Vitamin E

Abstract

Vitamin E, the collective name for a group of fat-soluble compounds with distinctive antioxidant activities, is found naturally in some foods, added to others, and available as a dietary supplement. Naturally occurring vitamin E exists in eight chemical forms (alpha-, beta-, gamma-, and delta-tocopherol and alpha-, beta, gamma-, and delta-tocotrienol), but alpha-tocopherol is the only form that meets human dietary requirements. Vitamin E functions as an antioxidant, protecting cells from the damaging effects of free radicals that may contribute to the development of cardiovascular disease and cancer. In addition, vitamin E is involved in immune function, cell signaling, regulation of gene expression, and other metabolic processes. Alpha-tocopherol also inhibits the activity of protein kinase C, an enzyme involved in cell proliferation and differentiation in smooth muscle cells, platelets, and monocytes that helps blood vessels resist components that may adhere to their surface. Vitamin E also increases the expression of two enzymes that suppress arachidonic acid metabolism and may inhibit platelet aggregation. Numerous foods provide vitamin E, with nuts, seeds, and vegetable oils among the best sources of alpha-tocopherol. Significant amounts of alpha-tocoperol are also available in green leafy vegetables and fortified cereals. Most vitamin E in American diets is in the form of gamma-tocopherol from soybean, canola, corn, and other vegetable oils and food products. Naturally sourced vitamin E is referred to as RRR-alpha-tocopherol (or d-alpha-tocopherol); the synthetically produced form is all rac-alpha-tocopherol (or dl-alpha-tocopherol). The RDA for vitamin E varies by age, ranging from 4 mg for young infants (<6 mo) to 15 mg for those 14 years or older. One mg vitamin E (alpha-tocopherol) is equivalent to 1 mg RRR-alpha-tocopherol or 2 mg all rac-alpha-tocopherol. Vitamin E is listed on the new Nutrition Facts and Supplement Facts labels in milligrams rather than in the traditional international units (IUs). FDA required manufacturers to use the new labels starting in January 2020, but companies with annual sales of less than $10 million may continue to use the old labels until January 2021. One IU of natural RRR-alpha-tocopherol is equivalent to 0.67 mg of alpha-tocopherol; 1 IU of synthetic all rac-alpha-tocopherol is equivalent to 0.45 mg alpha-tocopherol. Supplements typically provide only alpha-tocopherol, although “mixed” products containing other tocopherols and tocotrienols are also available. A given amount of synthetic alpha-tocopherol (all rac-alpha-tocopherol, often labeled as “DL” or “dl”) is only half as active as the same amount (by weight in mg) of the natural form (RRR-alpha-tocopherol; often labeled as “D” or “d”). Most vitamin E–only supplements provide 67 mg or greater (100 IU of natural vitamin E), which is higher than the RDA. In addition, alpha-tocopherol in dietary supplements and fortified foods is often esterified to prolong shelf life. These esters (alpha-tocopheryl acetate and succinate) are hydrolyzed and absorbed by the body as efficiently as alpha-tocopherol. The Food and Nutrition Board at the Institute of Medicine of The National Academies suggests that mean intakes of vitamin E among healthy adults are probably higher than the RDA but cautions that low-fat diets might provide insufficient amounts unless people make their food choices carefully by, for example, increasing their intakes of nuts, seeds, fruits, and vegetables. Vitamin E deficiency is rare, and overt deficiency symptoms have not been found in healthy people who obtain little vitamin E from their diets. Because fat is required in the digestive tract to absorb vitamin E, people with fat-malabsorption disorders are more likely to become deficient. Symptoms include peripheral neuropathy, ataxia, skeletal myopathy, retinopathy, and impairment of the immune response. Patients with Crohn’s disease, cystic fibrosis, or an inability to secrete bile from the liver into the digestive tract sometimes require water-soluble forms of vitamin E, such as tocopheryl polyethylene glycol-1000 succinate. Ensure that patients understand that high doses of alpha-tocopherol supplements can cause hemorrhage and interrupt blood coagulation, as well as inhibit platelet aggregation. Doses of up to 1,000 mg/d (1,500 IU/d of the natural form or 1,100 IU/d of the synthetic form) in adults appear to be safe, although data are limited. Because vitamin E may interact with several types of medications, including anticoagulant and antiplatelet meds, simvastatin and niacin, and chemotherapy and radiotherapy drugs, patients should consult with their physician before taking vitamin E supplements. Claims have also been made about vitamin E’s potential to prevent and treat disease, including heart disease, cancer, eye disorders, and cognitive decline. Ensure patients understand that further work is needed to confirm these claims and the safety of vitamin E supplementation.