HomePublic HealthHydroxic Acid : The Love And Hate

Hydroxic Acid : The Love And Hate

What is Hydroxic Acid and does it used for

According to the position of the hydroxyl (OH) and (COOH) parts in the chemical structure, it can be divided into α-type and β-type (AHA and BHA) organic carboxylic acids.

These two categories exhibit almost similar biological effects, have been widely used all over the world, and have existed as ingredients for skin and cosmetics for centuries.

They are accepted by dermatologists, physicians, beauticians and consumers, and have long been proven to have good effects.

Chemical structure and natural source of AHAs

HAs range from simple aliphatic compounds to complex molecules. Many of these compounds are originally derived from nature and are therefore commonly referred to as fruit acids.

A large number of synthetic sources provide us with a way to obtain analogs of the response structure. At present, most of the AHAs used in dermatology and beauty are chemically synthesized.

According to the number of carboxyl groups, it can be characterized by chemical groups. According to their configuration, AHAs exist in different stereoisomeric structures, called enantiomers 1 and d or R and S. Some common AHAs naturally exist in the form of enantiomer enrichment.

About Glycolic Acid

Glycolic acid (2-hydroxyacetic acid) is a component of sugarcane juice. Lactic acid (2-hydroxypropionic acid) was first isolated in 1780.

The l-lactic acid produced by the microbial lactic acid bacteria is responsible for the taste and smell of yogurt.

In addition, it is the final product of epidermal anaerobic metabolism and appears on the skin surface in the form of low concentrations of sweat components.

The other enantiomer, d-lactic acid, also called muscle lactate, is formed during anaerobic muscle contraction and is also found in apples, ergots, artichokes, and tomatoes.

The extract of bitter almond is hydrolyzed to obtain mandelic acid (2-hydroxy-2-mandelic acid). Malic acid (2-hydroxy-1,4-succinic acid) was first isolated from immature apples in 1785.

Tartaric acid (2,3-hydroxy-1,4-succinic acid) was first isolated in 1769. It is widely distributed in plants, especially the leaves of grapes and wine.

Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) was first separated from lemon juice in 1784. Pineapple and other citrus fruits also contain this substance.

Biological activity of hydroxic acid

Summary: the stronger the acidity of the fruit acid, the better the effect and the greater the irritation.

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The biological activity of hydroxic acid is attributed to the acid strength of the compound. This physicochemical property is measured by the dissociation of protons in solution, and is expressed by its pKa.

The lower the pKa of a hydroxic acid, the stronger its acid strength: for every unit of pKa decrease, the acid strength increases tenfold.

The pH of the formulation varies with the nature and concentration of the hydroxic acid. In order to avoid irritation as much as possible, it is best to design a cosmetic formulation with a pH close to the physiological pH range of the skin.

This can be achieved by partial neutralization and the addition of effective buffers. However, AHA products with a neutral pH seem to have little effect on the skin.

Clinical indications of hydroxic acid

The clinical indications of hydroxic acid are diverse. The direct effect of AHAs and BHAs on the stratum corneum (SC) is beyond doubt, even in the presence of Sjogren, ichthyosis and similar conditions.

Some pigmentation problems also benefit from the same compounds. Hyperkeratosis in acne-prone patients usually improves.

In the field of benign tumors, high-concentration preparations can effectively treat keratosis and viral warts. The effect of hydroxic acid on solar skin diseases is more complicated, involving various mechanisms to promote the regeneration of aging skin.

Product regulatory of hydroxic acid

Most of the above effects are related to the dosage of hydroxic acid. When the active compound content in the formula is less than 4%, it is defined as a low concentration category.

Medium concentration is suitable for 4%-12% range, high concentration is higher dose.

From a regulatory point of view, the U.S. Food and Drug Administration (FDA) considers hydroxic acids to be cosmetic ingredients with a maximum concentration of 10% and a formula pH of 3.5 or higher.

Above this concentration or below this pH, they belong to the category of drugs

Effects on the cohesion and function of stratum corneum cells

Summary: Desmosomes are used to connect keratinocytes, and the loosening of desmosomes makes the epidermis easier to fall off.

During the formation and maturation of the stratum corneum (SC), the combination of desmosomes and cells forms tight junctions of keratin desmosomes.

The number of desmosomes gradually decreases in the transition from the inner layer of the skin to the surface of the skin.

In the case of dryness, scaly and ichthyosis, due to the continuous presence of desmosomes in the outer layer of SC, irregular accumulation of keratinocytes and scaly and exfoliation of the skin result in damage to orderly desquamation.

About Salicylic Acid

Salicylic acid is a long-established BHA used to improve dry skin and scaly conditions. Although this compound has little or no effect on normal SC at low and medium concentrations, there is evidence that the complete degradation of keratinocytes contributes to various dryness and ichthyosis.

Therefore, using the term “keratinizer” to describe this compound seems to be a misnomer, while the term “releasing agent” is more appropriate and specific.

Spalling and corrosion effect

Summary: high concentrations of fruit acids not only exfoliate, but also cause cell death. Although the effect is good, but also pay attention to side effects.

When high concentrations of AHAs are applied to the skin, it may cause exogenous epidermolysis and cause necrosis and shedding of keratinocytes.

Such damage leads to chemical exfoliation, which mainly depends on the damage to the skin. The farther the pH value during use is from the physiological pH value, the greater the risk of adverse events, and patients often have tolerable burning and itching sensations.

Whitening effect

Summary: The whitening effect of fruit acid is obvious, and it may be more effective when combined with other whitening agents.

Glycolic acid peeling is a useful auxiliary treatment for melanosis of the epidermis, such as melasma and acne scars.

The combination of 10% glycolic acid and 2% hydroquinone can improve this disease. There is also a combination of 5% glycolic acid and 2% kojic acid.

Some people think that AHAs have a similar effect as penetration enhancers, accelerating the conversion of the epidermis.


Summary: pay attention to sunscreen after use of AHA.

Most of the adverse reactions after the application of hydroxy acid are tingling sensations, without any other clinical and histopathological signs of irritation.

However, high concentrations can cause severe redness, swelling (especially in the eye area), burning, blistering, bleeding, rash, itching, and discoloration of the skin. Their long-term effects are unclear.

Some AHA users react more strongly to sunlight. In fact, it has been reported that subjects who received AHA products under ultraviolet radiation had twice as much cell damage in the area where they used AHA products than in the control group.

The US Food and Drug Administration stated that the hepatitis A virus can be absorbed by the skin at different rates.

The US Food and Drug Administration and the Cosmetic Ingredient Review (CIR) evaluated the safety of salicylic acid as a cosmetic ingredient.

They concluded that products containing salicylic acid should contain sunscreen, or recommend consumers to use other sunscreen products on the instructions.

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