, sulphate and methyl derivatives) which are released in the sys-
temic circulation for subsequent delivery to organs and excretion by the urine.
More complex polyphenols, especially oligomeric, and polymeric structures,
reach the colon almost unchanged, where they are metabolized by the gut mi-
crobiota together with conjugates excreted into the intestinal lumen through
the bile. Here, they undergo microbial enzyme transformations, including
C-ring cleavage, decarboxylation, dehydroxylation, and demethylation. The re-
sult is the generation of less complex compounds such as phenolic acids and hy-
droxycinnamates (Corrêa et al., 2019).
Polyphenols and gut microbiota modulation
The human gut is an ecosystem of around 1013–1014 bacterial cells, participating
in several metabolic functions that the host cannot fulfil by itself. Microbiota
that colonize the distal regions of the colon represent the highest concentration
of microorganisms found in human body, as well as the most diverse. A har-
monious balance in their composition has been associated with maintaining
health and a higher life expectancy accompanied by a satisfactory quality of
life (Nicholson et al., 2012). The mechanisms by which the phenolic compounds
modulate the gut microbiota still remain to be elucidated, but may involve di-
rect and indirect interactions. Phenolic compounds could directly stimulate or
inhibit bacterial growth. Inhibition is closely related to the antimicrobial prop-
erties of these compounds and stimulation presumably associated with the ca-
pacity of the bacteria to metabolize them (Etxeberria et al., 2013). It could be
said that polyphenols possess a selective bacteriostatic or bactericidal effect,
inhibiting the growth of a wide range of potentially pathogenic bacteria and
slightly affecting or even promoting the beneficial microbial population.
Some microbiota members are preferred to others due to efficacy they
have shown in ameliorating the gut ecosystem with positive effects at the lo-
cal and systemic levels. For this reason, most studies have focused on the ef-
fects of polyphenols on Bifidobacterium and Lactobacillus, which have been
observed to contribute to human health at different levels (Gibson, 2008). They
enhance gut barrier function, stimulate the host immune system, prevent di-
arrhoea or allergies, contribute to activation of provitamins, and modulate li-
pid metabolism (Burcelin et al., 2012; Gibson, 2008). However, there are other
bacterial species associated with negative implications, such as Clostridium dif-
determining their biological activity in vivo (Manach et al., 2005).
The bioavailability of dietary polyphenols is, in general, low. Small
amounts of their intake (about 5-10 %) may be absorbed in the small intestine,
mainly those with monomeric and dimeric structures. The released aglycones
enter the enterocytes by passive diffusion. Once absorbed, polyphenols reach
the liver through the portal circulation. Here, they undergo biotransformation
via phase I (oxidation, reduction and hydrolysis) and phase II (conjugation) re-
actions. These transformations produce water-soluble conjugated metabolites
(glucuronide, sulphate and methyl derivatives) which are released in the sys-
temic circulation for subsequent delivery to organs and excretion by the urine.
More complex polyphenols, especially oligomeric, and polymeric structures,
reach the colon almost unchanged, where they are metabolized by the gut mi-
crobiota together with conjugates excreted into the intestinal lumen through
the bile. Here, they undergo microbial enzyme transformations, including
C-ring cleavage, decarboxylation, dehydroxylation, and demethylation. The re-
sult is the generation of less complex compounds such as phenolic acids and hy-
droxycinnamates (Corrêa et al., 2019).
Polyphenols and gut microbiota modulation
The human gut is an ecosystem of around 1013–1014 bacterial cells, participating
in several metabolic functions that the host cannot fulfil by itself. Microbiota
that colonize the distal regions of the colon represent the highest concentration
of microorganisms found in human body, as well as the most diverse. A har-
monious balance in their composition has been associated with maintaining
health and a higher life expectancy accompanied by a satisfactory quality of
life (Nicholson et al., 2012). The mechanisms by which the phenolic compounds
modulate the gut microbiota still remain to be elucidated, but may involve di-
rect and indirect interactions. Phenolic compounds could directly stimulate or
inhibit bacterial growth. Inhibition is closely related to the antimicrobial prop-
erties of these compounds and stimulation presumably associated with the ca-
pacity of the bacteria to metabolize them (Etxeberria et al., 2013). It could be
said that polyphenols possess a selective bacteriostatic or bactericidal effect,
inhibiting the growth of a wide range of potentially pathogenic bacteria and
slightly affecting or even promoting the beneficial microbial population.
Some microbiota members are preferred to others due to efficacy they
have shown in ameliorating the gut ecosystem with positive effects at the lo-
cal and systemic levels. For this reason, most studies have focused on the ef-
fects of polyphenols on Bifidobacterium and Lactobacillus, which have been
observed to contribute to human health at different levels (Gibson, 2008). They
enhance gut barrier function, stimulate the host immune system, prevent di-
arrhoea or allergies, contribute to activation of provitamins, and modulate li-
pid metabolism (Burcelin et al., 2012; Gibson, 2008). However, there are other
bacterial species associated with negative implications, such as Clostridium dif-
