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Physiology of the Skin: A New Look at Vitamin A

By Peter T. Pugliese, MD
Posted: April 23, 2008, from the May 2008 issue of Skin Inc. magazine.

Vitamin A, or retinol, is one of the most effective and versatile treatment modalities available to the skin care professional. It can be used as a powerful service for aging, photo-damage, acne, pigmentation problems and rosacea. The key to using vitamin A is understanding how it works on—and in—the skin. Having well-rounded knowledge about vitamin A requires comprehension of the basic chemistry of retinoids, the chemical forms of vitamin A and how to use them, and the cellular effects of vitamin Aa. After all, it is used to have a fundamentally cellular effect on the skin. In this article, the chemistry of vitamin A will be covered, along with the different forms available to the skin care professional, and the various uses of vitamin A for multiple skin conditions. The term “vitamin A” will refer to all forms of vitamin A. The retinol shall be used for one specific form of vitamin A—the alcohol.

The chemistry
     Vitamin A is most commonly found in nature as an alcohol form, but only in animals. Carotenoids that contain vitamin A, such as beta-carotene, are found in plants such as dark or yellow vegetables, and carrots. These are precursors of vitamin A. If anyone tries to sell you a “natural plant vitamin A,” there is no such chemical. In some plants, such as carrots, carotenoids make up a large group of compounds. As you can see in Figure 1, beta-carotene is the precursor in the plant of vitamin A.
      There are four major forms of carotenes based on the ring structures at the ends of the molecule. Beta-carotene has beta rings at both ends; epsilon-carotene has an epsilon ring at both ends; alpha-carotene has one beta ring and one epsilon ring at each end; gamma-carotene has only one beta ring; and delta-carotene has only one epsilon ring.
     More importantly, notice in Figure 1 how vitamin A is produced from beta-carotene by animals. The point is that vitamin A comes in many forms, all of which must be made chemically, either in animals or manufactured otherwise. For example, retinol is converted to retinyl palmitate and stored in the liver. You can buy retinyl palmitate from several manufacturers as an ingredient, or you can buy it as
a finished skin care product.

Chemical forms of vitamin A
     The common, readily available forms of vitamin A include retinol, retinyl palmitate, retinyl acetate and retinoic acid. Retinol and the other two compounds known as esters—the palmitate and the acetate—will be discussed.
     A fourth form—the aldehyde form, retinal—is available and will be touched on briefly since it is new and not fully evaluated at this time.

     Retinol. Retinol is the most common form of vitamin A. It is the alcohol form, having an OH on the last carbon. The OH stands for oxygen and hydrogen, and is called a hydroxyl group. All alcohols must have one or more hydroxyl groups. You can see the structure in Figure 1.

     Retinyl palmitate. This is an ester, which is a special chemical formed when an alcohol is combined with an organic acid. Most perfumes are based on the use of esters. Retinyl palmitate is formed when retinol, an alcohol, is combined with palmitic acid, a 16-carbon fatty acid. When combined and heated, the alcohol group on the vitamin A combines with the acid group on the fatty acid. This is diagrammed in Figure 2.

     Retinyl acetate. The two most common esters of retinol are palmitic and acetic. Retinyl acetate is formed by combining retinol with acetic acid.

     Retinoic acid. The acid form of vitamin A is called retinoic acid, and is available by prescription only. It is the form of vitamin A used by the body at the cellular level. See Figure 3 to see the forms of vitamin A.

Vitamin A and the body
     Treatment ingredients are used because the normal body level is low and you are replacing what is missing, or you are adding a substance to modify or change an existing condition that is not desirable. If the body had everything it needed and every system was running perfectly normal, each person would be great, beautiful and perfectly healthy. Sadly, this is not the case; people need help, and one of the substances that can help is vitamin A.
     To understand what vitamin A actually does in the skin in order to be able to use it advantageously, examine the pathway from food intake to the cellular use of vitamin A.