What is Vitamin A and how does it work?

The human diet contains two sources for vitamin A: preformed vitamin A (retinol and retinyl esters) and provitamin A carotenoids.

- Preformed vitamin A is found in foods from animal sources, including dairy products, eggs, fish, and organ meats.

- Provitamin A carotenoids are plant pigments that include beta-carotene, alpha-carotene, and beta-cryptoxanthin. The body converts provitamin A carotenoids into vitamin A in the intestine via the beta-carotene monooxygenase type 1 BCMO1 enzyme, although conversion rates may have genetic variability. Other carotenoids in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A and are referred to as non-provitamin A carotenoids; they might have other important activities not involving vitamin A formation.

The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells. Retinyl esters and provitamin A carotenoids are converted to retinol after uptake into the lumen (for retinyl esters) or absorption (for provitamin A carotenoids). Retinol is then oxidized to retinal and retinoic acid, the two main active vitamin A metabolites in the body. Most of the body’s vitamin A is stored in the liver in the form of retinyl esters.

Recommended amount
The amount of vitamin A you need depends on your age and sex. Average daily recommended amounts of preformed vitamin A and provitamin A carotenoids are listed below in micrograms (mcg) of retinol activity equivalents (RAE).

- Birth to 6 months: 400 mcg RAE
- Infants 7–12 months: 500 mcg RAE
- Children 1–3 years: 300 mcg RAE
- Children 4–8 years: 400 mcg RAE
- Children 9–13 years: 600 mcg RAE
- Teen males 14–18 years: 900 mcg RAE
- Teen females 14–18 years: 700 mcg RAE
- Adult males: 900 mcg RAE
- Adult females: 700 mcg RAE
- Pregnant teens: 750 mcg RAE
- Pregnant adults: 770 mcg RAE
- Breastfeeding teens: 1,200 mcg RAE
- Breastfeeding adults: 1,300 mcg RAE

How harmful is if excess vitamin A?
Getting too much preformed vitamin A (usually from supplements or certain medicines) can cause severe headache, blurred vision, nausea, dizziness, muscle aches, and problems with coordination. In severe cases, getting too much preformed vitamin A can even lead to coma and death.

If you take too much preformed vitamin A while pregnant, it can cause birth defects in your baby, including abnormal eyes, skull, lungs, and heart. If you are or might be pregnant or breastfeeding, you should not take high-dose supplements of preformed vitamin A.

High intakes of beta-carotene do not cause the same problems as preformed vitamin A. Consuming high amounts of beta-carotene can turn the skin yellow-orange, but this condition is harmless and goes away when you eat less of it. However, several studies have shown that smokers, former smokers, and people exposed to asbestos who take high-dose beta-carotene supplements have a higher risk of lung cancer and death.

The daily upper limits for preformed vitamin A include intakes from all sources—food, beverages, and supplements—and are listed below. These levels do not apply to people who are taking vitamin A for medical reasons under the care of a doctor. There are no upper limits for beta-carotene and other forms of provitamin A.

- Birth to 12 months: 600 mcg
- Children 1–3 years: 600 mcg
- Children 4–8 years: 900 mcg
- Children 9–13 years: 1,700 mcg
- Teens 14–18 years: 2,800 mcg
- Adults 19 years and older: 3,000 mcg

What happen if deficiency in Vitamin A?
Vitamin A deficiency is rare in Western countries but may occur. Conditions that interfere with normal digestion can lead to vitamin A malabsorption such as celiac disease, Crohn’s disease, cirrhosis, alcoholism, and cystic fibrosis. Also at risk are adults and children who eat a very limited diet due to poverty or self-restriction. Mild vitamin A deficiency may cause fatigue, susceptibility to infections, and infertility. The following are signs of a more serious deficiency.

Xerophthalmia, a severe dryness of the eye that if untreated can lead to blindness, nyctalopia or night blindness, irregular patches on the white of the eyes, dry skin or hair.

6 BENEFITS FROM VITAMIN A

Protects Your Eyes From Night Blindness and Age-Related Decline
Vitamin A is essential for preserving your eyesight. It is needed to convert light that hits your eye into an electrical signal that can be sent to your brain.

In fact, one of the first symptoms of vitamin A deficiency can be night blindness, known as nyctalopia. The vitamin A is a major component of the pigment rhodopsin that is found in the retina of your eye and extremely sensitive to light. People with this condition can still see normally during the day, but have reduced vision in darkness as their eyes struggle to pick up light at lower levels.

In addition to preventing night blindness, eating adequate amounts of beta-carotene may help slow the decline in eyesight that some people experience as they age. Age-related macular degeneration (AMD) is the leading cause of blindness in the developed world. Though its exact cause is unknown, it’s thought to be the result of cellular damage to the retina, attributable to oxidative stress.

Lower Your Risk of Certain Cancers
Cancer occurs when abnormal cells begin to grow or divide in an uncontrolled way. As vitamin A plays an important role in the growth and development of cells, its influence on cancer risk and role in cancer prevention is of interest to scientists.

In observational studies, eating higher amounts of vitamin A in the form of beta-carotene has been linked to a decreased risk of certain types of cancer, including Hodgkin’s lymphoma, as well as cervical, lung and bladder cancer. Yet, though high intakes of vitamin A from plant foods have been associated with a reduced risk of cancer, animal foods which contain active forms of vitamin A aren’t linked in the same way.

Similarly, vitamin A supplements haven’t shown the same beneficial effects. In fact, in some studies, smokers taking beta-carotene supplements experienced an increased risk of lung cancer.

At the moment, the relationship between vitamin A levels in your body and cancer risk is still not fully understood. Still, current evidence suggests that getting adequate vitamin A, especially from plants, is important for healthy cell division and may reduce your risk of some types of cancer.

Supports a Healthy Immune System
Vitamin A plays a vital role in maintaining body’s natural defenses. This includes the mucous barriers in eyes, lungs, gut and genitals which help trap bacteria and other infectious agents.

It’s also involved in the production and function of white blood cells, which help capture and clear bacteria and other pathogens from bloodstream.

This means that a deficiency in vitamin A can increase your susceptibility to infections and delay your recovery when you get sick. In fact, in countries where infections like measles and malaria are common, correcting vitamin A deficiency in children has been shown to decrease the risk of dying from these diseases.

Reduces Your Risk of Acne
Acne is a chronic, inflammatory skin disorder. People with this condition develop painful spots and blackheads, most commonly on the face, back and chest. These spots occur when the sebaceous glands get clogged up with dead skin and oils. These glands are found in the hair follicles on your skin and produce sebum, an oily, waxy substance that keeps your skin lubricated and waterproof. Yet, the exact role that vitamin A plays in the development and treatment of acne remains unclear.

Supports Bone Health
The key nutrients needed for maintaining healthy bones as you age are protein, calcium and vitamin D. However, eating enough vitamin A is also necessary for proper bone growth and development, and a deficiency in this vitamin has been linked to poor bone health.

In fact, people with lower blood levels of vitamin A are at a higher risk of bone fractures than people with healthy levels. Additionally, a recent meta-analysis of observational studies found that people with the highest amounts of total vitamin A in their diet had a 6% decreased risk of fractures. Yet, low levels of vitamin A may not be the only problem when it comes to bone health. Some studies have found that people with high intakes of vitamin A have a higher risk of fractures as well.

This means that currently, the link between vitamin A and bone health is not fully understood, and more controlled trials are needed to confirm what has been seen in observational studies. Bear in mind that vitamin A status alone does not determine your risk of fractures, and the impact of the availability of other key nutrients, like vitamin D, also plays a role.

Promotes Healthy Growth and Reproduction
Vitamin A is essential for maintaining a healthy reproductive system in both men and women, as well as ensuring the normal growth and development of embryos during pregnancy.

Rat studies examining the importance of vitamin A in male reproduction have shown that a deficiency blocks the development of sperm cells, causing infertility. Likewise, animal studies have suggested that vitamin A deficiency in females can impact reproduction by reducing egg quality and affecting egg implantation in the womb.

In pregnant women, vitamin A is also involved in the growth and development of many major organs and structures of the unborn child, including the skeleton, nervous system, heart, kidneys, eyes, lungs and pancreas.

Yet, though much less common than vitamin A deficiency, too much vitamin A during pregnancy can be harmful to the growing baby as well and may lead to birth defects. Therefore, many health authorities recommended that women avoid foods that contain concentrated amounts of vitamin A, such as pâté and liver, as well as supplements containing vitamin A during pregnancy.

Food sources
The form of vitamin A will depend on the source (Trusted Source). For example, ready-made retinol, which is the active form of vitamin A, only comes from animal sources. The richest sources of retinol include: organ meats, such as liver; fatty fish, such as tuna and herring; milk and cheese; eggs

Plant-based foods contain carotenoids, which are antioxidant forms of vitamin A. The body converts these into retinol as it needs. Carotenoid is an orange pigment that contributes to the color of certain fruits and vegetables. They include: pumpkin, carrots, squash, sweet potato, red peppers, cantaloupe, apricot, mango

Plant foods that are rich in beta-carotene include dark green leafy vegetables, such as broccoli, spinach, turnip greens.

Did you know? 
Sometimes people take cod liver oil for vitamin D but may not be aware it is also very high in vitamin A, beyond the RDA at 192%, in the same synthetic form as in supplements (retinyl palmitate). So if you use cod liver oil, make sure your other supplements do not contain extra vitamin A. Other forms of fish oil supplements typically do not contain significant amounts of vitamin A, but always check the label.

There have been claims that vitamin A (in the form of retinol or retinyl palmitate) added to some sunscreens, moisturizers, and lip balms can cause vitamin A toxicity or cancer if used excessively. However, there has not been evidence to date to support this. Vitamin A in topical creams is not absorbed into the bloodstream and therefore would not contribute to toxic levels. The concern with cancer stemmed from studies in mice conducted by the FDA. The results showed increased oxidative stress (a potential precursor to cancer) in cancer cells exposed to retinyl palmitate and ultraviolet light. After review of these and other studies, a statement from the American Academy of Dermatology asserted, “Based on the current available data from in vitro, animal and human studies, there is no convincing evidence to support the notion that retinyl palmitate in sunscreens causes cancer.” They cited the high susceptibility of mice to skin cancer after ultraviolet exposure, even in the absence of retinyl palmitate, and therefore the results of these animal studies should not be applied to humans.

Retinoids in skin creams can cause skin to become highly sensitive to bright light, so it is advised to apply vitamin A creams at night and to avoid strong sun after their use. 

Compiled and penned by Crocus Media