Vitamin A

Basic Information

• Chemical Properties
  • Active vitamin A forms (retinol, etc.) are lipid-soluble, meaning they require fat for absorption and are stored in the liver and adipose tissue.
  • Provitamin A carotenoids are water-soluble in plant matrices and are broken down into retinol in the small intestine, with conversion efficiency varying by individual and dietary factors.
  • Vitamin A is sensitive to heat, light, and oxygen; prolonged cooking or exposure to air can reduce its bioavailability in foods.
   
• Food Sources
  • Preformed vitamin A (direct active forms): Found in animal products, including liver, dairy products, eggs, and fatty fish like salmon.
  • Provitamin A carotenoids: Abundant in colorful plant foods, such as carrots, sweet potatoes, spinach, kale, mangoes, and apricots. Beta-carotene is the most common and efficiently converted form.
   

Physiological Functions

1. Vision Health

    

   Vitamin A is a core component of rhodopsin, a light-sensitive protein in the retina’s rod cells. It supports low-light and color vision; deficiency can lead to night blindness and, in severe cases, irreversible corneal damage (xerophthalmia).
    
2. Immune System Regulation

    

   It maintains the integrity of mucosal barriers (e.g., respiratory, gastrointestinal, and urinary tracts) and supports the function of immune cells like lymphocytes. Adequate intake enhances resistance to infections and reduces the risk of infectious diseases.
    
3. Cell Growth and Differentiation

    

   Retinoic acid regulates gene expression related to cell growth and differentiation, which is essential for embryonic development, tissue repair, and the maintenance of healthy skin, hair, and nails. It is widely used in skincare products to treat acne and reduce signs of aging.
    
4. Reproductive Health

    

   Vitamin A plays a vital role in reproductive processes, supporting sperm production in males and normal fetal development, placenta formation, and lactation in females.
    

Deficiency and Toxicity

• Deficiency Symptoms
   
  Mild deficiency may cause night blindness, dry skin, and increased susceptibility to infections. Severe, long-term deficiency can lead to xerophthalmia (dry eyes progressing to corneal ulceration and blindness), growth retardation in children, and impaired reproductive function. Deficiency is more common in regions with limited access to animal products and diverse fruits/vegetables.
• Toxicity
   
  Unlike water-soluble vitamins, excess vitamin A accumulates in the liver, leading to hypervitaminosis A. Acute toxicity (from very high doses in a short time) can cause nausea, vomiting, headache, and blurred vision. Chronic toxicity (from long-term high-dose supplementation) may result in bone pain, hair loss, liver damage, and birth defects in pregnant women. Provitamin A carotenoids have low toxicity risk, as the body limits their conversion to retinol; excess intake may only cause harmless skin discoloration (carotenemia).

Recommended Intake