1.

Introduction

Striving to remain “forever young” is a basic characteristic of human beings. More than any other organ, the skin reflects our age to the environment. Even in ancient times, humans endeavored to reduce skin aging by the application of oils, plant extracts, and face masks from different types of soil.^{1, 2}

Skin aging is determined by individual genetic factors^{3, 4} and by the action of free radicals.⁵ It cannot be inhibited. Today, premature aging has increased due to the interaction of environmental influences on the skin.^{6, 7} The most important environmental factor causing skin aging is the UV irradiation of the sun.⁸ UV light produces different types of free radicals in the human skin.⁹ These highly reactive molecules damage the skin on cellular and molecular levels. Sunburn, immunosuppression, premature skin aging, and also skin cancer can be the consequences.^{8, 10, 11, 12, 13} The human organism has developed a defence mechanism against the destructive action of free radicals in the form of antioxidant molecules.¹⁴ These molecules neutralize free radicals before they can damage the tissue. Typical cutaneous antioxidant substances are vitamins A, C, and E; carotenoids; and enzymes. Most of the antioxidants cannot be produced by the human organism, therefore they must be uptaken by nutrition, e.g., by fruit and vegetables, which contain high amounts of antioxidants.¹⁵ Carotenoids are one of the most important antioxidants of the human skin.¹⁶

In a previous long-term study during $1\mathrm{yr}$ , 10 volunteers were investigated continuously concerning their antioxidative potential of the skin and their individual lifestyle.¹⁷ Volunteers who consumed high amounts of fruit and vegetables had higher carotenoid levels in their skin than other volunteers. Stress factors, such as nicotine and alcohol consumption, illness, and sleeplessness reduced the antioxidative potential. Additionally, the cutaneous antioxidative potential could be reduced by UV and IR irradiation.^{18, 19} Comparing the antioxidative potential of volunteers of the same age, Darvin ²⁰ demonstrated that individuals with high concentrations of the carotenoid lycopene in the skin had fewer furrows and wrinkles, which meant, they looked younger for their age than those with low lycopene concentrations. Additionally, it was found that carotenoids can be considered as marker substances for the whole antioxidative potential of the skin,¹⁷ as different antioxidant substances protect each other from the destructive action of free radicals, forming a protection chain.¹⁴

Also, animal products such as the yolk of the hen’s egg can be efficient sources of these essential substances.²¹ One marker for ecological husbandry of hens according to VO EWG $2092∕91$ (Decree of the European Commission of ecological agriculture and species-appropriate animal husbandry) is the “permanent access to grassland,” Hesterberg ²² observed a twofold concentration of carotenoid antioxidants in eggs laid by hens living with permanent access to grassland, in comparison to eggs laid by hens living in barns or batteries without access to green fodder. Furthermore, the carotenoids were not destroyed during boiling the eggs.²² In principle, the high concentration of antioxidants in egg yolk is not surprising, because it is uptaken by the chicken before it hatches and serves as nutrition and protection during the first days of its life.²³ Therefore, nature has developed a highly efficient protection system with the yolk of an egg.

Nevertheless, it was still unclear whether these positive properties of the egg remain during consumption by humans, and whether this leads to an increase in the antioxidative potential of the skin. Therefore, this study was carried out at the Center of Experimental and Applied Cutaneous Physiology, Charité–Universitätsmedizin Berlin, Germany, to investigate whether the uptake of eggs leads to an increase in the amount of carotenoids in the human skin.

The determination of the carotenoids in eggs and in the human skin was performed by resonance Raman spectroscopic measurement.^{24, 25, 26}

2.

Materials and Methods

2.4.

Determination of Carotenoids in the Human Skin

An argon laser with an excitation wavelength of $488\mathrm{nm}$ was used to determine the cutaneous carotenoids.²⁸ At $488\mathrm{nm}$ , the most prevalent carotenoid substances, beta-carotene, lutein, and lycopene have almost the same absorption coefficients.²⁵ The laser radiation was transferred by optical fibers to a hand piece containing the optical system. This hand piece can be utilized to measure the concentration of the carotenoids in the skin on all body sites²⁶ [see Fig. 1a ] or even in the yolk of the egg in a cuvette [see Fig. 1b]. The corresponding resonance Raman signal produced by the excitation wavelengths was transferred by additional fibers from the skin surface to the basic station containing the spectrometer and the control unit. The time for one measurement was $5\mathrm{s}$ . The measurements were repeated three times on the same skin area to obtain an average value. Subsequently, the average cutaneous carotenoid concentration was determined. The standard deviation of these three measurements was always less than 10%. The measurements were performed on the palm and the forehead of the volunteers.

Fig. 1

Raman spectroscopic system for the noninvasive determination of the carotenoid concentration in (a) the human skin in vivo and (b) the yolks of eggs in a cuvette.

3.

Results

Typical Raman spectra of the cutaneous carotenoids measured on the palm are presented in Fig. 2a . Figure 2b presents Raman spectra obtained from the yolk of eggs. The Raman bands of the carotenoids at 1525, 1163, and $1005{\mathrm{cm}}^1$ can be clearly recognized in both samples.

Fig. 2

Typical resonance Raman spectrum of the carotenoids in (a) human skin and (b) the yolks of eggs.

The volunteers were requested not to change their lifestyle during the period of investigation, excluding the uptake of two boiled eggs per day. Parameters of the volunteers were measured daily during a period of $8\text{days}$ : the measurements started $3\text{days}$ before the egg consumption and continued for the $5\text{days}$ during egg uptake. The kinetics of the carotenoid concentration in the skin of the palm and the forehead during this period are presented in Figs. 3a and 3b, correspondingly.

Fig. 3

Initial levels and kinetics of the carotenoid concentration in the skin on different body sites before and during the uptake of ecological eggs: (a) palm and (b) forehead.

The volunteers started with different individual levels of carotenoids in their skin, which varied at a maximum by factor 3. The concentration of the carotenoids had already increased on the second day after the start of the egg diet in both body regions. Table 1 summarizes the average concentration of the carotenoids in the palm of the volunteers during the $3\text{days}$ before and during the last $3\text{days}$ (days 6 to 8) of the egg diet. Table 2 summarizes the carotenoid values measured on the forehead of volunteers. For all volunteers, a significant increase $p<0.05$ of the carotenoid level was observed. Days 4 and 5 were considered as transitional periods and were therefore not included in the calculation.

Table 1

Average carotenoid concentration in the skin of the palm of volunteers during the 3days before and during the last 3days (days 6 to 8) of the egg diet.

Table 2

Average carotenoid concentration in the skin of the forehead of volunteers during the 3days before the last 3days (days 6 to 8) of the egg diet.

The average increase in the carotenoid concentration was $16\pm 2\%$ for the palm and $21\pm 2\%$ for the forehead region.

4.

Discussion

As suggested by Darvin,,²⁰ high concentrations of antioxidants in the human skin can be considered a good protection strategy against premature skin aging. Most of the relevant antioxidant substances have to be uptaken by nutrition. Fruit and vegetables are well-known sources for delivery of antioxidant substances.¹⁴ Hesterberg ²² demonstrated that also eggs, and in particular ecological eggs, contain high amounts of carotenoids. This is not surprising as the yolk of an egg represents the basis for the nutrition and protection system of the newly-hatched chicken during the first days of its life.²³ Carotenoids can be considered as marker substances for the complete antioxidative potential of the human organism.¹⁷

The results of this study demonstrate that after consumption of eggs, their carotenoids are transported to the skin, where they increase the antioxidative potential up to approx. 20%. Moreover, the present investigation shows that preparation does not necessarily decrease the antioxidants contained in foodstuffs. This is also known for tomato ketchup, where the lycopene concentration is considerably higher than in tomatoes themselves.²⁹ During the boiling process, the membranes of the cells are destroyed and carotenoids can be released, which simplifies their digestion.

Measurements were performed only with ecological eggs, whose initial concentration of carotenoids is two times higher than that of conventional eggs.

On the other hand, it must be taken into consideration that eggs also contain harmful cholesterol, which can have a negative impact on the human organism³⁰ if levels are increased.

Therefore, the intention of this study was not to recommend exorbitant egg consumption to prevent skin aging but to demonstrate that the valuable ingredients of our nutrition are transported and accumulated in the skin. This means that in the case of egg uptake, ecological eggs from hens kept on pasture should be preferred, to also receive a benefit for the skin. In general, the uptake of considerable amounts of fruit and vegetables should be preferred to increase the antioxidative potential of the human organism and also of the skin. This can be considered a good protection strategy against skin aging.