Chapter 45 Vitamin A Deficiencies and Excess
Overview of Vitamins
Vitamins are essential organic compounds that are required in very small amounts (micronutrients) and are involved in fundamental functions in the body, such as growth, maintenance of health, and metabolism. A vitamin can have several functions. Because our bodies cannot biosynthesize vitamins, vitamins must be supplied by the diet or as supplements. The dietary reference intakes (DRIs) for infants and children are summarized in Table 41-5. Vitamins are not chemically similar. Based on their chemical properties, they are classified as either water-soluble or fat-soluble; these 2 groups are handled differently by the body. The water-soluble vitamins (except vitamin C) are members of the B complex.
Deficiency states in developed countries are rare, except in some impoverished populations (Chapter 43) or after mistakes in food preparation or with fad diets, but they are common in many developing countries and are often associated with global malnutrition (Chapter 43). In the clinical setting, vitamin deficiencies can also occur as complications in children with various chronic disorders or diseases. Information obtained in the medical history related to dietary habits can be important in identifying the possibility of such nutritional problems. Except for vitamin A, toxicity from excess intake of vitamins is rare. The food sources, functions, and deficiency and excess symptoms of the vitamins are summarized in Tables 45-1 and 48-1.
Vitamin A
Absorption, Transport, Metabolism, Storage
The body acquires vitamin A either as preformed vitamin A (usually as esters) or as provitamin-A carotenoids. In the USA, grains and vegetables supply approximately 55% and dairy and meat products supply approximately 30% of vitamin A intake from food. Vitamin A and the provitamins-A are fat-soluble, and their absorption depends on the presence of adequate lipid and protein within the meal. Chronic intestinal disorders or lipid malabsorption syndromes can result in vitamin A deficiency. Ingested and absorbed provitamins-A are bioconverted to vitamin A molecules in the small intestine by the carotene cleavage enzyme dioxygenase; β-carotene provides twice the vitamin A activity of the other provitamins-A. Further processing in the enterocyte involves the esterification of vitamin A to retinyl palmitate for incorporation into chylomicrons, which are released into lymph and transported via the circulation to the liver for storage or to other tissues. The vitamin A content in the liver is low at birth, but it increases 60-fold during the first 6 mo of life. If the growing child has a well-balanced diet and obtains vitamin A from foods that are rich in vitamin A or provitamin-A (see Table 45-1), the risk of vitamin A deficiency is small. However, even subclinical vitamin A deficiency can have serious consequences.
