Introduction

Vitamin A is a fat-soluble life-essential group of compounds of animal and vegetal origin characterized by an unsaturated isoprenoid chain structure. It is a term referring to derivatives of β-ionone that have the biological activity of all-trans-retinol. All vitamin A forms share a similar structure and physiological functions in an organism. These compounds can also be classified as retinoids, including compounds with a common structure of four isoprenoid units being of either a natural or synthetic origin.

Biological function

Vitamin A is the primary signaling molecule in the visual process in which 11-cis-retinal, a component of the visual pigment rhodopsin in rod cells and iodopsin in cone cells, is converted by light to all-trans-retinal, eliciting a response in the visual cortex of the brain. Vitamin A also participates in cell-to-cell communication, cellular differentiation, embryologic development, spermatogenesis, and the immune response. Vitamin A induces mucous cell differentiation, alters keratin expression, and is required for normal mucociliary differentiation[1].

Importance

Deficiency results in squamous metaplasia of the respiratory tract epithelium; without protective mucus, the respiratory tract is prone to infection. Other early signs are loss of appetite, diarrhea, weakness, and retarded growth. With prolonged deficiency, xerophthalmia, night blindness, corneal damage, and retinal degeneration can occur. Vitamin A in excess can be toxic; signs include skin dryness and pigmentation, alopecia, anorexia, weakness, leukopenia, hypoplastic anemia, enlarged liver and spleen, hepatocellular damage, bleeding lips and gums, stiffness in joints, and pruritis Vitamin A is a known teratogen. All-trans and 13-cis retinoic acid have teratogenic effects similar to those of retinol, including embryolethality, skeletal abnormalities, and craniofacial malformations consisting of external ear defects, mandibular hypoplasia, cleft palate, and temporal bone deformities, hypoplasia of the thymus, and cardiac malformations in cynomolgus monkeys. Many of these defects can be traced to abnormalities of the embryonic hindbrain and its associated neural crest cells, contributing to the first and second pharyngeal arches.

Food sources

The human body is not able to produce vitamin A, and therefore, it is necessary to obtain it from the diet either as prepared vitamin A or in the form of provitamin A carotenoids. Vitamin A may be found in foods of animal origin or may result from the metabolism of carotenoids found in plant foods[2].

There are more than 50 provitamin A carotenoids, but only β-carotene, α-carotene, and β-cryptoxanthin are present in significant amounts in the human diet. These carotenoids have been identified in all groups of photosynthetic organisms, bacteria, fungi, and many animals. β-Carotene is the most abundant in the diet. It is mostly ingested through red and orange vegetables and partially through the same colored fruits and green vegetables. In Europe, carrots, spinach, and tomato products are the main contributors to β-carotene intake, while β-cryptoxanthin is most commonly taken from various citruses and citrus juices. β-Cryptoxanthin, as well as other carotenoids, occurs in plants both as free and esterified with fatty acids (lauric, myristic, palmitic), and these esters contribute to total vitamin A content due to their comparable bioavailability. Rich sources of cryptoxanthin or its esters include, in addition to the already-mentioned citruses (satsuma mandarins, tangerines, clementines, mineolas, and oranges), persimmons, chili peppers and red peppers, papaya, sea buckthorn, loquat, mango, and apricots. Other sources of provitamin A carotenoids also include various medicinal plants and herbs, cereals, and specific vegetable oils.

Studies often use high doses of vitamin A. However, such high doses can be toxic. A doctor should monitor any high-dose therapy (any dose approaching the level of 10,000 IU for an adult or above the recommended daily allowance for a child).

Daily dietary intakes for vitamin A are[3]:

Pediatric

Infants, birth to 6 months: 400 mcg

Infants, 7 to 12 months: 500 mcg

Children, 1 to 3 years: 300 mcg

Children, 4 to 8 years: 400 mcg

Children, 9 to 13 years: 600 mcg

Boys, 14 to 18 years: 900 mcg

Girls, 14 to 18 years: 700 mcg

Adult

Men, 19 years and older: 900 mcg

Women, 19 years and older: 700 mcg

Pregnant women, 14 to 18 years: 750 mcg

Pregnant women, 19 years and older: 770 mcg

Breastfeeding women, 14 to 18 years: 1,200 mcg

Breastfeeding women, 19 years and older: 1,300 mcg

References

[1] Vitamin A - an overview | ScienceDirect Topics https://www.sciencedirect.com/topics/neuroscience/vitamin-a

[2] Alejandro Carazo. “Vitamin A Update: Forms, Sources, Kinetics, Detection, Function, Deficiency, Therapeutic Use and Toxicity.” Nutrients (2021).

[3] Vitamin A (Retinol) Information | Mount Sinai - New York https://www.mountsinai.org/health-library/supplement/vitamin-a-retinol