Vitamin E occurs in at least eight molecular forms (tocopherols or tocotrienols); in humans the most biologically active form has generally been considered to be alpha-tocopherol, which is also the most common. All forms exist as light yellow, viscous oils. The best sources are vegetable oils. Other sources include green leafy vegetables, wheat germ, some nuts, and eggs. Vitamin E is necessary for the maintenance of cell membranes. It is essential to normal reproduction in some animals, but there is no evidence that it plays a role in human reproduction. It is a potent antioxidant; numerous studies have pointed to a protective effect against arterial plaque buildup and cancer. It is helpful in the relief of intermittent claudication (calf pain) and in preventing problems peculiar to premature infants. In large doses, it has an anticoagulant effect. The recommended daily dietary allowance for adults is 10 mg (tocopherol equivalents) for men and 8 mg for women, but nutritionists and physicians sometimes recommend higher doses for disease prevention.