Vitamin E and bone health: Not just alpha-tocopherol

Abstract

Vitamin E collectively refers to eight chemically distinct isoforms: a, ß, ? and d tocopherols and tocotrienols. Because a-tocopherol is the most abundant form of vitamin E in human tissues, most studies of vitamin E's antioxidant effect on bone have only investigated the effects of a-tocopherol and many supplements do not contain the other isoforms. Despite this, alpha-tocopherol shows mixed results in animal and human studies of bone, with the most positive outcomes obtained under conditions of oxidative stress. Some studies even show a detrimental effect but because so few human studies measure baseline or follow-up blood levels, it is impossible to determine whether the study population were in deficiency or excess. The only human intervention study to measure blood levels showed that supplementation caused an excess, which was dose-dependently associated with suppression of vitamin K-dependent proteins, thereby increasing risk of osteoporosis. It is likely that a-tocopherol will prove to have a U-shaped dose/benefit curve, as with some other fat-soluble vitamins. The remaining isoforms show greater promise than alpha-tocopherol for bone health, particularly because supplementation of a-tocopherol can suppress their bioavailability. In particular, ?-tocopherol and a- and ?-tocotrienol have shown improved results for bone relative to a-tocopherol.