Even though all alpha hydroxy acids (AHA) are derived from one major subgroup with a related chemical structure, each is derived from a different source. Further each of these acids has a slightly different action and uses
GLYCOLIC ACID: This is the most common and widely used acid for skin rejuvenation. The acid is derived from sugar cane but can also be made in the laboratory. It has a small chemical structure and can easily penetrate the skin. Glycolic acid is an exfoliant or a peeling agent. It has the ability to break up and remove the dead layers on top of the skin. This exfoliation is a useful property when it is used to treat blackheads, acne scars and sun spots discolorations. After its use, the skin is usually smooth, fresh and has a youthful appearance. It also causes moisturizing of the skin to some degree. Rarely the acid has been used to treat dark skin discolorations. This bleaching action is mild. Glycolic acid is a common ingredient in many cosmetic creams and lotions and when used regularly may help reverse some of the skin changes seen with age.
CITRIC ACID: This acid also has been touted as a skin rejuvenating agent. Unlike the other acids, it is more of an anti oxidant like vitamin C. It is postulated that when citric acid is used regularly, it may increase the skin tone by increasing collagen production (highly unlikely though). Some anecdotal reports indicate that it may also act like a bleaching agent and help treat various skin discolorations. The role of citric acid in the treatment of skin disorders is not fully established. Topical application of the acid most likely does not do much. Consuming it orally like vitamin C may offer some anti oxidant activity. Whether it has any medical properties is highly unlikely.
LACTIC ACID: This acid can be obtained from sour milk and is frequently used by cosmetic physicians as an exfoliating agent. The acid is frequently combined with the other acids to soften the skin. Various concentration of this acid are available and used to rejuvenate the skin.
MALIC ACID: Malic acid is derived from both apples and grapes. It is not used to treat any skin disorders but is frequently combined with some topical hydrocortisone to treat dry skin and itching. The acid has been touted to treat many medical disorders including fibromaylgia. Except for its bitter taste, it is unlikely that this acid has any medical benefits
TARTARIC ACID: Tartaric acid is obtained after fermentation of wine. It is widely used in the food industry and frequently found as an ingredient in numerous cosmetic products. There are numerous claims that the acid can improve facial appearance, smoothen skin, rid of fine lines and act as an exfoliant. All these claims remain hypothetical and untested
The above AHA are found in combinations in many cosmetic products. While each has a different action not all have medicinal value. There is some evidence that glycolic acids do have exfoliative and peeling properties. Most users of these products claim that the AHA can lead to smoother, softer a youthful skin.