Vitamin A: Functions, Health Benefits, and Drawbacks

Vitamin A was first discovered in 1914, and it is an essential nutrient with wide-ranging roles in human health.

Alongside vitamins D, E, and K, vitamin A is one of the four fat-soluble vitamins.

In this article, we examine the functions and health benefits that vitamin A can have.

Additionally, we also look at how much we require and some potential drawbacks and side effects.

How Much Vitamin A Do We Need?

To maintain adequate levels of vitamin A, researchers determined that the human body requires 6.7 mcg of the vitamin per kilogram of body weight (1).

Recommended dietary intakes have been developed based on this, and you can see these dietary reference values in the following table (2);

The ‘RAE’ abbreviation stands for retinol activity equivalents, which we will explain shortly.

Deficiency

Unfortunately, vitamin A deficiency is rife, and it is a severe public health problem around the world (3).

Vitamin A deficiency is an issue for most countries. However, the issue is particularly prevalent in India, where 62% of preschool children are deficient in the vitamin (4).

Vitamin A in Animal Foods vs. Plant Foods

First of all, vitamin A comes in two distinct forms (3);

• Preformed vitamin A: true vitamin A is also known as retinol, and it is only available in animal foods. Liver offers more retinol than any other food.
• Vitamin A precursors: various plant foods contain carotenoids such as beta-carotene and alpha-carotene. These compounds can convert to vitamin A (retinol) inside the body. We can find these compounds in certain plant foods with green, yellow, orange, and red pigments.

Preformed vitamin A is extremely bioavailable, and the human body absorbs approximately 70-90% of dietary retinol (5).

On the other hand, the conversion of carotenoids to retinol can be very unpredictable, and absolute absorption figures are variable. Bioavailability can be as low as 5-10% (6).

For this reason, foods that provide retinol are the optimal source of vitamin A.

To better communicate the difference between preformed vitamin A and carotenoids, ‘retinol activity equivalent’ (RAE) ratings have been set for all food sources of the vitamin (7).

For the best food sources of both retinol and carotenoids, see these 20 foods high in vitamin A.

Functions and Benefits of Vitamin A

Vitamin A has many different roles within the body, and it plays a key role in vision and healthy growth.

Here is a summary of the main functions (and benefits) the nutrient offers.

1) Vitamin A Enhances Immune Response

Research demonstrates that vitamin A plays an integral part in the body’s immune system.

For example, randomized controlled trials and systematic reviews have shown that high-dose vitamin A may reduce the mortality rate (and improve recovery) from numerous diseases in children. These diseases include malaria and acute pneumonia (8).

Additionally, retinol regulates many different cells and genes, many of which are essential for an optimal immune response (9).

Furthermore, research shows that vitamin A helps to regulate optimal T cell (lymphocyte) reactions in the body (10).

2) Plays An Important Role In Vision

Vitamin A is crucial for our vision. Retinal, a derivative of vitamin A, combines with opsin (a protein in the eye) to form a compound called rhodopsin (11).

When light hits the eye, rhodopsin converts it into electrical signals sent to the brain. The brain interprets these signals and outputs them as vision (12).

Vitamin A also helps us to adapt our vision to the dark.

For this reason, insufficient levels of the vitamin can cause an issue known as ‘night blindness.’ Night blindness refers to being unable to see anything in dim conditions when visibility is low (13, 14).

3) May Reduce the Risk of Cancer and Cardiovascular Disease

The evidence is not conclusive, but some research suggests that vitamin A may be protective against cancer and cardiovascular disease.

For example, there are “significant associations” between low levels of vitamin A and cardiovascular mortality in elderly individuals (15).

Additionally, another study followed 441 Australian adults with type 2 diabetes for five years. Notably, those with the highest plasma (blood) levels of vitamin A had the lowest risk of cardiovascular mortality (16).

Furthermore, some studies suggest that vitamin A status may have an impact on cancer risk;

• Retinol can help to regulate cell proliferation (17).
• Observational studies suggest that high intake of vitamin A sources may lower pancreatic cancer risk (18).
• Low blood levels of retinol are associated with reduced survival rates in colorectal cancer patients (19).

However, it is worth remembering that an association does not necessarily equal causation. More research is necessary to understand the impact of vitamin A status on chronic disease fully.

4) Regulates Gene Expression

Gene expression refers to the way our body uses information in our genes to manufacture structural components such as proteins.

Vitamin A plays a vital role in regulating gene expression (20, 21).

In vitro (test tube/cell culture) studies also show that retinol can induce the expression of genes responsible for apoptosis (22, 23).

Apoptosis is otherwise known as ‘programmed cell death,’ it helps to kill bad cells, and thus plays a key role in cancer prevention (24).

5) Vitamin A Has Antioxidant Functions

Carotenoids are thought to have antioxidant properties, and they may help to reduce oxidative stress in the body (25, 26).

Over the long-term, oxidative stress can lead to various chronic diseases.

However, it appears that vitamin A can also have pro-oxidant effects in excessive amounts (27, 28).

We will examine this issue in more detail a little later on.

6) Key Factor For Healthy Growth

The benefits of vitamin A for healthy growth and development have long been known.

On this note, the first discovery of vitamin A in the early 20th century came after researchers identified a previously unknown essential factor for growth (29).

The vitamin is essential for the normal growth and development of children (30, 31).

7) May Promote Healthy Bones

There is a link between blood levels of vitamin A and skeletal health (32).

For example, one study, featuring 5288 people, found that bone mass density was higher, and fracture risk was lower in individuals with the highest vitamin A intake (33).

However, it is worth noting that excessive blood levels of vitamin A may harm bone health. These negative effects have been seen in individuals with vitamin D deficiency (34).

8) May Protect Against Age-Related Macular Degeneration

As earlier discussed, vitamin A has important functions for our visual health. Research also suggests that it may help to prevent age-related macular degeneration.

Macular degeneration is a progressive eye disease that can impair vision or even result in loss of central vision (35).

Interestingly, carotenoids such as lutein and zeaxanthin may have a protective role against the condition.

On this note, a systematic review and meta-analysis found that high dietary intake of these carotenoids was “significantly related with a reduction in risk” of severe age-related macular degeneration (36).

9) Vitamin A Has Synergistic Effects With Vitamin D

Vitamin A and vitamin D share influence in several areas, such as skeletal health and immune response.

Furthermore, they may have a synergistic effect in certain areas.

For example, one study suggests that vitamin A and vitamin D may have a synergistic mechanism on causing apoptosis (cell death) in prostate cancer cells (37).

Another study focusing on the synergy between the two vitamins looked at their effect on lung cancer risk. This study found that vitamin A “may assist vitamin D in preventing lung cancer among smokers” (38).

Potential Drawbacks and Side Effects

Although vitamin A is an essential nutrient with a range of benefits, it can also cause some problems.

Here is a look at some of the potential concerns surrounding (excess amounts of) vitamin A.

1) Can Be Toxic In Excessive Amounts

Vitamin A is a fat-soluble vitamin that can build up over time in our bodies. For this reason, prolonged and excessive intake of vitamin A can accumulate in the liver (39).

As the vitamin builds up in excessive amounts, levels of free vitamin A can increase. Free and unbound vitamin A can cause various problems such as headaches, nausea, and vomiting (40).

In worse cases, this can result in alopecia (baldness), joint pains, bone damage, and even liver damage (41).

These adverse issues are otherwise known as hypervitaminosis A.

However, it is rare for vitamin A toxicity to occur from dietary sources of the nutrient. Generally speaking, the condition usually occurs from overdosing with vitamin A supplements (42).

To reduce the risk of vitamin A toxicity, tolerable upper levels for the nutrient have been published by the Food and Nutrition Board at the Institute of Medicine of the National Academies.

You can find these tolerable upper intake levels below (43);

2) Excess Intake May Impair the Effectiveness of Vitamin D

As previously mentioned, vitamins A and D interact in various ways within the body. However, it is also possible that an excessive intake of vitamin A may impede the body’s responsiveness to vitamin D.

For example, vitamin D receptors in the body require something called a retinoid X receptor (RXR) protein to bind elements that respond to vitamin D (44).

Animal studies show that when animals receive large doses of retinol, the retinol dominates these available RXR proteins (45, 46).

Studies looking into this have noted that high vitamin A levels may decrease calcium absorbance by impairing calcium response to vitamin D (47, 48).

Calcium is vital for bone health, and some human and animal studies suggest that excessive vitamin A intake may have adverse effects in this regard.

In a meta-analysis and systematic review, both low and high levels of vitamin A in the blood were associated with increased risk of hip fracture in older adults (49).

Furthermore, overdosing mice with vitamin A leads to decreased bone mass (50).

3) Carotenoids Have Pro-Oxidant Effects At High Doses

Although carotenoids can have antioxidant properties, studies show that high doses can have pro-oxidant activity (51).

It appears that particularly in areas with high oxygen pressure, such as in the lungs, carotene can act as a prooxidant and cause oxidative stress (52).

There also appears to be a combined negative effect between carotene and cigarette smoke.

Numerous studies in this area have found a statistically significant link between beta-carotene supplementation and lung cancer in smokers (53, 54, 55).

However, it should be noted that these links are with beta-carotene supplements and not natural food sources of vitamin A.

Final Thoughts

As shown in this article, vitamin A is a vital nutrient for our health, and it has numerous important functions and benefits.

However, just like with many things in nutrition, too much of a good thing isn’t necessarily better.

To put it another way; both insufficient intake and overconsumption of vitamin A have links to adverse health effects.

For more on essential vitamins, see this guide to foods high in vitamin C.