Did you know that our body can’t produce a host of vitamins essential for proper functioning? One such vitamin is vitamin A, one of four fat-soluble vitamins besides vitamins D, E, and K.
Since the body’s ability to synthesize this vitamin is minimal, the best way to get vitamin A is through vitamin A foods. Vitamin A comes in the form of retinoids, including retinol, retinal, and retinyl esters. It plays a critical role in eyesight, the immune and reproductive system, and cellular metabolism.
This article will enlighten you on vitamin A forms, vitamin A absorption into the bloodstream, the most important food sources, vitamin A dietary supplements, deficiency, and toxicity risks.
Vitamin A Absorption
Before the body utilizes this nutrient obtained from vitamin A food sources, it has to go through several processes. According to scientific studies, the absorption of this nutrient most likely occurs in the upper part of the small intestine.
The following paragraphs will explain the various stages vitamin A passes through before it’s absorbed into the bloodstream in detail.
Research shows that apical vitamin A intake occurs via simple diffusion. Studies point out that particular receptors are responsible for the transport of retinol and retinyl esters.
One of the receptors responsible for saturable carrier-mediated transport of retinol is the STRA6 (Stimulated by Retinoic Acid 6). This 74 kDa multi transmembrane transporter has a high affinity for retinol-binding protein (RBP).
This protein can be found in the intestine during early development. However, there’s still no scientific evidence to prove its existence in adults. Still, due to its structural similarities with RBP, it can be assumed to be responsible for the uptake of micellar retinol and retinol bound to lactoglobulin.
Another recently identified retinol receptor could play a role in apical retinol uptake. The receptor named RBPR2 is similar to STRA6, but unlike STRA6, this receptor that aids the cellular uptake of retinol into the enterocytes is found in adults.
After the vitamin A from food gets into the enterocytes, it’s converted to the esterified form by LRAT (Lecithin Retinol Acyltransferase) and ARAT (Acyl-CoA Acyl Transferase). Retinyl esters don’t go through this process as they’re already esterified.
Studies on laboratory mice showed that LRAT plays a significant role in retinol esterification. Namely, research on LRAT-deficient laboratory mice showed that this enzyme has a substantial role in retinol esterification. The primary ester forms created in this process include retinol palmitate, linoleate, and stearate.
Transporting retinol from the enterocytes into the surrounding blood vessels in the intestinal villi involves retinol-binding proteins. The most important retinol-binding protein is CRBPII (Cellular retinol-binding Protein II), present in the enterocytes of the small intestine.
It’s one of the soluble proteins in the jejunum, accounting for about 1% of the total proteins in the cytosol of intestinal cells. Furthermore, CRBPII is one of the most important proteins for vitamin A intake, as it’s involved in vitamin A transport and metabolism. Studies involving laboratory mice on a retinoid deficient diet have shown an increased expression of CRBPII.
Upon consuming vitamin A from vitamin A food sources, most of the retinyl esters and carotenoids are incorporated into chylomicrons. They’re circulated from the plasma into the lymph, which is an important process for optimum retinol uptake.
Studies show that the body can produce retinol in its free form during fasting state via Caco-2 cells. Furthermore, it’s possible to reverse retinol deficiency effects in abetalipoproteinemia patients unable to secrete chylomicrons by massively supplementing with retinol. This adaptive technique shows that there are other routes responsible for the transport and storage of retinol besides the chylomicron pathway.
Recommended Dietary Intake
Dietary reference intake (DRI) refers to the set of reference values prescribed by the Food and Nutrition Board for regulating and planning vitamin A intake per day.
The table below represents the vitamin A recommended daily intake:
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Vitamin A Food Sources
As mentioned before, the body’s ability to synthesize vitamin A is limited. Therefore, external sources such as vitamin-A-rich foods in your diet are the best source of vitamin A. You can find vitamin A in fruits and vegetables, as well as in animal products.
Animal products are the best sources of retinol in food, while plant-based foods are the best source of beta carotenoids. Liver and fish oils contain the highest concentration of vitamin A among animal sources. Dairy products (milk, cheese, butter, sour cream, etc.) and eggs are also good animal sources of this vitamin.
Vitamin A foods chart indicates that tomatoes, carrots, spinach, and broccoli are among the top plant-based food sources of vitamin A, especially provitamin A.
Let’s look at the detailed descriptions of the top food plant and animal food sources of vitamin A.
Animal Sources of Vitamin A (Retinol Foods)
Animals store plenty of vitamin A in the liver. Therefore, animal liver, especially the beef liver, is among the top food sources of retinol. Research shows that one serving (3 oz) of pan-fried beef liver contains as much as 6,582 mcg of vitamin A or 731% of the recommended daily value.
Fish oils are also one of the top animal foods with retinol. Research shows that only one tablespoon of cod liver oil provides as much as 4,080 mcg of vitamin A.
Cod liver oil is also an excellent source of cholecalciferol. Furthermore, fish oils are excellent sources of polyunsaturated fatty acids, especially omega-3 fatty acids.
Plant Sources of Vitamin A (Beta Carotene Foods)
Sweet potatoes are excellent sources of beta-carotene. A single baked sweet potato provides up to 1,400 mcg of vitamin A. This is over 150% of the daily recommended intake.
This food rich in vitamin A is very helpful in preventing age-related macular degeneration. For this reason, individuals at risk or suffering from this eye condition are advised to consume more of this vegetable. What’s more, studies show that sweet potatoes might also play a role in preventing the spread of malignant cells.
Additionally, consuming sweet potatoes is an excellent way of getting vitamin A for vegetarian diet followers.
Carrots are another excellent plant source of vitamin A. They’re amongst the top vitamin A foods rich in beta carotene. According to vitamin A content of foods charts, half a cup of unprocessed raw carrots provides around 459 mcg of vitamin A, which is about 51% of the daily value.
Carrots are not only rich in vitamin A. They also contain dietary fibers, which promote proper gastrointestinal function.
Research shows that individuals who consume plenty of carrots can have the beta carotene deposited in their body resulting in yellow skin. The skin becomes yellow due to the color of beta carotene and resembles jaundice symptoms. However, there’s a way to make the distinction between the two. Namely, in jaundiced patients, the sclera of the eyes turns yellow, which is not the case in individuals who consume excessive amounts of carrots.
It’s widely known that beans are an excellent source of proteins. However, that’s not the only benefit they provide. It’s less known that beans are also excellent food sources of vitamin A. A cup of boiled beans can deliver about 66 mcg of vitamin A.
Black-eyed peas can also be an excellent dietary source of iron. Therefore they’re often recommended to patients with iron deficiency. Research also shows that bean consumption supports heart health and potentially reduces blood pressure.
Finally, this is also a fantastic source of essential nutrients and vitamin A for vegan diet followers.
Spinach, the “queen of vegetables”, is on the top of the vitamin A vegetables list. Half a cup of cooked spinach contains 64% of the daily value of vitamin A (573 mcg).
Besides being a rich source of vitamin A, spinach also contains other beneficial nutrients like iron and magnesium. It also contains polyunsaturated fatty acids, which play a massive role in optimum cardiovascular function.
Mango is one of the top vitamin A fruits, so including a mango salsa or tropical fruit salad could be a great way to increase your vitamin A levels. A whole raw mango contains 112 mcg of vitamin A. This is 12% of the DV.
Additionally, mangoes are a good source of antioxidants and dietary fiber and promote good gastrointestinal function.
Cantaloupe is a melon species with sweet orange flesh belonging to the genus Cucumis. Half a cup of this melon provides around 135 mcg of vitamin A.
Cantaloupe is one of the vitamin A foods that’s also an excellent source of vitamin C, which makes it a great ally in fighting against various infections and diseases.
Snacking on some dried vitamin A fruits can also increase your vitamin A levels, as these sweet treats are an excellent source of vitamin A. A serving of ten dried apricot halves offers 63 mcg of vitamin A.
Additionally, dried apricots are a high-calorie food item containing plenty of sugar, so you should pay attention to the amount you consume.
Facts have shown that a serving of ten dried apricot halves offers 63 mcg of vitamin A.
Pumpkin pie can also serve as an excellent source of vitamin A with one serving providing as much as 488 mcg (54% DV) of this essential nutrient.
Pumpkin, just like carrots and other vitamin A fruits and vegetables, is rich in beta carotenes. However, like dried apricots, pumpkin pie is also rich in calories, so you should consume it with caution.
Besides being a great source of vitamin C and lycopene, tomato juice contains a remarkable amount of vitamin A. Namely, 3/4 cup of tomato juice provides 42 mcg (5% DV) of vitamin A.
Additionally, tomato juice could be an excellent option for vegans looking for sources of vitamin A in fruit and vegetables.
Forms of Vitamin A in Food Sources
Diet is the primary source of vitamin A. This nutrient comes in two forms: preformed vitamin A (retinol and retinyl esters) and provitamin A carotenoids (beta-carotene, alpha-carotene, and beta-cryptoxanthin).
Animal sources are the primary food sources of retinol and retinyl esters. Foods that contain retinol include dairy products, fish, and meat. Carotenoids, on the other hand, are mainly obtained from plant sources.
Vitamin A Supplements
Daily Vitamin A intake statistics show that over a quarter of the general population uses vitamin A supplements, the majority of them being children and the elderly.
This is because food with vitamin A is sometimes not enough to provide the recommended daily intake of vitamin A. In such cases, and especially in cases of vitamin A deficiency, vitamin A supplements are of great help.
You can supplement for vitamin A using vitamin A supplements or a multivitamin complex that usually provide this vitamin in the retinyl acetate or retinyl palmitate form. Some vitamin A supplements contain both retinyl esters and beta carotene, while others provide either retinyl esters or beta carotenes.
The concentration of vitamin A in supplements widely varies. It typically ranges from 50 to 3,000 mcg RAE (2,500–10,000 IU) in multivitamin supplements. For this reason you should find out what’s the vitamin A recommended intake for your health.
Vitamin A Deficiency
Though vitamin A deficiency is rare in the US, the lack of this vitamin remains a hard nut to crack in developing countries. Low standard of living and restricted access to retinol-rich foods contribute to the persistence of deficiency.
The WHO statistics reveal that about 190 million children of preschool age and 19.1 million pregnant women in the world have plasma vitamin A concentrations under 0.70 mcmol/L. In other words, they’re vitamin A deficient.
Vitamin A nutrition facts show that symptoms of vitamin A deficiency in developing countries occur during periods of high demand. They start exhibiting in infancy and persist during growth and development. Infants, children, pregnant women, and lactating mothers are generally most affected by vitamin A deficiency.
Infancy vitamin A deficiency occurs due to decreased exposure of infants to colostrum (breast milk). Lacking vitamin A can trigger diarrhea in young children that further increases the severity of retinol deficiency.
Vitamin A deficiency also causes a progressive eye disease called xerophthalmia. Individuals with xerophthalmia have dry cornea and conjunctiva. Initially, there’s conjunctival xerosis ( severe conjunctiva dryness) and night blindness, which progresses to corneal xerosis (severe cornea dryness) and, much later, keratomalacia (a condition in which the cornea becomes soft and cloudy).
According to vitamin A nutrition statistics from the WHO, vitamin A deficiency is one of the top causes of preventable child blindness worldwide. As many as 250,000–500,000 vitamin-A-deficient children worldwide become blind every year. What’s more, about half of them die within one year of losing their vision.
Additionally, vitamin A deficiency affects the immune system and decreases the body’s ability to fight infections. Studies show that vitamin A deficiency increases the risk of childhood death due to severe infection. Even subclinical vitamin A deficiency can pose serious health problems to infants. Namely, it can increase their risk of developing respiratory tract infections, gastrointestinal infections and cause problems with bone formation—all diminishing the possibility of survival. This is why adequate vitamin A intake is essential.
Finally, vitamin A deficiency negatively affects the maternal survival rate. It may also cause complications with pregnancy and lactation.
Vitamin A Toxicity
The human body stores unused vitamin A in the liver. The excessive accumulation of this vitamin can have adverse effects on the body.
Vitamin A toxicity (hypervitaminosis A) is a condition occurring due to excessive vitamin A intake. It’s usually related to excessive consumption of dietary supplements rather than a high intake of food rich in vitamin A.
Acute and Chronic Vitamin A Toxicity
Vitamin A toxicity may be acute or chronic. Acute toxicity occurs after taking a hefty dose of vitamin A. Symptoms usually develop within 24 hours and require immediate medical attention. Chronic toxicity, on the other hand, is a result of the prolonged accumulation of vitamin A in the body.
It’s important to note that a daily vitamin A intake of over 10,000 IU during the first eight weeks of pregnancy may cause developmental malformations and congenital defects such as encephalitis.
Vitamin A Toxicity Symptoms
Symptoms usually depend on the type of toxicity (whether it’s acute or chronic). Besides a range of nonspecific symptoms, blurred vision, bone pain, and skin changes such as rash or dry and rough skin may signal vitamin A toxicity.
Vitamin A nutrition facts reveal that chronic toxicity can cause liver and even brain damage in severe cases.
Diagnosing Vitamin A Toxicity
Blood tests are the most common way of diagnosing vitamin A toxicity.
Treating Vitamin A Toxicity
Vitamin A toxicity is mainly treated by reducing the dosage or stopping the intake of the vitamin altogether. Symptoms commonly disappear within a few weeks.
Vitamin A is an essential nutrient you can get from food sources or supplements. There’s quite a lot of animal and plant vitamin A foods on the market. However, developing countries with restricted access to such foods due to poverty are prone to suffering from vitamin A deficiency.
The best way to meet the recommended dietary allowance of vitamin A is by eating a balanced diet made of fruits, vegetables, fortified cereals, and animal products high in vitamin A. Alternatively, you can take supplements to meet the daily requirements.
How much vitamin A per day should I take?
The required amount of vitamin A depends on many factors like your age, health status, etc. The standard recommended dietary allowances for vitamin A are the following:
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Pregnant and lactating women require higher doses of vitamin A since these are periods of high nutritional demand for this nutrient. However, vitamin A doses shouldn’t be exceeded during pregnancy to avoid toxicity that could cause certain congenital defects in newborns.
What foods contain vitamin A?
There are many plant and animal food sources of vitamin A. Animal food sources contain retinyl esters, while plant food sources contain beta carotene. Liver oil, beans, spinach, fish oils, carrots, etc., are only some of the vitamin A food sources.
What foods are highest in vitamin A?
Vitamin A is not difficult to obtain through diet since there are many vitamin A foods available on the market. They can be sourced from both plants and animals. For example, top animal food sources include liver, fish, and dairy products. Top plant food sources, on the other hand, include carrots, cantaloupe, and broccoli. When it comes to fruit, mango serves as a great source of this nutrient, serving you with about 75% of vitamin A in an average-sized fruit. Finally, vitamin-A-fortified cereals could also be a great source of this vitamin.