HOW DOES BIOLOGICAL SEX INFLUENCE OUR SKIN?
When differences in phenotypes or traits clearly exist between different sexes, these traits are considered to besexually dimorphic. Some common examples of sexually dimorphic phenotypes include height, weight, muscle tone, disease prevalence, and even the way a disease manifests. In the case of mental health conditions like ADHD or depression, research has consistently shown that females are diagnosed with adolescent ADHD less than males however, females generally experience higher rates of depression [1]. It’s also important to note that these conditions may have different symptoms in males vs. females although they are often treated with the same variety of approaches [2,3]. Sexual dimorphism may be part of the reason why a specific therapy works well for some people but not for others. With this in mind, let’s turn to the skin.How does biological sexinfluence skin quality, the prevalence of skin conditions, and possibly even treatment response?
PHYSIOLOGICAL DIFFERENCES IN MALE AND FEMALE SKIN
The stratum corneum (SC), or outermost layer of the epidermis, is generallythicker in males than in females over the entire body, including the face [4]. While this is widely accepted, the data on skin hydration is less clear since environmental factors like sun exposure, hydration, diet, and air quality have the potential to significantly affect transepidermal water loss (TEWL) [5]. TEWL is a common parameter used to assess the health of one’s skin by measuring the ability of the skin barrier to retain water. Interestingly,one study of 300 mostly Caucasian volunteers found thattransepidermal water loss (TEWL) is lower in males than females but that with age, this difference disappears [6]. The same study also found that (1) SC hydration in females stays steady with age while male SC hydration declines beginning at the age of 40 and (2) males almost always produce more sebum than females, with female sebum and collagen production declining with age [7]. So, how do these subtle but significant differences in male and female skin affect aging, the prevalence of skin conditions, and how we treat those conditions?
SEXUALLY DIMORPHIC SKIN DIAGNOSES
There are a number of skin conditions and diagnosesthat exhibit sexual dimorphism. It has been consistently documented that males are at higher risk of developing cutaneousmelanoma, the most deadly form of skin cancer (lifetime risk in males: 1/67 and females: 1/86) [8]. Furthermore, when females do develop melanoma, they have a higher survival advantage than males [9]. And interestingly, UV exposure is a better predictor of melanoma in males than in females although it is not well understood why [10].Adult acne but not adolescent acneis more prevalent in females than males [11]. In contrast,psoriasis is more common and more severe in adult males than infemales out of a cohort of over 5000 people [12]. These are just a few examples of sexual dimorphism in dermatology.
There are twomainbiological mechanisms that partially explain the sexually dimorphic nature of these conditions:genomic mosaicism andhormone signaling. Because females have 2 X chromosomes, one must be silenced to prevent over-expression of genes located on the X-chromosome [13]. The silencing process happens randomly and somewhat inefficiently compared to our body’s ability to silence 1 copy of our other chromosomes (autosomes). This means that females are mosaics, sometimes expressing a gene from mom’s X chromosome and other times, from dad’s X chromosome. This increase ingenetic heterogeneity (and redundancy)lowers the risk of deleterious mutations in females since there is a “back-up” copy of chromosome X that is still functional at low levels. Secondly,females have greater levels of estrogen(which is often considered protective and pro-collagen forming)and a more robust immune response, creating auniquecellular microenvironment forhomeostatic cell function and in the context of disease [14]. The way in which just these 2 mechanisms influence hundreds of downstream signaling mechanisms is extremely complex and is an active area of research today. So other than helping us better understand our skin type and risk for developing many skin conditions, how does this knowledge help us optimally care for our skin?
SEXUALLY DIMORPHIC SKIN AND SKIN CARE
The vast majority of skin care products contain the same active ingredients for men and women but are simply marketed differently.A few more things to note about basic sex differences in skin are that males have (1) larger keratinocytes, (2) larger pore sizes, and (3) lower pH than females, all of which are known to affect absorption of topical agents[16]. Another main distinguishing factor is that much of the male skin care routine revolves around hair removal and caring for skin after hair removal [17]. In this regard, using a moisturizer with niacinamide has been shown to reduce post-shave TEWL. Ifresearch regarding sex differences in skin quality and disease riskis just starting to become more widely understood, then research abouttreating skin based on sex-specific molecular factors is truly in its infancy [15].
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