Vitamin A: Benefits, Side-Effects, Dosage, Research and Sources

Have you ever wondered if the old-age advice that eating carrots can help you see in the dark is accurate? This advice, indeed, has some truth!

The primary nutrient of carrots is beta-carotene, a precursor to Vitamin A. Beta-carotene, which is responsible for the characteristic orange colour of carrots, helps your eyes adjust during the night or in dim conditions.

Vitamin A can’t cure eyesight problems or give you that superpower of night vision. However, eating appropriate amounts of food containing beta-carotene can optimise your eye health.

Vitamin A refers to fat-soluble retinoids called retinyl esters and retinol. Vitamin A is involved in growth and development, cellular communication, immune function and male and female reproduction [1].

It also supports cell growth and differentiation and has a critical role in forming organs such as the eyes, lungs and heart.

Further, vitamin A is a vital component of rhodopsin, a retina protein sensitive to light and responds to the amount of light entering the eye. Rhodopsin is also involved in the differentiation and functioning of the cornea and conjunctival membranes.

What are the food sources of Vitamin A?

Vitamin A can be sourced from the food that we eat. The first type of vitamin A is called preformed vitamin A. Examples of preformed vitamin A include retinyl esters and retinol. Food sources of the preformed vitamin include the following:

Eggs
Fish
Dairy products
Organ meats

Examples of organ meat include cooked beef liver, lamb liver, goose liver pate and liver sausage. Since vitamin A is a fat-soluble vitamin stored in the liver, animal liver remains an excellent source of vitamin A.

Fishes rich in vitamin A include cooked king mackerel, cooked trout, cooked salmon, cooked bluefin tuna and cod liver oil. However, the amounts of vitamin A in fish are lower than in animal livers.

Meanwhile, goat cheese, butter, Limburger cheese, cheddar cheese, camembert cheese, Roquefort cheese and eggs are excellent sources of vitamin A. In addition, canned clams, cream cheese, canned oysters and whole milk also contain vitamin A.

The second form of vitamin A is called provitamin A carotenoids. Carotenoids are plant pigments that are converted into vitamin A once ingested and carried to the small intestine in the body. The small intestine also serves as the primary site of absorption of vitamin A.

The main carotenoids present in the human diet include the following:

Alpha-carotene
Beta-carotene
Beta-cryptoxanthin

Other carotenoids present in food are as follows:

Lutein
Lycopene
Zeaxanthin

Vegetables rich in vitamin A include baked sweet potato, baked butternut squash, cooked collard greens, cooked kale, cooked turnip greens, cooked carrots, raw sweet red pepper, cooked swiss chard, cooked spinach, raw romaine lettuce.

Fruits high in vitamin A are cantaloupe, mango, grapefruit, watermelon, papaya, tangerine, apricot, guava, nectarine and passion fruit.

Provitamin A and retinyl esters are converted to retinol once taken into the intestine and absorbed in the body. Once immersed in the small intestine, retinol is converted or oxidized to retinal and retinoic acid. Vitamin A is mainly stored in the liver as a retinyl ester.

What is the recommended dosage for Vitamin A?

Dietary guidelines [2] recommend that men take 900 mcg of vitamin A daily. The recommended dosage for women is 700 mcg, while for children and adolescents, the recommended dosage is 600 mcg daily.

Deficiency in vitamin A can lead to the following:

Skin problems
Diarrhoea
Night blindness
Dry eyes

One of the common symptoms of vitamin A deficiency includes impaired vision. A lack of vitamin A in the diet can lead to early signs of vision impairment during infancy and early childhood.

In extreme cases, long-term vitamin A deficiency leads to xeropthalmia or hyperkeratinization of the ocular epithelial tissues.

This hyperkeratinization can eventually lead to permanent total blindness. Xeropthalmia and, ultimately, total blindness is common in developing countries.

Although vitamin A is highly beneficial to the body, taking high doses is toxic to the body. Hence, you must take this vitamin according to the recommended dosage.

What are the benefits of Vitamin A?

Vitamin A has many benefits. These benefits range from reducing the risk of night vision problems to treating psoriasis and acne.

Treatment of psoriasis, acne and other skin disorders

Prescription pills and creams containing retinoids, synthetic forms of vitamin A, are used to treat psoriasis and severe acne.

Psoriasis is a skin disorder that causes silvery scales and patches of thick red skin. These patches are found typically on the lower back, knees, scalp, elbows, palms and soles of feet.

However, these patches can also be present in the moth, toenails and fingernails. Acne, on the other hand, is a skin condition where dead skin cells and oil clog hair follicles. Blockages of the pores produce whiteheads, blackheads, cysts and different types of pimples.

Cream retinoids also treat other skin disorders, such as premature ageing and warts. Evidence from the literature [3] has shown that when retinoids are combined with antioxidants, these can minimize wrinkles and the appearance of fine lines.

However, taking pills with retinoids should only be done under a doctor’s supervision. Isotretinoin, which is an oral medication for acne, has been found to have serious side effects.

Pregnant women or women of childbearing age not taking birth control pills should not take isotretinoin. Isotretinoin is shown to cause cleft palates and other disorders of the face in unborn children.

Protection against age-related decline and night blindness

Vitamin A is vital for preserving your eyesight. This vitamin is necessary for converting light that enters your eye into electrical signals sent to the brain. A deficiency of vitamin A can lead to nyctalopia or night blindness.

Vitamin A is a significant component of rhodopsin, a pigment in the eye’s retina and extremely sensitive to light. When there is a deficiency of rhodopsin, this makes it difficult for the eyes to see at night.

People with night blindness or nyctalopia can still typically see during the day. However, during nighttime, their reduced vision limits their ability to see even faint light during the dark.

Besides preventing night blindness, sufficient amounts of vitamin A or beta-carotene can slow down the age-related decline of eyesight.

One of the age-related eye disorders includes age-related macular degeneration (AMD), the leading cause of blindness worldwide.

Although the exact cause of AMD is not yet fully known, it is believed that oxidative stress of the retina may contribute to this condition.

However, there is some good news. A study [4] found that people over 50 who demonstrated some degeneration in their eyesight could benefit from antioxidant supplementation that includes beta-carotene.

Intake of these supplements reduced their risk of AMD. Although this is promising, it should be noted that beta-carotene supplementation was taken along with antioxidants.

This observation is supported by an extensive review [5], which showed that beta carotene alone is insufficient to reduce AMD risk.

Promoting a healthy immune system

Vitamin A is also well known to play an essential role in maintaining the body’s immune system or natural defences against infections and other diseases.

Vitamin A has been shown to keep the gut, lungs, eyes and genitals healthy. These organs likewise serve as significant barriers to infectious agents.

In addition, this vitamin is also essential in producing white blood cells, which are part of the immune system that heap clear or capture bacteria and other pathogens in the bloodstream.

When you lack vitamin A, you are at increased risk of infections and could experience poor recovery from illnesses or diseases.

Hence, vitamin A is necessary to protect you from infections. For example, in countries where children are highly susceptible to malaria or measles, correcting vitamin A deficiency reduces the children’s risk of getting measles or malaria [6].

Improvement of bone health

Calcium, vitamin A and protein are crucial nutrients for optimal bone health. However, the body also needs sufficient vitamin A for the proper development and growth of bones. A deficiency in vitamin A has been associated with poor bone health.

A meta-analysis of observational studies [7] has revealed that individuals with high levels of vitamin A sourced from their food are 6% less likely to experience bone fractures than their counterparts with only low levels of vitamin A from their diet.

However, it is acknowledged that low vitamin A levels are not the only reason for poor bone health or increased risk of bone fractures.

Some studies indicate that people with high intakes of vitamin A supplementation may be at risk of bone fractures.

It is noteworthy that vitamin A from food sources do not have toxic levels compared to vitamin A supplementation, where very high levels (i.e. higher than recommended daily allowance) are harmful to the body.

Even so, the findings of the observational studies from the meta-analysis [7] could not establish a cause-and-effect relationship.

The association between bone health and vitamin A has yet to be fully understood. More controlled clinical trials are required to confirm the relationship between vitamin A and bone health observed in observational studies.

It should be noted that deficiency in vitamin A alone does not lead to bone fractures. Other factors such as old age, lack of calcium and vitamin D also increase your risk of a bone fracture.

Lowering the risk of certain forms of cancer

Cancer is a condition that increases the risk of mortality or premature death worldwide. Cancer occurs when cells in the body mutate and multiply excessively.

This abnormal growth and spread of abnormal cells to different body parts could lead to premature death.

Many scientists have begun studying the role of vitamin A in cancer growth and prevention. It is well known that vitamin A plays a crucial role in the development and growth of cells.

One study [8] has shown that increased intake of vitamin A in food has been linked to reduced risk of Hodgkin’s lymphoma.

Other studies have also shown that eating beta-carotene sourced from food reduces the risk of cancer, such as bladder, cervical and lung cancer.

It would appear that eating foods rich in vitamin A are essential in preventing certain forms of cancer.

A randomized, controlled clinical trial [9] has shown that amongst women, taking beta-carotene supplementation of 50 mg on alternate days did not lead to a reduced risk of cancer and cardiovascular disease compared to women who only took a placebo.

This clinical trial showed that amongst healthy women, supplementation with vitamin A (beta-carotene) did not show any benefit or harm for a limited period on the incidence of cardiovascular disease or cancer.

The findings also suggest that vitamin A supplementation may not be linked to reduced cancer risk. This contrasts with vitamin A sourced from food, which is linked to a lower risk of certain forms of cancer.

At the moment, eating food rich in beta-carotene could help reduce the risk of certain forms of cancer.

However, the exact relationship between vitamin A levels in the body and cancer incidence is not yet fully understood.

Despite this observation, current research suggests that eating foods high in vitamin A is essential for healthy cell division and might reduce cancer risk.

Promotion of healthy growth and development of the reproductive system

Vitamin A has been linked to healthy development and normal growth of embryos during pregnancy. Further, this vitamin is vital for maintaining a healthy reproductive system.

Animal model studies [10] revealed that low vitamin A levels could cause infertility by blocking the development of sperm cells.

The same review indicated that retinoic acid (RA) is important in generating functional sperm cells. When RA levels are low, this leads to the production of abnormal sperm cells.

The effects of vitamin A are also seen in egg cell production in animal studies. The findings of these studies suggest that a lack of vitamin A results in reduced egg quality, affecting the implantation of eggs in the womb [10].

In pregnant women, it is well established that vitamin A is necessary for the growth and development of several structures and organs of the unborn child.

These include organs such as the heart, lungs and kidneys and body systems such as the nervous and skeletal systems.

While a deficiency in vitamin A can lead to poor development of embryos or the production of defective sperm and egg cells, too much vitamin A can also be harmful during pregnancy. Too much vitamin A can lead to congenital disabilities.

Hence, many health organizations and healthcare practitioners advise women of childbearing age or pregnant to avoid food containing concentrated amounts of vitamin A such as the liver.

Pregnant women are also advised to avoid vitamin A supplementation during pregnancy to avoid the risk of too much vitamin A levels in the liver and blood.

Side-effects

Unlike water-soluble vitamins, Vitamin A is a fat-soluble vitamin stored in the body, such as the liver. Consumption of too much vitamin A can lead to toxic levels.

Too high of vitamin A levels in the body is called hypervitaminosis. Hypervitaminosis A may be due to too many vitamin A supplements or eating preformed vitamin A from the food we eat.

Symptoms associated with hypervitaminosis A include the following:

Dizziness
Headaches
Pain
Death

It should be noted that excessive intake of preformed vitamin A from the food that we eat or from our diet is rare. In comparison, overconsumption of vitamin A supplements or medications is common.

A review [11] showed that eating food or plants rich in provitamin A does not carry the same risk as vitamin A supplementation in causing hypervitaminosis A.

Conversion of provitamin A from the plants to its active form is regulated. Hence, vitamin A toxicity from food plants is often rare.

Current research

Oesophageal cancer risk and vitamin A

A recent meta-analysis [12] reviewed 14 observational studies that examined the link between vitamin A intake and beta-carotene on the risk of oesophagal cancer.

Beta-carotene is converted to vitamin A in the body. Findings revealed that participants with the highest intake of beta-carotene found in vegetables or plants had the lowest rate of oesophagal cancer.

The same pattern was observed in those who took vitamin A supplementation. There was an inverse relationship between the intake of vitamin A supplementation and the risk of oesophagal cancer.

However, the effects were not as high as those of those who took high levels of beta-carotene from their food.

The findings of this study demonstrated that the intake of vitamin A from the food we eat is linked to a lower risk of oesophagal cancer instead.

Although vitamin A supplementation reduces the risk of oesophagal cancer, obtaining vitamin A from food sources would appear more beneficial.

Vitamin A and depression

Depression is a complex mental health condition influenced by several environmental and genetic factors. Social isolation is another factor that is often linked to depression.

The burden and risk of depression are also associated with lifestyle factors such as nutrition, diet and exposure to heavy metals.

Although the underlying mechanism remains unclear, previous meta-analyses have shown an inverse relationship between healthy dietary patterns and depression.

Diets high in fish, fruits, vegetables and whole grains and low in processed food and red meat are associated with a reduced risk of depression.

A greater intake of micronutrients and bioactive phytochemicals such as beta-carotene is linked to a reduced risk of depression as both these micronutrients and phytochemicals exhibit antioxidant and anti-inflammatory properties.

Antioxidants are needed to ensure cells’ proper functioning and prevent early cellular ageing. Further, food high in antioxidants also helps manage oxidative stress, which can cause inflammation and neuronal damage.

A meta-analysis [13] examined 25 observational studies that recruited 100,955 participants. The study’s findings showed that dietary beta-carotene and dietary vitamin A were associated with a reduced risk of depression.

This means that eating food rich in vitamin A, ranging from fish to vegetables, is linked to a reduced risk of depression.

The inverse relationship of vitamin A with depression is essential since this adds to evidence that vitamin A can play a role in improving one’s mental health.

The meta-analysis likewise showed that dietary beta-carotene intake is associated with more significant benefits than dietary vitamin A.

As seen in the meta-analysis, dietary beta-carotene is associated with a 37% decreased risk of depression. In comparison, dietary vitamin A was only associated with a 17% reduced risk of depression.

A subgroup analysis of the study revealed that the findings appeared specific to women. Amongst men, there were no differences in the amount of dietary intake of vitamin A or beta carotene between those who were depressed and men in the control group who were not suffering from depression.

Understanding the general roles of vitamin A may help explain why this is linked with a reduced risk of depression. For instance, vitamin A is found to be critical for the development and function of the brain.

It also influences neuroprotection and neurogenesis. As a provitamin, beta-carotene is converted to vitamin A once ingested in the body.

Beta-carotene is an antioxidant that helps address oxidative stress in the body. Meanwhile, oxidative stress is associated with an increased risk of depression.

The general role of vitamin A may explain its link to depression. However, the exact mechanisms of how vitamin A can reduce depression are not yet fully understood.

Key take-home message

Vitamin A is an essential micronutrient for optimal vision, healthy bones and a healthy immune system.

This micronutrient optimizes cellular growth and functioning and promotes the early growth and development of embryos in the womb.

Due to its importance for growth and development, a deficiency of this vitamin is linked to several diseases and health conditions, which include impaired vision or permanent blindness.

However, hypervitaminosis of this micronutrient can also lead to toxicity, especially in pregnant women. Hence, the recommended daily dose must be followed to prevent toxicities or deficiencies.

Talking to your doctor or healthcare practitioner is essential in helping you decide which food sources to take for optimal levels of vitamin A in your body.

Consider vitamin A supplementation when you have vitamin A deficiency. Always consult your doctor to ensure you take the recommended daily allowance for this micronutrient.

When planning to get pregnant, talk to your doctor about the correct vitamin A supplement dosage to prevent the risk of congenital abnormalities of the embryo or unborn child.

To date, there are many vitamin A supplements on the market. Comparing these supplements and presenting them to your physician would help you make the right choice.

Finally, vitamin A supplementation is also linked to longevity as it can help in promoting optimal cell growth and development. The proper functioning of the cells is critical in promoting longevity and a healthy lifespan.