Vitamins A, D, E, and K are fat-soluble. As a class, fat-soluble vitamins are responsible for regulating protein synthesis. These vitamins require carrier proteins for transport. Vitamins A and D are carried by specific plasma proteins, and vitamins E and K are carried by plasma lipoproteins, mostly low-density lipoproteins. All of the fat-soluble vitamins are converted to active forms.
Deficiencies in these vitamins during infancy and childhood often result in numerous complications. However, the medical manifestations of fat-soluble vitamin deficiency are preventable if recognized early and treated appropriately.
Vitamin A is present in food. It is hydrolyzed in the small intestine by a pancreatic esterase and an intestinal hydrolase. Bile salts are required for activation of these enzymes. Human vitamin A requirements range from 10 to 30 mcg retinol per kilogram of body weight, although infants have higher requirements.
Vitamin A deficiency in infants causes night blindness, Bitot spots (keratinization of the cornea), xerophthalmia (dry eyes), keratomalacia, corneal opacities, growth failure, hyperkeratosis, reduced resistance to infection, and death. Laboratory tests may show a mild leukopenia. Clouding of the cornea in a child who has vitamin A deficiency is considered a medical emergency and requires parenteral administration of high doses of the vitamin.
Vitamin D is required to prevent rickets in children and osteoporosis and osteomalacia in adults. Rickets once was considered the most common disease of early childhood. With adequate light exposure, there is no need to supplement with vitamin D. Indeed, vitamin D is a facultative vitamin