**2.2. Antioxidant status**

Due to the general consumer rejection of synthetic food additives there is a growing interest in studies of natural additives as potential antioxidants. Research on the antioxidative properties of herbs and spices showed that they are effective in retarding the process of lipid peroxidation in oils and fatty foods (summarised by [12]). Herbal phenolic compounds also improved the oxidative stability of animal derived products such as poultry meat, pork, rabbit meat and eggs (reviewed by [1]). Furthermore, research with rats [13] and poultry [14] suggested that dietary phytogenic supplements may improve the antioxidative status of the animals, reducing intestinal cell damage and sustaining the integrity of the intestinal mucosal layer. These supplements acted as effective free radical scavengers and also influenced the *in vivo* antioxidant defence systems in the animal. In addition, diets supplemented with turmeric, curcumin, green tea, grape seed proanthocyanidins and society garlic, which all possess antioxidative properties, reduced small intestinal lesion scores, lowered oxidative stress and improved weight gains during coccidial infection (summarised by [6]). All these compounds may exert their anticoccidial activity by protecting infected tissues from oxidative damage and therefore reducing the severity of coccidiosis.

#### **2.3. Immune status**

to exhibit growth promoting and/or therapeutic properties [1, 2]. Initially, PE were extensively studied because of the adverse effects they have when ingested by animals [3]. However, the use of PE as an alternative to in-feed antibiotics to prevent the risk of developing pathogens resistant to antibiotics and to satisfy consumer demand for a poultry food chain free of drug residues has gained recent interest [4]. Antibiotics have been added to poultry diets to maintain health and production efficiency in the last few decades [5]. The withdrawal of in-feed antibiotics as growth promoters have increased the risk of bacterial disease, especially in growing poultry [6]. The ability of PE to contribute to the health of the host is well documented [1]; however, the exact mechanisms by which PE exerts its effects remain speculative. As documented, PE are composed of a diverse group of natural products [7]. However, while some are nutritionally valuable, others have no nutritional value or even possess antinutritional properties. This is likely due to PE varying widely in their chemical structures [3]. Since the effects of PE depend to a great extent on the chemistry of the compounds, it is impossible to have a uniform explanation of their mode of action. This chapter provides a brief overview of the main benefits of adding selective PE to poultry diets. Specifically, it describes the mode of action of carvacrol, cinnamaldehyde and capsaicin when fed to broiler chickens as a standardised commercial mix.

It has been hypothesised that PE additives may stabilise overall digestive functions in the gastrointestinal tract of poultry, however, the available literature does not provide a consistent picture. Numerous feeding trials involving dietary supplementation with various PE have been reviewed [1, 6]. Regarding growth performance (assessed primarily as feed efficiency) and nutrient digestibility, the effect of added PE was beneficial in 11 of the studies; no effects

The information on the effect of PE on dietary metabolisable energy (ME) is also inconsistent. Some authors found an increase in dietary ME in response to PE [8, 9], others [10, 11] did not. There is also a discrepancy in the published data in the differences in ME and growth performance of birds fed dietary PE. Recent studies [10, 11, 8] found an improvement in bird growth performance but not in dietary ME when various PE were fed to poultry. It has been reported [9] that there is a parallel improvement in dietary ME and feed efficiency when feeding a standardised commercial mix of carvacrol, cinnamaldehyde and capsaicin to broiler chickens.

Due to the general consumer rejection of synthetic food additives there is a growing interest in studies of natural additives as potential antioxidants. Research on the antioxidative properties of herbs and spices showed that they are effective in retarding the process of lipid peroxidation in oils and fatty foods (summarised by [12]). Herbal phenolic compounds also improved the oxidative stability of animal derived products such as poultry meat, pork, rabbit meat and

were observed in 17 and there was a detrimental effect in 18 of the reviewed studies.

**2. Effects of dietary plant extracts when fed to poultry**

**2.1. Bird growth performance and dietary available energy**

104 Phytochemicals - Source of Antioxidants and Role in Disease Prevention

**2.2. Antioxidant status**

Immunomodulation is described as a change of the indicators of cellular and humoral immunity and nonspecific defence factors [15]. Immunomodulation can present as immunosuppression (substances that inhibit the immune system) or immunostimulation (substances that activate or induce the mediators or components of the immune system), thus regulating or altering the scope, type, duration or competency of the immune response [16, 17].

It has been speculated that the benefit of using PE in animal diets is associated with reduced intestinal inflammation in part from a reduction of proinflammatory cytokines. One study [18] reported that cinnamaldehyde suppressed the lipopolysaccharide-induced production of tumour necrosis factor (TNF), interleukin 6 (IL-6) and IL-1, thus suggesting that the inclusion of cinnamaldehyde could show suppressive effects on the production of various types of inflammatory cytokines. Similarly, [19] also found that a mixture of capsicum and turmeric oleoresins was an effective phytonutrient against clinical signs of experimental avian necrotic enteritis when supplied in dietary form. Research by Lee et al. [20] suggested that immunomodulatory effects are responsible for improved weight gain, oocyst shedding, increased interferon gamma (IFN-γ) and IL-15, when powder from oriental plum (a plant rich in phenolic compounds) was fed to coccidia challenged birds. Furthermore, supplementation with Chinese mushroom and herb extracts resulted in enhancement of both cellular and humoral immune responses in *Eimeria tenella* infected chickens [21].

There is strong evidence that PE have antimicrobial properties, being able to reduce the proliferation of pathogenic organisms at minimal inhibitory concentrations of 0.05–5 microliters per millilitre *in vitro*, and at higher concentrations (0.5–20 microliters per gram) in food [22]. While these levels are unlikely to be met in animal feedstuffs, and therefore not the primary use of PE in feed, there is evidence that PE have effective antimicrobial action against pathogens common in poultry production. These include *Escherichia coli* and *Clostridium perfringens* [23–25]. Pathogenic microorganisms in the gut are able to trigger immune responses in the gastrointestinal tract. This results in inflammation of the intestine contributing to poor gut health. In addition to reducing pathogenic challenge, PE may also possess direct immunomodulation properties. For example, it is known that cinnamaldehyde in particular is involved in gene regulation, including antigen presentation, humoral immune response and inflammatory disease [26].
