Chapter 38 - Vitamin Metabolism

Abstract

The word vitamin is used to describe any of the heterogeneous group of organic molecules that are required in small quantities for normal growth, reproduction, and homeostasis, which the human body is unable to synthesize in adequate amounts. The group includes the fat-soluble vitamins A, D, E, and K, and the water-soluble vitamins B-complex and C. Vitamins are generally needed in catalytic quantities and do not function as structural elements in the cell. In addition to nutritional inadequacy, vitamin deficiency may result from malabsorption, effects of pharmacological agents, and abnormalities of vitamin metabolism or utilization. Thus, in biliary obstruction or pancreatic disease, the fat-soluble vitamins are poorly absorbed despite adequate dietary intake because of steatorrhea. Absorption, transport, activation, and utilization of vitamins require the participation of enzymes or other proteins whose synthesis is under genetic control. Dysfunction or absence of one of these proteins can produce a disease that is clinically indistinguishable from one caused by dietary deficiency. Vitamin D is synthesized in the skin, provided radiant energy is available for the conversion. This pathway is adequate to supply the body's need for vitamin D, provided exposure to sunlight is adequate. Physiological age-related changes in the elderly can affect the nutritional status. Decreased active intestinal transport and atrophic gastritis impair the absorption of vitamins and other nutrients. Reduced exposure to sunlight can lead to decreased vitamin D synthesis and various drugs may impair both appetite and absorption of nutrients. Loss of night vision (nyctalopia) is an early sign of vitamin A deficiency, and clinical features of well-developed deficiency include epidermal lesions, ocular changes, growth retardation, glandular degeneration, increased susceptibility to infection, and sterility.