A review of the application of polyphenols in poultry

In poultry meat production birds face several challenges, all of which disturb the normal functioning of the organism, with the gastrointestinal tract being the most affected. This results in impaired absorption of nutrients, leading to reduced performance and increased mortality. Previously general practice was to feed antibiotics at sub-therapeutic levels to enable birds to cope with the challenges during growth (Wati et al. 2015). With the European Union passing legislation to ban the use of in-feed antibiotics in 2006, many other countries are pursuing this path, Australia included. This has been the main driving force to seek alternative ‘natural growth promoters’.

One of the main challenges Australian antibiotic free producers will face surrounds intestinal health, specifically prevention and control of coccidiosis and necrotic enteritis (Cervantes, 2015). Removing ionophore anticoccidials and antibiotic feed additives is certain to cause problems in controlling coccidial parasites and bacterial organisms, particularly Clostridium perfringens (Van der Sluis, 2000). Ideally, an alternative to in-feed antibiotics should have the same beneficial effects when included in the diets; it is generally accepted that in-feed antibiotics and growth promotors elicit some antibacterial actions and thereby reduce the incidence and severity of subclinical infections (Wati et al. 2015). Polyphenols are almost ubiquitous in plants, with certain polyphenols such as quercetin common among all plants, whereas other polyphenols are specific to particular food plants (Manach et al. 2004). Antimicrobial activity and immune enhancement are two major properties possessed by polyphenolic compounds that are essential for the health and wellbeing of poultry and which make polyphenols an ideal candidate as an alternative to antibiotics and ionophores. The objective of the present paper is to present an overview of polyphenols, their potential mode of action and the role that they can play in replacing in-feed growth promoters in broiler nutrition.

Polyphenols (phenolic compounds or phytogenic feed additives) constitute one of the most extensive groups of chemicals in the plant kingdom, with more than 8,000 compounds being isolated and described (Surai, 2014). Polyphenols are products of secondary metabolism in plants, and they arise biogenetically from two main synthetic pathways; the shikimate pathway and the acetate pathway (Harborne, 1989). Natural polyphenols can range from simple molecules such as phenolic acid to highly polymerised compounds such as tannins. They occur predominately in conjugated form, with one or more sugar residues linked to hydroxyl groups, although direct linkages of the sugar molecule to an aromatic carbon atom also exist (Manach et al. 2004; Bravo, 1998). The associated sugars can be present as monosaccharides, disaccharides or even as oligosaccharides, with glucose being the most common (Bravo, 1998).

Polyphenols are classified into different groups as a function of the number of phenol rings they contain, and on the basis of structural elements that bind these rings to one another. The main classes include; flavonoids, phenolic acids, tannins, stilbenes and lignans. Of these groups, flavonoids are the most widely abundant, with more than 4,000 varieties identified. Yang et al. (2009) further classified polyphenols based on their biological origin, formulation, chemical description and purity, and devised four main categories:

1. Herbs – products from flowering, non-woody and non-persistent plants

2. Botanical – entire or processed parts of a plant, e.g. roots, leaves, bark

3. Essential oils – hydro-distilled extracts of volatile plant compounds

4. Oleoresins – extracts based on non-aqueous solvents

There are numerous factors that affect the phenolic content of plants, including but not limited to; part of the plant used, ripeness, time of harvest, environmental factors, geographical origin, processing and storage (Huyghebaert et al. 2011; Windisch et al. 2008). Polyphenolic content in cereal grains is usually less than 1% of the dry matter, with the exception of sorghum (Sorghum bicolor) cultivars, which have as much as 10% phenolic content. Isoflavones are the main phenolic compound found in legumes, with the darker legumes such as red kidney beans and black beans tending to have higher polyphenolic content (Hermann, 1988).

Coccidiosis infection caused by Eimeria spp. is a common disease challenge facing poultry producers. Annually, this disease causes a global loss of 2.4 billion US dollars (Muthamilselvan et al. 2016). Anticoccidial chemicals, coccidiocides, coccidiostats and ionophores have long been used as mainstream strategies to control coccidiosis (Chapman et al. 2010). Despite the effectiveness of these products, there is a push towards banning and/or limiting their use; therefore, natural products are emerging as a potential way to combat coccidiosis. Currently, there are at least four commercially available plant-based products on the market, many of which contain phenolic compounds. In the literature, there are numerous studies demonstrating the positive effects of phenolic compounds on the prevention of coccidiosis infection. Jang et al. (2007) demonstrated that green tea polyphenols significantly inhibited the sporulation process of coccidian oocycts. This was further supported by Molan and Faraj (2015) who observed that the selenium and polyphenolic compounds in green tea extract were necessary for inactivation of the enzymes responsible for coccidian sporulation. Molan et al. (2009) investigated the used of pine bark extracts (Pinus radiata), a natural rich source of condensed tannins, on Eimeria spp. oocyst development. They reported a significant reduction of infectious oocysts in the environment, due to the ability of the condensed tannins to penetrate the oocyst and interfere with the endogenous enzymes responsible for sporocyst formation. Naidoo et al. (2008) compared the use of 4 plant phenolic compounds (Combretum woodii, Vitis vinifera, L. Artemisia afra and Tulbaghia violacea) with Toltrazuril, a veterinary grade anticoccidial used as a positive control. Dietary inclusion of Combretum woodii at 160 mg/kg was shown to be highly toxic to the birds, whereas dietary treatments containing Tulbaghia violacea (35 mg/kg), Vitis vinifera (75 mg/kg) and Artemisia afra (150 mg/kg) produced FCRs comparable to the anticoccidial drug. Birds that were fed Tulbaghia violacea showed a decreased in oocysts per gram of faeces compared to infected birds receiving no treatment. In addition to decreased shedding of oocysts, the lower levels were maintained for the duration of time birds were fed Tulbaghia violacea and were then seen to increase after treatment ceased. The promising results of polyphenols warrant further research into the use of these plant extracts as therapeutic or prophylactic anticoccidial agents.

Gut health has recently become a subject of interest in poultry research. The gastrointestinal tract is the pivotal organ which mediates nutrient uptake and use by the bird. The gut is also the main site of potential exposure to pathogens (Yengi and Korver, 2008). When gut function is impaired, digestion and absorption are affected, which in turn compromises health and performance of poultry.

A wide range of spices, herbs and their extracts with phenolic content are known to exert beneficial effects within the gastrointestinal tract (Chrubasik et al. 2005). Stimulation of digestive secretion, bile and mucus as well as enhanced enzyme activity is proposed to be one of the core modes of action of polyphenolic compounds (Platel and Srinivasan, 2004). As polyphenols come from a wide variety of plants, which vary in composition and content of active ingredient, it makes it difficult to understand the mode of action of all phenolic compounds, with some modes of action potentially only being possible when a defined combination of ingredients are available (Grashorn, 2010). However, there has been a lot of speculation as to the possible mechanisms through which polyphenols exert their beneficial effects on the gut (Windisch and Kroismayr, 2007). These include:

1. Modulation of the cellular membrane of microbes leading to membrane disruption of the pathogens.

2. Increasing the hydrophobicity of the microbial species which may influence the surface characteristics of microbial cells and thereby affect the virulence properties of the microbes.

3. Stimulating the growth of favourable bacteria such as lactobacilli and bifidobacteria in the gut.

4. Acting as an immunostimulatory substance.

5. Protecting the intestinal tissue from microbial attack.

Studies have shown that essential oils containing phenolic compounds enhance the activity of trypsin and amylase in broilers. Jang et al. (2007) tested the use of a commercially available blend of essential oils (29% active ingredients including thymol) at different inclusion rates against antibiotics. They observed that, when the fed was supplemented with essential oils and lactic acid, there was a significant increase in trypsin activity compared to the diet supplemented with antibiotics. Total and specific amylase activities were also observed to be significantly increased compared to the antibiotic dietary treatment. However, the dietary treatment group using only essential oils did not stimulate digestive enzyme activity, which is consistent with the findings of Lee (2002).

Saponin is a broad classification term used for amphipathic glycosides. In order for glycosides to be absorbed they must be hydrolysed into their corresponding aglycone, commonly taking place in the large intestine by caecal microflora (Manch et al. 2004). Saponins have been proposed to reduce intestinal ammonia formation, and thus pollution of the housing and environment (Francis et al. 2002). Studies by Killeen et al. (1998) and Duffy (2001) both observed that active phenolic compounds in Yucca schidigera were able to lower intestinal urease activity and enzymes involved in the metabolic urea cycle in rats, and Nazeer et al. (2002) further confirmed that Yucca extracts reduced intestinal and faecal urease activity when fed to broilers. Further research is warranted to clarify the potential use of saponins as feed additives in poultry feed.

Despite the varying results reported in the literature, the use of polyphenols for their antimicrobial and antioxidant properties in animal health is promising, with a wide application across multiple disease and health challenges. Future research into the antimicrobial and gut health properties of polyphenols in production animals is needed. As metabolic activity differs widely among polyphenols, safety needs to be assessed separately for individual polyphenol products. Studies on the interactive effect between polyphenols and enzymes are limited and further investigation is warranted if polyphenols are to use be as an alternative to in-feed antibiotics.