Vitamin A

Retinol - vitamin A alcohol

retinyl ester, vitamin A ester

retinal, vitamin A aldehyde

retinoic acid, vitamin A acid.

b-Carotene is absorbed by the intestinal lymphatics;

Retinol is esterified inside mucosal cells and is stored in the liver as retinyl palmitate

this in turn is hydrolyzed to free retinol for transport to its site of action.

Zinc is required for this mobilization.

Carotenemia may result in yellow discoloration of the skin but not of the sclera. - likely to occur in children with liver disease, diabetes mellitus, or hypothyroidism and congenital absence of enzymes that convert provitamin A carotenoids.

Etiology.

colostrum and breast milk furnish large amounts of the vitamin.

Breast milk and cow's milk are sources of vitamin A.

Other foods -vegetables, fruits, eggs, butter, liver

Deficient diets commonly cause disease by age 2–3 yr. Vitamin A deficiency also results from inadequate intestinal absorption, - chronic intestinal disorders or fat malabsorption.

Pathology.

The human retina contains rods & cones

The rods are sensitive to light of low intensity,

cones to colors and to light of high intensity.

Retinal is the photosensitive pigment in both rods and cones.

The visual pigments in rods (rhodopsin) and in cones (iodopsin)

in darkness -  All-trans retinal isomerizes to 11-cis form. It combines with opsin to form rhodopsin. Energy from light reconverts 11-cis retinal to the all-trans form

this energy exchange, transmitted via the optic nerves to the brain, results in visual sensation.

retinitis pigmentosa may be related to a defect in retinol-binding protein.

Retinoids are essential for cell differentiation

Vitamin A has a role in keratinization, cornification, bone metabolism, placental development, growth, spermatogenesis, and mucus formation.

changes in epithelium - proliferation of basal cells, hyperkeratosis, and the formation of stratified, cornified squamous epithelium.

Epithelial changes in the respiratory system may result in bronchiolar obstruction. Squamous metaplasia of the renal pelves, ureters, urinary bladder, enamel organs, and pancreatic and salivary ducts may lead to an increase in infections in these areas.

Clinical manifestations of Vitamin –A deficiency

Eye, Anemia, skin, urinary tract, intracranial

Ocular The posterior segment of the eye is first affected, = impairment of dark adaptation resulting in night blindness.

Later, drying of the conjunctiva (xerosis conjunctivae) and of the cornea (xerosis corneae) is followed by wrinkling and cloudiness of the cornea (Keratomalacia)

Dry, silver-gray plaques may appear on the bulbar conjunctiva (Bitot spots) and photophobia.

Retardation of mental and physical growth and in apathy.

Anemia with or without hepatosplenomegaly

The skin is dry and scaly, and follicular hyperkeratosis may at times be found on the shoulders, buttocks, and extensor surfaces of the extremities.

The epithelial metaplasia of the urinary tract may contribute to Pyuria and hematuria. Increased intracranial pressure with wide separation of cranial bones at the sutures

Hydrocephalus, with or without paralyses of the cranial nerves

Diagnosis.

Dark adaptation tests may be helpful.

Dry conjunctivae can be detected by microscopic examination of the conjunctiva.

The plasma carotene concentration falls

Prevention

Infants - 500 mg of vitamin A daily

Older children and adults, 600–1500 mg of vitamin A

At 9 months – 1lakh units with measles vaccine

Every 6 months 2 lakh units until third birthday

Treatment

2 lakh units on day 1, day 2 and 14

Hypervitaminosis A

Acute hypervitaminosis A may occur in infants after ingesting 100,000 mg or more.

symptoms are nausea, vomiting, drowsiness, and bulging of the fontanel.

Diplopia, Papilloedema, cranial nerve palsies, -pseudo tumor cerebri

Toxicity has occurred – during Vitamin a supplementation with measles vaccine administration

Chronic hypervitaminosis A -after ingestion of excessive doses for several weeks or months.

Child has anorexia, pruritus, and a lack of weight gain.

Increased irritability, limitation of motion, and tender swelling of the bones.

Alopecia, seborrheic cutaneous lesions, fissuring of the corners of the mouth, increased intracranial pressure, and hepatomegaly

Craniotabes and desquamation of the palms and soles are common.

X ray -- hyperostosis affecting long bones; - the middle of the shafts

congenital malformations may occur in infants of mothers consuming large amounts of oral retinoids used in treating acne.

Serum vitamin A level is elevated

Hypercalcemia or liver cirrhosis occurs occasionally.