What can you absorb through your skin?

While some skincare ingredients just sit on the topmost layer of the skin, others can penetrate more deeply – potentially delivering more benefits, but also more risks.

In March 2021, American social media influencer Mikayla Nogueira, whose followers number in the millions, appeared on TikTok to test a toner and serum combination containing a cocktail of fashionable ingredients, which promised to tighten pores, hydrate skin, and instantly "reveal a dewy glow". 

In the minute-long video, Nogueira applies the products to her makeup-free face, before proclaiming: "That was the best foundation application I've ever had in my life!". The post quickly went viral. 

The example is emblematic of a movement that has gained momentum in the skincare industry of late: ingredient-led beauty, where customers seek out particular ingredients to address specific skincare concerns. The toner Nogueira tested, for instance, contains the chemical exfoliants polyhydroxy acid and beta-hydroxy acids, while the serum comprises vitamin B3 or niacinamide, reported to brighten dull skin. (Although it is worth noting that many scientific studies on the benefits of such ingredients involve researchers employed by beauty companies that seek to sell them in products).

There's a smorgasbord of other ingredients that are often found in skincare products, alongside various claims about their benefits: vitamin A or retinol for anti-ageing; vitamin C to brighten and protect skin against environmental damage; hyaluronic acid to diminish dryness, reduce wrinkles and fine lines; vitamin E to heal and soothe – the list goes on.

Searches for specific ingredients on online luxury retailer Net-a-Porter reportedly grew by nearly 700% between 2020 and 2022, while "What does retinol serum do?" ranked among one of the most-Googled skincare questions in the UK last year. Over in the US, a 2022 survey revealed that 61% and 48% of millennial and Gen X beauty shoppers, respectively, look for specific ingredients when buying skincare products.

But can the skin even absorb such ingredients and soak up their purported benefits, or is it all a marketing shtick designed to empty our pockets? And do they need to be absorbed in order to work? As scientists learn more about what can and cannot pass through the skin barrier, the implications aren't limited to skincare – they are leading to a new understanding of the best ways to deliver medications, from painkillers to vitamins. 

Skin is the largest organ in our bodies, comprising up to 22 sq ft (2 sq m) in surface area and weighing as much as 8lb (3.6kg) in adults. It comprises three distinct layers, although skincare companies are mostly interested in the top two.

The outermost layer, or epidermis, is the skin that we can see and touch. "It's made up of cells called corneocytes that act as bricks," explains cosmetic chemist Ron Robinson, founder of the skincare brand BeautyStat. "These bricks are tightly bound by mortar-like fats such as ceramides, cholesterol, and fatty acids," 

Skin's middle layer, the dermis, which makes up 90% of skin's thickness, "is where all the fun stuff happens", says Mary Sommerlad, a London-based consultant dermatologist. It consists of connective tissue, blood vessels, hair follicles, and oil glands, but also collagen and elastin. "These are magic proteins that we need to keep our skin smooth to avoid wrinkles and fine lines, and what makes our skin feel plump – it's like the holy grail of anti-ageing," she says.

In truth, "the majority of biomolecules never penetrate the skin barrier", says Greg Altman, founder of Evolved by Nature, a Massachusetts-based company that makes sustainable soap and skin sprays.

That's because skin is our body's first line of defence against the outside world.

"Skin is designed to keep things out, and it does that very well," says Zoe Draelos, a clinical and research dermatologist at Duke University in North Carolina, who has been in the field for more than 35 years.

"It regulates your temperature and water balance, it protects you from disease, it maintains a microbiome balance of healthy organisms," she says. "Skin does so many things that are vital to life that it has to be a barrier. It can't be a wholly easily penetrated structure."

Substances such as oils, silicones, and waxes found in many moisturisers are too big to slip through the epidermis. Instead, they remain on top of the skin, acting as emollients – moisturising agents – by sealing water in, which makes skin feel soft and smooth, says Robinson.

For a molecule to cross the skin barrier, journey through the epidermis, and enter into the dermis, it must possess properties similar to the skin's surface. It must be lipophilic (or fat-loving) with a slightly acidic pH (between 4.6 and 5.5) – and have a molecular weight smaller than 500 Daltons.

However, there are ways to circumvent these limitations. Sometimes cosmetics companies break a big molecule down into nanoparticles, or encase a water-loving compound in a layer of fat or oil. Or it's possible to use microneedles to poke holes in the skin's surface or chemicals, such as glycolic acid, to make it more penetrable. It is worth noting that many scientific studies on the benefits of certain ingredients involve researchers employed by beauty companies that seek to sell them in products.

"If you want to penetrate [the skin] a little deeper, you can tweak your product formulation," says Draelos. However, doing so comes with certain risks. "Once something reaches the dermis, it's going to get absorbed into the systemic circulation," she says. "And cosmetic products aren't supposed to do that," she says.

Still, some ingredients do. Retinol, for instance, has been found to pass through melanocytes, the bottom-most layer of cells on the epidermis which are responsible for protecting skin from harmful ultraviolet radiation, inhibiting their growth and making users more susceptible to sun damage. 

Parabens, used as a preservative and to prevent contamination, are another example, with some studies suggesting they may affect hormone production. But Sommerlad says "the jury is [still] out" on whether this is the case and dermatologists generally consider these ingredients safe to use, although pregnant women might want to avoid using them as a precaution.

The trouble is that "skincare is caught between science and marketing", says Altman, compounded with the fact that the beauty industry is largely self-regulating. There's still a lot scientists don't know about which ingredients can be absorbed into the skin, how deep they go, and their potential risks, he says.

"Although it's well-known that various skincare formulations can positively impact skin health, the precise mechanisms underlying these effects aren't always fully elucidated," says Sebastian Björklund, an associate professor of physical chemistry at Sweden's Malmö University. Which is why he is now leading a study to investigate the topic, specifically focusing on vitamins, with the first results expected later this year.

Designing ingredients that can absorb into the dermis and beyond – as opposed to the more superficial penetration of the epidermis – is "actually very tricky", says Sommerlad. "Not many drugs can easily cross the trans-epidermal route because the chemistry behind it is quite difficult."

That's one reason why transdermal patches for vitamins haven't quite taken off. Adhesive stick-to-skin patches have been used to deliver nicotine, pain-relieving fentanyl, and pregnancy-preventing hormones for decades, offering users a convenient way to receive medication at a constant dose. Inspired by this success, the US military's combat feeding research and engineering programme began exploring in 2003 whether similar patches could be used to deliver vitamins and micronutrients to soldiers in combat.

But two decades on, scientific evidence to support the use of such patches remains sparse. There are no known results from the US military's study, and no large-scale clinical trials have ever been conducted. In a small study published in 2019, researchers found that gastric bypass patients who used a multivitamin patch for a year post-surgery were twice as likely to be deficient in vitamin D, and had lower serum concentrations of B2, B12, folate, and ferritin, compared with those who took an oral multivitamin.

"Vitamins are much more effectively absorbed from the inside," says Draelos. "Topically (through the skin) is not a replacement." Moreover, maintaining a healthy, well-balanced diet paired with sufficient water intake is essential for healthy skin, she says. "You need to have the raw building blocks for your body to generate the vitamins, minerals, fats, and proteins that are naturally found in skin," she says.

When it comes to deciding what skincare products to purchase, dermatologists like Draelos and Sommerlad offer the following advice: for those with healthy skin, use a gentle cleanser and a good moisturiser that suit your skin type; protect yourself with sunscreen, preferably one with vitamin C or other antioxidants; and adopt a "less is more" approach by using products with fewer ingredients, especially if you have sensitive skin.

"The other piece of advice I give to my patients is to handle your skin like a silk scarf," says Draelos. "You wouldn't rip, pull, pinch, or ring a silk scarf – it would ruin it. Your skin is the same way."

For those concerned about ageing skin, Sommerlad recommends adding a retinoid to the routine above which will help boost collagen and elastin levels. "Even then, I would say it's a good idea to speak to a dermatologist who can guide you because it's a minefield and you don't want to spend lots of money on things that either just damage your skin barrier, or don't do what you want them to do." 

But ultimately experts agree that skin, if healthy, is more than capable of looking after itself. "A lot of people think that you have to do something to your skin to make it healthy or make it look good," says Draelos. "But actually the skin will be beautiful as long as it's healthy just by itself. So in some ways, the less you do, the better," she says.

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