Vitamin A — Nutrition Sources, Health Benefits, and Deficiency

Arpi Gasparyan | Обновлено: Март 25, 2024

Introduction

Vitamin A is a group of fat-soluble retinoids with the biological activity of retinol and retinyl esters. It is a vital nutrient worldwide; in 2017, over 250 million people were estimated to have low retinol levels and related eye diseases.

Vitamin A has many systemic bodily functions, such as immune function, epithelial cell integrity, gene transcription, growth and development, skin health, and reproduction. However, its primary function is to keep the eyes healthy and prevent night blindness and other related disorders (1, 2).

Types & Physiological Functions

Dietary vitamin A comes in two forms (2, 3):

Preformed vitamin A - retinol and retinyl esters (palmitic acid, oleic acid, stearic acid, and linoleic acid). They are mainly of animal origin and are found in dairy products, eggs, fish, and animal organs.
Precursors to vitamin A - provitamin A carotenoids. They are derived from colorful fruits and vegetables and include beta-carotene (β-carotene), alpha-carotene (α-carotene), and beta-cryptoxanthin. They are converted into vitamin A in the intestines via the action of an enzyme named BCMO1.

Other carotenoids, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A and are otherwise called non-provitamin A carotenoids (2).

The table below summarizes the physiological roles of different types of vitamin A in the body (1).

Type of Vitamin A	Function
Retinol	Cell transport and reproduction
Retinal	Vision
Retinoic acid	Cell differentiation, immune function, gene transcription, and reproduction
Retinyl esters	Storage

Vision

In the eyes, retinol is transported to the retinal epithelial cells located at the back of the eye, converted to retinyl esters, and stored. When needed, retinyl esters undergo chemical reactions and form retinal, which binds to a protein called opsin and forms rhodopsin, the visual pigment (visual purple). Rod cells containing rhodopsin can detect very small amounts of light, making it possible to see in the dark (night vision). When light passes through the lens, it is sensed by the retina, converted to a nerve impulse, and interpreted by the brain; thus, inadequate retinol levels will result in impaired dark adaptation or night blindness (4).

Immunity

Vitamin A is essential for the normal functioning of the immune system. The skin and mucosa of the airways, digestive, and urinary tracts function as a barrier against infections. Vitamin A is required to maintain the integrity and function of these cells. Moreover, vitamin A also participates in the development and differentiation of white blood cells, playing regulatory roles in innate and adaptive immune systems (4, 5).

Growth and development

Vitamin A is essential for the development of the embryo and fetus. Retinoic acids are required for the development of limbs and most internal organs and the regulation of expression of the growth hormone gene (4).

Reproductive health

Vitamin A is essential for maintaining the male genital tract and sperm formation. Vitamin A status is also essential for women, both at the time of conception and during pregnancy, and is a critical determinant in reproductive outcomes. Its deficiency can lead to failure of reproduction or pregnancy loss, whereas its excess may have teratogenic effects on the fetus, leading to birth defects and even spontaneous abortion (4, 6).

Red blood cell production

Stem cells, the precursors of all blood cells, require vitamin A to differentiate into red blood cells. Additionally, vitamin A helps to mobilize the stored iron to the developing red blood cells for incorporation into hemoglobin (4).

Skin and hair

Vitamin A (retinoic acid) regulates the stem cells of hair follicles and melanocyte stem cells, leading to the normal functioning of the hair cycle, wound healing, melanocyte differentiation, and proliferation (7).

Absorption & Metabolism

Various forms of vitamin A are hydrolyzed in the presence of gastric acid and pancreatic enzymes, forming micelles with other nutrients. The micelles are absorbed by the duodenal mucosal cells.

Retinyl esters and provitamin A carotenoids are converted to retinol, which is then oxidized to retinal and retinoic acid, the two primary active metabolites of vitamin A in the body (1, 2).

Most vitamin A is stored in the liver as retinyl esters in lipid-containing stellate cells. With sufficient vitamin A intake, over 90% of the body's vitamin A is stored in the liver (8).

Retinol and carotenoid levels are commonly measured in blood samples; however, they are not always reliable indicators of vitamin A status. The main reason is that blood vitamin levels do not decrease until vitamin stores in the liver are almost depleted. Several infections and some factors may also affect blood retinol concentrations (2).

Recommended Intakes

According to the 2020-2025 Dietary Guidelines for Americans, the recommended daily value (RDV) of vitamin A RAE is 700 mcg for women and 900 mcg for men (8).

The RDVs for vitamin A are in micrograms (mcg) of retinol activity equivalents (RAE), accounting for preformed vitamin A and provitamin A carotenoids.

The table below shows the RDV for different age groups (2, 8).

Age	2-3	4-8	9-13	14-18	19-50	51+
Male	300 mcg	400 mcg	600 mcg	900 mcg	900 mcg	900 mcg
Female	300 mcg	400 mcg	600 mcg	700 mcg	700 mcg	700 mcg
Pregnancy				750 mcg	770 mcg
Lactation				1200 mcg	1300 mcg

Food Sources of vitamin A

As mentioned above, most preformed vitamin A comes from animal products, whereas most provitamin A carotenoids come from green, orange, and yellow fruits and vegetables.

Below, we’ll see some major sources of vitamin A in each food per 100g serving size. The RDVs are calculated at 900 mcg as the recommended value.

Food	Vitamin A RAE (per 100g)	The RDA
Beef liver, cooked	9442 mcg	1049%
Cod liver oil	1350 mcg (in 1tsp or 4.5g)	150%
Sweet potato, baked	961 mcg	107%
Carrots, raw	835 mcg	93%
Spinach, boiled, drained	524 mcg	58%
Pumpkin, boiled	288 mcg	32%
Bell pepper, red, raw	157 mcg	17%
Eggs, whole, hard-boiled	149 mcg	17%
Ice cream, vanilla	118 mcg	13%
Salmon, cooked	69 mcg	8%
Paprika	56.6 mcg (in 1tsp)	6.3%
Mango	54 mcg	6%
Tomato, raw	42 mcg	5%
Broccoli	31 mcg	3%
Beef, loin, 0% fat	2 mcg	0%

To learn about other foods rich in vitamin A, visit our “Foods high in Vitamin A” page.

Dietary Supplements

Vitamin A is available as a dietary supplement as retinyl acetate or retinyl palmitate, provitamin A carotenoid (beta-carotene), or a combination of these (9).

The daily intake of vitamin A from food and supplements should not exceed 1500 mcg. Vitamin A supplements are not recommended if you eat liver weekly (10).

Health Benefits and Risks of Vitamin A

Cancer

The relationship between vitamin A and cancer risk remains unclear. Several large studies have shown that higher vitamin A intake from foods and supplements is associated with a decreased risk of lung, pancreatic, oral cavity, laryngeal, esophageal, ovarian, and bladder cancers, glioma, and non-Hodgkin lymphoma. In contrast, other studies have found no positive associations between vitamin A intake and the risk of liver, colorectal, and prostate cancers and non-Hodgkin lymphoma.

The Carotene and Retinol Efficacy Trial, including over 18,000 male and female participants with a current or past history of smoking or exposure to asbestos, and The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study, including over 29,000 male smokers, have found that daily supplementation with beta-carotene, alpha-tocopherol, and retinyl palmitate increases the risk of lung cancer and death from lung cancer (2).

Several other studies have found no associations between smokers supplementing with different types of vitamin A and overall cancer and lung cancer risks. Supplementation with vitamin A does not seem to increase lung cancer risk in non-smokers (2).

Supplementing with different types of vitamins A (retinol, alpha-carotene, beta-carotene) and E has been studied to increase the risk of prostate cancer, especially in current and past smokers and regular alcohol consumers (11).

Age-Related Macular Degeneration

Age-related macular degeneration (AMD) leads to vision loss in older people. Oxidative stress is a major risk factor for AMD, and supplementing with antioxidants, such as beta-carotene, lutein, zeaxanthin, vitamins E and C, and minerals, such as zinc and selenium, might help in disease prevention or treatment (2, 12).

Epidemiological data has shown that people who consume lots of fruits and vegetables rich in vitamin A and provitamin A carotenoids have a decreased risk for any stage of AMD; however, several other studies have not found any significant associations between vitamin A-rich foods intake and reduced risk of AMD (12).

Measles

Measles is an acute and highly contagious disease of viral etiology. For decades, the American Academy of Pediatrics and WHO have recommended vitamin A for managing the disease. Many studies in areas where vitamin A deficiency is common have shown that supplementing with age-appropriate doses of vitamin A may significantly reduce the risk of complications and death from measles (13).

Vitamin A Deficiency: Risk Groups & Symptoms

Plasma or serum retinol levels <0.70 μmol/L indicate non-symptomatic vitamin A deficiency in children and adults, whereas levels <0.35 µmol/L indicate severe deficiency (14).

Some groups of people at risk of vitamin A deficiency

Preterm infants, as they have low liver stores of vitamin A during the first year of life
Infants, children, and pregnant and lactating people in low to middle-income countries
People with cystic fibrosis who have pancreatic insufficiency, leading to decreased absorption of fat and fat-soluble vitamins
People with gastrointestinal disorders leading to malabsorption, such as Crohn’s disease, ulcerative colitis, celiac disease, short bowel syndrome, chronic pancreatitis, and cirrhosis (2).

The primary symptom of vitamin A deficiency is xerophthalmia, ranging from milder stages of night blindness and Bitot spots to irreversible blindness. Bitot spots are small, white, foamy patches on the eye's surface, specifically on the conjunctiva.

Chronic vitamin A deficiency may also be associated with birth defects, such as abnormal lung development, an increased risk of anemia, and increased measles and infectious diarrhea severity.

Vitamin A deficiency may lead to frequent gastrointestinal, pulmonary, and urinary infections. The skin manifestations may be xeroderma (dry skin) and phrynoderma (follicular hyperkeratosis) (2, 14, 15).

Vitamin A Toxicity

Vitamin A toxicity (hypervitaminosis A) can be acute, chronic, or teratogenic. The acute toxic dose is 25,000 IU/kg (international units per kg), whereas the chronic toxic dose is 4000 IU/kg daily for 6-15 months. Acute vitamin A toxicity is rare and likely caused by isotretinoin (2, 15, 16).

Acute vitamin A toxicity may present with severe headache, blurred vision, nausea, vomiting, dizziness, irritability, muscle pain, coordination problems, and, in severe cases, bone pain, drowsiness, and even coma and death. Toxicity may also affect the skin and mucosal layers, causing dry lips, mouth, eyes, nose, dry skin, pruritus (itching), palms and soles peeling, and fingertips fissuring (2, 16).

Chronic vitamin A toxicity may cause dry, cracked skin, hair loss, brittle nails, muscle and joint pain, fatigue, loss of appetite, headache, nausea, and enlarged fatty liver. Chronic toxicity can affect various system organs, such as the bones (osteoporosis), liver (fatty liver, liver injury, fibrosis with collagen deposits), skin (exfoliative dermatitis and epidermal hyperplasia), nervous system, and mental health. It may also cause abnormal liver tests, hypertriglyceridemia, leukocytosis, low platelet count, anemia, increased blood urea nitrogen, creatinine, and imbalance of electrolytes (2, 16).

Excess vitamin A and some topical retinoid medications, such as isotretinoin and etretinate, cause teratogenic vitamin A toxicity, which is associated with congenital birth defects (malformations) affecting the central nervous system, eyes, skull, lungs, urinary tract, and heart (2, 16).

Excess beta-carotene causes carotenodermia, a harmless condition characterized by yellow-orange skin that can be reversed by discontinuing beta-carotene intake (2).

Vitamin A Contraindications

Vitamin A is teratogenic for the fetus; thus, it is contraindicated during pregnancy and breastfeeding. It should not be taken by people allergic to this class of medications (17).

Vitamin A is prescribed with caution to people with liver and kidney disease, alcoholism, and acne. Isotretinoin may help manage acne for selected people; however, it is teratogenic and toxic and should be strictly monitored in the case of prescription by the medical provider (17, 18).

Vitamin A Interactions With Medications

Vitamin A may interact with certain medications: orlistat (Alli, Xenical), bile acid sequestrants (cholestyramine, colestipol, colesevelam), and IBAT inhibitors decrease vitamin A’s absorption.

Retinoids (such as acitretin, bexarotene, and isotretinoin) may increase the risk of vitamin A toxicity.

Tetracycline antibiotics (such as doxycycline) may increase the risk of pseudotumor cerebri and lead to permanent vision loss (19).

Summary

Vitamin A has two forms: preformed vitamin A or retinol and retinyl esters and provitamin A carotenoids (beta-carotene, alpha-carotene, and beta-cryptoxanthin). Lycopene, lutein, zeaxanthin, or non-provitamin A carotenoids are not converted into vitamin A.

Preformed vitamin A is found in animal products, whereas provitamin A is stored in colorful fruits and vegetables.

Vitamin A is required for vision, immunity, epithelial cell integrity, gene transcription, growth and development, skin health, and reproduction.

Vitamin A supplementation benefits during age-related eye disorders and measles but increases the risk of lung cancer in ever-smokers and prostate cancer.

Preterm infants and people with cystic fibrosis and gastrointestinal, pancreatic, and liver diseases are at risk of developing vitamin A deficiency, leading from night blindness and Bitot spots to irreversible blindness.

Vitamin A is contraindicated during pregnancy and breastfeeding and prescribed with caution to people with liver and kidney disease, alcoholism, and acne.

References

Автор статьи: Arpi Gasparyan

Специальность: General Medicine at YSMU

Обновлено: Март 25, 2024

Проверено Victoria Mazmanyan