Vitamin E is known for its antioxidant properties that can protect sensitive compounds from oxidative degradation, however, at elevated concentrations vitamin E acts as a pro-oxidant that accelerates the oxidative degradation process. This is understood in terms of the mechanism of radical formation that is responsible for the bioactivity of vitamin E (Ouchi et al., 2009; Chapman et al., 2009).

In the case of an unsaturated fatty acid such as linolenic acid that undergoes oxidation by a peroxyl radical intermediate, the vitamin E can interact with this structure to prevent oxidation of the fatty acid by deactivating the intermediate fatty acid peroxyl radical. However, the process leads to the formation of a vitamin E radical. This vitamin E radical is an activated form of vitamin E that can now act as a pro-oxidant. When the vitamin E radical interacts with another unsaturated fatty acid the oxidative degradation process of the fatty acid can be initiated. In biological systems the vitamin E radical would be deactivated by another compound such as vitamin C which would regenerate the antioxidant form of vitamin E. Such coupled pathways provide a mechanism to deactivate the vitamin E radical and maintain the antioxidant activity of vitamin E.

Antioxidant compounds such as vitamin E are needed to protect unsaturated lipid compounds from oxidation. Highly unsaturated lipids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are particularly susceptible to oxidation due to their degree of unsaturation. The oxidation of unsaturated lipids generates malodorous compounds and leads to off-flavours and the loss of bioactivity of the original lipid structures. DHA and EPA have shown numerous health benefits when included in the diet. These benefits include improved nervous system development and cardiovascular health (Harris, 1989; Leaf, 1990; Martinez, 1995). This has generated interest in their use as feed additives (Rising et al., 1990; Kim et al., 1993; Watanabe, 1993; Enjalbert et al., 1997). Fish oils are a rich source of polyunsaturated fatty acids and especially of the more highly unsaturated fatty acids such as DHA. However, when fish oils are used in the feed formulations of laying hens to improve the nutritional value of the eggs there is some decrease in sensory quality reported as fishy off-flavours (Van Elswyk et al., 1995; Van Elswyk, 1997; Gonzalez-Esqerra and Leeson, 2000). This can be alleviated by using marine algae as the primary source of the polyunsaturated fatty acids rather than the fish oil (Herber-McNeill and Van Elswyk, 1998). Similar studies with flaxseed as a source of linolenic acid (ALA) have shown that the polyunsaturated fatty acids do accumulate in the eggs, however, there are anti-nutritional compounds in the flaxseed that have negative effects on production (Bean and Leeson. 2003). When isolated ALA was fed to Shaver hens a reduction in hatchability was observed that could be off-set by supplementing the diet with soybean oil (Muma et al., 2006).

The ability of vitamin E to prevent the oxidative degradation of the unsaturated lipids DHA and ALA was investigated in a series of laboratory experiments at elevated temperature. Mixtures of DHA, ALA, and vitamin E were incubated at 70°C to accelerate the oxidative process. Samples were analyzed periodically by gas chromatography with the following results. After 72 hours mixtures with 0.25 mg vitamin E/mg lipid retained 79.6 per cent of the initial amount of DHA and 94.2 per cent of the ALA. When vitamin E was present at 2.5 mg/mg lipid the amount of DHA decreased to 62.5 per cent and ALA decreased to 87.4 per cent. Mixtures that contained 5 mg vitamin E/mg lipid retained 55.9 per cent DHA and 77.0 per cent ALA. Control samples without vitamin E retained 43 per cent DHA and 52.6 per cent ALA. These results show that vitamin E acted as an antioxidant to limit the degradation of these polyunsaturated fatty acids. However, the protective effect clearly decreased with increased levels of vitamin E. This experiment was not extended to include mixtures with higher levels of vitamin E. where the pro-oxidant behaviour might be observed. If vitamin E acted as a pro-oxidant then the amount of DHA and ALA remaining would be lower than the controls that contained no vitamin E. This relationship needs to be given some consideration when formulating feeds with higher levels of vitamin E.

The addition of polyunsaturated acids to poultry feeds provides a straightforward approach to improve the nutritional quality of poultry meat and eggs. However, the highly unsaturated structure of these lipid compounds makes them susceptible to oxidative degradation which results in a loss of bioactivity and the development of volatile degradation compounds. Antioxidants can limit the oxidative degradation of unsaturated compounds when included in the feed formulation. Vitamin E possesses antioxidant properties through the ability to deactivate lipid radicals and is recommended as a feed supplement at 10 IU/kg feed for broilers by the National Research Council (NRC) in 1994. However, the beneficial amount of vitamin E in feed can vary with a number of factors including growth stage and environmental stress. Vitamin E can improve feed quality, production, and nutrition but should be evaluated carefully for optimal results.

References

• Bean, L.D. and S. Leeson. 2003. Long-term effects of feeding flaxseed on performance and egg fatty acid composition of brown and white hens. Poult. Sci. 82:388-394.
• Enjalbert, F., M.C. Nicot, C. Bayourthe, M. Vernay and R. Moncoulon. 1997. Effects of dietary calcium soaps of unsaturated fatty acids on digestion, milk composition and physical properties of butter. J. Dairy Res. 64:181–195.
• Chapman, T.M., H.J. Kim and D.B. Min. 2009. Prooxidant activity of oxidized alpha-tocopherol in vegetable oils. J. Food Sci. 74:C536 – C542
• Gonzalez-Esqerra, R. and S. Leeson. 2000. Effect of feeding hens regular or deodorized Menhaden oil on production parameters, yolk fatty acid profile, and sensory quality of eggs. Poult. Sci. 79:1597-1602.
• Harris, W.E. 1989. Fish oils and plasma lipid and lipoprotein metabolism in humans: a critical review. J. Lipid Res. 30:785-807.
• Herber-McNeill, S.M. and M. E. Van Elswyk. 1998. Dietary marine algae maintains egg consumer acceptability while enhancing yolk color. Poult. Sci. 77:493-496.
• Kim, Y.K., D.J. Schingoethe, D.P. Casper and F.C. Lundens. 1993. Supplemental dietary fat from extruded soybeans and calcium soaps of fatty acids for lactating cows. J. Dairy Sci. 76:197–204.
• Leaf, A. 1990. Cardiovascular effects of fish oils. Beyond the platelet. Circulation. 82:624-628.
• Martinez, M. 1995. Polyunsaturated fatty acids in the developing human brain, erythrocytes and plasma in peroxisomal disease: therapeutic implications. J. Inher. Metab. Dis. 18(Suppl. 1):61-75.
• Muma, E., S. Palander, M. Nasi, A-M. Pfeiffer, T. Keller and J.M. Griinari. 2006. Modulation of conjugated linoleic acid-induced loss of chicken egg hatchability by dietary soybean oil. Poult. Sci. 85:712-720.
• Ouchi, A., M. Ishikura, K. Konishi, S. Nagaoka and K. Mukua. 2009. Kinetic study of the prooxidant effect of alpha-tocopherol. Hydrogen abstraction from lipids by alpha-tocopheroxyl radical. Lipids 44:935–943.
• Rising, R., P.M. Maiorino, R. Mitchell and B. L. Reid. 1990. The utilization of calcium soaps from animal fat by laying hens. Poult. Sci. 69:768–773.
• Van Elswyk, M.E., P.L. Dawson, and A.R. Sams, 1995. Dietary menhaden oil influences sensory characteristics and headspace volatiles of shell eggs. J. Food Sci. 60(1):85–89.
• Van Elswyk, M.E., 1997. Comparison of n-3 fatty acids sources in laying hen rations for improvement of whole egg nutritional quality: A review. Br. J. Nutr. 78(Suppl. 1):S61–S69.
• Watanabe, T. 1993. Importance of docosahexaenoic acid in marine larval fish. J. World Aquacult. Soc. 24:152–161.

This article was first published on Feedinfo.com

April 2010