Detrimental is through the inactivation of digestive

effects of grape polyphenols

Irrespective of
a great deal of information claiming health-promoting properties of various
polyphenols, it should be mentioned that not all flavonoids are necessarily
beneficial, and their physiological effects depend on a range factors including
type, concentration, absorption and metabolic transformation, etc. Indeed, the
dual role of this substance by producing either toxic or beneficial effects
seems also to depend on doses and/or the experimental cell type (Hodek et al.

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           It should be mentioned that in some
aforementioned studies, the authors did not pay much attention to the
detrimental consequences of grape extract feeding. In particular, decrease
chicken weight gain and increase feed conversion ratio (FCR)
, while vitamin E supplementation improved FCR. Furthermore, inclusion GPC into
the chicken diet decreased fat digestibility, while vitamin E supplementation
improved fat digestibility (Brenes et al. 2008). In other studies, it
has been observed that condensed tannins could bind biliary salts, a limiting
factor for efficient fat digestion in poultry (Krogdahl,1985), with a
concomitant reduction in their absorption and an increase in the faecal
excretion in mice (Roy and Schneeman, 1981). Another mechanism whereby
nutrients are rendered less digestible by polyphenols is through the inactivation
of digestive enzymes. For example, proanthocyanidin extracts from bean greatly
inhibited digestive enzymes (trypsin,a-amylase and lipase) in young chicks
(Longstaff and McNab, 1991). Moreno et al. (2003) also demonstrated in
vitro the inhibitory effects of GSE on fat-metabolizing enzymes and lipoprotein
lipase. There is some evidence to show that polyphenols can inhibit a range of
enzymes including ?-glucosidase and pancreatic lipase (You et al. 2011),
?-amylase and ?-glucosidase activity (Yilmazer-Musa et al. 2012),
alpha-amylase and alpha-glucosidase (McDougall et al. 2005). The
inhibition of digestive enzymes may be explained with the ability of condensed
tannins to form insoluble complexes with proteins in the gastrointestinal tract
(Griffiths, 1986;Horigome et al. 1988).