Vitamin E is available in natural and chemically synthesized forms. Natural vitamin E is mainly used in pharmaceuticals and chemicals, food and beverages, and health and beauty products. The feed industry mostly uses chemically synthesized vitamin E, namely DL-a--tocopherol. Natural vitamin E has 8 analogues, including 4 tocopherols and 4 tocotrienols. All eight analogues contain a 6-chroman ring and a side chain. The more methyl groups on the chroman ring, the higher its biological activity. Scientists have measured the vitamin activity of different tocopherols and tocotrienols through various experiments. Because the functions being studied differ, their observed relative activities also differ.
Among all currently available forms of vitamin E, the a-tocopherol isomer has the highest activity. It is followed by β-tocopherol, with an activity of 12%-40%. The activities of several other compounds are much lower, such as: γ-tocopherol, 3%- 20%; β-tocopherol, 0.3%～1%; a-tocotrienol, 17%～29%; β-tocotrienol, 1%～5%; β-tocotrienol has essentially no vitamin activity.
The most widely used commercial vitamin E in the feed industry is DL-α-tocopheryl acetate with a content of 50%. In layer production, the actual amount of vitamin E added varies considerably because of vitamin E’s broad range of functions and the different objectives nutritionists have for its physiological functions in layer feed. Research findings on the vitamin E requirements of layers differ substantially among countries and researchers. This is mainly because, when studying vitamin E requirements in layers, the choice of evaluation criteria can lead to widely varying results, even within the same laboratory.
The recommended addition level for 1 is 5—10 mg/kg. The target function is to prevent vitamin E deficiency in layers.
Vitamin E deficiency in laying hens is clinically characterized mainly by encephalomalacia in chicks, exudative diathesis, and white muscle disease. With the widespread use of compound vitamins for livestock and poultry, vitamin E deficiency in laying hens has become very rare in commercial layer production.
The US NRC recommends adding 5～10 mg]kg of vitamin E for laying hens, while Feedstuffs in the US recommends 6 mg/kg. China's recommended amount in its nutrient requirements standard for laying hens is 5 mg/kg. This indicates that adding 5～10 mg/kg of vitamin E to layer feed is generally sufficient to meet normal growth requirements and effectively prevent typical vitamin E deficiency.
Recommended addition level: 2 to 10 one 30 mg/kg. The target function is to promote reproductive performance in laying hens and increase the laying rate.
The Ministry of Agriculture’s “Specifications for the Safe Use of Feed Additives” recommends a vitamin E dosage of 10 to 30 mg/kg in feed for laying hens. DSM’s recommended vitamin E dosage for OVN laying hen feed is 20. 30 mg/kg.
Vitamin E is closely related to the reproductive function of laying hens (as its chemical name, tocopherol, indicates). It can promote the function of gonadotropin-secreting cells in the anterior pituitary, increase gonadotropin secretion, regulate reproductive function, improve ovarian function, and increase the number of follicular cells, thereby enhancing the reproductive and egg-laying capacity of laying hens.
Vitamin E can also affect the synthesis of steroid hormones and prostaglandins, promote sperm formation in breeding roosters and enhance sperm motility, improve the fertilization rate of breeding hens, and promote embryonic development. When the vitamin E content in the diet is insufficient, the laying rate, egg fertilization rate, and hatchability of laying hens may decrease; more weak chicks may hatch, and chick survival rates may be low.
Some researchers believe that adult chickens do not show typical clinical and pathological symptoms even when consuming low doses of vitamin E, but the hatchability of hatching eggs declines markedly, and embryo mortality is more likely to occur early in incubation (on day 4 of incubation). This is because days 3–4 of incubation coincide with the formation of the allantois. The embryo transitions from anaerobic to aerobic respiration, making it prone to producing large amounts of oxygen free radicals. These cause oxidative damage, leading to embryo mortality.
3 Recommended addition amount: 50–200 mg/kg. The target functional positioning is antioxidant.
Vitamin E is listed in the Vitamins and Vitamin-like Substances category and the Antioxidants category of the Catalogue of Feed Additives (2013).
Vitamin E has strong antioxidant properties, which are primarily manifested in three ways: scavenging free radicals, blocking lipid peroxidation, and enhancing the activity of glutathione peroxidase.
The integrity of cell membranes and organelle membranes is an important foundation for cells to perform their physiological functions. The special localization of vitamin E in biological membranes enables it to scavenge lipid peroxyl radicals very efficiently. Vitamin E binds closely to the highly hydrophobic hydroxyl region of biological membranes, positioning the center of the chromanol ring right at the cell membrane surface. The chromanol ring here is highly versatile: on the one hand, it can capture free radicals and terminate chain reactions; on the other hand, it can capture and collect other lipid-soluble (such as coenzyme Q) and water-soluble (such as vitamin C and glutathione) reducing substances. When the cell membrane is attacked by free radicals, vitamin E can capture the free radicals, causing the phenolic group on its benzopyran ring to lose a hydrogen atom and form a tocopherol radical, which then reacts further with another free radical to form the non-radical product tocopherol. This prevents lipid chain reactions from continuing and protects the cell membrane. Vitamin E can exert antioxidant activity also because vitamin E radicals can be reduced by other reducing molecules in the body back to their original active form. These reducing substances mainly include vitamin C, glutathione, NADPH, and coenzyme Q.
Current research results indicate that oxidation reactions mainly occur in the cell membrane system. Adding high doses of vitamin E to the diet can increase the vitamin E content in the mitochondria and microsomes of laying hen tissue cells, thereby enhancing the stability of the antioxidant response in subcellular membrane structures. When the vitamin E content in the blood of laying hens increases, plasma SOD activity decreases significantly, which can effectively improve the body's antioxidant capacity.
As an antioxidant, vitamin E can protect cells from oxidative damage caused by reactive oxygen species and stop free radical reactions, thereby protecting the polyunsaturated fatty acids in cell membrane phospholipids and plasma lipoproteins from attack by oxygen free radicals. Therefore, vitamin E can ensure the integrity of cell membrane structure and function and resist stress damage caused by oxygen free radicals.
Vitamin E, as a natural biological antioxidant, can independently or in cooperation with other antioxidants protect polyunsaturated fatty acids in biomembrane phospholipids and plasma proteins. Its antioxidant mechanism is as follows: the phenolic hydroxyl group of tocopherol reacts with an organic peroxide group to form a stable lipid peroxide and a tocopherol peroxide group. This effectively interrupts the peroxidation chain, protecting fats from free-radical attack. The resulting lipid peroxide groups are then metabolized through non-radical reactions. The resulting tocopherol peroxide groups can be reduced back to tocopherol by reacting with hydrogen-atom donors, such as reducing agents including thiols and glutathione.
Vitamin E primarily participates in the body's free radical metabolism by inhibiting lipid oxidation in the body and repairing damage to membrane structures caused by free radicals.
Studies by Wang Xianhui et al. demonstrated that adding 200 mg/kg of vitamin E to the diet was most effective in improving the body's antioxidant capacity.
Additionally, for every increase of 1% in the fat content of layer feed, the amount of vitamin E added should increase by 30 mg/kg.
Recommended addition amount: 4–150 to 300 mg/kg; the target functional positioning is to promote immune F function and enhance stress resistance and disease resistance.
Research has found that adding a large dose of vitamin E to laying hen feed can raise antibody levels in the hens and enhance the phagocytic function of phagocytes. Levels of some cytokines associated with the immune response increase. The rate of mitosis in T cells and B cells increases, and the function of neutrophils and macrophages is enhanced.
Vitamin E can also enhance immunity by acting together with enzymes and peroxides on cyclooxygenase to reduce the rate of prostaglandin E2 synthesis. Vitamin E regulates immunity by reducing the release of adrenal corticosteroids. It helps resist stress. Vitamin E enhances animals’ immune capacity, in part by protecting cell membranes and organelle membranes from oxidative damage, maintaining the integrity and stability of cells and organelles, and ensuring normal cell function.
On the other hand, vitamin E can regulate the synthesis of cellular modulators, prostaglandins, thromboxane, and growth-promoting substances. Vitamin E can enhance the activity of macrophages in the body and promote the oxidative metabolism of neutrophils in the peripheral blood system.
Vitamin E also plays an important role in the development of the immune system. It promotes the proliferation of lymphocytes, particularly helper T lymphocytes, as well as the coordinated action of T and B lymphocytes and the proliferation of macrophages. Vitamin E can promote the development of the bursa of Fabricius in laying hens. It activates B-lymphocyte proliferation and participates in the conversion of antibody synthesis from IgG to IgM. Wen Jie et al. reported that adding vitamin E significantly increased the HI antibody titer in chicken serum, indicating that vitamin E supplementation can improve humoral immunity in chickens.
In addition, vitamin E has a generational effect on the activation of humoral immunity in chickens. When hens are fed high levels of vitamin E, maternal antibody levels in their offspring chicks increase. Cellular immunity is mainly mediated by T cells. Vitamin E can increase the number of peripheral blood T lymphocytes, which has a certain role in maintaining and enhancing cellular immunity.
Vitamin E can promote the maturation and differentiation of T lymphocytes, playing a key role in cellular immune regulation. High levels of vitamin E in the diet can increase the lymphocyte transformation rate in chicken blood. However, high doses of vitamin E can also reduce prostaglandin production, thereby decreasing intracellular adenylate cyclase activity and lowering the cAMP concentration in lymphocytes, which in turn reduces cortisol production.
For thyroid hormones, vitamin E may regulate their secretion by enhancing the body's immune function. It may also act directly on a particular site in the hypothalamus, anterior pituitary, or thyroid to affect their secretion and thereby regulate the body's immune function. Liu You et al. confirmed through heat stress trials that vitamin E can significantly reduce serum cortisol levels and increase thyroid hormone T3 and T4 concentrations, thereby affecting immune function. Heat stress trials also confirmed that vitamin E supplementation significantly increased thyroid hormone levels and significantly reduced serum cortisol levels in chickens, thereby alleviating heat stress in the body. Feeding chickens high doses of vitamin E was confirmed to significantly increase Newcastle disease hemagglutination inhibition antibody titers.
Adding high doses of vitamin E to layer diets significantly increases serum IgG levels, lymphocyte responsiveness, and the immune response to Newcastle disease. After chicks are injected intraperitoneally with sensitized red blood cells, supplementing their diets with different doses of vitamin E (10～300 mg/kg) increases their IgG and IgM antibody titers in a dose-dependent manner. Adding vitamin E at 150 mg/kg to hen diets raises the level of passively transferred IgM in their chicks and can stimulate the chicks to synthesize antibodies themselves. Vitamin E plays an important role in the immune system of poultry. It can effectively promote the growth and development of the spleen, thymus, and bursa of Fabricius, the immune organs of young poultry, and enhance humoral and cellular immune responses. This improves poultry health, reduces the occurrence of increased mortality and decreased production performance caused by various stress factors during production, and improves economic benefits. Adding vitamin E to diets can increase blood immunoglobulin levels and enhance the ability to produce antibodies against various vaccine antigens, as reflected in increased antibody titers and significantly higher antibody levels. Research has confirmed that broilers fed diets with high vitamin E content may preferentially produce CD4+ lymphocytes. CD4+ lymphocytes are the main helper cells in antibody production, demonstrating that vitamin E can enhance cellular immune responses in poultry.
DSM believes that the optimal vitamin E supplementation level for laying hens to achieve optimal immunity is 200 mg/kg. Bollengier's research found that, to prevent the effects of heat stress during hot seasons on laying hen performance and egg quality, the optimal vitamin E supplementation level in laying hen feed is 250 mg/kg. Adding 300 mg/kg of vitamin E to chick diets helps protect against bacterial and viral infections. For laying hens aged 140 days, feeding 300 mg/kg of vitamin E before vaccination against Newcastle disease can greatly improve the immune response and antibody titers. Experimental results show that, under stress conditions, adding 300 mg/kg of vitamin E to the diet can alleviate heat stress in laying hens and significantly reverse the decline in egg production.
5 Recommended addition amount: above 300mg/kg. The target function is to increase vitamin E deposition in egg yolks and improve egg quality.
Research has shown that the deposition of α-tocopherol in egg yolks increases linearly as the level of vitamin E supplementation in the diet increases. At the same time, adding vitamin E to the diet can significantly increase the α-tocopherol content in the tissues of laying hens and improve the nutritional quality of eggs. Sunder et al. reported that when different doses of vitamin E (0, 100, 1000, 10000 and 20000 mg/kg) were added to the diets of laying hens, tissue and egg-yolk α-tocopherol deposition increased significantly with increasing doses, with the greatest deposition in the egg yolk, followed by the liver and muscle. The high deposition of β-tocopherol in egg yolks is beneficial to embryo development during hatching of fertilized eggs. In addition, the deposition of α-tocopherol in the egg yolk, heart and liver also provides an effective means of producing functional eggs and other functional foods.
6 Conclusion
As scientific research into animal vitamin nutrition continues to deepen, further physiological functions of vitamin E in layer feed will be explored and discovered. To determine the optimal amount to add to compound feed, factors such as the layer breed, rearing environment, basal nutritional level, and the precise intended function of vitamin E should be considered together.