Hypovitaminosis A: contemporary scientific issues.

Contemporary scientific issues relating to vitamin A can be divided into three groups dealing with metabolism, physiology, and public health and policy. Current metabolic issues include intestinal interactions with other nutrients and food components, the pathways and control of carotenoid metabolism, the mechanisms underlying transport homeostasis, the functions of binding proteins, the actions of nuclear receptors in gene expression, and the pathways, mechanisms and control of isomerization reactions. Contemporary physiologic issues include the role of retinoids in embryogenesis, mechanisms involved in vision, the role of vitamin A in the immune response, therapeutic applications of retinoids, and mechanisms underlying the toxicity of retinoids. Public health and policy issues include the effects of vitamin A on mortality and morbidity among children in various societies, appropriate means for the assessment of vitamin A status, and suitable criteria for setting recommended dietary intakes.