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Questions and Answers
Which of the following foods is NOT a good source of vitamin A?
Which of the following foods is NOT a good source of vitamin A?
Retinal is the primary form of vitamin A used in the visual cycle.
Retinal is the primary form of vitamin A used in the visual cycle.
False
What is the role of retinol-binding protein (RBP) in vitamin A transport?
What is the role of retinol-binding protein (RBP) in vitamin A transport?
It transports retinol from the liver to extrahepatic tissues.
Vitamin A can be found in plant foods as __________, which can be converted into retinal.
Vitamin A can be found in plant foods as __________, which can be converted into retinal.
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Match the following active forms of vitamin A with their descriptions:
Match the following active forms of vitamin A with their descriptions:
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What is one of the earliest signs of vitamin A deficiency?
What is one of the earliest signs of vitamin A deficiency?
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Excessive intake of vitamin A can result in a condition known as hypervitaminosis A.
Excessive intake of vitamin A can result in a condition known as hypervitaminosis A.
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What dermatologic problems are effectively treated with retinoic acid?
What dermatologic problems are effectively treated with retinoic acid?
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Severe vitamin A deficiency leads to __________, characterized by dryness of the conjunctiva and cornea.
Severe vitamin A deficiency leads to __________, characterized by dryness of the conjunctiva and cornea.
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Match the following symptoms or conditions with their corresponding effects related to vitamin A deficiency or toxicity:
Match the following symptoms or conditions with their corresponding effects related to vitamin A deficiency or toxicity:
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Study Notes
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
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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.
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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.
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Description
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.
