Nutrition Lecture 42: Vitamin A Importance

Lecture 42: Fat-Soluble Vitamins 2 - Importance of Vitamin A

• Specific Objectives: Students will understand vitamin A functions, explain visual cycles, and discuss dietary deficiency effects.

Food Sources of Vitamin A

• Animal Sources: Liver, cream, butter, cheese, and egg yolk are excellent sources.
• Vegetable Sources: Carrots, papaya, mango, pumpkins, and spinach contain beta-carotene, a precursor to vitamin A.

Active Forms of Vitamin A

• Retinol: A primary alcohol.
• Retinal: An aldehyde derived from retinol oxidation.
• Retinoic Acid: An acid derived from retinal oxidation.
• β-carotene: Found in plant foods, it's oxidatively cleaved in the intestine to yield two molecules of retinal.

Absorption and Transport

• Transport to the Liver: Dietary retinol esters are broken down to release retinol and free fatty acids in the intestinal lumen. These retinols are re-esterified to long-chain fatty acids and secreted as components of chylomicrons that travel to the liver and adipose tissue. Retinyl ester is stored there.

• Release from the Liver: When needed, retinol is released and transported to extrahepatic tissues by retinol-binding protein (RBP). The RBP complex attaches to specific receptors on cells in peripheral tissues, permitting retinol entry.

Visual Cycle

• Vitamin A's Role: Vitamin A is essential for visual pigments in rod and cone cells.
• Rhodopsin: The visual pigment in rod cells in the retina consists of 11-cis retinal bound to the protein opsin.

Vitamin A Functions

• Vision: Retinal is the active form essential for normal vision.
• Growth: Retinoic acid plays a role in tissue growth and differentiation.
• Reproduction: Retinol is necessary in males for spermatogenesis and in females to prevent fetal resorption. Deficiency can cause impotence.
• Antioxidant: Carotenoids in fresh vegetables may reduce cancer incidence.

Dietary Deficiency Effects

• Night Blindness: One of the earliest signs, the visual threshold increases, making dim-light vision difficult. Prolonged deficiency causes irreversible visual cell loss.
• Xerophthalmia: A severe vitamin A deficiency. It's characterized by a dry conjunctiva and cornea, and untreated, can lead to corneal ulceration and eventually blindness.

Other Effects

• Acne and Psoriasis: Retinoic acid, or its derivatives, are used in treatments.

Vitamin A Toxicity

• Hypervitaminosis A: Excessive intake creates a toxic syndrome. Amounts exceeding 7.5 mg/day of retinol should be avoided.
• Chronic Symptoms: Early signs include dry, itchy skin, an enlarged and potentially cirrhotic liver and neurological signs (intracranial pressure mimicking brain tumor). Excessive intake is particularly hazardous to pregnant women.

This quiz explores the critical role of Vitamin A in human health. Students will learn about its functions, food sources, active forms, and the physiological impact of deficiency. Prepare to discuss how Vitamin A affects visual cycles and overall metabolic functions.