Multifunctional activity of vitamin E in animal and animal products: A review
Abstract
Vitamin E is an essential nontoxic fat-soluble micronutrient whose effects on livestock performance and products can be attributed to its antioxidant and nonantioxidant properties. Although it is needed in small quantity in the diet, its roles in livestock production are indispensable as it is required in boosting performance, nutritional qualities, and yield of animal and animal products. The dietary or oral supplementation of vitamin E is essential in reducing lipid oxidation in muscle, egg, and dairy products as well as lowering cholesterol concentrations and improving antioxidant status of livestock. Evidence has shown that bioavailability of vitamin E–enriched animal products could serve as an invaluable nutritional benefit to consumers; especially those in regions of limited resources where vitamin E deficiencies pose a risk that may be detrimental to some cellular activities of the body and on human health. It is therefore important to redirect research on the impact of vitamin E supplementation as antioxidant on livestock performance and animal products.
1 INTRODUCTION
Animal products (meat, milk, and eggs) represent one of the most frequently consumed foods in human nutrition because of their desirable taste, high nutritional value, availability, and accessibility. Their intake is important for growth, sound health, and smooth functioning of the body. For most countries, animal products are valuable sources of complete protein, including essential amino acids, fatty acids, and many other essential micronutrients, such as iron, zinc, calcium, vitamin A, E, and B1, as well as other bioactive compounds, which are often deficient in other food sources (Schonfeldt, Pretorius, & Hall, 2013). Besides these qualities, animal products are easily susceptible to oxidation and quality deterioration. This deterioration usually begins during the ante-mortem stage and it continues through the post mortem condition (Falowo, Fayemi, & Muchenje, 2014) due to the inability of inherent endogenous antioxidant to combat free radicals that are generated during the process.
However, the susceptibility of animal products to oxidation and quality deterioration can be reduced by supplementing them with antioxidants such as vitamin E. It has been revealed that supplementing animals with vitamin E (α-tocopherol) can slow down the oxidation of lipids and improve oxidative stability of muscle and dairy food (Gallardo, Manca, Mantecón, Nudda, & Manso, 2015). Basically, vitamin E is a member of a class known as the fat-soluble vitamins which includes vitamin A, D, E, and K. Vitamin E plays an important antioxidant role by acting as a chain-breaker and free-radical scavenger in muscle cell membranes and tissues while boosting the immune system (Azzi & Zinggl, 2005). Within tissues, vitamin E is mainly concentrated in the unsaturated phospholipid bilayer of the cell membranes where it inhibits lipid oxidation by functioning as a free-radical scavenger (Birben, Sahiner, Sackesen, Erzurum, & Kalayci, 2012; Florou-Paneri, Giannenas, Christaki, Govaris, & Botsoglou, 2006).
The supplementation of vitamin E in the diet of livestock has been reported to have positive effect on the performance of carcass and meat quality characteristics as well as protection of ruminal epithelium against action of ruminal content in ruminant animals (Macit et al., 2003). Both dietary and oral supplementation of vitamin E has been shown to immensely improve livestock performance and their products (Figure 1). Vitamin E supplementation in animal diet has been reported to be effective in reducing lipid oxidation in meat (Gallardo et al., 2015; Macit et al., 2003), egg (Bölükbaşi, Erhan, Keleş, & Koçyiği, 2007), and milk (Santos et al., 2016), as well as lowering serum and egg yolk cholesterol concentrations and improving antioxidant status of the animal (Sahin et al., 2006; Santos et al., 2016). Other reports also revealed that dietary vitamin E can increase the level of sex hormones (Franchini et al., 1991), hatchability, and fertility (Ipek & Dikmen, 2014) of livestock. Supplementation of vitamin E reduced the incidence of retained fetal membranes and the incidence and/or duration of clinical mastitis (Bourne, Wathes, Lawrence, Mc Gowan, & Laven, 2008).
With these abovementioned benefits of vitamin E, a growing number of studies aim to promote vitamin E bioavailability in animal product, knowing that its deficiency could be detrimental to cellular activities and human health. In addition, the perspective of vitamin E supplementation as antioxidant in animal production could be another feasible alternative to reduce the formation of heterocyclic aromatic amines and polycyclic aromatic hydrocarbons, which have been found to be mutagenic (i.e., they cause changes in DNA that may increase the risk of cancer) in cooked muscle food (Viegas, Amaro, Ferreira, & Pinho, 2012). Therefore, the objective of this study is to highlight the recent results on importance of vitamin E in animal and animal products.
2 ORIGIN AND STRUCTURE OF VITAMIN E
Vitamin E is an efficient nontoxic and widely consumed lipid-soluble vitamin because of its antioxidant capacity and multiple health benefits. It can be found in two forms: natural or synthetic. The natural vitamin E was first discovered in 1922 by Herbert Evans and Katherine Bishop when they isolated an uncharacterized lipid-soluble compound from green leafy vegetables which is required to prevent a fetal resorption in pregnant rats during reproduction (Evans & Bishop, 1922). Despite the discovery, it was only until 40 years after that the vitamin was established as essential nutritional factors that are vital in very small amounts in human nutrition. Since then the role of vitamin E has been extensively studied in humans and animals.
Vitamin E was originally termed as tocopherol (obtained from Greek words tokos and phero, meaning to bear children), but it is now commonly referred to as α-tocopherol (Peh, Tan, Liao, & Wong, 2016). Since its discovery, seven additional vitamin E isoforms of vitamin E have been identified in food with various chemical activities in biological system. Basically, vitamin E can be divided into groups of tocopherols and tocotrienols. Tocopherols are known to possess a saturated phytyl chain while tocotrienols contain unsaturated phytyl chain with three double bonds at positions 3’, 7’, and 11’ (Yoshida, Niki, & Noguchi, 2003). The individual tocopherol and tocotrienol are subdivided into four isomers (alpha (α), beta (β), delta (σ), and gamma (ϒ), with each one having a hydroxyl chromanol ring with varying number and position of methyl groups in their chemical structures as seen in Figure 2 (AOR, 2012). Tocopherols has been demonstrated to possess three chiral centers which is responsible for the existence of eight stereoisomers in each tocopherol, whereas each tocotrienol has only two stereoisomers because of the lack of chiral centers in their side chains (Shahidi & de Camargo, 2016).
In nature, vitamin E is mainly synthesized by plants and photosynthetic organisms including algae and some cyanobacteria (Peh et al., 2016; Reboul, 2017). According to Peh et al. (2016), tocopherols can be found in food source such as almonds, avocados, hazelnuts, peanuts, sunflower seeds, oregano, poppy seeds, pecans, pistachios, sesame seeds, walnuts, edamame, raspberries, and also food oils such as corn, peanut, and soybean oil. Animal tissues tend to contain low amounts of α-tocopherol, the highest levels occurring in fatty tissues. The bioavailability of vitamin E in plant sources may be influenced by the species of plant, its maturity at harvesting, and the conditions under which it is stored and processed (Finch & Turner, 1996).
On the other hand, the synthetic vitamin E (all-rac-a-tocopherol) was first synthesized in 1938 (Karrer, Fritzsche, Ringier, & Salomon, 1938) and consist of four side-chain isomers. According to Fu, Htar, Silva, Tan, and Chuah (2017), it was reported that all rac-a-tocopherol are synthesized from the reaction of 2, 3, 5-trimethylhydroquinone and racemic isophytol under acidic condition. The reaction has been reported to yield eight stereoisomers which can be grouped into 2R configurations (RRR, RSR, RRS, and RSS) and 2S configurations (SSS, SRS, SSR, and SRR). However, various attempts to synthesize tocotrienols have been challenging as compared with tocopherols, as it only produced low-to-moderate yields and purity (Netscher, 1996). In general, synthetic vitamin E is the most commonly used vitamin E as supplement both in human and animal because of its stability, availability, effectiveness, and low cost compared with natural vitamin E (Vagni, Saccone, Pinotti, & Baldi, 2011). Although study has indicated that natural vitamin E may exhibit higher biological activity than synthetic vitamin E (Lauridsen, Jensen, Skibsted, & Bertelsen, 2000), Weiss, Hogan, and Wyatt (2009) in their study with dairy cows revealed that natural vitamin E has higher bioavailability (about 1.36 times greater) than that of synthetic vitamin E.
The metabolism (degradation and absorption) of vitamin E is believed to occur in the small intestine together with fat in the presence of bile salts (Cassano, 2012; Weber, 2009) and not in the stomach (Reboul, 2017). Most time, vitamin E is often hydrolyzed in the duodenum by pancreatic lipases which is incorporated into micelles, and then transported via the lymphatic system into the blood that carries it to the liver to be used or stored there (Cassano, 2012; Weber, 2009). Several factors have been reported to influence the metabolism and bio-accessibility of vitamin's E including food matrix and type, mode of processing (natural, thermal, or high pressure treatment), and presence of fat or phospholipids (Reboul, 2017). Whereas the content of vitamin E in food matrix (forage, plant, and pasture land) may depend on factors such as stage of maturity, edaphic type, plant species, climate, and climate change (Maiorano & Tavaniello, 2016).
3 ANTIOXIDANT MECHANISM OF VITAMIN E
The release of free radicals begins with rapid uptake of oxygen during tissue oxidation. Free radicals attack all major components of tissue, especially the unsaturated fatty acids (UFA). The oxidation of UFA is generally destructive because it proceeds as a self-perpetuating chain reaction involving three major stages via initiation, propagation, and termination (Figure 3).
The initiation reaction can be catalyzed by heat, light, and transition metals to produce alkyl radicals (Chan, 1987). In the propagation step, the alkyl radicals react with singlet oxygen at a very high rate to produce a peroxyl radical. The peroxyl radical can thereafter react with another unsaturated fatty acid to produce a lipid hydroperoxides, and a new alkyl radical which is rapidly converted into another peroxyl radical. In a further reaction, the peroxyl radical meets and combines with another radical to form inactive products to terminate the reaction.
In the presence of efficient antioxidant like vitamin E, the lipid peroxyl radicals that have been produced at propagation stage can be intercepted to terminate the lipid peroxidation chain reactions. Strictly, the antioxidant activity of vitamin E is due to its ability to release its hydroxyl groups to combat free radicals and terminate the reaction, while itself is reduced to α-tocopheroxyl radical (α-TO•). The resultant tocopheroxyl radical formed is known to be relatively stable under normal circumstances, but insufficiently reactive to initiate lipid oxidation itself, without a proper regeneration into another α-tocopherol through a redox reaction by other co-antioxidants such as vitamin C and coenzyme Q10 also known as ubiquinol (Cassano, 2012).
However, in the absence of the co-antioxidants, the tocopheroxyl radical is decomposed into an inactive product (vitamin E quinone) or react with other radicals such as alkyl radicals and peroxyl radical or undergo self-coupling to generate 6-O-lipid-oe-tocopherodimers, α-tocopherylquinone, and dimers/trimmers (Figure 3) or form a pro-oxidants. In general, vitamin E can work synergistically with vitamin C to quench free radicals and other reactive species, thus enhancing their antioxidant properties in cells and tissues (Pham-Huy, He, & Pham-Huy, 2008).
4 APPLICATION AND ROLES OF VITAMIN E IN LIVESTOCK
4.1 Ruminants
4.1.1 Goat
The role of vitamin E in livestock production is multifacet as it acts as an antioxidant and immune enhancement to livestock (Schneider, 2005). Conversely, research evidence on the use of vitamin E to improve the performance of goat seems to be scanty. Nonetheless, some authors have reported the use of vitamin E in the improvement of goat and its product. In the study by Possamai et al. (2018), it was reported that Boer-Saanen kids fed dietary vitamin E levels (DL-α-tocopherol acetate) at 50, 150, and 450 mg/kg had higher live body weight gain than the control group. In addition, reports on the use of vitamin E to promote chevon meat quality at 100 mg/kg inclusion rate has also been reported (Table 1).
| References | Treatment | WHC (%) | DL (%) | CL (%) | a* | L* | TBARS |
|---|---|---|---|---|---|---|---|
|
Lee et al. (2008) (Cattle, 1,080 mg/head/day) |
Control | 47.07 | 1.65 | 31.88 | 15.90 | 37.74 | — |
| Vitamin E | 46.68 | 1.15 | 26.64 | 17.89 | 38.23 | — | |
| Significant | NS | * | * | * | NS | — | |
|
Macit et al. (2003) (Lamb, 15 mg/kg) |
Control | — | 0.85 | — | 13.61 | — | — |
| Vitamin E | — | 0.54 | — | 17.66 | — | — | |
| Significant | — | * | — | * | — | — | |
|
Sethy et al. (2014) (Goat, 100 mg/kg) |
Control | 29.54 | — | — | 2.38 | — | 0.72 |
| Vitamin E | 33.98 | — | — | 3.56 | — | 0.51 | |
| Significant | * | — | — | * | — | * | |
|
Atay et al. (2009) (Lamb, 45 mg/kg) |
Control | — | — | — | 12.40 | — | 1.70 |
| Vitamin E | — | — | — | 15.96 | — | 0.60 | |
| Significant | — | — | — | * | — | * | |
|
Ortuño, Serrano, and Bañón (2015) (Lamb, 450 mg) |
Control | — | — | — | — | 46.90 | 4.56 |
| Vitamin E | — | — | — | — | 42.20 | 0.44 | |
| Significant | — | — | — | — | * | * |
- Abbreviations: a*, Redness; CL, Cooking loss; DL, Drip loss; L*, Lightness; NS, Not significant; TBARS, Thiobarbituric and reactive substances; WHC, Water holding capacity.
- * Significant (p < .05).
4.2 Sheep
Sheep which represent another type of small ruminant animal has received tremendous boost in terms of their performance as a result of vitamin E supplementation added to their diet. Recent reports have shown that dietary supplemental of vitamin E in sheep diet have positive effects on the growth performance and the quality of their meat products (Zhao et al., 2013).
Jose, Jacob, Pethick, and Gardner (2018) in their study found a significant higher total live weight in lambs fed diet containing vitamin E diet at 175.7 mg/kg (α-tocopherol acetate) than those fed the control diet, over the 6-week feeding period. The same author also observed that lamb fed diets containing 27, 135, 247.5, and 360 mg vitamin E/kg grew faster at an average of 1.35, 1.15, 1.29, and 0.99 kg/week, respectively, and had higher mean slaughter weights than control (0.39 kg/week) grazing only on green pasture. Maiorano et al. (2016) reported that lamb injected with a total dose of 1,350 mg vitamin E for a period of 15–64 days of age had lower slaughter weight, weight gain, and carcass weight compared with those of the control group. In another study, Zhao et al. (2013) found that lamb fed diet containing vitamin E supplementation at 100, 200, and 2000 IU/day over a feeding period of 130 days has lower average daily weight gain compared with control groups. This may be due to high intake of vitamin E in the diet which may suppress growth of the animals since the recommended minimal requirement for normal growth and health of sheep by the Agricultural Research Council (ARC, 1980) has been stipulated to be between 10 and 15 mg/kg dietary DM. However, Atay, Gökdal, Eren, Çetiner, and Yikilmaz (2009) found no significant difference in the average daily weight gain and feed conversion efficiency of Karya male lambs fed diet containing 45 mg/lamb/day vitamin E during the fattening period compared with the control group.
Furthermore, different studies have shown significant effect of vitamin E supplementation on the color stability of lamb meat during cold storage (Tables 1 and 2). Dietary inclusion of vitamin E supplementation has been reported to delay met-myoglobin formation in lamb meat under modified atmospheric condition (Kerry, O'Sullivan, Buckley, Lynch, & Morrissey, 2000) and increased oxymyoglobin formation of lamb meat under modified atmosphere during 14 days storage (Lauzurica et al., 2005). Myoglobin is the main protein responsible for meat color along with hemoglobin and cytochrome C (Mancini & Hunt, 2005). Meat discoloration originates from the oxidation of ferrous myoglobin derivatives to metmyoglobin (Zeferino et al., 2016). Guidera, Kerry, Buckley, Lynch, and Morrissey (1997) found that the myoglobin oxidation (conversion into met-myoglobin) was slower in semi-membranous cuts stored for 6 days from lambs supplemented with 900 mg of vitamin E/kg DM when compared with lambs supplemented with 18 mg/kg DM. The reason for this has been linked to vitamin E’s role in protecting oxymyoglobin from the effects of oxidizing lipids (O’Grady, Monahan, Fallon, & Allen, 2001). It has also been reported that vitamin E supplementation at 1,000 mg kg−1 day−1 in lamb diet can favorably improve color stability than control treatment during cold storage (Guidera et al., 1997), whereas other studies (Atay et al., 2009; Macit et al., 2003) have found no differences in meat color of lamb fed diet containing vitamin E compared with control groups. These differences in findings of different authors could be attributed to breed, age at slaughter, storage period, packaging, and amount of vitamin E supplementation (Atay et al., 2009).
| References | Treatment | C14:0 | C16:0 | C18:0 | C18:2 | C18:3 | SFA | PUFA |
|---|---|---|---|---|---|---|---|---|
|
Bellés et al. (2018) (1,000 mg/kg) |
Control | 2.47 | 22.34 | 13.59 | 8.74 | 0.57 | 38.86 | 13.49 |
| Vitamin E | 1.26 | 19.51 | 10.94 | 11.14 | 1.98 | 36.08 | 16.01 | |
| Significant | * | * | * | * | * | * | * | |
|
Muíño et al. (2014) (300 mg/kg) |
Control | 2.94 | 20.44 | 14.00 | 8.92 | 1.20 | 40.80 | 15.91 |
| Vitamin E | 1.09 | 16.76 | 11.30 | 10.63 | 2.70 | 36.60 | 16.03 | |
| Significant | * | * | * | * | * | * | NS | |
|
Berthelot, Broudiscou, and Schmidely (2014) (286 mg/kg) |
Control | 2.96 | 15.40 | 12.40 | 10.65 | 0.11 | 29.70 | 16.90 |
| Vitamin E | 0.99 | 15.51 | 9.98 | 13.51 | 1.84 | 26.10 | 20.50 | |
| Significant | * | NS | * | * | * | * | * | |
|
Zhao et al. (2013) (180 mg/lamb/day) |
Control | 1.84 | 24.24 | 20.73 | 3.57 | 0.17 | 48.76 | 6.70 |
| Vitamin E | 1.81 | 21.03 | 16.62 | 6.96 | 0.91 | 43.30 | 9.96 | |
| Significant | NS | * | * | * | * | * | * | |
|
Kasapidou et al. (2012) (500 mg/kg) |
Control | 0.43 | 16.80 | 13.60 | 10.70 | 1.10 | 35.68 | 12.94 |
| Vitamin E | 0.10 | 12.95 | 9.10 | 16.28 | 2.98 | 31.80 | 15.10 | |
| Significant | * | * | * | * | * | * | * |
- Abbreviations: C14:0, Myristic acid; C16:0, Palmitic acid; C18:0, Stearic acid; C18:2, Linoleic acid; C18:3, Linolenic acid; NS, Not significant; PUFA, Polyunsaturated fatty acid; SFA, Saturated fatty acid; Vit E, Vitamin E.
- * Significant (p < .05).
Conversely, vitamin E supplementation has been reported to reduce lipid oxidation and decreased metmyoglobin formation, as well as improving the color stability of meat (Belles et al., 2018). López-Bote, Daza, Soares, and Berges (2001) in a study found a significant decreased in TBARS values of refrigerated meat from sheep supplemented with 1,000 mg of vitamin E/kg (at 0.45 mg/kg muscle) compared with the control group (at 3.1 mg/kg muscle) after 9 days of cold storage at 4°C. The protective effect of dietary vitamin E supplementation against lipid oxidation under different temperature conditions in mutton has also been reported (Lauzurica et al., 2005). By inhibiting the oxidative process, vitamin E reduces the quantity of generated oxidative products (e.g., singlet oxygen, hydroxyl radical, ferryl radical, ferrous–dioxygen–ferric complex, etc.) that may deteriorate the quality of meat and meat products including taste, aroma, color, and texture (Decker, Faustman, & Lopez-Bote, 2000; Kennedy, Stewart-Knox, Mitchell, & Thurnham, 2005). In the study by Strohecker, Faustman, Furr, Hoagland, and Williams (1997), it was found that there was a delay in lipid oxidation during storage in lamb meat from sheep supplemented with 2000 mg of vitamin E per day for 49 days.
Dietary supplementation of vitamin E in sheep diet have been used to increase the polyunsaturated fatty acid (PUFA) content and decrease the saturated fatty acid (SFA) content of mutton. The study of Liu, Lanari, and Schaefer (1995) revealed that, supplementation of 20, 100, 200, 1,000, 2,000, or 2,400 IU/head/d vitamin E in wool sheep diet for 12 months significantly reduced SFA content and increased monounsaturated FA (MUFA) content in the Longissimus lumborum and Gluteus medius muscles compared with control. In addition, the same authors also found that, the supplementation of 20, 100, 200, 1,000, 2,000, or 2,400 IU/head/d vitamin E significantly increased cis 9 trans-11-conjugated linoleic acid (c 9t 11-CLA) content in the Longissimus lumborum compared with the control group. Similarly, Belles et al. (2018) reported that dietary vitamin E supplementation (1,000 mg of DL-α-tocopheryl acetate/kg of basal diet) lamb diet caused higher percentage of polyunsaturated fatty acids in fresh meat than the control group. Hence, it could be said that vitamin E supplementation has great potential in preserving the quality of fresh mutton.
4.3 Cattle
4.3.1 Meat
Beef is one of the widely consumed meats in the world. Due to its extensive acceptability by consumers, research on improving the production of cattle and its products cannot be overemphasized. Over the years, vitamin E supplementation has been shown to impact positively on cattle growth performance, carcass characteristics, health status, as well as nutritional composition of livestock products. In studies with large ruminant animal, Yang, Brewster, Lanari, and Tume (2002) found that cattle reared on pasture and fed with 2,250 mg vitamin E/day for 132 days showed a reduction in its final body weight and increased carcass weight compared with the control group. Similarly, Descalzo et al. (2005) found that cattle reared on pasture and supplemented with vitamin E at 450 mg/head/day had reduced daily weight gain, and higher carcass yield and plasma α-tocopherol level compared with cattle fed grain and the same vitamin E supplement. However, cattle reared on pasture or grain with vitamin E had improved performance than those fed diet without vitamin E supplement.
In contrast, Cano, Montano, Salinas-Chavira, and Zinn (2015) reported no significant difference in feedlot Holstein steer calves fed diet containing 0, 250, or 500 IU/d of vitamin E during 56 days feeding trial, whereas Mir et al. (2003) found that steers fed diet containing 500 IU/d of vitamin E had lowered live weight and average daily weight gained compared with the control group. Similarly, Plascencia, Montano-Gomez, Salinas-Chavira, Torrentera-Olivera, and Zinn (2018) in their study found that Holstein steers fed diet containing vitamin E level at 0, 250, and 500 IU had no significant difference in the average daily weight gain, dry matter intake, gain efficiency, and carcass characteristics after 312-day feeding trials. The discrepancies in growth performance response of livestock supplemented with vitamin E from different studies may be attributed to the effect of age and breeds, other dietary ingredients, animal stress, or supplementation period of the different trials (Cano et al., 2015; Decker et al., 2000).
Furthermore, the supplementation of vitamin E to meat-producing animals has been showed to increase muscle α-tocopherol concentrations (Lauridsen & Jensen, 2012; Mitsumoto, Ozawa, Mitsuhashi, & Koide, 1998), improve color, and lipid stability in fresh meat for several days (Yang et al., 2002). The improvements in color stability have been associated with α-tocopherol-mediated mechanism to reduce lipid and myoglobin oxidation (Surai, 2002). Lee, Panjono, Kim, and Park (2008) reported an increase in redness (a*) of beef from cattle fed vitamin E supplement compared with control group (Table 3). In addition, Lee, Kim, Liang, and Song (2003) in their study also reported that meat from Korean cattle fed diet containing vitamin E at 1,000 IU/head/day for 6 months prior to slaughter revealed higher redness (a*) than the control group, with the M. longissimus having higher values than the M. semimembranosus during retail display (3 ± 1°C, 1,200 lux). The same authors also found that the TBARS values for two beef muscle were significantly lower in 1,000 IU group than in the control group, depicting the antioxidant capacity of vitamin E to decrease lipid oxidation and improve oxidative stability during display and storage.
| References | Treatment | C14:0 | C16:0 | C18:0 | C18:2 | C18:3 | SFA | PUFA |
|---|---|---|---|---|---|---|---|---|
|
Ferrinho et al. (2018) (450 mg) |
Control | 2.96 | 22.54 | 12.53 | 5.67 | 0.34 | 39.83 | 11.64 |
| Vitamin E | 3.07 | 19.28 | 13.47 | 8.58 | 0.43 | 33.91 | 14.90 | |
| Significant | NS | * | NS | * | * | * | * | |
|
Hollo et al. (2016) (50 mg/head/day) |
Control | 3.01 | 28.50 | 20.49 | 2.45 | 0.03 | 53.00 | 3.83 |
| Vitamin E | 1.98 | 24.14 | 17.43 | 5.88 | 0.04 | 48.36 | 10.86 | |
| Significant | * | * | * | * | NS | * | * | |
|
Machado Neto et al. (2014) (2,250 mg) |
Control | 2.32 | 22.50 | 17.30 | 9.36 | 0.54 | 43.50 | 13.30 |
| Vitamin E | 1.04 | 19.21 | 14.11 | 13.60 | 0.92 | 43.90 | 17.90 | |
| Significant | * | * | * | * | * | NS | * | |
|
Mir et al. (2003) (450 mg/head/day) |
Control | 3.00 | 33.19 | 13.22 | 13.90 | 1.90 | 52.20 | 20.00 |
| Vitamin E | 3.10 | 30.83 | 10.37 | 21.00 | 2.85 | 52.11 | 27.90 | |
| Significant | NS | * | * | * | * | NS | * |
- Abbreviations: C14:0, Myristic acid; C16:0, Palmitic acid; C18:0, Stearic acid; C18:2, Linoleic acid; C18:3, Linolenic acid; NS, Not significant, PUFA, Polyunsaturated fatty acid, SFA, Saturated fatty acid.
- * Significant (p < .05).
In another study, Eikelenboom, Hoving-Bolink, Kluitman, Houben, and Klont (2000) reported that beef cattle fed diet containing 2025 mg vitamin E/day for 136 days prior to slaughter had significant lowered lipid oxidation (TBAS values), but no significant effect on muscle color during cold storage compared with control group. The lack of significant differences in treatments has been attributed to presence of α-tocopherol levels in muscle of animal in control group (longissimus thoracis: 2.1 and psoas major: 3.2 μg/g muscle), probably caused by relatively high natural vitamin E uptake from the control diet.
More so, different authors have also reported on the effect of vitamin E supplementation on fatty acid composition and quality parameters of meat from ruminant animal across the world (Tables 1 and 3). Their findings showed a decreasing trend in proportions of Myristic (C14:0), Palmitic (C16:0), and Stearic (C18:0) acids, and a significant increase in contents of Linoleic (C18:2) and Linolenic (C18:3) acids in the muscle tissue of animals fed diets supplemented with vitamin E. It was found that animal fed higher levels of vitamin E supplement had lower concentrations of saturated fatty acids (SFA) and higher concentrations of polyunsaturated fatty acids (PUFA), than those animals under control group. Likewise, it also suggests that the PUFA were protected from oxidation by α-tocopherol supplementation (Li et al., 2015; Surai & Sparks, 2000; Taşdelen & Ceylan, 2017). The higher concentration of PUFA found in vitamin E group may be related to its antioxidant activity (Table 3). Unsaturated fatty acids are known to be more susceptible to oxidation; so the inclusion of dietary vitamin E in the basal diet of livestock feed may have protected PUFA from these reactions (oxidation), which result in higher PUFA percentage in these samples more than the control group.
Conversely, the biological mechanism of vitamin E supplementation affecting fatty acid profiles of intramuscular fat may be due to the ability of vitamin E to modify ruminal pathways of PUFA bio-hydrogenation in both dairy (Bell, Griinari, & Kennelly, 2006; Pottier et al., 2006) and beef cattle (Juárez et al., 2011), acting either as an inhibitor or bacteria producing trans-10C18:1 or as an electron acceptor for Butyrivibrio fibrisolvens (Juárez et al., 2010). However, some authors (Bellés et al., 2018; Rebolé et al., 2006) have reported that there are no effect of vitamin E on the intramuscular fat content and fatty acid profile which suggests the need for more future research around this subject matter.
From human nutritional point of view, Myristic and Palmitic acids deserve more attention. According to I. Hollo, Hollo, and Csapo (2016), Lauric (C12:0), C14:0 (Myristic), and C16:0 (Palmitic) acids are the primary fatty acids associated with increased plasma, low-density lipoprotein, and cholesterol concentrations in humans. The authors further observed that C16:0 is the main end product of de novo fatty acid synthesis which can be elongated to Stearic (C18:0) acid. Fat deposition in the muscle tissue is as a result of absorption, de novo synthesis, and oxidation of fatty acids (Sanz, Lopez-Bote, Menoyo, & Bautista, 2000). Studies showed that dietary vitamin E decreases hepatic lipogenesis and increases the metabolic rhythm and inhibits de novo fatty acid synthesis (Barroeta, 2007; Sanz et al., 2000).
4.3.2 Milk
Another interesting aspect to discuss in this study borders on the effect of vitamin E on ruminant milk. The effect of vitamin E on milk quality has been investigated in several studies due to the important role milk plays in our daily diet. The inclusion of vitamin E in livestock diet could be said to have both direct and indirect effect on milk quality. The direct effect of vitamin E included in animal diet relates to its ability to improve the oxidative stability of milk. Conversely, the indirect influence of vitamin E on milk quality is observed in its ability to reduce both the levels of somatic cell counts (SCC) and the activity of proteolytic enzyme plasmin in milk.
Talking about the direct effect of vitamin E on milk quality, several studies have reported the influence of vitamin E (in animal diet) in terms of improving the oxidative stability of milk (Al-Mabruk, Beck, & Dewhurst, 2004; Charmley & Nicholson, 1993; Charmley, Nicholson, & Zee, 1993). However, in the study by Slots, Skibsted, and Nielsen (2007), it was reported that livestock supplemented with a high (261 mg of vitamin E/kg of DM in the form of all-rac α-tocopheryl acetate) vitamin E concentration had higher susceptibility to oxidation than those supplemented with a lower concentration (26.1 mg of vitamin E/kg of DM in the form of all-rac-α tocopheryl acetate). Report on the optimal vitamin E concentration that is required to enhance the oxidative stability of milk has not yet been fully established (Politis, 2012). This area of research requires more investigation in order for scientist and dairy farmers to be able to establish a valid standard for the optimal allowance of vitamin E inclusion in the diet of cows. It was suggested that the supplementation of cow feed with 900 mg/day of vitamin E may help to improve herds producing milk with off-flavors; although findings for controlled studies of this nature are lacking (Politis, 2012). However, according to Smith, Hogan, and Weiss (1997), the inclusion of vitamin E (450 mg/day of supplemental vitamin E when fed stored forages) in the diet of lactating cows may slightly increase the vitamin E content of milk and may also help in plummeting oxidative flavor problems.
Another aspect of our discussion is on the indirect effect of vitamin E supplementation (in livestock feed) on milk quality. This relates to the secondary effects of vitamin E supplementation (i.e., reduction in intramammary infections) on milk and does not necessarily mean that it has a direct mechanism through which it enhances milk quality. The indirect effect of vitamin E supplementation on milk quality in this review will be discussed in relation to how it affects somatic cell counts (SCC) and plasmin level in milk.
One of the ways through which the quality of milk is evaluated is through the load of somatic cell counts in the milk. A high SCC in bulk milk shows an indication of herds with many infected cows and the milk produced is generally considered to be of low-grade quality. In the study by Baldi et al. (2000), it was reported that, the supplementation (oral administration of 1,800 mg/cow per day from day 14 before up to day 7 after parturition) of vitamin E suppresses the load of SCC by 20% to 30%, compared with the values of the control experiment that received only 900 mg/cow per day during the same study interval. Likewise, Politis et al. (1996), in their findings, reported that herds orally supplemented with vitamin E (2,700 mg/cow per day at late gestation period up to 8 weeks in lactation, plus one injection of 4,500 mg a week before calving) lowers the level of SCC/ml of milk for the first 4 weeks of lactation in cows.
Several other researchers have reported the indirect influence of the inclusion of vitamin E in herds as it associates with the reduction in SCC per ml of milk in cows (Batra, Hidiroglou, & Smith, 1992; Nyman et al., 2008). However, contrast still exist in the findings of different researchers regarding the association that exist in the supplementation of herds with vitamin E and how it helps to lower the load of SCC in milk. Some studies have reported no association between the supplementation of vitamin E and SCC levels in milk (Jukola, Hakkarainen, Saloniemi, & Sankari, 1996; Ndiweni, Field, Williams, Booth, & Finch, 1991; Persson, Sandgren, Emanuelson, & Jensen, 2007). A critical look at the reason for these variations in the findings of these different authors could be attributed to difference in the location of study, amount of vitamin E dosage administered to cows, age or physiological status of animals, and the management of studied animals among other factors. These differences in research findings still possess a strong debate on whether or not these evidences should be accepted as a fact or left for further investigations.
Furthermore, another indirect effect of vitamin E supplementation of livestock feed on milk quality is seen in the level of plasmin present in milk. The supplementation of vitamin E may help to lower the plasmin level of cow milk which will in turn help to enhance quality dairy product made from the milk. The process of proteolysis during the manufacturing of cheese may be affected by high plasmin level in cows’ milk; of which may result in low cheese yield and poor-quality cheese product (Politis, 2012). In addition, high plasmin level decreases the stability of long-life milk (UHT milk) and this is linked with the production of bitter flavors of milk (Ismail & Nielsen, 2010). In the study by Politis, Bizelis, Tsiaras, and Baldi (2004), it was reported that vitamin E supplementation in dairy cows resulted in the reduction in plasmin levels in milk by 30%. Conversely, it could be noted that milk possessing lower levels of plasmin has an increased ability to tolerate dairy processing for the manufacturing of basically all dairy products.
In general, dairy cows raised on pasture do not normally require vitamin E supplementation. Notwithstanding, a supplementation (900 mg/d for nonlactating cows and 450 mg/d for lactating cows) of vitamin E may be required to significantly improve their performance, such as reducing intramammary infections, retained placenta, milk somatic cell counts (primary indicator of milk quality), clinical mastitis, and metritis in dairy herds (Hogan, Weiss, & Smith, 1993). In addition, as it could not always be feasible to ascertain the vitamin E content of the basal diet in most country, the universal recommendation is that dry cows should be supplemented with 900–2700 mg of vitamin E/day (Smith et al., 1997). Likewise, worthy of note is that herds with a high incidence of mastitis should be supplemented with 2,700 mg/day to achieve blood vitamin E values higher than 3 mg/ml for their optimum performance (Politis, 2012).
5 NONRUMINANT
5.1 Pig
Pork meat is one of the most widely eaten meat worldwide. Efforts to improve its production with the use of health promoting antioxidants from natural or synthetic sources (supplemented to pig's diets) is been highly considered by researchers, industries, and livestock farmers. Vitamin E supplements have been shown to have positive effects on pig growth performance, particularly in improving average daily gain and feed efficiency. It has also been reported that direct supplementation of vitamin E in swine diet during the growing and finishing periods can improve pork quality (oxidative stability and sensory quality) during storage and display (Table 4). Asghar et al. (1991) in their study reported that pigs fed diets supplemented with 90 and 180 mg vitamin E/kg feed exhibited significant improvement in daily body weight gain and feed conversion efficiency in the early growth phase, but with advance in age, the growth curves of pigs fed the higher levels of vitamin E became parallel to that of the control group. Feeding of pigs with diet containing 100–200 mg/kg of vitamin E (dl-α-tocopherol acetate) has been reported to effectively delay the onset of lipid oxidation in fresh whole-muscle pork cuts, precooked pork, and cured pork products (Guo et al., 2006). The addition of vitamin E at 40 and 200 UI/kg in pig diet has been revealed to significantly increase the level of monounsaturated and total unsaturated fatty acid proportions in neutral lipids of muscle and adipose tissues (Guo et al., 2006).
| References | Treatment | DL (%) | a* | L* | TBARS |
|---|---|---|---|---|---|
|
Li et al.(2015) (400 mg/kg) |
Control | 2.25 | 3.90 | 46.77 | — |
| Vitamin E | 2.17 | 4.62 | 45.30 | — | |
| Significant | NS | NS | NS | — | |
|
Bahelka, Nürnberg, Küchenmeister, and Lahučký (2011) (500 mg/kg) |
Control | 4.24 | 2.54 | 9.02 | 0.27 |
| Vitamin E | 3.02 | 3.41 | 9.11 | 0.19 | |
| Significant | * | * | NS | * | |
|
Ľahučký, Bahelka, Küchenmeister, Vašíčková, and Ender (2006) (500 mg/kg) |
Control | 3.42 | 2.73 | 47.20 | 0.22 |
| Vitamin E | 3.04 | 3.63 | 48.30 | 0.15 | |
| Significant | * | * | NS | * | |
|
Guo et al., 2006 (200 mg/kg) |
Control | 1.02 | 10.80 | 43.50 | 0.82 |
| Vitamin E | 0.52 | 11.60 | 43.80 | 0.36 | |
| Significant | * | NS | NS | * | |
|
Lahučký, Bahelka, Novotna, and Vašíčková (2005) (500 mg/kg) |
Control | 4.86 | — | 48.67 | 0.25 |
| Vitamin E | 4.12 | — | 48.58 | 0.15 | |
| Significant | * | — | NS | * |
- Abbreviations: a, Redness; DL, Drip loss; L, Lightness; NS, Not significant; TBARS, Thiobarbituric and reactive substances.
- * Significant (p < .05).
Specifically, Guo et al. (2006) observed a lower proportion of C16:0 and C18:0 in raw meat from pigs fed 400 mg of vitamin E/kg of diet. Likewise, Lauridsen et al. (2000) in their study observed that fatty acid composition in the mitochondria of the Psoas major muscle of vitamin E supplemented pigs had a greater proportion of C18:3 and C20:1 at the expense of C16:0 and C18:0. With these findings, it is evident that dietary vitamin E supplementation can be used to improve lipid stability of muscle food. In addition, other studies have shown that dietary supplementation of vitamin E can decrease the drip loss in pork during cold storage (Guo et al., 2006; Monahan et al., 1994; Offer & Knight, 1998). However, different authors have reported low influence of vitamin E supplementation on color attributes and stability of pork during storage and retail display (Li et al., 2015).
5.2 Rabbit
Rabbit meat is a common food that is gaining ground in terms of consumption in most part of Western and African countries (Dalle Zotte & Szendrő, 2011). Judging from the nutritional stand point, rabbit meat is model for all kinds of consumers. According to Karppanen and Mervaala (2006), rabbit meat is especially suitable in Western countries for people whose diet is mostly rich in fats and sodium, thereby exposing them to dangerous health problems such as hypertension, cardiovascular diseases, and obesity. Meat from rabbit is well furnished with proteins, B vitamins, and minerals but it is low in sodium, fat, and cholesterol with a fairly high energy content (789 kJ/100 g meat, average carcass value), which is mostly coming from its protein contents (Dalle Zotte & Szendrő, 2011). Conversely, supporting research that will propagate rabbit meat production is worth exploring.
Cardinali et al. (2015) observed in their study that the supplementation of vitamin E at 150 ppm in New Zealand White rabbit's diet significantly increased feed intake, feed conversion ratio, average daily weight, live weight, and carcass weight after 80 days of feeding trial compared with the control treatment. Ebeid, Zeweil, Basyony, Dosoky, and Badry (2013) also observed that California growing rabbit supplemented with dietary vitamin E had significant higher feed intake, final body weight, daily gain, and dressing percentage, but reduced feed conversion ratio than the control group fed with basal diet. Having established the nutritional importance of rabbit meat, more research needs to be done to promote large production of the animal, and subsequently, awareness should be made for more people to consume its meat because of its massive nutritional and health benefits.
6 BIRDS
6.1 Poultry
6.1.1 Meat
It should be mentioned that poultry cannot synthesize vitamin E on its own; therefore, vitamin E requirements must be met in poultry diet (Chan & Decker, 1994). Vitamin E is one of the well-established, safe, and essential micronutrient needed in poultry for optimum health, growth performance, fertility, and normal physiological functions including egg production and quality parameter (Khan et al., 2011). Chicken fed diet with α-tocopheryl acetate at 170 mg/kg was reported to produce higher body weight gain, feed intake, and improved feed conversion ratio compared with the control group and other treatments (Florou-Paneri et al., 2006). Botsoglou et al. (2004) in their trial observed that broiler chicken fed α-tocopheryl acetate at 200 mg/kg compared with control and other treatment did not show any significant difference in final body weights, daily weight gains, daily feed intakes, and feed conversion ratios, after 42 days of feeding. Guo, Tang, Yuan, and Jiang (2001) found that supplementation of chicken diet with vitamin E at 50 and 100 mg/kg increased daily body weight gain and improved feed utilization during the growing period of 0–3 weeks of age compared with the control group. However, Niu, Liu, Yan, and Li, (2009) found no significant difference in body weight and feed intake of chicken fed vitamin E at 100 and 200 mg/kg compared with control, although the feed conversion was significantly higher at 100 mg/kg vitamin E supplementation than the other treatment group. Similarly, Puthpongsiriporn, Scheideler, Sell, and Beck (2001) reported that the performances of laying hens were not influenced by supplemental vitamin E at 0.9, 22.5, 40.5, or 58.5 IU of vitamin E/kg. In another study, Lin, Chang, Yang, Lee, and Hsu (2005) found that Cockerels fed diet containing 40, 80, and 160 mg/kg showed higher total body weight and body weight gain after 52 weeks of age compared with those fed vitamin E diet.
In relation to meat quality, vitamin E has been used to improve chicken meat (Tables 5 and 6). Conversely, Lee et al. (2008) in their study found that Water holding capacity (WHC) and L* (lightness) did not differ significantly, while drip loss, a* (redness), and cooking loss were affected by vitamin E supplementation. Water holding capacity of meat is directly related to the intramuscular lipids and moisture content of the meat (Huff-Lonergan & Lonergan, 2005). The inability of meat to bind water leads to decrease in muscle tenderness and increase in cooking loss (Zhang, Wang, Zhou, & Wang, 2012). Conversely, in the study by Zhang et al. (2012), it was reported that dietary supplementation of vitamin E at 30 and 60 mg/kg can reduce the rate of surface discoloration in chicken meat. Thiobarbituric and reactive substances (TBARS; mg malonaldehyde/100 g tissue) values in meat samples from the control group were found to be higher than those of the vitamin E–treated group (Table 5). According to Tasdelen and Ceylan (2017), TBARS levels of thigh and breast muscle from birds fed diet without vitamin E or 100% rapeseed oil without vitamin E supplementation displayed an increased susceptibility to lipid peroxidation while dietary combination of oil and vitamin E at 300 mg/kg markedly decreased the susceptibility to oxidation. These findings indicate that dietary inclusion of vitamin E can effectively increase stability of PUFA-enriched broilers meat across the oxidative damage (Surai & Sparks, 2000).
| References | Treatment | DL (%) | a | L | TBARS |
|---|---|---|---|---|---|
|
Lahučký et al. (2006) (200 mg/kg) |
Control | 4.72 | 5.21 | 40.40 | 1.58 |
| Vitamin E | 2.09 | 5.52 | 40.09 | 0.41 | |
| Significant | * | NS | NS | * | |
|
Niu, Min, and Liu (2018) (200 mg/kg) |
Control | 12.77 | 10.30 | 45.29 | 0.28 |
| Vitamin E | 11.54 | 11.34 | 45.20 | 0.18 | |
| Significant | * | * | NS | * | |
|
Zdanowska-Sąsiadek et al. (2016) (200 mg/kg) |
Control | 14.90 | 0.60 | 51.60 | — |
| Vitamin E | 11.80 | 1.38 | 50.50 | — | |
| Significant | * | * | NS | — | |
|
Zhang, Zhong, Zhou, Du, and Wang (2009) (60 mg/kg) |
Control | 2.58 | 7.49 | 55.40 | — |
| Vitamin E | 1.27 | 7.99 | 54.70 | — | |
| Significant | * | NS | NS | — | |
|
Tasdelen and Ceylan (2017) (300 mg/kg) |
Control | — | — | — | 1.13 |
| Vitamin E | — | — | — | 0.99 | |
| Significant | — | — | — | * |
- Abbreviations: a, Redness; DL, Drip loss; L, Lightness; NS, Not significant; TBARS, Thiobarbituric and reactive substances.
- * Significant (p < .05).
| References | Treatment | C14:0 | C16:0 | C18:0 | C18:2 | C18:3 | SFA | PUFA |
|---|---|---|---|---|---|---|---|---|
|
Taşdelen and Ceylan (2017) (Chicken, 300 mg/kg) |
Control | — | 13.53 | 4.68 | 27.54 | 3.39 | 19.30 | 36.12 |
| Vitamin E | -— | 12.05 | 4.12 | 32.54 | 5.29 | 17.33 | 39.43 | |
| Significant | — | * | NS | * | * | * | * | |
|
Surai and Sparks (2000) (Chicken, 300 mg/kg) |
Control | — | 14.12 | 5.00 | 32.00 | 3.46 | 19.90 | 33.60 |
| Vitamin E | — | 12.06 | 3.87 | 38.99 | 5.37 | 16.97 | 37.88 | |
| Significant | — | * | * | * | * | * | * | |
|
Li, Zhao, Chen, Zheng, and Wen (2009) (Chicken, 200 mg/kg) |
Control | 2.59 | 19.56 | 11.87 | 21.30 | 0.89 | 34.45 | 34.81 |
| Vitamin E | 1.48 | 17.55 | 9.70 | 24.86 | 1.15 | 31.18 | 38.90 | |
| Significant | * | * | * | * | * | * | * | |
|
Guo et al. (2006) (Pig, 200mg/kg) |
Control | 2.98 | 26.01 | 15.96 | 7.03 | 2.14 | 39.37 | 32.45 |
| Vitamin E | 1.68 | 23.73 | 12.81 | 9.24 | 4.79 | 35.38 | 38.63 | |
| Significant | * | * | * | * | * | * | * |
- Abbreviations: C14:0, Myristic acid; C16:0, Palmitic acid; C18:0, Stearic acid; C18:2, Linoleic acid; C18:3, Linolenic acid; NS, Not significant; PUFA, Polyunsaturated fatty acid; SFA, Saturated fatty acid.
- * Significant (p < .05).
Furthermore, Bou, Grimpa, Baucells, Codony, and Guardiola (2006) observed that the proportion of C16:0 was decreased and C18:3 was increased by feeding chickens with vitamin E at 225 mg/kg and thus a significant increase was obtained in the ratio of PUFA to SFA (Table 6). In contrast, O'neill, Galvin, Morrissey, and Buckley (1998) fed chickens with diets containing 6% tallow or olive oil with vitamin E at 200 mg/kg and found no effect on fatty acid composition of the meat.
The inclusion of vitamin E up to 20,000 mg/kg in poultry diet was demonstrated not to negatively impact their performance (Weber, 2009). However, nutrient in the presence of inadequate feed or impaired absorption or utilization of vitamins in poultry production can lead to specific deficiency disorders, low productivity, and other related diseases (Weber, 2009). In some studies, birds fed diet containing varying level of vitamins E (alpha-tocopheryl acetate) have been reported to gain improved feed intake, body weight, feed efficiency, egg production and quality, nutrient digestibility, immune response, and antioxidant status (Yardibi & Gülhan Türkay, 2008).
6.1.2 Egg
The study of Yardibi and Gülhan Türkay (2008) revealed that the addition of vitamin E in layer diet can increase egg productivity by preventing liver damage, which is important for egg yolk protein synthesis. Similarly, Bollengier-Lee, Mitchell, Utomo, Williams, and Whitehead (1998) reported a tremendous increase of 7% and 4% in egg production of hens under heat stress fed diet containing 125 and 500 mg/kg of vitamin E, respectively, for 4 weeks compared with birds given 10 mg/kg of vitamin E. Likewise, Çiftci, Nihat Ertas, and Guler (2005) in their study indicated that White Leghorn fed with diet containing vitamin E (200 mg/kg) under a chronic heat stress showed improved egg production, egg weight, and egg yolk compared with control bird. In another study, Yardibi and Gülhan Türkay (2008) found that layers diet supplemented with 50 mg/kg vitamin E under an heat-stressed condition (35°C) had higher Haugh unit, egg shell thickness, egg weight, and egg-specific gravity compared with the control group. Puthpongsiriporn et al. (2001) also reported a significant increase in egg mass, egg yolk, and Haugh unit of layer fed diet supplemented with vitamin E (58.5 mg) during heat stress. Asli, Hosseini, Lotfollahian, and Shariatmadari (2007) in their study reported that the inclusion of 200 mg of vitamin E in diet of laying hens exposed to heat stress (33°C) greatly improved their yolk percentage. It has also been reported that laying hens fed diet containing 54 mg/kg vitamin E improved yolk and albumin solids, foam stability, and angel cake volume during heat exposure (Kirunda, Scheideler, & McKee, 2001).
In general, vitamin E has been shown to protect ovarian follicles in poultry from oxidative damage and facilitates the release of vitellogenin as a precursor of the yolk from the liver to increase egg production (Weber, 2009). Supplementing vitamin E in the poultry diet was reported to increase the antioxidant (vitamin E) content and improve the oxidative stability of the egg yolk, particularly in the presence of fat during storage (Weber, 2009). The vitamin E content of egg is believed to act as chain-breaking antioxidant to protect long-chain polyunsaturated fatty acids in the membranes of cells and thus maintain their bioactivities (Nimalaratne & Wu, 2015). Therefore, the intake of egg rich in vitamin E can help reduce oxidative damage.
Other egg components that can act as antioxidant include ovalbumin, ovotransferrin, phosvitin, selenium, and carotenoids (Nimalaratne & Wu, 2015). On average, the vitamin E content of egg is estimated at around 1.1 mg which is equivalent to 8.5% of the Recommended Dietary Allowance (RDA) (Scheideler, Weber, & Monslave, 2010; Surai, MacPherson, Speake, & Sparks, 2000). However, studies have shown that this can be increased up to 150% RDA by enriching the diet more with vitamin E, without causing off-flavor in the egg yolk (Nimalaratne & Wu, 2015; Surai et al., 2000).
Scheideler et al. (2010) in their study supplemented α-tocopherol at 45, 90, or 135 mg/kg in layer diet and found a progressive increase in α-tocopherol content and other nutrients in the yolk of fresh eggs and eggs stored for 2 weeks. In another study, Jiang, Zhang, and Shan (2013) found that layer birds fed diet containing 200 mg/kg vitamin E had higher α-tocopherol concentration of egg yolk and serum compared with the control group. The same author also reported that supplementation of vitamin E at 200 mg/kg significantly decreased malondialdehyde values (oxidation) and increased glutathione peroxidase and total superoxide dismutase concentrations (endogenous antioxidants) of the egg yolk and serum compared with the control group. Ward (2017) also reported that increasing dietary vitamin E content from 0 to 200 mg/kg can significantly elevate yolk vitamin E content from 2.52 to 3.60 µg/g of yolk. However, factors such as species, nutrition, heat stress, and ambient temperature could influence the egg vitamin E content of laying birds. Surai, Ionov, Kuchmistova, Noble, and Speake (1998) in their study found that chicken fed with the same concentration of vitamin E diet produced egg with higher vitamin E content compared with turkey, duck, and goose.
6.2 Quail
Quail meat and its products (e.g., egg) is another nutritious delicacy that is being consumed in recent times in some parts of the world including Japan and some parts of Africa because of their delightful taste, and good source of nutrients, vitamins, and minerals; hence its production should be promoted for consumption. One of the ways researchers have tried to increase the production of quail meat and its product is in the use of vitamin E to supplement their diet.
In the study by Ipek, Avci, Yerturk, Iriadam, and Aydilek (2007), it was observed that Japanese quails fed vitamin E supplement at 25 mg/kg under heat stress conditions had significantly higher final body weight, feed consumption and feed conversion ratio than those in the control treatment after 35 days of feeding trial. In terms of animal products, Sahin, Sahin, and Onderci (2002) found that supplementing vitamin E (at the level of 250 or 500 mg/kg) in quail's diet increased egg production, egg weight, egg-specific gravity, egg shell thickness, and Haugh unit in Japanese quails reared under conditions of heat stress (34°C). Likewise, supplementing quail diet with a combination of dietary lycopene and vitamin E (100 mg/kg lycopene plus 250 mg dl-α-tocopheryl-acetate/kg diet) reduced serum and yolk cholesterol concentrations and improved the antioxidant status of quails (Sahin et al., 2006). Improving the production of quail meat and its products with the inclusion of vitamin E supplements in their diet stands to promote its availability for human consumption.
7 FUTURE PERSPECTIVES
The use of vitamin E in the diet of livestock has proven to be of immense benefits in animal husbandry and their products. However, the bioavailability of vitamin E and the manifestations of its deficiency can be affected by antioxidant nutrients such as selenium and vitamin C (Combs Jr & McClung, 2016). The synergetic interaction between selenium and vitamin E nutrients may enhance the production of glutathione peroxidase, which is an important aspect of the antioxidant pathway. Thus, selenium nutrients should be considered when developing a feeding program especially with vitamin E inclusion to improve the oxidative stability of muscle and dairy food during storage. Selenium could be supplemented as part of the mineral program either as loose mineral, tubs, blocks, or various protein supplements.
Likewise, recent research interest is tending toward the supplementation of dietary selenium and vitamin E above the recommended levels under precise conditions implicated in curbing the growing oxidative challenge on livestock. However, great caution is required while designing such strategies, as the underlying oxidative status of farm animals may have implications for the outcome of such strategies.
Although largely known to be an antioxidant substance, vitamin E can likewise be a pro-oxidant. Vitamin E can even perform nonantioxidant functions including as a regulator of gene expression and as a signaling molecule. Equally, since the nomenclature, vitamin E includes a group of eight structurally related tocopherols and tocotrienols, individual isomers have different tendencies with respect to these novel, nonconventional functions. Important to note in future studies is the role particular beneficial effects of the individual isomers have to be considered when evaluating the physiological impact of dietary vitamin E or supplements (principally containing merely the alpha-tocopherol isomer) in livestock experimental trials.
Furthermore, little or no study has demonstrated the toxicity level of this vitamin E supplementation (alpha-tocopherol) in ruminants and there maybe need to conduct research in this regard. However, reports have shown that excessive supplementation of alpha-tocopherol maybe be detrimental to the other vitamin E forms in human. However excessive dietary vitamin E has been reported to lower the activities of antioxidant enzymes in red blood cells of rats fed salmon oil (Eder, Flader, Hirche, & Brandsch, 2002). Residual effect of vitamin E supplement on oxidative stability of meat and dairy products during thermal processing and cold storage has not been fully studied. Many authors have found that feeding livestock can increase the antioxidant content of their products; however, there is need to consider the behavior and residual content of vitamin E in muscle and dairy food after thermal application and processes.
Study has also shown that vitamin E supplementation had a beneficial effect in lowering the concentration of aldehydes, which are considered to be responsible for rancid off-flavors, in fresh meat. Moreover, the effect of dietary vitamin E supplementation on formations of volatile (especially aldehyde) compound during cooking has not been fully examined. In addition, it has been revealed that increased cooking temperature increases the formation of volatile compounds including aldehydes which is the most abundant compounds in cooked samples (Domínguez, Gómez, Fonseca, & Lorenzo, 2014). More so, the use of natural antioxidant has been reported to provide a strong inhibitory effect against the formation of heterocyclic aromatic amines in cooked meat products at high temperature (Viegas, Santos, Barreto, & Fontes, 2012). Therefore, application of vitamin E supplementation in livestock production as antioxidants could be a feasible solution to reduce the formation of heterocyclic aromatic amines and polycyclic aromatic hydrocarbons, which have been found to be mutagenic (i.e., they cause changes in DNA that may increase the risk of cancer) in cooked meat, with a consequent of being beneficial to human health. Study should be directed to the evaluation of the effect of vitamin E on formation of heterocyclic aromatic amines and polycyclic aromatic hydrocarbons in meat during cooking at high temperature. Furthermore, policy makers, researchers, and industries need to collaborate in novel research to produce antioxidant-enriched animal foods (including meat, milk, and their products) due to the fast demand for healthy animal protein and functional foods.
8 CONCLUSION
The incorporation of vitamin E into livestock diets is beneficial to lipid oxidative stability thus preserving muscle, egg, and dairy food quality, to boost human health after consumption. Evidence has shown that bioavailability of vitamin E–enriched animal products could serve as an invaluable nutritional benefit to consumers; especially those in regions of limited resources where vitamin E deficiencies pose a risk that may be detrimental to some cellular activities of the body and on human health. It is therefore essential to focus research on the impact of vitamin E supplementation as antioxidant on livestock performance and animal products.
ACKNOWLEDGMENT
The authors wish to thank Govan Mbeki Research Development Centre (GMRDC); Agricultural Rural Development Research Institute, University of Fort Hare and the South African System Analysis Centre (SASAC, 2018), for their intellectual impact and financial assistance.
