Acids have never been more popular in skincare. Also known as AHAs and BHAs (alpha- and beta-hydroxy acids), these chemical exfoliants have made their way into most people's skincare routines, replacing the face scrubs we used in the past.
But in certain circles, acids are actually considered harmful—or at least, something you shouldn't use very often—because of the notion that they thin (and age!) the skin.
Is that true? In this tutorial, you will learn how acids work and if they cause skin thinning, whether you should be worried about depleting your skin cells, and the best gentle acids for your skin.
Why Are Acids Thought to Cause Skin Thinning?
There are two reasons why acids are thought to make your skin become thinner.
1. The Way Acids Work Is Misunderstood
First is the idea that thinning the skin is the way AHAs and BHAs work.
"With acids, you really have to be careful," says celebrity facialist Joanna Vargas in an interview with Yahoo. "The skin thins as we age already, so a big risk is peeling away the layers of skin faster than collagen can grow. If you use [acids] more frequently, like in a daily cleanser, you risk thinning out the skin over time. Once a week is more than enough."
Annemarie Gianni of Annemarie Skin Care feels the same way. "Salicylic acid may help with acne temporarily, but over the long term can dry and thin your skin," she writes on her blog.
Anecdotally, this way of thinking seems to be widespread amongst skin therapists. One reader wrote to me: "I've been told by two different estheticians that I should be exfoliating my face as little as once every three months... and that by exfoliating, I'm thinning my dermis."
2. The Theory of Limited Cell Division
The other school of thought is that acids thin the skin because our cells can only renew themselves so many times before they die... forever.
Perhaps the most famous proponent of this point of view is Dr. Barbara Strum, a celebrity skin doctor and the founder of Dr. Barbara Sturm Skincare.
"The 'good' acids like hyaluronic acid or citric acids support the skin with moisture, strong anti-oxidative effects and other valuable benefits," she told Refinery29. "The acids that damage the skin, however... accelerate the cell renewal and cause a long-term effect of the skin thinning out, as the cells cannot divide infinitely."
This theory has also made the rounds amongst estheticians and on Reddit.
One Redditor wrote: "I went to the beautician and... we ended up talking about my routine and my use of acids. She freaked.... Apparently you should only be using an AHA at most once every two weeks. When you exfoliate with an AHA you're essentially speeding up the rate at which your cells divide.... But fibroblasts have a limit on how many times they can actually divide. So at first everything looks great... but later in life your skin gets very saggy or droopy and thin because your fibroblasts have hit their dividing limit and their division becomes very sluggish, and suddenly an important part of your skin isn't holding it together so well anymore."
How Do Acids Exfoliate Your Skin?
If the above statements have you worried about the acids in your skincare routine, hold up. Before you ditch them, let's go over the facts.
How AHAs Work
AHAs (alpha-hydroxy acids) are a family of acids derived from sugarcane, milk or fruit, and include glycolic, lactic, citric, malic, mandelic and tartaric acids.
Researchers (see here, here and here) hypothesize that AHAs work by creating acidic conditions within the skin cells. This allows calcium ions to flow in, eventually overloading the cells and leading to cell death—and therefore a "forced" exfoliation.
How BHAs Work
BHAs (beta-hydroxy acids) are derived from willow tree bark, wintergreen leaves or sweet birch bark. There's just one BHA, salicylic acid, but derivatives such as betaine salicylate can have a similar effect.
Unlike AHAs, BHAs don't cause cell death—instead, they exfoliate by softening the protein bonds that hold skin cells together. When these bonds are weakened, dead skin cells are easily shed off.
Both AHAs and BHAs exfoliate the skin surface, but BHAs have the added ability to penetrate into pores. There, they exfoliate the pore lining, which loosens clogs and allows oil to flow out more freely.
Do Acids Really Thin Your Skin?
To recap, we know that AHAs and BHAs both remove surface dead skin cells, although they work in different ways.
But does that mean they're making your skin thinner overall? To answer that, we need to understand how acids affect the different layers of skin.
Human skin has three main layers:
- Epidermis: The top layer of skin, which acts as a barrier from bacteria, irritants and allergens, and where cellular renewal continuously takes place. The outer layer of the epidermis is known as the stratum corneum.
- Dermis: The middle layer, which houses structural proteins, blood vessels, sebaceous glands and hair follicles.
- Hypodermis: The subcutaneous fat layer at the bottom.
How Acids Affect the Stratum Corneum
So here's the thing. At the low concentrations you find in over-the-counter skincare products, acids do indeed thin the skin... but only this uppermost layer, which consists of dead cells.
For example, this study found that 2-5% glycolic acid thinned the stratum corneum without any change to the epidermis.
This is a good thing, because it means the acid is doing its job of sloughing off old, dull, clogging dead cells to reveal the fresh new cells underneath. "It exfoliates the outermost dead layer of the skin, which is called the stratum corneum, and improves the reflection of light on the skin," says Dr. Gretchen Frieling in an interview with Marie Claire.
How Acids Affect the Epidermis
As for the epidermis overall, acids have been shown to either produce no change or to even thicken it.
At the typical low amounts, AHAs and BHAs are unlikely to affect the epidermis at all. Just like the example above with glycolic acid, this study (cited here) found that the use of 0.5% and 2% salicylic acid caused "thinning of the corneal layer [stratum corneum] without any change in the thickness of the epidermis."
At higher doses—which should only be administered under the care of a medical professional—both AHAs and BHAs have been shown to thicken the epidermis.
This study found that a 50% glycolic acid peel produced "a thinning of the stratum corneum, an enhancement of the granular layer [within the epidermis], and an epidermal thickening" after five weeks. Another study found that 25% glycolic, lactic or citric acid "caused an approximate 25% increase in skin thickness" after six months. The same goes for BHAs: a weekly 30% salicylic acid peel was "associated with [a] thickened epidermal layer" after six weeks, according to this study.
How Acids Affect the Dermis
What's more, acids can have a thickening effect on the dermis, the middle layer of tissue where collagen and elastin fibers can be found.
This study reported that AHAs evoke dermal changes including increased collagen production and increased dermal thickness. And this study showed that salicylic acid produced a significant increase in collagen and elastin thickness after six weeks.
Again, this applies to professional-strength treatments rather than weaker formulas for home use.
What’s Wrong With the “Hayflick Limit” Theory of Cell Division?
But what about the notion that acids thin our skin because our cells can't keep on dividing and regenerating forever?
This theory of cell division is known as the "Hayflick limit," and was conceived by Dr. Leonard Hayflick, an anatomist, in the 1960s. He proposed that human cells only have the capacity to divide about 50 times before they deteriorate and die.
By this logic, any type of exfoliating would be bad because we only have a set number of skin cells, and we'd be depleting them faster than they'd slough off naturally—eventually leaving us with thin, weak, aged-looking skin. In other words, we'd be sacrificing our future skin health in order to look good for the moment!
Fortunately, the Hayflick limit has been disproven.
Hayflick's theory was based on just one type of cell, a lung fibroblast, in a culture dish, and his observation that they quickly deteriorated. But "other researchers, simply by changing a single factor, caused great increases in the longevity of the cultured cells," says Ray Peat, PhD. "Simply using a lower, more natural oxygen concentration, the cells were able to undergo 20 more divisions. Just by adding niacin, 30 more divisions; vitamin E, 70 more divisions. Detailed investigation of skin cell growth showed that cells in the lower layer of the skin divide at least 10,000 times in a normal lifetime, and similar processes occur in the lining of the intestine."
In other words, acids or not—none of us are going to run out of skin cells.
Dr. Peat lists more than 40 scientific references on this, by the way. So why does he think the Hayflick limit theory has persisted in the scientific community for so many decades? "Since then, facts that came out because of the Freedom of Information Act, cause me to believe that a financial motive guided [Hayflick's] thoughts about his cultured fibroblasts."
The Best Gentle Acids for Your Skin
Needless to say, I don't consider it harmful to incorporate a mild acid in your at-home skincare routine, as often as daily. I think it's best to use a leave-on product, rather than a cleanser, since the latter is only on your skin for a few moments—not enough time to have much of an effect. The key is to choose an acid treatment at the appropriate strength for your skin, and use it at a frequency that you can tolerate without experiencing irritation.
Here are some of my favourite gentle acid exfoliants:
The Ordinary Lactic Acid 5% + HA
With 5% lactic acid, The Ordinary Lactic Acid 5% + HA is ideal for beginners and sensitive skin. Lactic acid is one of the mildest acids, and it's bolstered by hyaluronic acid for extra hydration and Tasmanian pepperberry to calm the skin. See my review here.
Biossance Squalane + Lactic Acid Resurfacing Night Serum
Biossance Squalane Lactic Acid Resurfacing Night Serum has 10% lactic acid in a squalane-based hydrating serum—so it's gentle on the skin, but strong enough to transform its texture and luminosity.
Juice Beauty Blemish Clearing Serum
Contrary to its name, Juice Beauty Blemish Clearing Serum isn't just for acne. Most skin types can benefit from its low 0.75% dose of salicylic acid, which exfoliates and deep-clean pores without causing dryness or inflammation.
INNBeauty Project Down to Tone Life Changing Toner
INNBeauty Project Down to Tone Life Changing Toner is the closest match I've seen for the famous Lotion P50. This version is not only cheaper and more widely available, it contains a similar mix of lactic, malic and salicylic acids, gluconolactone, niacinamide and vinegar.
Farmacy Deep Sweep 2% BHA Pore Cleaning Toner
For oilier skin, Farmacy Deep Sweep 2% BHA Pore Cleaning Toner gives you all the benefits of salicylic acid without stripping your skin. Unlike most treatments at this strength, this formula is free of drying alcohol, and has added skin conditioners like arginine (an amino acid).
COSRX BHA Blackhead Power Liquid
Of course, I can't mention acids without recommending COSRX BHA Blackhead Power Liquid, my all-time favourite. What makes this unique is its active ingredient, betaine salicylate, which is gentler and more hydrating than salicylic acid. See my review here.
Paula’s Choice Skin Perfecting 8% AHA Gel Exfoliant
Although glycolic is the most aggressive acid, Paula's Choice Skin Perfecting 8% AHA Gel Exfoliant should be tolerable for most skin. Besides its 8% dose, it has green tea, chamomile and hyaluronic acid in an alcohol-free, fragrance-free and silicone-free gel.
Whether glycolic, lactic or salicylic, acids only thin out your uppermost layer of dead skin cells—which is exactly what you want. They don't make your skin thinner overall.
However, your skin might appear thinner, if you've been overdoing it.
If it's too strong for your skin, or if you apply it too often, any acid can cause barrier damage. Skin looks and feels tight, raw and inflamed, and it's vulnerable to irritants and bacterial invaders. Without that protective barrier, you're also more likely to get sun damage, premature aging and discolourations.
Again, it's of utmost importance to choose a mild acid—I generally prefer BHAs instead of AHAs, since their large molecule sizes make them less irritating, and they have anti-inflammatory properties. Either way, you should always monitor your progress and back off if you notice side effects such as excessive redness, flaking or sensitivity.
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