01 - Ingredient Science

Skincare is often discussed as though individual ingredients possess simple, predictable powers: one ingredient hydrates, another brightens, another builds collagen, another exfoliates. Biology is considerably more complicated. A topical ingredient does not act in isolation; its effect depends on its chemical structure, concentration, stability, formulation, ability to reach its biological target, the condition of the skin, frequency of application, duration of use, and the interaction between the ingredient and the skin's own physiology. The most useful way to understand skincare ingredients, therefore, is not to ask whether an ingredient is "good" or "bad," but to ask what it is capable of doing, through which biological mechanism, under what conditions, and with what level of evidence.

Among the most extensively discussed cosmetic actives are Retinoids, Peptides, Ceramides, Vitamin C, Niacinamide, AHAs, and BHAs. They belong to very different chemical and functional classes, and treating them as interchangeable "actives" obscures important differences in how they interact with the skin.

Retinoids comprise a family of vitamin A-related compounds that influence cellular behaviour through retinoid receptorsand downstream changes in gene transcription. Their biological effects are therefore fundamentally different from those of a simple moisturizer. The best-characterized member, tretinoin, is all-trans retinoic acid, the biologically active form of retinoic acid used therapeutically. Other topical retinoids, including retinol and retinaldehyde, require metabolic conversion within the skin before producing their principal retinoic-acid-related effects. This distinction is important because different retinoids have different conversion requirements, stability, potency and irritation profiles.

Retinoid activity affects keratinocyte proliferation and differentiation, epidermal organization and aspects of the extracellular matrix. With sustained use, topical retinoids can improve clinical manifestations of photoaging, including fine wrinkles and irregular pigmentation. Evidence for tretinoin is particularly strong, with randomized controlled trials demonstrating improvements in several measures of photoaging over months of treatment. Retinoids are not, however, biologically neutral. Increased epidermal turnover and alterations in the stratum corneum can initially produce dryness, scaling and irritation, and inappropriate use can compromise barrier function. This is why concentration alone cannot be used to predict a product's practical effect: formulation, molecular stability, delivery, frequency of use and individual tolerance all matter.

Peptides represent a different category. A peptide is a short chain of amino acids, and cosmetic peptides are generally incorporated into formulations because particular sequences may interact with cellular or extracellular processes. Some have been investigated for effects associated with collagen synthesis, extracellular-matrix signalling, inflammation or skin repair. The critical scientific issue is that "peptide" is not a single active ingredient with one mechanism. Sequence, molecular size, chemical modification, stability, concentration and delivery all influence biological activity. A peptide demonstrated to influence a cellular pathway in vitro cannot automatically be assumed to produce the same effect when applied to human skin. Consequently, claims surrounding peptides should be evaluated at the level of the specific peptide and formulation rather than the category as a whole. Clinical evidence exists for some peptide-containing formulations, but the evidence base is considerably more heterogeneous than the blanket marketing language surrounding "collagen-boosting peptides" might suggest.

Ceramides operate closer to the fundamental architecture of the epidermal barrier. They are sphingolipid-derived lipidsthat form part of the intercellular lipid matrix of the stratum corneum, together with cholesterol and free fatty acids. This lipid organization contributes to the skin's ability to regulate water movement and resist environmental insults. Ceramide-containing formulations are therefore not simply "hydrators" in the conventional sense. Their relevance lies in supporting the physical organization and function of the epidermal barrier. The particular ceramide species, their proportions relative to other lipids, the vehicle carrying them and the overall formulation are important variables. A product containing a ceramide somewhere in its ingredient list should not automatically be equated with a clinically optimized barrier-repair formulation. The biology is more sophisticated than the ingredient name alone.

Vitamin C is another example of an ingredient whose reputation is justified by legitimate biological chemistry but whose performance depends heavily on formulation. Vitamin C, particularly L-ascorbic acid, is a water-soluble antioxidant involved in numerous biological processes. In skin, topical vitamin C has been investigated for its ability to neutralize reactive oxygen species, participate in antioxidant protection, influence collagen-related processes and affect pigmentation pathways. It has therefore attracted considerable interest in photoaging and photoprotection research.

The problem is chemical stability. L-ascorbic acid is readily susceptible to oxidation, and the amount of vitamin C printed on a label does not tell the entire story. pH, aqueous environment, oxygen exposure, light, temperature, packaging and formulation chemistry can influence stability and therefore the amount of intact active ingredient available to the skin. Derivatives of vitamin C have consequently been developed to improve stability or formulation characteristics, although conversion to biologically active vitamin C and clinical efficacy vary among derivatives. A scientifically responsible discussion of vitamin C therefore has to distinguish between the molecular identity of the ingredient, its concentration, its chemical stability and evidence from human studies.

Niacinamide, the amide form of vitamin B3, illustrates another important principle: an ingredient can influence multiple biological pathways simultaneously. Topical niacinamide has been associated with improvements in epidermal barrier function, reductions in transepidermal water loss (TEWL), effects on pigmentation, sebum production and some manifestations of photoaging. It also participates indirectly in cellular metabolism through its relationship to NAD+ and related coenzymes, although the biological consequences of topical application depend on the concentration and tissue context. Research has also shown that topical niacinamide can support the production of important epidermal lipids, including ceramides, cholesterol and free fatty acids.

AHAs, or alpha-hydroxy acids, include compounds such as glycolic acid and lactic acid. Their primary cosmetic significance is their ability to influence corneocyte cohesion and epidermal desquamation, thereby altering the appearance and texture of the skin. Depending on the particular acid, concentration, pH and exposure conditions, AHAs can improve surface roughness, pigmentation and some signs of photoaging. Their effects are not simply equivalent to "removing dead skin." Hydroxy acids can influence processes within the epidermis and, with appropriate formulations and sustained use, have been investigated for effects extending beyond superficial exfoliation.

BHAs, most notably salicylic acid in skincare, are chemically distinct from AHAs. Salicylic acid is a beta-hydroxy acidwith keratolytic and comedolytic properties. Because of its physicochemical characteristics, it can be particularly useful in formulations intended for acne-prone or oily skin, where follicular plugging and excess sebum contribute to disease processes. Its activity, like that of other acids, depends on formulation parameters rather than percentage alone. The presence of an acid at a particular numerical concentration does not guarantee an equivalent biological effect across different products.

The distinction between these ingredient classes becomes particularly important when they are combined. Skincare products frequently contain multiple active ingredients because biological processes overlap. Retinoids, Vitamin C, Niacinamide, AHAs, BHAs, Ceramides, and Peptides may all appear within the same broader skincare regimen, but more ingredients do not necessarily mean better results. The skin has a finite capacity to tolerate chemical and physical stress, and irritation itself can produce inflammation, barrier disruption and undesirable pigmentation in susceptible individuals. Contemporary evidence increasingly supports the importance of tolerability and formulation when selecting active ingredients rather than simply maximizing the number of actives used.

This is particularly relevant because an ingredient's mechanism of action does not automatically predict its clinical outcome. Laboratory studies can demonstrate receptor binding, antioxidant activity, enzyme inhibition, collagen-related signalling or changes in cultured cells, but human skin is a living, multilayered biological system. The stratum corneumpresents a formidable permeability barrier; enzymes can metabolize compounds; molecules can degrade; formulations can alter penetration; and biological responses vary between individuals. Consequently, the strongest evidence for a skincare claim comes from well-designed human clinical studies using the actual ingredient, concentration and formulation being evaluated.

This distinction also explains why ingredient percentages should be interpreted cautiously. A higher concentration is not synonymous with greater efficacy. Increasing concentration can increase biological activity, but it can also increase irritation, instability or other adverse effects. Conversely, a lower concentration may be entirely appropriate when an ingredient is potent, when long-term tolerability is important, or when the formulation has been engineered to improve delivery. Concentration is therefore one variable within a larger system rather than a universal ranking of product quality.

The same principle applies to "natural" and "synthetic" ingredients. Chemical origin does not determine biological safety or efficacy. A molecule produced by a plant and an identical molecule produced synthetically have the same molecular structure; conversely, a complex botanical extract may contain dozens or hundreds of compounds whose concentrations and biological activities can vary according to species, cultivation, extraction method and processing. Botanical ingredients can possess genuine pharmacological or cosmetic activity, but claims should be evaluated according to the evidence for the specific preparation rather than the appeal of the plant's name.

Ultimately, ingredient science is the study of relationships: molecule to formulation, formulation to skin, skin to biology, and biology to measurable outcome. Retinoids demonstrate how a vitamin-derived molecular class can alter cellular signalling and tissue architecture. Peptides demonstrate the complexity of sequence-specific biological communication. Ceramides demonstrate the importance of lipid organization in barrier function. Vitamin C demonstrates how chemical stability can determine whether an apparently powerful antioxidant remains biologically available. Niacinamidedemonstrates how one molecule can influence several aspects of epidermal physiology. AHAs and BHAs demonstrate that chemical exfoliation is fundamentally a matter of controlled interaction between molecules, the epidermis and the formulation that delivers them.

The scientifically meaningful question is therefore not simply, "What ingredients are in this product?" It is, "What are these molecules capable of doing, how are they delivered, what evidence supports those effects in human skin, and what conditions are required for the claimed benefit to occur?" That is the foundation of evidence-based skincare—and the difference between understanding an ingredient and merely recognizing its name on a label.

References

Al-Niaimi, F. and Chiang, N.Y.Z. (2017) ‘Topical vitamin C and the skin: mechanisms of action and clinical applications’, Journal of Clinical and Aesthetic Dermatology, 10(7), pp. 14–17. 

Babilas, P., Knie, U. and Abels, C. (2012) ‘Cosmetic and dermatologic use of alpha hydroxy acids’, Journal der Deutschen Dermatologischen Gesellschaft, 10(7), pp. 488–491.

Goh, C.F. et al. (2024) ‘Niacinamide: a review on dermal delivery strategies and clinical evidence’. Journal of Drug Delivery Science and Technology.

Michalek, I.M., Lelen-Kaminska, K. and Caetano Dos Santos, F.L. (2019) ‘Peptides stimulating synthesis of extracellular matrix used in anti-ageing cosmetics: are they clinically tested? A systematic review of the literature’, Archives of Dermatological Research.

Nukaly, H.Y. et al. (2026) ‘Oral and topical peptides for skin aging: systematic review and meta-analysis of randomized controlled trials’, Frontiers in Medicine, 13, 1618306.

Sitohang, I.B.S., Makes, W.I., Sandora, N. and Suryanegara, J. (2022) ‘Topical tretinoin for treating photoaging: a systematic review of randomized controlled trials’, International Journal of Women’s Dermatology, 8(1), e003. 

Tanno, O., Ota, Y., Kitamura, N., Katsube, T. and Inoue, S. (2000) ‘Nicotinamide increases biosynthesis of ceramides as well as other stratum corneum lipids to improve the epidermal permeability barrier’, British Journal of Dermatology, 143(3), pp. 524–531. 

Vasques, L.I., Vendruscolo, C.W. and Leonardi, G.R. (2023) ‘Topical application of ascorbic acid and its derivatives: a review considering clinical trials’, Current Medicinal Chemistry, 30(29), pp. 3272–3286.