What is Vitamin A and Why Should You Care?

What is Vitamin A

What Is Vitamin A?

Vitamin A is present in the body as retinoids, or molecules related to a family of chemical compounds called retinol. These molecules are responsible for proper growth, reproduction, vision and the preservation of epithelial linings (such as the intestinal tract, airways, esophagus and skin - basically anything open to the external environment!).

What is Vitamin A in Terms of Its Derivatives?

This is a common area within this topic where the facts about vitamin A tend to be slightly skewed. The main idea I'm trying to convey here is that the term "vitamin A" encompasses a multitude of molecules that each have their own function(s). For example, a cream containing cis-retinoic acid has a completely different function and purpose than a cream containing trans-retinoic acid, even though they're both under the umbrella of "vitamin A".

These vitamin A derivatives are called retinoids (and carotenoids when referring to beta-carotene) are responsible for quite a few physiological processes. Since most people have heard about some of these derivatives, I thought it would be appropriate to define them here for clarity and completeness. This is optional info! (It's slightly more complicated.)

The Retinoids

Beta-Carotene: This a dietary source of vitamin A, present in any orange or green vegetables. Beta-carotene is absorbed directly into the intestinal cells, where it's ultimately converted to retinol and retinyl esters for transport to the liver.

Retinyl Esters: This is another one of our dietary sources of vitamin A, present in some animal products. It consists of a retinol molecule (below) with fatty acids attached. It's transported with fatty acids to the liver via chylomicrons (these are little "balls" of fats and cholesterol that are metabolized and sorted in the liver). After reaching the liver, the retinyl esters are either stored or converted to retinol, which is shuttled to various needy tissues via retinol binding proteins (RBP) present in the blood.

Retinol: This is a retinyl ester without the fats attached, also present in the diet. It's directly absorbed in the intestine, converted to retinyl esters (by simply attaching a fatty acid) and sent to the liver via chylomicrons. Again, the retinyl esters can either be stored, or converted to retinol to be shipped out to tissues in need. Just as a side note, retinol and beta-carotene are used in a multitude of dietary supplements (these are the two that are normally obtained in the diet).

Retinal: One of the places to which retinol is shipped is to the eyes, where it's oxidized to retinal. There's a protein in the rods of the eye, called opsin which binds to cis-retinal, forming a complex known as rhodopsin. In response to light, opsin is removed from the complex and cis-retinal isomerizes to trans-retinal. Finally, this molecule spontaneously reconverts to cis-retinal, sending a signal through the optic nerve. This process happens millions of times each second, producing an image on the brain. So, without vitamin A, how could you see? You can't!

Retinoic Acid: After retinol leaves the liver, it can also end up in the epithelial tissues. In these cells, retinol is converted to retinoic acid. Retinoic acid then enters the nucleus of the cell, where it promotes genes for cell division (growth), differentiation (the type of tissue to be made) and the production of keratin-like proteins responsible for the durability of skin, bones, teeth and other surface tissues. Retinoic acid, as implied here, is especially useful in dermatology and the production of skin care products and prescription drugs.

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