A number of compounds share the activity of vitamin E including α-, β-, γ- and δ-tocopherol and α-, β-, γ- and δ-tocotrienol. While the tocopherols have a saturated phytyl tail attached to a chromanol head, the tocotrienols have an unsaturated tail that causes the structure to kink. This affect the biological properties of the tocotrienols, particularly their ability to interact within membranes. The main forms of vitamin in plasma are α- and γ-tocopherol, with the former being the most biologically active isomer. The plasma concentrations of vitamin E reflects dietary intakes. All mammals required vitamin E, which functions as the primary lipid soluble antioxidant in cell membranes. Vitamin E deficiency is rare in humans because the vitamin is stored in adipose tissue. During periods of low intake a net release of vitamin E from adipose tissue maintains plasma levels above that which would cause deficiency. However, animals fed vitamin E deficient diets for long periods develop reproductive abnormalities. Other functions for vitamin E beyond its role as an antioxidant have been hypothesised, but little research has been conducted in this regard.