Vitamin A - Function, Benefits, Sources, Deficiency

Vitamin A, a fat-soluble vitamin, is essential for human health, playing a critical role in vision, immune defence, reproduction, and cellular communication. It includes two primary types, retinoids (active forms found in animal-derived foods) and carotenoids (precursors found in plants). Both forms are necessary for the body’s optimal functioning, with a deficiency or excess leading to significant health implications.

Chemical nature and types of vitamin A

Vitamin A refers to a group of unsaturated organic compounds, including: 

• Retinoids: Retinol, retinal, retinoic acid (active forms in animal sources). 
• Carotenoids: Beta-carotene, alpha-carotene, and others, which the body converts into retinol. 

These compounds are fat-soluble, requiring dietary fats for absorption. They are stored in the liver and released into the bloodstream as needed.

Vitamin A functions

Vitamin A is essential for maintaining healthy vision, immune function, and cell growth. It also plays a crucial role in skin health and the proper functioning of various organs. Some of the key functions of vitamin A are as mentioned below:

• Vision and eye health: Vitamin A is crucial for maintaining healthy vision, as it helps form rhodopsin, a protein in the retina that enables low-light and color vision. The role of vitamin A in vision and eye health are:

• Rhodopsin formation: Vitamin A forms rhodopsin, a pigment in the retina that enables vision in low-light conditions. 
• Corneal maintenance: It keeps the cornea—the transparent front part of the eye—moist and healthy, preventing conditions like xerophthalmia and blindness. 
• Phototransduction: Retinal, a vitamin A derivative, converts light into electrical signals essential for vision.

• Immune function: Vitamin A is essential for maintaining epithelial integrity, acting as a barrier against pathogens, and regulating immune responses, ensuring optimal white blood cell activity across the respiratory, gastrointestinal, and genitourinary tracts.
• Cell growth and differentiation: As a regulator of gene expression, retinoic acid ensures proper growth, repair, and differentiation of epithelial cells in tissues such as the skin, lungs, and intestines. 
• Reproductive health: In men, vitamin A supports spermatogenesis, the production of healthy sperm, and in women, it facilitates normal ovulation and embryonic development during pregnancy.
• Bone and skin health: Vitamin A promotes bone remodeling and enhances skin repair by stimulating keratinocyte differentiation and reducing keratin buildup.
  

Dietary recommendations of vitamin A

The Recommended Dietary Allowance (RDA) for vitamin A daily requirement, expressed in retinol activity equivalents (RAE), varies by age, gender, and physiological state: 

• Adult men: 900 mcg RAE/day 
• Adult women: 700 mcg RAE/day 
• Pregnant women: 770 mcg RAE/day 
• Lactating women: 1,300 mcg RAE/day

Infants and children have lower requirements, but their needs increase with age.

Vitamin A benefits

Vitamin A offers a wide range of health benefits, contributing to overall well-being. Here are some of its key benefits.

• Supports vision: Adequate vitamin A prevents night blindness and may reduce the risk of age-related macular degeneration (AMD), a leading cause of blindness among older adults. 
• Boosts immune system: Vitamin A deficiency weakens immunity, increasing susceptibility to infections like measles and respiratory illnesses. Its Supplementation has been shown to reduce mortality in children.
• Promotes healthy skin: Vitamin A helps maintain skin integrity, reduces acne, and manage conditions like psoriasis by regulating cell turnover.
• Enhances growth and development: it is essential for skeletal growth in children and foetal organ development during pregnancy, reducing the risk of birth defects. 
• Antioxidant properties: Provitamin A carotenoids, particularly beta-carotene, neutralize free radicals, lowering oxidative stress and potentially reducing the risk of chronic conditions like cardiovascular disease and cancer. 

Vitamin A sources

Vitamin A is an essential nutrient that plays a crucial role in maintaining immune function, good vision, and overall health. It comes in two forms: provitamin A carotenoids (beta-carotene) and preformed vitamin A (retinol). The following are some of the key dietary sources of vitamin A:

Provitamin A Carotenoids (Beta-Carotene) Sources

Plant-based foods:

• Orange and yellow vegetables: Carrots, sweet potatoes, pumpkins, and butternut squash are excellent sources.
• Dark leafy green vegetables: Spinach, kale, and collard greens provide high levels of beta-carotene.
• Bright-coloured fruits: Mangoes, papayas, and apricots are rich in carotenoids. 
• Fortified foods: Almost all breakfast cereals and infant formulas are commonly fortified with vitamin A. Margarine and cooking oils are fortified in some regions to combat deficiencies.

Preformed Vitamin A (Retinol) Sources

Animal-based foods

• Liver: Beef, fish and chicken liver are the richest sources. A small portion can meet the RDA. 
• Dairy products: Milk, cheese, yogurt and butter provide moderate amounts of retinoids. 
• Eggs: Egg yolks are an accessible source of vitamin A. 
• Fish oils: Cod liver oil is highly concentrated with retinol.
• Fatty fish (salmon, herring): These are good sources of vitamin A, also excellent sources of omega-3 fatty acids, which are good for heart health.

Vitamin A supplements

Vitamin A supplements are easily available and serve as a controlled source of nutrition.

Tips for Incorporating Vitamin A-Rich Foods into Your Diet

• Eat a variety of colorful fruits and vegetables: Aim for at least 5 (five) servings per day.
• Include dairy products in your diet: Choose low-fat or fat-free options.
• Opt for fatty fish: Consume them at least twice a week.
• Consider fortified foods: Check labels for vitamin A content.
• Cook vegetables with a little fat: This helps your body absorb vitamin A better.

Note:

• Too much vitamin A can be harmful: It's important to consume vitamin A in moderation.
• Consult a general physician: If you have any concerns about your vitamin A intake or overall health.
• By incorporating a variety of vitamin A-rich foods into your diet, you can support your overall health and well-being.
  

WHO classification of vitamin A deficiency

Vitamin A deficiency can manifest clinically as xerophthalmia, which includes night blindness, Bitot’s spots, and more severe conditions like corneal xerosis and keratomalacia. Night blindness, especially common during pregnancy in developing countries, is an early clinical sign. Retinol, the primary form of vitamin A in the blood, reflects liver stores but is tightly regulated and may not correlate with dietary intake. Serum retinol levels are used to assess subclinical deficiency; levels below 0.70 μmol/L indicate subclinical deficiency, while levels below 0.35 μmol/L signal severe deficiency.

Causes of vitamin A deficiency

• Poor dietary intake, especially in developing countries. 
• Malabsorption disorders like celiac disease, Crohn’s disease, or chronic pancreatitis. 
• Chronic illnesses or infections, which increase the body’s vitamin A requirements. 
• Alcoholism, which depletes liver stores of vitamin A. 

Vitamin A deficiency disorders

• Night blindness: The earliest sign, where individuals struggle to see in low light. 
• Xerophthalmia: Dryness of the eyes, progressing to corneal ulcers and blindness. 
• Keratinization: Thick, scaly skin due to impaired epithelial cell function. 
• Immunodeficiency: Increased susceptibility to infections like respiratory illnesses and diarrhea. 
• Growth retardation: In children, it impairs growth and development.

Vitamin A deficiency treatment

Vitamin A deficiency (VAD) can be treated with vitamin A supplements and improved dietary intake. Supplements, often given as a single high-dose treatment (100,000–200,000 IU for children, 25,000–50,000 IU for adults), are common in areas with high deficiency rates. Dietary sources rich in vitamin A include liver, eggs, dairy, and fish, as well as beta-carotene-rich foods like carrots, sweet potatoes, and leafy greens. Additionally, fortified foods and nutrition programs can help prevent and treat VAD, ensuring long-term health benefits. 

Global impact

Vitamin A deficiency affect lakhs of children and pregnant women in low-income countries, contributing to preventable blindness and child mortality. 

Management strategies

• Vitamin A capsules: High-dose vitamin A capsules or vitamin A tablets are distributed in regions with endemic deficiency. 
• Dietary diversification: Encouraging the consumption of vitamin A-rich foods. 
• Food fortification: Adding vitamin A to staples like rice, sugar, and cooking oil.
  

Vitamin A overdose and toxicity

Excessive intake of vitamin A can lead to overdose and toxicity, which may cause a range of harmful health effects such as:

Hypervitaminosis A

Hypervitaminosis A refers to the condition caused by an excessive intake of vitamin A, leading to toxicity. This can occur from consuming high doses of vitamin A supplements or large amounts of foods rich in preformed v itamin A (retinol), such as liver.

• Causes
• Overuse of vitamin A supplements. 
• Excessive consumption of liver or fortified foods. 
• Prolonged use of retinoid-based medications. 

• Symptoms of toxicity
• Acute toxicity: Symptoms appear within hours or days of large doses, including nausea, vomiting, dizziness, and blurred vision. 
• Chronic toxicity: Long-term overconsumption causes dry skin, hair loss, bone pain, hepatotoxicity, and intracranial pressure (pseudotumor cerebri). 
• Teratogenic effects: Excess vitamin A during pregnancy can cause congenital disabilities, including neural tube defects and craniofacial malformations. 

Carotenemia

Excessive beta-carotene can cause carotenemia, a harmless condition where the skin turns yellowish orange. Unlike retinoids, it does not lead to toxicity since conversion to active vitamin A is tightly regulated.

Striking the right balance of vitamin A

Balancing vitamin A intake is essential to leverage its health benefits without risking toxicity. This can be achieved through: 

• Dietary awareness: Including diverse sources of vitamin A, particularly in regions prone to deficiency. 

• Cautious supplementation: Ensuring supplements are used judiciously, particularly in populations with sufficient dietary intake. 



• Public health interventions: Targeted fortification programs in communities with widespread deficiency.
  

Balancing vitamin A: Addressing deficiency and preventing toxicity

Vitamin A is a cornerstone nutrient for human health, supporting vision, immunity, cellular function, and growth. While its deficiency remains a significant global health issue in low-income regions, overconsumption poses risks in populations with easy access to supplements and fortified foods. 



Efforts to combat vitamin A deficiency through dietary diversification, supplementation, and food fortification have proven effective in improving public health outcomes. At the same time, awareness of the risks of toxicity is crucial to preventing adverse effects. Maintaining a balanced intake is the key to harnessing the full potential of this vital nutrient, ensuring optimal health across diverse populations.