Zoya Formulates · Science Concepts
How Actives Work
Retinoids and hydroxy acids are two groups of activ ingredients often used in skin renewing and rejuvenating formulations that genuinely change how your skin is built, not just how it looks on the surface. This page will walk you through the science step by step, in plain language, so it actually feels readable.
Retinoids
What Are Retinoids?
Retinoids are vitamin A derivatives (compounds made from vitamin A). This family includes retinol, retinaldehyde, and retinoic acid (also called tretinoin). They are very commonly recommended for numerous skin concerns as it is one of the most closely studied ingredient groups in dermatology, with decades of clinical research showing that they can change how skin cells behave at the level of gene activity. It is because of this that it’s often sighted as the golden standard . It originally was prescribed for acne patients but has since been shown to help with numerous signs of aging…like a chemical 2 for 1 !I
| Retinoid | Prescription Required | Converts To | Potency |
|---|---|---|---|
| Retinol | No | Retinoic acid (2-step) | Moderate |
| Retinaldehyde | No | Retinoic acid (1-step) | Moderate–High |
| Retinoic acid (tretinoin) | Yes (most countries) | Already active | High |
| Retinyl esters | No | Retinoic acid (3-step) | Mild |
Mechanism
Receptor Activation
Retinoic acid is fat-soluble(lipophilic) which means it can travel through the cell membrane on its own without a carrier protein(think of this like a vehicle, you need a car to get to certain places). Once inside it moves into the nucleus often referred to as the control center of the cell and binds to nuclear receptors called Retinoic Acid Receptors(RAR) which then binds to another receptor Retinoid X Receptors(RXR), think of these as project supervisors essentially doing the foundational planning before the actual work begins . These receptors then pair up and form what is called a heterodimer (a fancy word for a molecular complex) and attach to specific DNA sequences called Retinoic Acid Response Elements. That attachment turns certain genes on or off, so retinoids literally call the shots and change what your skin cells are making!!
Collagen
Collagen Synthesis and MMP Inhibition
Collagen Synthesis
When RAR (Retinoic Acid Receptors) are activated, they basically tell the cell to switch on genes that make type I and type III collagen which are the two main structural proteins that keep skin firm, bouncy and smooth. Retinoids also stimulates fibroblasts (the collagen-producing cells in the dermis) to multiply, and recue the activity of enzymes that break collagen doen.. Over time, this thickens the dermis and softens the look of fine lines, resulting in better aging over all.
MMP Inhibition
MMPs (matrix metalloproteinases) are enzymes that naturally break down collagen…like demolition enzymes. This is a normal cellular occurance.. but UV exposure and ageing speed this process up. Retinoids dial down MMP-1, MMP-3, and MMP-9 by using the RAR pathway to suppress the genes that code for them. The result is less collagen is broken apart, so the skin’s structural matrix stays intact for longer.
💡 You can think of this as a two-way switch: retinoids turn up collagen production and turn down collagen breakdown at the same time.
Side Effects
Retinoid Draw Backs.
Retinoid Dermatitis
Retinoid dermatitis often called the retinoid uglies is a predictable adjustment phase, not an allergy. It usually shows up as dryness, flaking, redness, and short-term sensitivity to other products or even touch. This happens because retinoids speed up cell turnover faster than your skin barrier can keep up at first. Think of it like construction workers working faster on a specific part than the rest of the site is ready for, while the work is being completed the general environment it not prepared resulting in temporary chaos . The reaction most often peaks around weeks two to four, then settles as the barrier adapts. You can usually manage it by starting slowly (every 2–3 nights), applying to fully dry skin, and sealing with a gentle occlusive moisturiser on top.
Photoinactivation
Retinoic acid is chemically unstable when exposed to UV light. A photoisomerisation reaction known as a light driven shape change in the molecule, converts all-trans-retinoic acid into less active cis-isomers( again think of this like a different shaped molecule that becomes less active), which cuts down its effectiveness. That is why retinoids should be used at night and stored in opaque, airtight packaging. Retinol breaks down in a similar way but also oxidises when it meets air, so airless or anhydrous (water free) formulas keep it stable for longer.
Hydroxy Acids
The Hydroxy Acid Family
Hydroxy acids work differently than retinoids. Instead of primarily changing gene expression, they exfoliate by loosening the connections between dead skin cells on the surface layer of the skin (the stratum corneum), allowing those cells to shed more easily. Picture this like soaking an object in warm water lossening the glue to more easily remove a label .This ingredient family can be divided into different groups based on their chemical structure, including AHAs, BHAs and PHAs. These structural differences affect things like how well they dissolve in water or oil, their molecular size and how they behave in the skin, meaning each type works a little differently.
AHAs — Alpha-Hydroxy Acids
AHAs (alpha-hydroxy acids) mostly work on the skin surface by disrupting ionic bonds (charged chemical links) between corneocytes, the dead cells in the outer layer. Glycolic acid (from sugarcane) has the smallest molecular weight, so it can travel the deepest. Lactic acid (from milk fermentation) is gentler and also acts as a humectant, helping skin hold onto water. Mandelic acid (from almonds) has a larger molecule and works more slowly, which makes it a good fit for sensitive skin. Tartaric acid (from grapes), citric acid (from citrus fruits), and malic acid (from apples) complete the AHA group.
BHAs — Beta-Hydroxy Acids
BHAs (beta-hydroxy acids) are oil soluble, so they can move into the pore lining instead of staying only on the surface. Salicylic acid the main BHA used in skincare loosens corneocyte bonds inside hair follicles, which makes it especially helpful for comedones, oily skin, and acne. It also has mild anti-inflammatory effects, which is one key difference from AHAs.
Large molecule
PHAs — Polyhydroxy Acids
PHAs (polyhydroxy acids) have larger molecular structures than AHAs, so they do not sink as deeply into the skin. That limited penetration makes them much gentler. The most common PHAs are gluconolactone and lactobionic acid. Beyond exfoliation, PHAs also act as humectants and antioxidants, which is why they are often a good choice for sensitive or barrier-compromised skin.
Typical sources include: gluconolactone from glucose oxidation and lactobionic acid from milk whey.
PHA derivatives
Bionic Acids
Bionic acids are PHA derivatives that have an extra sugar (lactone) unit attached. This increases their molecular size even more and slows how quickly they penetrate the skin. That is why they are considered the gentlest exfoliating acids. Because they have multiple hydroxyl (–OH) groups, they also act as strong humectants. The two main bionic acids are lactobionic acid and maltobionic acid.
Common sources include: lactobionic acid from milk whey and maltobionic acid from maltose.
Comparison
AHA vs BHA vs PHA at a Glance
Not sure which acid is right for what your skin needs? This comparison shows how they line up side by side.
| Property | AHA | BHA | PHA |
|---|---|---|---|
| Solubility | Water soluble | Oil soluble | Water soluble |
| Molecular size | Small–medium | Small | Large |
| Penetration depth | Surface | Pore lining | Surface only |
| Best for | Texture, dullness, fine lines | Congestion, acne, oily skin | Sensitive, reactive, dry skin |
| Humectant effect | Lactic acid only | None | Yes (all PHAs) |
| Anti-inflammatory | No | Yes (salicylic acid) | Yes (some PHAs) |
Collagen Pathway
How Hydroxy Acids Stimulate Collagen
AHAs do more than just smooth the surface. By speeding up how quickly the stratum corneum renews itself, they set off a signalling cascade that resembles a controlled wound response. These signals reach fibroblasts in the dermis the cells responsible for producing collagen encouraging them to increase collagen production without causing actual injury.
Step 1
AHA loosens the bonds between corneocytes (dead skin cells) → the stratum corneum sheds more quickly (accelerated desquamation).
Step 2
The skin notices this faster cell loss and sends out a controlled wound-healing signal.
Step 3
Keratinocytes (the main cells in the outer layer of skin) release pro-inflammatory cytokines — including IL-1α and IL-1β — which act as chemical messengers that travel down into the dermis.
Step 4
Dermal fibroblasts respond by increasing production of type I and type III collagen.
Step 5
Over the course of weeks to months, the dermal matrix thickens, which softens fine lines and improves overall skin quality.
This pathway is different from the nuclear receptor mechanism used by retinoids, but the final outcome — more collagen — is the same.
Key Takeaways
Here are the main ideas, pulled together in one place.
- Retinoids work by activating nuclear receptors (RAR and RXR), which directly change gene transcription in skin cells.
- They increase collagen synthesis (type I and type III) while at the same time reducing MMP-driven collagen breakdown.
- Retinoid dermatitis is a predictable adjustment phase, not an allergy, and it usually settles with time and careful use.
- Retinoids should always be applied at night because UV light photoisomerises retinoic acid into less active forms.
- AHAs are water soluble and exfoliate mainly at the surface, while BHAs are oil soluble and can move into the pores.
- PHAs and bionic acids are larger-molecule, gentler acids that tend to suit sensitive or barrier-compromised skin better.
- Hydroxy acids stimulate collagen through a cytokine-signalling cascade triggered by faster desquamation (cell shedding), not through receptor binding.