Introduction: Why Vitamin A Matters:
Vitamin A are important but playing many roles when it comes to health and survival, the body requires them in a small dose as micronutrients. Vitamin A is special when it comes to vitamins because of the many different roles it has in areas like, vision, growth, immunity, and well-being. Vitamins do not provide energy like macronutrients like proteins, carbohydrates, and fats; instead, vitamins act as catalysts to biological reactions for the body to be at its optimum.
Vitamin A is referred to as the “eye vitamin” because of its well-established role in vision especially in the prevention of night blindness. However, vitamin A contributes much more than vision – it plays a role in immune system regulation, cellular growth, reproduction, and skin health. No other vitamin has the duality of being the prevention of disease and danger of overdose like vitamin A does, which may be one of the reason its a very interesting nutrient in human nutrition.
In this article, we will review vitamin A including: chemical forms of vitamin A, physiological roles, whole food sources of vitamin A, recommended intakes of vitamin A, global deficiencies in vitamin A, risk for vitamin A toxicity, and finally it’s role in medicine and public health.
What is Vitamin A?
Vitamin A is a group of fat-soluble retinoids, not a single compound. Vitamin A retinoids include the following:
•Retinol, the active form that is stored in the liver
•Retinal, an essential component for vision
•Retinoic acid, which regulates gene expression and cell differentiation
•Retinyl esters, the storaged form of retinoids in animal tissues.
There are two main types of Vitamin A in the diet:
- Preformed Vitamin A (Retinoids): This form is obtained from animal products such as liver, fish oils, dairy, and eggs, and is easily absorbed and used in the body.
- Provitamin A (Carotenoids): These come from plants, such as beta-carotene as found in carrots, sweet potatoes, and leafy green vegetables, and will be converted to retinol in the body.
The efficiency of conversion of carotenoids to retinol varies tremendously from person to person based on genetics, dietary fat consumption.
Roles of Vitamin A in the Body
Vitamin A is a nutrient with multiple roles. Its roles can be grouped into six main roles, including:
1) Vision and Eye Health:
- Retinal (an active form of Vitamin A) combines with the protein opsin to form the visual molecule rhodopsin (the photopigment in the retina that mediates vision in the dark).
- The night blindness (an early symptom) associated with Vitamin A deficiency occurs when this process cannot function properly.
- If the deficiency continues for a long period, xerophthalmia (dry eyes) can occur as well as blindness (which is permanent).
2) Immune System Support:
- Vitamin A supports the ability to produce and function of white blood cells.
- Vitamin A helps facilitate the integrity of the skin and mucous membranes, the first line of defense against an infection.
- Children who are deficient in Vitamin A are more susceptible to measles, diarrhea, and respiratory infections.
3) Growth and Development:
- It is also essential for cell differentiation, the process where cells develop into their specific cell types.
- Furthermore, Vitamin A plays a role in the healthy growth and development of organs outwardly and inwardly, as during fetal growth.
- Children, who are low in Vitamin A often demonstrate poor growth.
4) Reproductive Health:
- In men, Vitamin A plays a role in sperm production.
- In women, it plays a role in promoting the development of the placenta and fetus.
- In pregnancy, index of a severe deficiency in pregnancy can be associated with maternal mortality. It can also lead to congenital birth defects.
- Skin and Cellular Health
• Retinoic acid was shown to regulate genes in epithelial cells. Thus, it is tied to maintaining epithelial tissues in the skin, lungs, and gastrointestinal tract.
• Topical retinoids (a derivative of Vitamin A) are considered high value in dermatoogy for treatment of acne, wrinkles and psoriasis.
Dietary Sources of Vitamin A:
Animal Sources (Preformed Vitamin A)
• Beef and chicken liver – among the highest sources
• Cod liver oil
• Animal products (milk, cheese, butter)
• Eggs (most concentrated in the yolk)
• Fish such as salmon, mackerel, and tuna
Plant Sources (Provitamin A Carotenoids)
• Carrots
• Sweet potatoes
• Spinach and kale
• Mangoes
• Pumpkin
• Red bell pepper
Fortified Foods:
In many countries, Vitamin A is found in added to:
• Margarine
• Breakfast cereals
• Infant formula
Supplements:
Vitamin A can be found in either capsule or multivitamin supplement forms. However, it is better to visit a physician to have a guided supplementation approach- all vitamin supplementation has the capability for toxicity.
Vitamin A Recommended Daily Allowance:
The Recommended Dietary Allowance (RDA) depends on age, gender and physiological status (e.g., pregnancy and lactation). Values are generally expressed in micrograms of Retinol Activity Equivalent (RAE).
• Infants (0 – 6 months): 400 mcg RAE
• Children (1 – 8 years old): 300 – 400 mcg RAE
• Males (14 years and older): 900 mcg RAE
• Females (14 years and older): 700 mcg RAE
• Pregnant women: 770 mcg RAE
• Lactating women: 1,300 mcg RAE
Vitamin A Deficiency:
Vitamin A deficiency is a major worldwide issue, especially in developing nations. The World Health Organization (WHO) estimates about 190 million preschool-age children and 19 million pregnant women globally with Vitamin A deficiency.
Signs of Deficiency:
• Night blindness
• Dry eyes (xerophthalmia)
• Corneal ulcers and blindness
• Dry skin and brittle hair
• Frequent infections due to reduced immunity:
• Reduced growth in stature in children at risk
Risk Groups:
• Children under 5 years old
• Pregnant and lactating women
• Populations living in regions where dietary diversity is low
• Patients with conditions related to fat malabsorption (e.g., cystic fibrosis, celiac disease)
Consequences of Deficiency:
• Increased mortality in children
• In blindness when severe
• In the body’s ability to fight off infection and disease:
• In pregnancy and childbirth complications.
Toxicity of Vitamin A :
Because vitamin A is a fat-soluble vitamin, excess amounts can be stored in the liver and accumulate to toxic levels that may pose a risk to health.
Causes of Toxicity :
• Prolonged supplementation with high-dose supplements
• Overconsumption of animal liver
• Inappropriate use of fortified food products
Signs and Symptoms of Toxicity
• Headaches, nausea, and dizziness
• Blurred vision
• Liver damage
• Pain in the bones and fractures
• Birth defects when consumed in higher amounts during pregnancy
Tolerable Upper Intake Level (UL)
• Adults: 3,000 mcg RAE/day
• Children: much lower based on age.
Vitamin A: medicine and public health:
1.Vitamin A Supplementation Programs:
In developing countries, programs for vitamin A supplementation are critical. High-dose capsules are given to children < 5 years old to prevent blindness and decrease mortality.
2.Contribution to Reducing the Childhood Death Rate:
It has been demonstrated that Vitamin A supplementation can decrease the child mortality rate by 23–34% in populations at risk of Vitamin A deficiency.
3.Cutaneous Applications:
Retinoids, of which Vitamin A is a derivative, are used in:
• the treatment of acne,
• the treatment of psoriasis, and
• cosmetic anti-aging skin creams.
4.Biofortification:
Projects such as Golden Rice (genetically modified rice that has been created to contain beta-carotene) seek to combat Vitamin A deficiency in populations that eat rice as a staple food.
Scientific Research and Future Perspectives:
Research continues in these areas of focus in regards to Vitamin A and its associated compounds:
• personlized nutrition – the role genetics play in Vitamin A metabolism,
• immune modulation – potential for use in decreasing inflammation and autoimmune disease,
• cancer prevention – some studies suggest carotenoids may decrease risk of some types of cancer, though the evidence is mixed,
• biotechnology – advancements in crop biofortification may allow for the eventual elimination of vitamin A deficiency globally.
The Story of Vitamin A:
The story of Vitamin A is an uplifting tale of how science changed during the early part of the twentieth century.
• In the 1800s, scientists observed that animals fed diets deficient in certain components developed diseases of the eye and died.
• By 1913, researchers had identified a factor in butter and egg yolk that was “fat-soluble factor A” that was associated with growth.
• Within the next few years, Vitamin A was identified and became one of the first vitamins to be defined.
The discovery was a huge milestone in nutrition because it showed that the microscopic, invisible compounds in food could govern life and health.
Vitamin A Compounds:
Vitamin A may not be a single compound, but a group of related fat-soluble compounds. There are two main types:
- Preformed Vitamin A (Retinoids):
• Retinoids include retinol, retinal, and retinoid acid
• Preformed vitamin A is derived from foods from animal sources including liver, dairy, and fish oils.
• These compounds are ready for immediate use by the body. - Provitamin C Carotenoids:
• These are plant-based pigments that the body can convert into vitamin A.
• The most well-known carotenoid is beta-carotene which is most commonly found in carrots, sweet potatoes, and leafy green vegetables.
• Carotenoids are also known to be antioxidants and a precursor for vitamin A.
Conclusion: Finding A Balance
Vitamin A is a double-edged sword: indispensable for vision, defense, and growth, and yet dangerous at excessive levels. Nutritional deficiency is still a prevalent and significant issue in many developing areas of the world, while vitamin A toxicity from supplements is a relevant public health concern in wealthier countries.
The best possible way to ensure balance is through balanced nutrition:
- Depend on a variety of natural food sources (as many colorful vegetables, fruits, dairy products, fish, and eggs combined as possible).
- Consider dietary supplements only in the case of medical necessity.
- Support global efforts to ameliorate nutrient deficiency through fortification and/or supplementation programs.
By taking an interest and understanding the science of Vitamin A, we will not only protect our eyes, but also support the immune system, grow, and promote overall longer-term health.
