Welcome to “chemicals and society” a new feature at UYBFS where we highlight the current understanding of the biologic effects and safety of some of the most common chemicals in today’s society.

 

Today’s Chemical: Vitamin A (retinal, retinol, retinyl esters, and retinoic acid)

 

What is vitamin A and how are people exposed to it?

“Vitamin A” is technically a group of very similar chemicals. The main forms in the body are retinal, retinol, retinyl esters, and retinoic acid. Vitamin A is required for normal development and function in all vertebrates (animals with a backbone) and some non-vertebrates (like worms and bugs). Since animals can’t make vitamin A from scratch, they require dietary sources – either from plants containing carotenes (you’ve probably heard of beta-carotene, but did you know there were alpha and gamma forms too?) or by eating other animals who had already produced vitamin A from a plant source. several forms of vitamin A (tretinoin, isotretinoin, and alitretinoin) are also FDA-approved drugs, used to treat severe acne and some rare skin cancers.

Vitamin A has many different functions in the body, including roles in the immune system, bone metabolism, and blood cell production. The retinal form acts as a chromophore (a light-absorbing compound) and is required for normal vision, while retinoic acid is required for normal embryonic development. It acts as a morphogen, controlling gene transcription (which genes get transcribed into proteins) to dictate the correct pattern in the early embryo. Retinoic acid is also required for normal skin cell development, which is why it is such an effective treatment for acne and other skin conditions.

 

What happens if you don’t get enough vitamin A?

You need vitamin A. Very few people in developed countries who eat “normal” diets have vitamin A deficiencies. Most affected people live in developing countries and eat diets low in meats and vegetables, or – despite eating an adequate diets – are unable to absorb sufficient vitamin A due to chronic diarrhea or other GI disorders. Vitamin A deficiency is the number one cause of childhood blindness, and can also lead to infections due to poor immune function. When mothers have insufficient vitamin A during pregnancy, birth defects can occur.

 

What happens if you get too much vitamin A?

It is not possible to get too much (that is, toxic amounts of) vitamin A from plant sources. This is because plant-based vitamin A is all in the form of precursors (like the carotenes) which can be safely stored. However, it is possible to eat toxic levels of vitamin A from animal sources, because these come as the already active retinal, retinol, and retinoic acid forms. Symptoms include headache, blurred vision, vomiting, and confusion. Really high doses could be fatal. For adults, the only way to get these dangerously high levels of vitamin A is to take a boatload of vitamin A supplements (please only take the recommended daily amount!), or to eat a food with extremely high levels of vitamin A. Since the liver stores vitamin A, this organ meat has the highest levels of it, Among animals people may eat, the polar bear, walrus, bearded seal, and moose have dangerously high levels of vitamin A. That’s right – eating a polar bear’s liver can literally kill you.

While it is rare for someone to meet their maker because they couldn’t resist the taste of polar bear liver, there is a real danger of too much vitamin A to the developing fetus. Recall that retinoic acid is a morphogen, tightly controlling the pattering in the early embryo. Well having too much retinoic acid around can throw this whole process into disarray and cause serious birth defects. For this reason, women who are pregnant or could become pregnant should not take any more vitamin A supplements than the RDA suggests, and they should avoid taking or even handling vitamin A-based drugs like tretinoin, isotretinoin, or alitretinoin.

 

So What’s the bottom line?

Vitamin A is essential for normal development and body functions. However, like most fat-soluble vitamins, more is not necessarily better. High doses of vitamin A are dangerous to the developing fetus, and very high doses can be dangerous to otherwise-healthy adults.  So make sure you get the recommended daily amount in your diet or via supplements, but don’t over do it. Also be careful if you are a pregnant woman and you know someone on a vitamin A drug. Finally, and perhaps most importantly of all, the next time you find yourself in the arctic living off only what you can hunt, do not eat the liver of the polar bear – it would be a shame to survive an encounter with a polar bear only to be done it by it’s liver.

 

 

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