Binding of polyphenolic compounds toboth dietary and endogenous protein, such as digestive enzymes and proteinslocated at the luminal side of the intestinal tract, could be a reason of thereduced apparent digestibility of protein in polyphenol-containing diets.
Polyphenolscan form complexes with protein due to the interaction of their reactivehydroxyl groups with the carbonyl group of protein. As a consequence of thiscomplexation, protein and amino acid digestibility were reduced by theinclusion in chicken diets of sorghum and faba bean polyphenols (Jansman etal. 1989; Ortiz et al. 1993). Some polyphenolic compounds can alsoreduce the absorption of iron,and possibly other trace metals, when included ina diet. For example, grape seed extract inhibited Zn absorption (Kim et al.2011) or heme iron absorption in a dose-dependent manner (Ma et al.2011);(Ma et al.
2010), altered microbial protein fermentation and/oramino acid metabolism in human (Jacobs et al. 2012) or glucose uptake byintestinal cells or folic acid absorption (Lemos et al. 2007). Ingeneral, effect of polyphenol compounds on absorption and assimilation ofdifferent nutrients depends on many different factors including type ofcompounds, its dosage, combination with other compounds, etc. (Martel et al.2010). However, results of GP utilization on growth performance in broilerswere inconsistent. The pronounced and significant growth depression wasobserved with the use of a GSE reported by Hughes et al.
(2005) and Lauand King (2003). whereas Hajati et al. (2015) reportedGSE supplementation at the levels of 300 mg/kg diet improved body weight ofbroilers. Also, Ebrahimzadeh et al. (2017) reported thatsupplemental GP up to 10 % did not impair performanceof broilerchickens.
Ingeneral, relatively high dietary concentrations of polyphenols by the additionof these ingredients reduced performance in chickens as well as other livestock(Jansman et al. 1989; Nyachotti et al. 1997).
On the other hand,there is a need for further research in this exciting area.