The science behind AHAs and BHAs in skincare
Modern skincare has made some amazing advances, with ingredients that can clear up acne breakouts, reduce hyperpigmentation, and turn back the clock on aging. But what’s really going on at a cellular level when you apply these ingredients to your skin? In this post we’ll explore the science behind alpha and beta hydroxy acids, and explain why they have such amazing effects.
What are alpha and beta hydroxy acids?
AHAs and BHAs are a group of natural acids found in fruits and other natural products. There are eight AHAs most commonly found in skincare products. These are:
- Lactic acid (from milk)
- Glycolic acid (from sugar cane)
- Citric acid (from citrus fruit)
- Malic acid (from fruit)
- Mandelic acid (from bitter almonds)
- Tartaric acid (from grapes)
- Hydroxycaproic acid (from royal jelly)
- Hydroxycaprylic acid (from animals)
If you see BHAs on the label of your skincare products, that means salicylic acid, which comes from willow tree bark. Fun fact, the active ingredient in willow bark, salicin, is also the source of aspirin!
These ingredients are used in skincare to boost skin shedding, accelerating the cellular turnover cycle. This leads to a fresher, brighter complexion. The same mechanisms that break down the bonds between the topmost layers of skin cells also work to clear pores and even out skin tone. AHAs/BHAs also have great hydrating properties, which helps to smooth skin and reduce the appearance of fine lines and wrinkles.
What do AHAs do for your skin?
AHAs and BHAs do many similar things, but they all have slightly different effects as a result of different molecule sizes, pH, and concentrations used. To understand how they all work, you first have to understand the makeup of your skin.
All about skin structure
Tomáš Kebert & umimeto.org / CC-BY-SA 4.0
Our skin is an incredibly complex organ, just like our heart or kidneys. It is made up of three layers, the epidermis, dermis, and subcutaneous tissue.
The epidermis is the outermost layer of skin, and its main role is to prevent bacteria from entering our bodies, and to keep moisture inside.
The dermis is the layer just below the epidermis. It holds our hair follicles, sweat glands, and connective tissues.
Finally the subcutaneous layer, also known as the fascia, contains fat and more connective tissues, which give our skin its structure.
You can see in the skin diagram that the epidermis and subcutaneous layers are much thinner than the dermis. The epidermis is the layer skincare products primarily work on. This layer is made up of five layers of its own:
- Stratum corneum (the outermost layer)
- Stratum lucidum
- Stratum granulosum
- Stratum spinosum
- Stratum basale (the deepest layer)
Of these, most skincare products work on the stratum corneum, also known as the “horny layer” (that’s literally what the name “stratum corneum” means in Latin). This layer is made up of 15-20 rows of flat cells called corneocytes. These cells have no nuclei or organelles, and for a long time were thought to be dead, although now scientists know that that’s only true for the very outermost of them.
The effect of AHAs on corneocytes
Corneocytes are held together by a lipid matrix containing ceramides, cholesterol, and fatty acids. AHAs are very small molecules — just 2-6 microns — and they can penetrate the topmost layers of corneocytes to affect the deepest layers of the stratum corneum, where new corneocytes are forming. They smooth down these cells and make the overall stratum corneum thinner, which improves the surface texture of skin. Thinner layers are also more flexible, which prevents them from cracking or wrinkling. This is particularly beneficial to dehydrated skin, which is more prone to cracking.
This happens whether or not we use products containing AHAs, because our bodies make our own AHAs. As a result, we can never be deficient in AHAs. However we also make other molecules, alpha acetoxy acids (AAAs), by adding an acetyl group to an alpha-hydroxy acid.
AAAs have the opposite effect to AHAs, increasing the strength of bonds between corneocytes and making skin thicker and more likely to crack. Because our skin is constantly being affected by these two types of molecules, supplementing AHAs through skincare can tilt the balance in our favor and improve the overall condition and appearance of our skin.
Another factor that impacts skin cracking is hydration. AHAs are great for making dehydrated skin more supple by reducing the thickness of the stratum corneum, but they are also powerful humectants, meaning they attract and hold moisture, hydrating your skin. This not only improves the appearance of skin, it also boosts cellular turnover — the process that replaces the old, dead cells in the outermost layer. Think about when you take a bath. When your skin is super hydrated, it’s easy to exfoliate the outer layer of skin. Although it’s not as easy to see, dehydration has a reverse effect, preventing old skin cells from shedding.
AHAs and ionic bonding
Corneocytes are held together by a process known as ionic bonding, which is very similar to magnetism. Negative and positive groups of molecules surrounding the cells attract each other and hold the cells together. The negative groups are made up of carboxy, sulfate, and phosphate, while positive molecules come from amino groups in amino acids. All these groups are found in the proteins, sterols, and lipids that coat the outer layers of corneocytes. That means your skin is held together by billions of ionic bonds at a molecular level!
There are three factors that affect the strength of these bonds.
- The distance between positive and negative groups
- The material the groups are suspended in
- The number (density) of groups
When the stratum corneum is hydrated, the cells swell and the outer layer coating them stretches thinner. This increases the distance between the group and reduces their density by spreading out the material containing them. That weakens the bonds holding cells together — just like moving two magnets farther apart — and makes skin shedding easier.
AHAs boost this process in two ways. Firstly, they hydrate the corneocytes, helping to weaken ionic bonds. But they also outcompete other molecules for the sulfate and phosphate that make up the negative sides of the ionic bonds. This reduces the number of available connectors to make the bonds in the first place.
Recap: AHAs and skin
In summary, our skin is made of several layers, and most skincare works on only the very outermost cells of the outermost layers. AHAs can penetrate deeper than other skincare ingredients because of their very small molecule size. At the base of the outermost layer, where new cells are forming, AHAs boost hydration and smooth the cells, creating more supple and even skin. This protects against cracks and wrinkles, and is especially effective at protecting dehydrated skin.
But that isn’t all AHAs do! Our skin cells are held together through ionic (positive/negative) bonds. AHAs outcompete other molecules for the negative groups that make up those bonds, reducing their numbers. They also lower the density of the bonds through increasing hydration. This weakens the bonds holding skin cells together, making them easier to shed.
The results of AHAs on skin
Now we know that AHAs have three major effects on our skin — they flatten the outer layers of cells, increase hydration, and weaken ionic bonds between cells. The effect these three processes have can be seen in the skincare results they deliver.
Reducing the thickness of the stratum corneum by smoothing the cells gives us a more even skin texture and lessens the appearance of fine lines and wrinkles. AHAs are less effective at reducing deep wrinkles because they don’t penetrate beyond the start of the topmost layer of cells.
By increasing skin shedding (a process known as “desquamation”), AHAs also improve skin tone and can reduce dark spots caused by hyperpigmentation. When old cells build up in the stratum corneum, they make skin appear dull and dry, and can worsen the appearance of an uneven complexion. Simply increasing the turnover rate of those cells can dramatically improve the skin’s appearance. Another great side effect of this process is that it reduces the likelihood of clogged pores, helping to control acne breakouts.
Using AHAs in your skincare products therefore provides lots of beneficial effects, and the best thing is they’re totally natural. Skincare companies often use synthetic (lab-made) versions of AHAs because they’re more stable, but they’re molecularly identical to AHAs found in fruit and sugar molecules. Your body already understands this process and converts food into AHAs to nourish your skin. By supplementing with AHAs in skincare, you’re just giving your cells a helping hand.
What do BHAs do for your skin?
Beta hydroxy acids are also great skincare ingredients. Although they sound the same as AHAs, they do have different effects, in part because of where in the skin they work. BHAs are fat soluble, whereas AHAs are water soluble. This is particularly important for your skin because the cells are held together by a layer of lipids (fats). That means BHAs can absorb easier into your skin, and work deeper than AHAs. BHAs are also more effective at treating acne than AHAs. To understand why, we first need to understand what causes acne to begin with.
What causes acne?
- Own work / CC BY 3.0
There are four underlying causes of acne — inflammation, sebum, blocked pores, and bacteria. Everything else contributes to one of these causes. For example hormones might trigger excess sebum production, or not washing your face leads to a buildup of dirt that blocks pores. BHAs directly target two of these causes — inflammation and sebum — and also help to clear blocked pores. That means using BHAs in your skincare products tackles three out of four of the causes of acne with just one product. That’s amazing!
How salicylic acid targets sebum production
Sebum, or skin oil, is produced from sebaceous glands. These glands are located inside hair follicles in the dermis, the middle layer of skin. BHAs can reach them because they’re fat soluble, so they can get through the epidermis and actually dissolve into the sebum itself. Once inside the sebaceous glands, they stop them from producing lipids by blocking a protein pathway known as SREBP-1.
SREBP-1 stands for sterol regulatory element-binding protein 1, and it regulates lipid and cholesterol production by sensing the levels of sterol in a cell. Salicylic acid disrupts this process by decreasing the mRNA levels in the amino acids that make up the protein. That means when you use skincare with BHAs, you’re flipping a switch that turns off oil production from the sebaceous glands. That’s great news for people with oily and acne-prone skin.
An awesome side effect of this process is a reduction in blocked pores. Many things can clog pores, from microscopic dirt and pollution you pick up when you go outside, to the powder from your makeup. However a big underlying culprit is skin oil. Not only can sebum itself clog pores, it also binds skin cells and stops them from shedding effectively, and also traps all the other particles that get on your skin and cause blackheads.
How BHAs target inflammation
The next thing salicylic acid does is suppress a cell regulatory pathway known as NF-kB (nuclear factor kappa light chain enhancer of activated B cells). This pathway controls inflammatory responses in cells, and BHAs specifically target this pathway is sebocytes, the cells that make up the sebaceous glands. By inhibiting this pathway, salicylic acid turns off the inflammatory response in these cells.
This is super important for the treatment of acne, because inflammation is always detected when acne is present. In fact, a study on the Role of Inflammation in the Pathology of Acne suggests that acne might even be an inflammatory condition.
Salicylic acid doesn’t just target the NF-kB pathway of sebocytes, however. It also triggers a cell death pathway known as AMPK (AMP-activated protein kinase). This is a fuel-sensing enzyme that tells cells to switch between anabolic and catabolic metabolism. When AMP is detected, cells are in an anabolic state where they proliferate and grow. When the enzyme doesn’t detect AMP, that means the cells have no energy, and it triggers catabolism instead. When this happens, cells stop growing and some are broken down to convert into energy for others.
BHAs hijack this pathway in sebocytes by increasing phosphorus levels in the cells, which confuses the enzyme and causes it to trigger catabolism. That sends out a message to sebaceous gland cells to stop replicating and to actually start breaking down. This leads to an increase in apoptosis (cell death) in sebocytes that decreases the overall size of the sebaceous gland, and therefore the amount of lipid the gland can produce.
Recap: BHAs and skin
In summary, acne has four primary causes: inflammation, sebum, blocked pores, and bacteria. Salicylic acid targets two of these causes directly, and one indirectly. It controls sebum production by disrupting a protein pathway that tells sebaceous glands to stop producing skin oils. This also reduces the likelihood of blocked pores caused by excess oil or trapped dirt.
Next, BHAs reduce the size of sebaceous glands by signalling the cells to break down. This limits the amount of skin oil the glands can produce, even if they become overactive. Therefore salicylic acid has long term effects that continue to control the underlying causes of acne even if you discontinue using BHAs in your skincare products.
The results of BHAs on skin
Because salicylic acid is fat soluble, it can penetrate much deeper into the skin than AHAs, reaching the dermal layer where hair follicles and sebaceous glands are found. That means it can work directly on the cells that control the underlying condition of your skin, not just on the very outer surface. And being fat soluble also means BHAs are more effective on oily skin, making them even more powerful as an anti-acne ingredient.
The effects of BHAs on lipids have been well documented. In fact, there is a direct correlation between the concentration of salicylic acid applied to the skin and the rate of lipid reduction. BHAs are also great for sensitive and inflamed skin because they are more easily absorbed.
The science of AHAs and BHAs
While the effects of alpha and beta hydroxy acids on the skin are clear, and what has been discovered so far about how they work is amazing, that isn’t the whole story. Researchers are still studying the precise mechanisms of these ingredients.
We know that when we use AHAs and BHAs in different combinations, pH, and concentrations, they have different effects. For example AHAs in low concentrations reduce the bonds between corneocytes, but in high concentrations they smooth the entire structure of the stratum corneum. Low concentrations of BHAs have a cleaning and anti-inflammatory effect, while in high concentrations salicylic acid can be used as a chemical peel, providing deep exfoliation.
However just because the science behind AHAs/BHAs is new, that doesn’t mean the ingredients are. Humans have actually been using them for centuries. Cleopatra (60 BC) took milk baths to stay looking youthful, and Madame de Pompadour (1721) used to bathe in sour wine for the same reason. While that might sound dumb today, they were both using alpha hydroxy acids — lactic acid from milk, and tartaric acid from grapes. When it comes to cutting edge skincare science, the old ways really might be the best!
Additional Resources:
- Applications of hydroxy acids: classification, mechanisms, and photoactivity
- Considerations in alpha hydroxy acid peels
- Effects of alpha-hydroxy acids on the human skin of Japanese subjects
- Hyperkeratinization, corneocyte cohesion, and alpha hydroxy acids
- Salicylic Acid Peels for the Treatment of Photoaging
- Salicylic acid as a peeling agent: a comprehensive review
- The science of hydroxy acids: mechanisms of action, types and cosmetic applications