Daytime Changes of Skin Biophysical Characteristics: A Study of Hydration, Transepidermal Water Loss, pH, Sebum, Elasticity, Erythema, and Color Index on Middle Eastern Skin.

BACKGROUND: The exposure of skin to ultraviolet radiation and temperature differs significantly during the day. It is reasonable that biophysical parameters of human skin have periodic daily fluctuation. The objective of this study was to study the fluctuations of various biophysical characteristics of Middle Eastern skin in standardized experimental conditions.
MATERIALS AND METHODS: Seven biophysical parameters of skin including stratum corneum hydration, transepidermal water loss, pH, sebum, elasticity, skin color, and erythema index were measured at three time points (8 a.m., 12 p.m. and 4 p.m.) on the forearm of 12 healthy participants (mean age of 28.4 years) without any ongoing skin disease using the CK MPA 580 device in standard temperature and humidity conditions.
RESULTS: A significant difference was observed between means of skin color index at 8 a.m. (175.42 ± 13.92) and 4 p.m. (164.44 ± 13.72, P = 0.025), between the pH at 8 a.m. (5.72 ± 0.48) and 4 p.m. (5.33 ± 0.55, P = 0.001) and pH at 12 p.m. (5.60 ± 0.48) and 4 p.m. (5.33 ± 0.55, P = 0.001). Other comparisons between the means of these parameters at different time points resulted in nonsignificant P values.
CONCLUSION: There are daytime changes in skin color index and pH. Skin color index might be higher and cutaneous pH more basic in the early morning compared to later of the day.

What was known?

Biophysical parameters of human skin have periodic daily fluctuations

Geographical or cultural environment may influence skin biophysical parameters.

Introduction

Skin performs a variety of functions, such as forming a protective physical barrier, controlling the passage of water and electrolytes and thermoregulation. The exposure of skin to ultraviolet radiation and temperature differs during the day. It is reasonable that biophysical parameters of human skin have periodic daily fluctuations.[1] Biological rhythms are described as physiological changes taking place in a period with a reproducible waveform.[2] Circadian clocks may regulate a range of different physiological responses according to cell types. Skin is a complex organ consisted of different cell types.[34] Skin is a complex organ consisted of different cell types. It has to be clarified whether expression levels for clock genes are different among the variety of cell types of the skin.[1]

Diurnal rhythms of biophysical skin parameters have been investigated in the past.[5] Le Fur et al. conducted a controlled study on healthy women and reported that skin temperature, sebum production, pH, skin hydration, and transepidermal water loss (TEWL) varied during the day.[6] Yosipovitch et al. also showed a significant daily variation in skin temperature and TEWL and demonstrated the correlation of detected changes, with the variations in skin blood flow.[7] Similarly, there are some evidence of daytime variation in skin color.[8]

It has also been demonstrated that the geographical or cultural environment may influence skin biophysical parameters.[9] However, as we know, little is known about daytime changes of skin parameters in West Asia and Middle East. In this study, we assessed the fluctuations of seven biophysical parameters of normal skin (hydration, TEWL, pH, sebum, elasticity, erythema, and color index) during the day in standardized experimental conditions in Tehran (the capital city of Iran).

Materials and Methods

Study population

This study was a cross-sectional study performed on 12 healthy volunteer participants with Fitzpatrick skin types of 3 and 4 in a skin clinic in Tehran, Iran. The participants neither have an ongoing skin disease nor have a history of skin disorder. The exclusion criteria also included pregnancy, breast-feeding, smoking, using oral contraceptives in the previous 3 months, and being on a medication for 15 days before the study. Measurements in female participants were performed in their luteal phase of menstrual cycle (28 ± 2 day cycles).[6]

The study was performed according to the Declaration of Helsinki and was approved by the Ethics Committee of the Center for Research and Training in Skin Diseases and Leprosy, Tehran University of Medical Sciences. All participants provided a written informed consent.

Measurements

Participants were asked to wash their hands with water only, 12 h before measurements and not to use any soap or detergent or cosmetic product so that they would not affect moisture, TEWL, and pH measurements.[1011] Measurements were performed on the right midforearm and at time points of 8 a.m., 12 p.m., and 4 p.m. Before each measurement, the participants rested in a room with climate control having a temperature of 22°C ± 2°C and relative humidity of 30%–40% for 30 min. The same investigator performed the same measurements in each participant. Hot beverages and spicy foods were not allowed during study time.[6] Participants were asked not to perform a strenuous exercise during the study day. Standardized meals were served at fixed hours. The participants maintained a social and ecologic synchronization with diurnal activities with light on at 7.00 a.m. (±1 h) and light off at midnight (±1 h) during the 24-h study. This schedule was close to their spontaneous individual behavior.

All measurements were performed using respective probes, Corneometer, TEWL meter, Mexameter, Sebumeter Cutometer, and pH meter (Courage and Khazaka Electronic GmbH, Cologne, Germany) by the same investigator.

Sebumeter

To determine the amount of absorbed sebum after the measurement, this probe uses the differences in light intensity through a plastic strip.[12] In our study, the collection time was 30 s and it was performed once in each site.

Corneometer

The high dielectric water constant is used for analyzing the water-related changes. The probe was put on the skin surface once, and the value was read and recorded. The measurements of water content will be shown in system-specific arbitrary units (AUs).[13]

Mexameter

In this instrument, 16 light-emitting diodes arranged circularly emit light at three wavelengths including 568 nm (green), 660 nm (red), and 880 (infrared). A photodetector is applied to measure the light reflected by the skin. An erythema index is computed from the intensity of the absorbed and the reflected light at, respectively, 568 and 660 nm.[14] It is a sensitive measurement that gives values on a broad scale (0–999) for melanin.

TEWAmeter

TEWL measurement is based on water diffusion in an open chamber and is measured as g/m2/h.[15] The probe is a small hollow cylinder (10 mm × 20 mm), and to measure TEWL, it was put on the skin for approximately 1 min so that the TEWL graphs would become stable.[10]

Cutometer

The skin is sucked into the probe with a controlled vacuum pressure (450 mbar, suction time 2 s, relaxation time 2 s). The probe with 2-mm diameter opening is used for measurements. The measurement is computerized and is based on the vertical deformation of the skin, and the results are expressed in AUs.[16] R2 was considered as the main parameter for determination of the skin elasticity since it describes the gross elasticity and mostly used in similar studies.

pH meter

The pH was measured using a flat glass electrode which was put on the skin once, and the value was read and recorded. A drop of DI water was used to get good contact. This method has been previously described by Yosipovitch et al.[1011]

Statistical analysis

The collected data were analyzed with SPSS 18 software (Released 2009, Chicago: SPSS Inc.). The Kolmogorov–Smirnov test was used for demonstrating whether the variables had a normal distribution. Variables were normally distributed and, therefore, were analyzed using repeated measure ANOVA and Bonferroni tests. P < 0.05 was considered statistically significant.

Results

Three participants were male and nine were female. The age range of participants was 23–39 (mean, 28.4) years.

pH

The mean and standard deviation (SD) of pH measurements at 8 a.m., 12 p.m., and 4 p.m. are presented in Table 1. The observed difference in skin pH was strongly significant (P = 0.001). Pair-wise comparisons showed that the significance was due to the significant difference between the pH at 8 a.m. and pH at 4 p.m. and also between the pH at 12 p.m. and pH at 4 p.m [Table 1].

Table 1

Comparisons for skin pH at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)

Skin color

The mean and SD of three measurements have been described in Table 2. The observed difference in skin color was statistically significant (P = 0.008). Performing pair-wise comparisons showed that the color index at 8 a.m. and 4 p.m. was significantly different (P = 0.025).

Table 2

Comparisons for skin color index at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)

Sebum content

The mean and SD of sebum at 8 a.m., 12 p.m., and 4 p.m. are described in Table 3. The observed differences were not statistically significant (P = 0.491). Performing pair-wise comparisons between measurements showed no significant difference between any pair of measurements.

Table 3

Comparisons for skin sebum at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)

Other skin barrier parameters

The mean and SD of other measured skin parameters including hydration, TEWL, erythema index, and elasticity are presented in Tables 4–7.

Table 4

Comparisons for transepidermal water loss at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)

Table 7

Comparisons for skin elasticity at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)

No significant difference was detected among the values of the mentioned parameters at 8 a.m., 12 p.m., and 4 p.m. P values for skin hydration, TEWL, erythema index, and elasticity at three measurements were 0.466, 0.186, 0.673, and 0.105.

Discussion

Daytime changes of biologic functions of skin have recently attracted increasing attention since they have a potential importance for drug delivery as well as choosing proper skin care products. In this study, for the first time, we investigated the fluctuations of biophysical parameters of Middle Eastern skin during the day. Several biophysical parameters have been assessed in this study, while just a few reports have covered such a wide range of skin parameters.

Human stratum corneum has a pH gradient from an acidic pH 4 to a neutral pH 6.5.[17] Interestingly, we found that skin pH declines through the day. In our study, the mean values of skin surface pH on the forearm of the study participants ranged from 5.33 to 5.72. The highest amount belonged to 8 a.m., while the most acidic skin surface was measured at 4:00 in the afternoon. The diurnal variation of skin surface pH is probably a reflection of diurnal variations in enzyme function in the human stratum corneum.[10] In contrast to our study, in a study done by Yosipovitch et al.,[10] the maximal value of pH in a 24-h span was obtained in the afternoon between 14:00 and 16:00. Le Fur et al. found out that skin pH of forearm ranges from 5.4 to 6.6 during a 24-h period. Furthermore, they realized that the skin surface acidity level of the forearm is not time-dependent.[6] In our study, relative peak-to-trough differences were statistically significant (P = 0.001) and were as follows: 5.72 at 8 a.m./5.33 at 4 p.m. In concordance with these findings, Yosipovitch et al. found a significant difference between the maximum and minimum amounts of pH of the forearm (P < 0.05: 5.44 between 2 p.m. and 4 p.m./4.87 around 8 p.m.)[10] Although the involvement of sweat-derived lactic acid and free amino acids of the stratum corneum is apparent in skin surface acidity,[18] but the entire mechanism is not well defined.[19]

Several pigments such as hemoglobin, melanin, bilirubin, and carotene determine the skin color. The skin color could be a good indicator of skin properties such as the barrier integrity.[20] We realized that color index of the skin significantly declines during the day that means the skin gets lighter in the afternoon compared with the morning. Furthermore, the maximum and minimum amounts have a statistically significant relationship (P = 0.025). The result supports Chilcott and Farrar's finding that reported a decrease in skin color of the forearm between 9 a.m. and 5 p.m.[8] On the contrary, Latreille et al. did not observe significant changes in skin color of the forearm.[21] Personal factors (age, sex, race, anatomical site, and skin surface properties), environmental factors (light conditions, temperature), and different procedures influence skin color.[22] However, the experimental conditions, the time points for the measurements, and the duration of the study of Latreille et al. were different from our study.

Erythema index showed a decline through the day from 252.32 to 247.80 in this study, but this difference was not statistically significant. The lowest and the highest levels of sebum content were reported at 8:00 and 12:00, respectively, although the difference was not statistically significant.

Several authors[62324] have demonstrated a sebum excretion peak around midday. Our data also report a prominent peak at noon, but no statistical significance was detected in sebum content at three measurements. It has been reported that increase in skin temperature caused an increase in sebum excretion in the range of 10% per 1°C.[25] Environmental changes were controlled well during our study, but we do not have any datum about skin temperature in our participants to check its possible effect on sebum excretion (since the skin temperature affected by several nonenvironmental factors such as blood pressure).[26] It also suggested that the role of regulation of androgen receptors was prominent in the regulation of sebum production.[24]

TEWL is a well-accepted in vivo indicator of skin barrier function.[27] In our study, no statistical significance was detected in TEWL at three measurements. This result supports the finding of a study conducted by Pinnagoda et al.[27] They reported that diurnal variations of TEWL are small and of no practical significance.

The lowest and the highest amounts of TEWL were measured at 4:00 p.m. and 12:00 p.m., respectively. Yosipovitch et al.[10] and Le Fur et al.[6] detected time-dependent changes for TEWL of the forearm. Both of these studies reported at least one TEWL peak around 4:00 p.m. Furthermore, Spruit[28] found that TEWL on the forearm skin was higher in the afternoon than in the morning. He suggested that the diurnal variation in TEWL of the skin is a function of diurnal variation in skin surface temperature. Of course in a study conducted by Le Fur et al., the 24-h TEWL of the forearm of study participants ranged from 5.9 to 10.4 g//m2/h; however, this range was from 3.16 to 4.56 in an 8-h span in our study.

The minimum level of TEWL was reported by Touitou et al.[29] at 14:00. Differences in the sample size, study protocols, (for instance, three times measurements during an 8-h period vs. 12-h and 24-h periods in other studies), as well as differences of the environmental temperature and relative humidity during the studies complicate the comparison of these data.

In concordance with a study conducted by Yosipovitch et al.,[10] we did not find time dependence of capacitance on the forearm. However, the maximum value of skin hydration reported by Yosipovitch et al. was at 4 p.m. since the lowest and the highest levels of skin water content were measured at 8:00 a.m. and 12:00 p.m., respectively.

In a study done by Le Fur et al., one peak was reported at 12:00 p.m., similar to our study.[6] They also detected the time-dependent capacitance of forearm, which was in contrast to our study.

Skin elasticity showed an increasing manner during the day. In the morning, we detected the lowest amount (0.212), and at 16:00, the maximum level (0.225) was reported. In concordance with our study, Tsukahara et al.[3031] in Japan reported that the elasticity of forearm skin increased during the day and showed the maximum amount in the afternoon. They compared the upper and the lower halves of the body and reported that their elasticity follows an opposite trend during the day. This suggests that skin thickness varies diurnally with dermal fluid volumes, which change in association with the shift of dermal fluid from the upper part of the body toward the lower limbs by gravity during the day.[2930]

Conclusion

The results showed that skin pH and skin color index in our study population vary during the day. These variations should be considered for future studies on these parameters and also may have clinical significance for the choice of skin care products.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

What is new?

Skin pH and skin color index in the Middle Eastern population vary during the day.

Table 5

Comparisons for erythema index at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)

Table 6

Comparisons for skin hydration at three different time measurements (1: Measurement at 8 a.m.; 2: Measurement at 12 p.m.; 3: Measurement at 4 p.m.)