## 2 Materials and Methods
#### 3.2.1 Serum Biochemical Parameters  
Serum lipid profile (cholesterol -CHOL triglyceride -TG high-density lipoproteins (HDL)) was evaluated in laying hens when clove bud powder (CBP; 0.2% and 0.4% of the diet) or VitE (200 mg/kg) were incorporated in diets with high (HFA) or low (LFA) n-6 to n-3 fatty acids ratio (n-6:n-3) [50]. The addition of CBP (0.2% of the diet) or VitE in an HFA diet decreased CHOL to the same extent (by 17.1% and 18.5%, respectively). However, the highest inclusion level of CBP compared with VitE and basal diets was more effective in decreasing (by an average of 27.90%) CHOL. By contrast, VitE, when incorporated in LFA diet, was more effective in decreasing CHOL (by 11.7%) compared with the unsupplemented dietary treatment. However, CBP addition in both HFA and LFA diets was more effective than VitE and unsupplemented treatments in reducing CHOL (by 16.0% and 25.8%, respectively). A linear decrease in serum CHOL by increasing the inclusion levels of CBP in laying hens' diet was observed. This could be related to the hypocholesterolem property that some plant molecules--usually found in essential oils--may exert [51; 52], mainly through the reduction of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoA), which is the rate-limiting enzyme involved in CHOL synthesis [53]. Moreover, CB essential oil contains several bioactive compounds, among which is eugenol, which was also demonstrated to possess antioxidant properties [54]. In broilers, the lower dose of GP (5%, 7.5%, and 10% of the diet) inclusion in an unspecified acid profile diet was not effective in decreasing total CHOL. The same results were observed using VitE (200 mg/kg). By contrast, GP (10.0% of the diet) compared with VitE or a basal diet increased HDL-C (by 50.1% on average). However, all the GP compared with control fed animals showed a significant decrease (by 31.0% on average) for LDL-C, whereas only the highest inclusion level of GP showed lower (by 28.6%) LDL-C compared with VitE [34]. As the authors suggested, the mechanism by which GP by-product could affect lipid metabolism is not well established yet. Nevertheless, the HDL-C increase in this study could be associated with the capacity of phenolic molecules contained in GP by-products to scavenge free radicals and consequently to reduce oxidized lipid products [5]. In contrast to these findings, the dietary inclusion of another vinery by-product in broilers, namely grape seed extract (GSE; 0.015-0.045% of the diet), did not have any significant effect on CHOL, HDL-C, or LDL-C parameters in comparison with basal and VitC (300 mg/kg) diets under thermoneutral conditions [40]. By contrast, under HS conditions, VitC and GSE (0.015% and 0.030% of the diet) fed animals, compared with the unsupplemented ones, showed lower CHOL (by an average of 8.4%) level. The lower level of GSE was not effective in decreasing CHOL concentration, whereas LDL proteins were lowered (by 27%) only with the highest inclusion amount in comparison with the basal diet. Similarly, the highest inclusion level of GSE compared with the basal diet resulted in lowering (by 10.7%) the very low density lipoproteins (VLDL). Based on these results, it seems that HS plays a key role in lipid metabolism of broilers, affecting also GSE impact, since stressors affect CHOL level in plasma of poultry by the stimulation of the adrenal grand, which induces lipolysis. Laying hens supplemented with green tea powder (GTP; 0.5% of the diet), GTE (1.5% of the diet), marigold powder (MP, 0.5% of the diet), or marigold extract (ME, 1.5% of the diet) presented lower serum LDL and CHOL levels in comparison with the unsupplemented animals, which expressed an effect similar to those consuming the VitE (200 mg/kg) diet. In addition, reductions (by 7.3% and 18.2%, respectively) of serum TG and HDL/CHOL were only observed for the GTE supplementation when compared with the basal diet [33]. Consistently, green tea (_Camelia sinensis_) polyphenols, such as catechins, were demonstrated to alter lipid metabolism in broilers' livers [55].