Vitamin E (α-Tocopherol) - Nutrition

What is Vitamin E (α-Tocopherol)?

Vitamin E is a category of essential fat-soluble vitamins referred to as tocopherols. There are four stereoisomers of tocopherols found in nature (α, β, γ, and δ). Of these, α-tocopherol has the highest biological activity. Similar to other fat-soluble vitamins, vitamin E is incorporated into mixed micelles along with dietary fat and absorbed by diffusion across the mucosal surface of the small intestine. Absorbed vitamin E is then incorporated into chylomicrons and released into the lymphatics for transport to the liver, though some absorption into the portal circulation occurs as well. Within the liver selective α-tocopherol-binding proteins will incorporate α-tocopherol into very low density lipoproteins (VLDLs); α-tocopherol-binding proteins have only limiting binding of β-, γ-, or δ-isomers. The resultant α-tocopherol laden VLDLs transport α-tocopherol throughout the body. Vitamin E is primarily excreted through bile in faeces, though significant amounts of the metabolite α-tocopheric acid can also lost through urine.

Why is it Important?

Vitamin E is an antioxidant that protects cellular membranes lipid peroxidation by free radicals. Roles in the body

1. Antioxidant: Reactive oxygen species (e.g., peroxide, superoxide, and nitric oxide radicals) are formed during normal cellular respiration. These free radicals can cause damage to membrane-bound polyunsaturated fatty acids (PUFAs) as well as deoxyribonucleic acid (DNA). Membrane and intracellular vitamin E is able to donate a hydrogen electron to help prevent or stop propagation of this cellular damage. Oxidized α-tocopherols can be regenerated within the cell by other antioxidant systems, such as glutathione and vitamin C.
2. Cell Signalling: Aside from its role as an antioxidant, α-tocopherol is also an inhibitor of protein kinase C in platelets. The presence of high concentrations of α-tocopherol in endothelial cells also down-regulates the intracellular and vascular cell adhesion molecules. The combination of these two effects can result in inhibition of platelet aggregation.

Consequences of Vitamin E Deficiency

Dogs:

Vitamin E deficiency in dogs can develop anorexia, reproductive failure, skeletal and endocardial muscle degeneration, retinal degeneration, dermatitis, and subcutaneous oedema. Dogs with concurrent intestinal disease affecting absorption of dietary fat (i.e., a protein-losing enteropathy) as well as dogs with liver disease are at a higher risk of developing relative α-tocopherol deficiencies despite adequate dietary intake.

Cats:

Clinical signs of vitamin E deficiency in cats and kittens include anorexia, depression, myopathy, and pansteatitis (i.e., painful nodular inflammation of adipose tissue). The level of vitamin E required to prevent clinical sign of deficiency is directly related to the level of dietary PUFAs.

Influence of Diet:

The metabolic requirement for vitamin E is dependent on the PUFA concentration in the diet as well as the degree of peroxidation that occurs during processing and storage. Diets high in fat and specifically high in long-chain omega-3 PUFAs will increase the tocopherol requirements in the diet. Recycling of α-tocopherol is also impaired with concurrent high intakes of vitamin C (a water-soluble vitamin that is not a dietary required for dogs and cats); supplementation with vitamin C may increase vitamin E requirements.

Toxicity

There are no published reports of vitamin E toxicity in dogs, though in cats high levels of dietary vitamin E can resulting in prolonged bleeding times. High dosage of vitamin E supplementation in people has also been associated with increased risk of mortality, though this effect has not been studied in dogs and cats.

Dietary Sources

Nuts, seeds and seed oils have high concentration of α-tocopherol. Mixed-tocopherols (i.e., combination of δ- and α-tocopherol) are used in commercial pet foods due to increased stability with processing and storage compared to α-tocopherol alone. The concentration of the biologically active α-tocopherol in mixed-tocopherol can range from 10-40% of the total vitamin E content. Mixed-tocopherols are effective at preventing lipid oxidation that occurs during the processing and storage of foods, but depending on the source may not provide adequate concentrations of α-tocopherol to the diet.

Diagnosing Vitamin E Deficiency

Confirmation of Vitamin E deficiency is made by measuring plasma α-tocopherol concentration, although this is not routinely available in most veterinary reference laboratories. Physical examination may reveal consist skin lesions and a diagnosis may be confirmed via biopsy of nodules. A suspicion of a deficiency may arise when there is a presence of clinical signs consistent with deficiency and evaluation of diet demonstrates a deficiency.