Vitamin E is required for growth and reproduction, it helps to maintain the structural tissue integrity, it supports development of the nervous system and contributes to disease resistance. Furthermore it enhances the nutritional value of meat and eggs.
By G. Litta, T K Chung and GM Weber,
DSM Nutritional Products, Switzerland
Vitamin E is present in natural form in the lipid fraction of feed ingredients, albeit it is susceptible to oxidation. Vitamin E cannot be synthesised by poultry and pigs and thus must be ingested via their diet. The commercial form of vitamin E for animal feed supplementation is all-rac-α-tocopheryl acetate, an ester which protects the
sensitive active α-tocopherol moiety against oxidation.
Metabolism of vitamin E
Vitamin E is well recognised and accepted as nature's most effective lipid-soluble, chain-breaking antioxidant, protecting cellular membranes from being attacked by lipid peroxyl radicals. It is actively absorbed in the gut: absorption depends on a number of factors but on average, achieves 42% expressed as α-tocopherol equivalents.
Vitamin E is packaged into lipid-bile micelles and is transported via the lymphatic pathway to the liver, where it is transiently stored. Vitamin E is deposited, in a dose-dependent way, in cellular and subcellular membranes (mitochondria, microsomes), which are rich in fatty acids and thus becomes an integral part of these structural elements, beneficially influencing the fluidity, the structural integrity and the functionality of biological membranes in all cells of the organism.
The symptoms of vitamin E deficiency are mainly related to disorders of the cellular membrane, due to the oxidative degradation of polyunsaturated fatty acids, and can have serious consequences. Vitamin E deficiency generally causes liver necrosis and is the reason for several species-specific disorders such as exudative diathesis (abnormal permeability of the capillary walls) and encephalomalacia ("crazy chick syndrome", a fatal aberrant development of the brain) in poultry. Sub-clinical vitamin E deficiency is hardly detectable but will result in retarded growth and impaired feed conversion as well as diminished fertility, higher susceptibility to infectious diseases, reduced stress resistance and impaired welfare of farm animals.
It has been observed that supra-nutritional supplementation of vitamin E has beneficial effects for example on heat stress.
Vitamin E requirements may increase under conditions of heat stress. In layers, subjected to high ambient temperatures, the basic vitamin E requirement was found to be higher. With chronic heat stress, a vitamin E supplementation of 500 mg/kg was necessary to alleviate the adverse effects on egg production and egg weight.
If the supplemented diet was provided before, during and after heat stress, a reduced dietary dose of 250 mg vitamin E per kg was optimum for alleviating the adverse effects of chronic heat stress in laying hens. Research showed that dietary supplementation of vitamin E at levels between 125 and 300 mg/kg minimised reduction in egg production, feed efficiency and eggshell density. When broiler chickens were subjected to heat stress, supplementing starter feeds with 200 mg vitamin E per kg was found to minimise peroxidation of erythrocytes in various tissues at 3 and 5 weeks of age.
The role of vitamin E for the functionality of the immune system has been extensively studied by several research groups and is based on the capability of vitamin E to prevent lipid peroxidation in membranes caused by lipid peroxyl radicals. Infectious diseases are an important factor in the production of free radicals for example as a consequence of macrophage function.
It was observed that 300 mg/kg vitamin E improved immune response and reduced mortality in poultry facing an E. coli challenge. Similar effects were obtained, with dosages between 100 and 300 mg/kg, in cases of colibacillosis, coccidiosis, and listeriosis in turkeys. Vitamin E also improved the immune response to vaccinations against Newcastle disease and bronchitis (Figure 1, see below). It has been shown that vitamin E promotes the phagocytic activity of macrophages, especially at thymus level and is involved in other immune mechanisms mediated by cells. Chicks from hens supplemented with 300 mg/kg vitamin E presented better humoral immunity and more active lymphocytes. Broilers infected with the virus causing malabsorption syndrome suffered less damage and recovered sooner if the breeders had received a higher vitamin E supplementation.
Vitamin E is necessary for the normal functioning of the reproductive system. There is evidence that the hormone regulation involved in the reproductive cycle of the birds is also involved in the metabolism of vitamin E. In breeders the vitamin E supplied in the ration, once absorbed, is transported to the liver and, from there, to the developing oocyte by very low density lipoproteins (VLDL). It has been demonstrated that there is a high efficiency in incorporating α-tocopherol from the maternal feed into the yolk of the chicken egg. The quantity of vitamin E in the egg increases linearly with the consumption of vitamin E by the hen. Supplementation of vitamin E above the minimum requirement improves ovulation during the last phase of the laying period, improving the bird's defence system and averting negative consequences on egg production also in situations of environmental stress.
Vitamin E protects against oxidation in sperm, egg yolk and embryonic tract. The supply of vitamin E in the ration should be continuous, as the hepatic reserve is insufficient to maintain an adequate vitamin concentration in all eggs, when laying rate is high. Inadequate vitamin E in the diet of breeders is detrimental to fertility, giving rise to the production of eggs with low hatchability and high embryonic mortality in the last phase of incubation due to failures related to the circulatory system.
Thus an adequate supply of vitamin E in the diets of breeders has a clear effect on their progeny. It is worth to remember that the immune, digestive and endocrine systems of newly hatched chicks are not completely developed until they are 7–10 days old. In particular, vitamin E is important for preventing lipid oxidation and stimulating immune defences during and after hatching. A direct relationship has been demonstrated between the level of vitamin E in the liver of a day-old chick and its viability. Chicks coming from breeders which consumed rations deficient in vitamin E presented exudative diathesis at hatching: in other words the deficiency had arisen during incubation. Supplying vitamin E in the breeder's diet is especially critical in turkeys since vitamin E absorption is low, so that not only the level in the egg but also the transfer to the embryo and the reserve in newly hatched turkeys is lower than those described in chickens. For these reasons, vitamin E supplementation in the rations of female breeder turkeys has important positive repercussions and is able to increase the vitamin E reserve in the progeny during the first 10 days of life.
Enrichment in eggs
An interesting concept to make animal derived food nutritionally more valuable is the enrichment of eggs with micronutrients. Eggs are a particularly nutritious food and represent a perfect vehicle for the transfer of high-quality nutrients to human subjects. When vitamin E, which is associated with health benefits for humans, was supplemented to laying hens at dietary levels of up to 400 mg/kg, alpha-tocopherol increased from the control level of 144 mcg/g of yolk to 477 mcg/g of yolk. When feeding very high doses of vitamin E (100, 1,000, 10,000 and 20,000 mg/kg) over a period of 20 weeks, vitamin E content of eggs increased from 1 to 4, 21, 46 and 51 mg per egg, which represents a considerable enrichment of eggs with vitamin E.
The beneficial effects of including high levels of α-tocopherol acetate in feed on the oxidative stability and sensory quality of meat have been extensively researched in poultry. Oxidation processes are responsible for the occurrence of unpleasant smells and flavours, for changes in the nutritive value (reduction in the meat's polyunsaturated fatty acid and fat-soluble vitamin content) and colour of the meat and even for the appearance of components which are potentially harmful to health, such as cholesterol oxides (COPs). The amount of published material on all of these aspects is vast and important aspects now seem clear such as optimum levels and period of supplementation and the effects on composition, oxidative stability and sensory quality of the meat.
Oxidative stability of meat
Most of the published studies on vitamin E and meat quality have been aimed at studying its effects on oxidative stability, which in poultry meat is lower than for beef or pork due to its higher polyunsaturated fatty acid content. Supplementation with high levels of vitamin E is even more important if the fat incorporated in the feed is altered by oxidation or heat. The risk of oxidation is also increased by the manufacturing processes of meat products (eg. freezing, mincing, adding salt, pre-cooking or meat irradiation) reducing its vitamin E content and accelerating oxidation, already intense in just three days. Vitamin E supplementation in feed is much more effective in maintaining oxidative stability than adding it to meat post-mortem since by that route it will not be physiologically and naturally incorporated in cellular membranes.
A significant negative correlation has been found between the thiobarbituric acid-reactive substances (TBARS) values obtained and vitamin E ingestion. Reduction of TBARS in fresh chicken meat normally ranges from 40% to 90% and from 39% to 66% in processed and/or precooked products, respectively. According to Barroeta (2007), the use of 200 mg/kg of vitamin E prevents 84–88% of TBARS development. Similar level of supplementation also reduces oxidised cholesterol compounds (COPs) in chicken meat by 50%. As the TBARS method has been criticised for not being sufficiently specific, using more sophisticated methods, such as the determination of the profile of volatile substances, it has been established that higher levels of vitamin E in feed reduce the concentration of secondary oxidation products (aldehydes and ketones) by around 50%. It is obvious that the efficacy of vitamin E supplementation increases in relation to dosage and period of administration.
Accordingly, it was demonstrated that 225 mg/kg for three weeks before slaughter is equivalent to 150 mg/kg in the last 32 days, which may be more economical (Figure 2, see below). A number of alternative ingredients have been tested including β-carotene, oregano oil, tea catechins and grape extracts. Although many ingredients succeeded in reducing TBARS values, the supplementation of vitamin E at a dosage of 200 mg/kg was superior by between 30% and 50%.
Nutritional value of the meat
The most immediate effect of supplementing vitamin E above the levels recommended for normal growth is the enrichment of poultry meat with this vitamin which improves its nutritional merit. The increase is more rapid and greater in the darker muscles of the thigh and leg which have a higher fat content. Vitamin E is deposited in cellular membranes, where the process of oxidation begins, and in subcellular organelles such as mitochondria and microsomes. The enrichment of meat with vitamin E is directly proportional to its levels in the diet and the length of the supplementation period. It has been experimentally calculated that every 100 mg/kg additional vitamin E in the diet increases the vitamin E content of meat to reach 7% of the recommended daily human intake.
Some studies have directly evaluated the effects of high levels of vitamin E on the sensory quality of meat using objective methods and/or panels of trained tasters. Vitamin E supplementation induced a lower drop in postmortem pH, which indicates a potential use in the prevention of pale, soft and exudative meat syndrome (PSE) a problem which is now more prevalent in turkeys than in broilers. Some trials have shown that drip losses diminished even when supplements were given only in the last 2–3 weeks of fattening.
Different authors found an improvement in meat taste using 160 mg/kg vitamin E compared to a level of 20 ppm. An improvement in flavour was detected with just 80 ppm and this continued up to 160 ppm. Continuous use of 200 ppm vitamin E instead of 50 ppm significantly improved perceptions of texture, succulence, flavour, abnormal flavours and general acceptability and that evaluation of flavour corresponded with the lowest concentration of aldehydes, especially hexanal. These effects are less pronounced in very fresh meat, but clearly evident with storage times of 7–12 days. An improvement in the colour stability of meat may also be expected although the meat of birds, especially breast meat, has lower pigment content.