Vitamin A 

Trans-retinol or retinol (alcohol-based structure) is considered the parent compound of vitamin A. However, vitamin A can come in two other forms: retinal (aldehyde-based form), or retinoic acid (acid from retinal).² Retinal can be reversibly converted into retinol. Retinol can be reversibly into retinyl esters but irreversibly converted into retinoic acid.² Previtamin A are known as carotenoids which can be converted into vitamin A in the human body.²

Digestion and Absorption: Pepsin and other proteases detach protein, and lipases detattach fats from the various vitamin A isomers to free retinol and carotenoids.² Some retinyl esters, retinol, and carotenoids are packaged into chylomicrons and enter lymph circulation. 

Circulation, Storage, and Excretion: 90% of vitamin A is stored in the liver. Vitamin A in the form of retinol combines with retinol-binding protein (RBP) to form holo-RBG that circulates within the plasma. Carotenoids are components of lipoproteins, and other vitamin A forms are also found within the plasma at lower concentrations.² Excretion occurs in bile (70%) and urine (30%).²

Functions:  

Eye health - All-trans retinol undergoes a series of reactions that result in signals within the brain’s optic center.

Cell Differentiation - Retinoic acid and all-trans-retinol bind to specific nuclear receptors (class of proteins involved in transcription of DNA) termed RXR and RAR, leading to specific gene expressions.² Vitamin A is notable for epithelial cell differentiation.²

Glycoprotein Formation - Vitamin A may play a role in glycoprotein synthesis. Glycoproteins help cells recognize and communicate with each other amongst other roles.²

Antioxidants - Only carotenoids (previtamin A) are associated with antioxidant capacity. Antioxidants bind with free radicals such as O₂ that can react with other molecules causing oxidative stress.² Beta-carotene (a type of carotenoid) is a well-known antioxidant.²

Other Roles - Vitamin A is also important for bone synthesis through the regulation of osteoclasts and osteoblasts.¹ Vitamin A also is involved in immune health.¹

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) for adult men and women is 900 and 700 micrograms/day.³

Deficiency - Vitamin A deficiency is known to cause night blindness due to the keratinization of cells. Night blindness is a symptom of xerophthalmia, an eye disorder related to a deficiency in vitamin A that can lead to permanent blindness.² Other deficiency symptoms include dry skin and hair, poor development of bones, and increased risk of infections.¹

Toxicity - Hypervitaminosis symptoms (lack of appetite, vomiting, nausea, headache, bone and muscle pain, etc) may develop if vitamin A is taken at ~10x the recommended level.² Birth defects and other pregnancy complications can arise from hypervitaminosis.² 

Dietary Sources of Vitamin A and Carotenoids

Orange, yellow, and dark green vegetables contain carotenoids.¹ Vitamin A is found in meat and dairy products. Vitamin A-fortified foods include milk and cereal products. Examples of vitamin A/carotenoid - containing foods include:

Vegetables - sweet potatoes, spinach, pumpkin 

Fruits - Cantaloupe, mango, apricots 

Meat/ poultry products - Liver, herring fish, egg

Dairy - milk, cheese, butter

Source(s)

1. Jerry Tortora and Bryan Dickerson, Principles of Anatomy & Physiology, 16th ed. (New Jersey: John Wiley & Sons, 2021).

2. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

3. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/