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...

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