Key lipophilic vitamins to health are b-carotene, cholecalciferol, and tocopherol. b-Carotene is cleaved to form retinal, which in turn gives rise to retinol and retinoic acid, both of which play vital roles in epidermal biology including cell proliferation and epidermal differentiation. In vitro and in vivo studies have demonstrated that of the retinoids tested those most effective in altering the levels of cellular retinoic acid-binding protein in the skin were also capable of significantly altering hair cycle dynamics.