## 2 Materials and Methods
#### 3.1.4 Nutrients Digestibility  
The PFAs exert different biological activities that are related to intestinal tract functions [12]. In fact, a lower nutrient digestibility could be related to the dietary content of polyphenols, which may affect both protein and fat digestibility.  
The GP (1.5% of the diet) compared with VitE inclusion in broilers diet did not modify apparent ileal fat digestibility (AIDF) [14]. Nevertheless, the dietary supplementation with GP (3% and 6% of the diet) compared with VitE reduced (by 3.9% on average) the AIDF, whereas the apparent ileal protein digestibility (AIDP) was not affected [14]. The lower AIDF might have been related to the increase of both hydrolysable polyphenols (HP) and condensed tannins (CT) intake, dependent on the use of GP. In fact, broilers fed with the highest inclusion level of GP compared to broilers receiving the lowest level had obviously higher HP (by 18.35%) and CT (by 289.4%) total intake. Moreover, the GP addition at high levels (3% and 6% of the diet) increased HP total intake (by 8.7% and 23.3%, respectively) compared with basal and VitE rich diets [14]. Ileal digestibility and fecal digestibility of HP (IDHP and FDHP, respectively) were reduced by increasing GP inclusion in broilers diet. The IDHP was lower (by 16.0% on average) compared with both control and VitE groups, whereas FDHP was lower (by 16.8%) compared with control and VitE treated animals [14]. The same findings were observed for the AIDP (by a mean value of 6.62%) in the UGS (6% of the diet) fed animals. Moreover, UGS dietary addition (3% and 6% of the diet) reduced ileal total extractable polyphenols (TEP) compared with the basal (by a mean proportion of 18.4%) and the VitE (by an average amount of 22.3%) fed diet [16].  
On the other hand, FGS did not significantly affect the above parameters. Broilers supplemented with FSE (0.1% of the diet) for 42 days exhibited greater apparent digestibility of crude protein (on average by 3.3%) and calcium (on average by 5.5%) compared with those consuming basal and VitC diets [36]. The RL (2.5% of the diet) compared with rosehip administration for 5 weeks reduced (by 9.1%) the nitrogen metabolizability of broilers diet and the metabolizable energy (by 6.4% on average) compared with the control, the VitE, and the nettle groups [17].  
A greater apparent digestibility of energy (by 3.9%) and phosphorus (by 12.2%) in FSE fed animals was found [36]. The dietary supplementation with OLE (0.4% of the diet) in HS broilers for 14 days lowered apparent digestibility of energy (by 8.5%) compared with VitE fed animals but not in comparison with those consuming the basal diet where this parameter was increased (by 2.7%). The OLE fed animals (0.4% of the diet) showed a reduction in apparent digestibility of crude protein (by 14.5%), ash (by 22.7%), and phosphorous (by 15.7%) [26].  
From the studies reported above, it appears that the only positive effects on protein and mineral digestibility were related to the use of _Forsythia suspensa_ extract, while negative effects on this parameter were exerted by grape pomace, unfermented grape skin, rosmears leaves, and olive leaves extract. As already highlighted, the lower digestibility might be related to the increase of both hydrolysable polyphenols and/or condensed tannins intake associated with the use of PFA.  
Effects on Metabolic, Hematological Parameters, and Antioxidant Status (Table S2 in Supplementary Materials)