ABSTRACT
Vitamins A and D are lipophilic substances required in a continuous supply for specialized but essential functions in the animal body. Though vitamins A and D carry out separate functions in the body, the two tend to be associated nutritionally because of their fat-soluble nature and because the two are often combined in foodstuff supplements and pharmaceutical formulations. More is known about the age-related changes in vitamin A absorption and transport than about the effects of aging on the metabolic enzymes. Thus, inadequate dietary vitamin D intake and inadequate solar exposure appear to contribute to low vitamin D status in the elderly. Endogenously produced vitamin D is vitamin D3, whereas dietary vitamin D can be either vitamin D3 of animal origin or vitamin D2 from plant sterols. However, mild vitamin A deficiency caused by a fall in the plasma Retinoblastoma Binding Protein-retinol level is usually associated with impairment of dark adaptation.
