Vitamin E and selenium (Se) are essential micronutrients that play a key role in the maintenance of the oxidative balance. The oxidative balance refers to the balance between the production of free radicals and the antioxidant capacity of the organism.

Free radicals are involved in numerous cellular functions and are necessary for oxide-reduction reactions, phagocytosis, collagen and prostaglandins synthesis, as well as for the functioning of various enzymes functioning, among others.

However, when the production of free radicals exceeds the antioxidant capacity of the organism, composed of enzymes, such as glutathione peroxidase (GSH-Px) or superoxide dismutase (SOD), and nutrients, such as vitamin E and selenium, a phenomenon known as oxidative stress takes place, which results in a wide variety of disorders and diseases.

1. Oxidative stress

Free radicals or reactive oxygen species (ROS) occur as a result of the aerobic metabolism of organisms and are involved in various cellular reactions. However, these highly reactive molecules can cause damage at different levels. At a tissue level, they react with polyunsaturated fatty acids in cell membranes.  At a cellular level, they react with DNA nucleotides. Finally, at a molecular level, they react with the sulphuric bonds of proteins.

Under physiological conditions, the production of free radicals is controlled by different antioxidant agents. Oxidative stress occurs when the production of free radicals exceeds the antioxidant capacity of the organism.

An antioxidant is any molecule capable of preventing, delaying or eliminating the oxidative damage. Antioxidants work by donating electrons to free radicals and reactive oxygen species (ROS), preventing them to react with other macromolecules.

The body has an antioxidant system composed of enzyme antioxidants, among which are superoxide dismutase (SOD), catalase and glutathione peroxidase (GHS-Px), and non-enzymatic antioxidants, such as vitamin E, selenium, vitamin C, glutathione, etc.

2. Structure and mechanism of action

The vitamin E comprises the set of two types of compounds, tocopherols and tocotrienols. Chemically, these compounds are composed of an hydroquinone nucleus with an isoprenoid in the secondary chain. α-tocopherol is the most biologically active compound and is the main component of commercial vitamin E.

Vitamin E is essential for growth, reproduction and the immune system. It is the main fat-soluble antioxidant associated with membranes, preventing the formation of free radicals from lipids and unsaturated fatty acids, avoiding the oxidative stress. It works by capturing the unpaired electron present in free radicals.

Lipids, especially phospholipids, are an important component of cell membranes. Vitamin E prevents oxidation and destruction of these components and ensures the structural integrity of cells.

Selenium is an essential micronutrient that is part of a set of enzymes known as selenoenzymes. In food, it is generally associated with aminoacids, mainly methionine and, in a smaller proportion, with cysteine. More than 25 selenoenzymes with antioxidant and anabolic function have been described. Among them, the glutathione peroxidase (GSH-Px), cytosolic enzyme that plays a key role in maintaining oxidative balance.

Selenium acts as a secondary antioxidant in the cell cytosol and mitochondrial matrix, destroying hydrogen peroxides and organic hydroperoxides through GSH-Px, enzyme of which it is cofactor. There are four isoforms of GSH-Px, each with a different location. The GSH-Px cellular or classic, present inside almost all cells, the extracellular or plasma GSH-Px, the GSH-Px phospholipid hydroperoxide, which prevents the peroxidation of the lipids that make up the cell membranes, and the gastrointestinal GSH-Px, important in reducing cholesterol hydroperoxides and in the protection from lipid hydroperoxides present in the food.

3. Nutritional requirements

To establish the vitamin E and selenium requirements in different animal species is a difficult task due to their interactions with other dietary components. The requirements vary depending on the dietary content of polyunsaturated fatty acids, antioxidants and amino acids, as well as management practices (which can increase the stress to which animals are subjected), breed, lineage and productive stage.

4. Related pathologies

Vitamin E and selenium deficiencies cause tissue alterations that primarily affect the muscle, the liver and the nervous tissue. The most common pathological consequences of the oxidative stress in porciculture and poultry are described below.

• Mulberry heart disease (MHD)

It is the most common disease related to vitamin E and selenium deficiency in swine production. It is characterized by the sudden death of animals, without previous symptoms. In certain cases, muscle weakness and dyspnoea may be observed. It is a fast-running disease (1-3 days) that affects pigs of all production stages. However, it is more common in piglets, especially those with good performance. Postmortem findings are characterized by extensive ecchymosis and bleeding at the cardiac level, with necrotic areas (whitish) along with pericardial and pulmonary edema.

• Fatty liver disease

It is a nutritional disease that mainly affects piglets under 3 months. Like MHD, it is characterized by sudden mortality with hardly any associated symptoms. In some cases dyspnoea, vomiting, diarrhoea, lettargia and jaundice may be observed prior to the death of the animals. Diagnosis is based on postmortem findings, which are characterized by wide necrotic areas of the liver capsule and surface, necrosis of hepatocytes, bleeding, liver paleness and pulmonary edema.

• Nutritional muscle dystrophy (NMD)

Nutritional muscle dystrophy or white muscle disease is a nutritional myopathy associated with vitamin E and selenium deficiency. In the case of pigs, the disease is usually present in conjunction with MHD and dietary hepatosis.

It mainly affects sows and piglets. Clinically, it is characterized by causing alterations in the locomotion of the piglets, observing the loss of muscle striations and the fragmentation of the muscle fibers along with mineral deposition. The most often affected muscles are the longissimus muscle, the adductors and the diaphragm.

In birds mainly affects the striated musculature of the breast and thighs, observing white-yellow muscle fibers due to Zencker’s characteristic necrosis. In certain cases, necrosis may be observed in the smooth musculature of the gizzard.

• Encephalomalacia

It is a disease that causes nervous symptomatology, associated with vitamin E and selenium deficiency in poultry. It usually occurs in 15 to 30-day-old birds, although it may occur earlier. It is characterized by ataxia, imbalance, clone spasms and rotated head. Postmortem findings show bleeding and coagulative necrosis in the cerebellum. In many cases, the presentation of disease is associated with high levels of polyunsaturated fat in the diet.

• Exudative diathesis

Exudative diathesis occurs because of the affection of the capillary wall of tissues due to vitamin E and selenium deficiency and is more common in young animals. The disease is characterized by the formation of gelatinous subcutaneous edemas, especially in the chest, abdomen and submandibular space.

5. Conclusions

Oxidative stress is a pathological state generated by an imbalance between the formation of free radicals and the antioxidant capacity of the organism, which results in tissue alterations at different organic levels.

The animal organism has an antioxidant system made up of enzymes and nutrients that maintain the oxidative balance. The intensive breeding conditions to which animals are subjected make it necessary to provide nutrients with antioxidant activity.

Vitamin E and selenium are the essential nutrients with the highest antioxidant activity and, therefore, their addition in the feed is essential to avoid the different pathologies associated with their deficiency, as well as to maintain adequate productive levels.