Introduction
As the largest and outermost human organ, skin is a protective barrier to exogenous substances and can be modulated by various internal and external factors, such as ethnic (Yamashita et al., 2012), age, gender (Firooz et al., 2012; Man et al., 2009), body site (Lee et al., 2013), seasonal variation, skin diseases (Darlenski et al., 2009) and etc. Also, region plays an important factor in determining the skin condition for the different of climate, UV radiation, temperature and humidity, daily activities and habits, etc.
After the reform and opening, China's cosmetics market is developing rapidly. China's huge population base brings the opportunity to the development of the cosmetics market. By the end of 2013, China's cosmetics market scale has reached $23.5 billion, which accounted for ten percent of worldwide cosmetics consumer market, becoming the world's third largest market after the United States and Japan. What's more, by the end of 2015, retail figures released by the national bureau of statistics showed that the domestic cosmetics retail sales grew by 8.8%, amounted to $31.5 billion, even higher than GDP growth rate. China has become the world's second largest cosmetics market after the United States. As a result of the wide area and varied regional characteristics of China, it is necessary to investigate the alterations of skin biology in different regions for the design of cosmetic with regional characteristics.
To explore the differences between skin physiological parameters of women at different age in different regions, we choose Beijing and Kunming as the comparison area. Beijing and Kunming respectively located in the northern and southern China, have two kinds of different climatic conditions (Table 1), living conditions, eating habits and etc.
Therefore, a specific study protocol was devised to collect comparable skin data in Beijing and Kunming women in order to determine and quantify the skin characteristics in each population. Six biophysical characteristics of normal skin (hydration, TEWL, sebum content, pH, MI, EI) were measured in a normal population in different cities. It contributes to providing practical application guidance for skin care habits.
Methods
1. Study design
This study was cross-sectional study about female skin feature in Beijing city and Kunming city. The test was implemented in May 2010 and the data were obtained in a closed environment maintained at a mean standard deviation (SD) temperature of 22±2℃, relative humidity of 50%±10%. Prior to the taking of measurements, subjects were allowed to wash their facial with cleanser then have 20 min rest to equilibrate to the indoor conditions. Before the test, the volunteers were informed the test procedure, and signed the informed consent. Experiments conformed to the declaration of Helsinki. In this study, left cheek was deemed as target area for skin biophysical parameter testing place.
2. Subjects
This test was only for Han Chinese females and volunteers were recruited who had lived in Beijing and Kunming, respectively, for at least five years. 150 women residing in Beijing and 173 women residing in Kunming were enrolled this study. It is reported that woman body state will change with seven periods in the Chinese great medical ancient book Huangdi Neijing (Tian, 2005), so we divided respective population into six groups with seven years (group 1, 21-27 years; group 2, 28-34 years; group 3, 35-41 years; group 4, 42-48 years; group 5, 49-55; group 6, 56-62), which was different with the existing population division.
Recruitment standards:
a) None of the subjects had significant skin disease such as psoriasis, eczema, etc., had taken any medications that could interfere with the study results, or had undergone any skin treatments (I.e. chemical peel, facial, microdermabrasion, collagen injection, retin-A or retinoid treatment, Botox therapy).
b) Women who were pregnant, lactating, or planning a pregnancy were excluded from entering the study.
The populations in 7-year age groups and their mean ages were shown in Table 2. They gave their written informed consent to participate in the study. The study protocol was approved by an internal review board.
3. Biophysical measurement
As showed in Figure 1, the biophysical parameters were measured on the left cheek (1) (Lee et al., 2013).
1) Skin hydration
The Corneometer® CM 825 (Courage+Khazaka electronic GmbH, Germany) was used to measure the skin electrical capacitance, its principle was based on the fact that the dielectric constant of water is 81 and that of dry skin is below this. A normal value of stratum corneum hydration was accepted as higher than 40 u (Lee et al., 2013).
2) Skin transepidermal water loss
Trans-epidermal water loss was measured by using Tewameter® TM300 (Courage+Khazaka electronic GmbH). It reflects the degree of evaporation of water through the skin. Values were plotted and TEWL expressed as g/m2/h (Hillebrand et al., 2001).
3) Sebum excretion rate
Skin sebum content was measured on the check by using Sebumeter SM810 (Courage+Khazaka electronic GmbH). The device measured variations in light intensity through a plastic strip. The values obtained were proportional to the quantity of sebum absorbed into the strip. The level of sebum was expressed as μg/cm (Wan et al., 2015).
4) Skin pH
Skin surface pH was measured with a flat glass electrode connected to a pH meter (pH 900; Courage+Khazaka electronic GmbH).
5) Skin erythema and skin melanin
Skin erythema and skin melanin index were measured with Mexameter MX18 (Courage+Khazaka electronic) (Lee et al., 2013). Similar to the Cutometer, the central portion of the probe of the Mexameter contains an elastic spring that ensures that constant pressure is applied to the skin. For each measurement the probe was held perpendicular to the skin and lightly touched to the skin surface, with no contact of the outer ring. This activated the light emitter resulting in an immediate display of the erythema and melanin index on the display console as arbitrary units (a.u.) such that the probe remained in contact with the skin for only a few seconds. Three individual measurements were taken with the Mexameter to give three hemoglobin and melanin indices and the mean was used for analysis. The Mexameter calibration was evaluated on a weekly basis as per the manufacturer's specifications (Nedelec et al., 2016).
All measurements were performed in triplicate, and the average of these three measurements was calculated and used for group statistics.
Results
1. Skin hydration
As shown in Figure 2, there were no significant differences in the skin hydration between these two cities in all groups. But the trend of the average moisture content in Kunming women was higher than Beijing in every age group.
2. Skin transepidermal water loss
As shown in Figure 3, TEWL values in Kunming were general lower than in Beijing for each age groups and significantly decreases at age in the 42-48-year-old age (p<0.05), 49-55-year-old age (p<0.05), and 56-62-year-old age group (p<0.05). The lowest TEWL with a measuring value of 9.61±1.98 g/m2/h was in 56-62-year-old age in Kunming.
3. Sebum excretion rate
As shown in Figure 4, sebum secretion rate in Kunming women was gradually reduced with increasing age while the sebum excretion was fluctuated in Beijing women. As can be seen in Figure 3, there was a significant lower of sebum secretion rate in Kunming women than that in Beijing in the 49-55-year-old age (p<0.01). The highest sebum was measured at 35-41-years-old in Beijing (38.46±13.43), whereas 56-62-years-old in Kunming showed the lowest mean sebum (8.19±2.83). The reasons for the high SD value were the individual large difference of skin sebum excretion rate.
4. Skin pH
As shown in Figure 5, the change of pH values in all age groups was not obviously, either in Beijing or in Kunming. In addition, there were no significant differences in the skin pH between Beijing and Kunming women for all groups, but the mean values of pH in Kunming was generally higher than Beijing in the each age group. The highest pH was found in group 35-42-years-old in Kunming (6.10±0.10), and the lowest pH was found in group 28-34-years-old in Beijing (5.87±0.56).
5. Melanin
Figure 6 showed the comparison of skin melanin among the Beijing and Kunming by age group. Skin melanin of Kunming was generally higher than Beijing. Specifically, the increase was very significant between the female subjects aged 28-34-year-old age and 42-48-year-old age (p<0.001). There are moderate significantly increasing in skin melanin of women came from Kunming women at age group 35-41-year-old age (p<0.01).The highest melanin index was measured at 42-48-year-old age in Kunming (208.10±15.56) and the lowest melanin index was measured at 28-34-year-old age in Beijing (145.13±103.24).
6. Erythema
Figure 7 showed the comparison of skin erythema among the Beijing and Kunming by age group. Skin erythema of Kunming was general higher than Beijing and significantly higher in 42-48-year-old age group (p<0.01). The highest erythema index was measured at 42-48-year-old age in Kunming (329.98±16.97) and the lowest erythema index was measured at 56-62-year-old age in Kunming (276.42±13.43).
Discussion
The primary objective of this study was to quantitatively measure the difference in skin biophysical parameters between Beijing and Kunming females so that it could help consumers improve skincare strategy and cosmetic enterprises devise cosmetics product more targeted. Generally, the findings of our study showed that Kunming women have lower sebum secretion and TEWL, higher hydration, MI and EI than Beijing women.
1.Skin hydration
Hydration refers to the water content of the stratum corneum and is one of the important skin properties for evaluation of stratum corneum barrier function (Hadi et al., 2016). The increase in skin hydration indicates intact skin barrier function. Our research showed that the skin hydration of females living in Kunming city was higher than that living in Beijing. The higher levels of skin hydration in Kunming city might owe to the higher rainfall amount, which was consistent with previous findings that skin surface hydration was found to be strongly affected by both environmental parameters such as temperature and relative humidity (Cravello & Ferri, 2008). Allan found that relative humidity (RH) effects the rate of desquamation by modulating the activity of the desquamatory enzymes, and specifically stratum corneum chymotryptic enzyme (SCCE) (Watkinson et al., 2001). Exposure to a dry environment leads to depletion of water from the peripheral stratum corneum layers.
2. Skin transepidermal water loss
Transepidermal water loss is one of the important indicators for judgment of skin barrier function. Elevated TEWL values are observed in a number of diseases with skin barrier abnormalities (Darlenski & Fluhr, 2012). TEWL measurement can be affected by several factors including amount of water intake as well as use of moisturisers (Hadi et al., 2016).
Beijing is windy when in the spring, which may be the reason why Beijing women have higher TEWL than Kunming women. The wind will take more away the skin's surface water, resulting in high transepidermal water loss in skin. This differences is more obvious in elderly group, maybe the reason is that with the age increasing, the skin barrier is abate (Roskos & Guy, 1989). Besides, airborne dust could reduce skin barrier function (Pan et al., 2015). Haze weather in Beijing may be another factor for the increase of transepidermal water loss.
3. Sebum excretion rate
Sebaceous lipids contribute to the integrity of the skin barrier supplying the stratum corneum with the hydration (Fluhr et al., 2003; Pilgram et al., 2001) providing photo-protection, particularly against UVB (Marques et al., 2002). We found sebum secretion decreased with the age, and sebum excretion retain Beijing were higher than Kunming. The difference of results may owe to geographic and living habits differences. The sebaceous glands activity is abate with the increase of age (Pochi et al., 1979), so the skin sebum excretion rate gradual declines. It was reported that ethnic differences in the structural properties of facial pores exist and enlarged pore sizes are associated with sebum levels (Sugiyama-Nakagiri et al., 2009). Because of the minority nationalities diversity in Kunming, ethnic intermarriage may contribution to the different of sebum excretion level. The sebum secretion is also related to life stress and emotions (Zouboulis & Böhm, 2004). The fast pace of life and irregular routine in Beijing exert more pressure to body, which may cause the disorder of endocrine and show more sebum secretion (Feng, 2012).
4. Skin pH
The skin pH is usually weak acid under the influence of physiological properties of stratum corneum such as deamination of filaggrin-derived histidine, phospholipid hydrolysis (Zlotogorski, 1987). It is reported that there is a negative correlation between skin hydration and skin pH (Kobayashi & Tagami, 2004; Pratchyapruit et al., 2007) which is reversed to our result. The results may be caused differences in the measurement device, race and age (Berardesca & Maibach, 2003; Rawlings, 2006). Further studies are required to be investigated.
5. Melanin and erythema
Melanin and erythema are also factors of skin color and the UV exposed regions have higher erythema and melanin index than protected skin (Kleesz et al., 2012; Kobayashi & Tagami, 2004; Schnetz et al., 1999). Because of the high altitude, Kunming has the stronger ultraviolet radiation than Beijing while the sun protection consciousness of Kunming women is not as stronger as Beijing women (Cheng, 2008). The major acute effects of UV irradiation on normal human skin comprised sunburn inflammation (erythema), tanning, and local or systemic immunosuppression. At the molecular level, UV irradiation causes DNA damage such as cyclobutane pyrimidine dimers and (6-4) photoproducts. Chronic exposure to UV irradiation leads to photoaging, immunosuppression, and ultimately photocarcinogenesis (Matsumura & Ananthaswamy, 2004).
Conclusion
The findings of our study testified that geographical influences skin condition. External circumstances and living habit were both factors which influence the skin physiological parameters. Skin hydration, sebum secretion, melanin, erythema and TEWL were all subject to regions variations. Kunming women show lower sebum secretion, higher hydration, TEWL, MI and EI than Beijing women. These findings provide a registry of data for healthy women living in Beijing and Kunming city. However, further studies are required to elucidate the effects of gender, facial area, and age on environment changes. These findings will have potential applications in dermatology and in the development of new facial skincare products, cosmetics, and topical medicines.