To prevent skin aging, adopting a skincare routine with sunscreen, retinol and hydration, ensuring a healthy lifestyle through sufficient rest and adequate water consumption, avoiding smoking and protecting yourself from excessive sun exposure, and a diet rich in antioxidants and Omega-3 fatty acids can contribute to skin elasticity. The active ingredients in nutricosmetics can be classified into two categories: those that replace the key structural elements in maintaining skin health, collagen and hyaluronic acid, and those that prevent tissue destruction through indirect mechanisms such as the antioxidant one, this category can include vitamins, minerals and natural compounds. A balanced diet that provides all categories of macro and micro-nutrients along with protection from UV radiation is the simplest way to slow down the aging process. However, as simple as it may seem, the lifestyle of contemporary man is characterized by speed, so the use of food supplements concentrated in functional ingredients has become a much easier option.

The term nutricosmetics was born by combining the concepts of nutrition and cosmetics. A nutrient is a substance that provides energy or allows the growth, repair or proper functioning of the body. Nutrients are divided into different categories, such as minerals, vitamins, proteins, carbohydrates, lipids and water. According to Regulation (EC) No. 1223/2009 of the European Parliament and of the Council of the European Union, “a cosmetic product means any substance or mixture intended to be placed in contact with the external parts of the human body such as the skin, hair, nails, lips and external genitalia, or with the teeth and oral mucosa with a view exclusively or mainly to cleaning them, perfuming them, changing their appearance and/or correcting body odours and/or protecting them or maintaining them in good condition”. Awareness of the importance of nutrition for general well-being and the growth of the concept of “beauty from within” have led to the emergence of a new field, namely new product categories: nutricosmetics and nutricosmetics. The term "nutricosmetics" defines natural health products administered orally that improve the function and appearance of human skin, nails and hair. They are agents with a protective or supportive effect used for beautification and/or for beneficial effects of personal hygiene. From the point of view of legislative aspects, nutricosmetics are food supplements, which implies that, in the territory of the European Union, they must be in accordance with Directive 46/2002/EC of the European Parliament and of the Council which establishes that "food supplements" are understood as food products whose purpose is to supplement the diet and which represent concentrated sources of nutrients (vitamins, minerals, amino acids, essential fatty acids, fibers and various plants and plant extracts) or other substances with a nutritional or physiological effect, alone or in combination, marketed in dosage form, respectively in pharmaceutical forms such as: capsules, tablets, lozenges or other similar forms (powder sachets, ampoules, spray bottles). Also, to ensure a high level of consumer protection and to facilitate their choice, products placed on the market must be safe and bear adequate and appropriate labelling [1].

The key factors driving the growth of this market can be considered as follows:

• increasing awareness of the link between nutrition and health, including physical appearance - consumers are becoming more perceptive to the concept of "beauty from within";
• aging of the population in developed areas of the world - they are willing to allocate more financial resources in an effort to maintain a youthful appearance and promote general well-being;
• continued research and development of nutritional active substances for use in nutricosmetics.

The global nutricosmetics market is expected to grow significantly, with a forecast for the period 2020-2025 estimating a compound annual growth rate (CAGR) of 8.63%. The largest share of this market is held by Europe, which is also the continent with the highest average age (42 years), with a significant share also in the Asia-Pacific region, especially Japan. But the North American nutricosmetics market is also growing. Being a relatively new field in the food supplement market, but with great growth potential, competition in developing effective products is fierce. The major research areas that have nutricosmetics as their objective can be divided as follows: identification and development of new ingredients; development of methods to prove the efficacy of ingredients or finished products. Analysis of nutricosmetics products can be done according to the main active ingredients used in the formulation of these products, the most common being: collagen, peptides, proteins, hyaluronic acid, vitamins, minerals, omega-3 fatty acids, as well as plant extracts [2].

Collagen as an ingredient in nutricosmetics products

Collagen is a protein present in all multicellular animals. To date, 28 types of collagen have been identified and characterized at the molecular level, consisting of 46 unique amino acid chains. Depending on the structure and supramolecular arrangement, there are 4 subfamilies from fibrillar to reticular. Of these, fibrillar collagen is the most abundant form in vertebrates, where it plays a structural role, contributing to the molecular architecture, shape and mechanical properties of tissues, such as the tensile strength of skin and the tensile strength in ligaments. Of the 28 existing types, type I collagen is found in the highest proportion (approximately 90% of the collagen in the body is type I) and is present in almost all connective tissues. It is a component of the interstitial matrix and a major structural protein of bones, skin, tendons, ligaments, sclera, cornea, blood vessels, and is also present in other tissues. Type I collagen comprises approximately 95% of the total collagen content of bone and approximately 80% of the total proteins present in bone, making it the tissue with the highest amount of type I collagen. The main types of collagen in the skin are I and III, but there are also different types of collagen, such as collagen VII, crucial for the integrity of the skin, even though it only constitutes approximately 0.001% of the total collagen in the skin. Collagen is in high demand in the cosmetic and biomedical industries due to its biocompatibility, bioactivity, low immunogenicity, and benefits for the skin. Along with elastin, collagen fibers are responsible for forming the main structural network of the skin, thus contributing to its smooth appearance. Since skin aging is associated with increased degradation of collagen fibers, it is a popular ingredient used as a nutricosmetic product, demonstrating benefits for human skin and connective tissues of cartilage. The aging process is viewed by researchers from two perspectives: intrinsic factors and extrinsic factors. With age, fibroblasts, the cells responsible for collagen synthesis, reduce their activity, so that its level decreases. In addition, the activity of enzymes responsible for collagen degradation is stimulated by UV radiation, which is one of the processes by which “photoaging” occurs. It is easy to understand that the balance is tilted towards the loss of collagen, which explains the lack of firmness, elasticity and structure of the skin of older people. It has been observed that after the age of 20, collagen production in the skin decreases by 1

Copper. Oral administration of copper contributes to skin rejuvenation because this mineral promotes the proliferation of keratinocytes and fibroblasts, is an important cofactor in the enzymatic reactions necessary for the binding of collagen to elastin by lysyl oxidase, regulates the production of collagen (types I, II and V) and components of elastin fibers produced by fibroblasts (elastin, fibrillin), is a cofactor for superoxide dismutase, inhibits oxidative cellular effects such as membrane damage or lipid peroxidation and is a cofactor for tyrosinase, an enzyme essential for the biosynthesis of melanin which is responsible for skin and hair pigmentation. The RDA of copper for an adult is 900 mcg, dietary intake is usually sufficient, a dose of 10 mg being the maximum tolerated dose. Foods rich in copper include shellfish, seeds and nuts, organ meats, whole grain products and chocolate. Chronic exposure to high doses of copper leads to liver damage and gastrointestinal symptoms (abdominal pain, cramps, nausea, diarrhea, and vomiting). Copper poisoning is rare in healthy people but can occur from drinking water with a high copper content (water distribution systems and household plumbing that allow copper to leach into the water) and in people with Wilson's disease, a rare, autosomal recessive disease [15].

Selenium. Selenium is an essential trace element in the human body, being a constituent of some enzymes with an antioxidant role, such as glutathione peroxidase, thioredoxin reductase, methionine sulfoxide reductase. Selenium is frequently found in nutricosmetic products because in the aging process, oxidative stress has significant implications, and selenium contributes to the improvement of inflammation caused by reactive oxygen species (ROS) and reduces the process of DNA damage. Numerous clinical studies have shown that the mechanisms by which selenium combats aging are related to its cellular activity. It has been observed that selenium supplementation, in vivo, significantly reduces malondialdehyde concentrations (a biomarker of oxidative stress) and improves the activity of superoxide dismutase, glutathione peroxidase and the antioxidant capacity to antagonize aging induced by D-galactose. Following clinical studies on selenium and its action on the aging process, it has been clarified that selenium supplementation has anti-inflammatory and anti-aging roles.

Although it is known that the RDA of selenium for adults is 55 mcg, more studies are needed to determine the ideal daily dose of selenium to obtain maximum skin benefit. The maximum tolerated dose is 400 mcg. Excessive selenium intake produces a garlic odor on the breath and a metallic taste in the mouth. The most common clinical signs of chronic selenium consumption are hair loss and brittleness to loss of nails. Other symptoms include skin and nervous system lesions, nausea, diarrhea, rashes, stained teeth, fatigue, irritability, and nervous system abnormalities [16].

Zinc. Zinc is an important cofactor for cellular activity and defense. It protects against lipid peroxidation, UV-induced cytotoxicity and ROS-induced oxidative stress. It plays a major role in wound healing and keratinocyte cell survival and also has an anti-inflammatory effect by inhibiting intercellular adhesion molecule 1, a pro-inflammatory marker of keratinocytes and decreasing nitric oxide production. Evidence of the importance of zinc in skin diseases is the very symptomatology of zinc deficiency in which skin lesions, delayed wound healing, hair loss, as well as other symptoms such as diarrhea, loss of appetite, growth retardation can be observed. The RDA for zinc is 11 mg for men and 8 mg for women, the main sources being oysters, seafood, red meat, nuts and cereals. The maximum tolerated dose is 40 mg. Adverse effects in case of acute zinc poisoning include nausea, vomiting, loss of appetite, abdominal cramps, diarrhea and headaches. Chronically high zinc intake can lead to mineral imbalances, such as decreased copper and iron levels, which can lead to decreased immune function. As with all over-the-counter products, vitamin and mineral dosages in nutricosmetics should be within the recommended daily allowances. Particular care should be taken in individuals who use multiple types of dietary supplements, which may contain some common ingredients, thus risking an intake that exceeds the maximum permissible doses [17].

Natural compounds

As part of the diet, plants represent an important source of nutrients, not only vitamins and minerals, but also various compounds with a significant role in human health. The effects of natural compounds on health have been known since ancient times, with plants representing important sources in the traditional medicine of each people. The multiple beneficial effects of natural compounds, but also economic considerations such as the availability of raw materials or the sustainability of sources are reasons why these substances are very often used in the food supplement industry. The implications of oxidative stress in the aging process are clinically proven; ROS are responsible for most pathological changes in human skin, such as: DNA damage, inflammatory response, reduced antioxidant production and activation / inhibition of various signaling factors that lead to collagen and elastin degradation. Wrinkles, age spots and loss of skin tone are some of the visual changes that reflect the aging process. Natural compounds used in nutricosmetic products are predominantly substances with antioxidant effects such as polyphenols or carotenoids [18].

Polyphenols. Polyphenols are a very diverse group of natural compounds consisting of three main categories: phenolic acids, flavonoids and nonflavonoids. These compounds are highly studied as anti-aging agents due to their antioxidant activity. A specific study on the potential uses of polyphenolic compounds in nutricosmetics was carried out by some researchers who analyzed polyphenol-rich extracts obtained from various species of maple, birch and pine. Their results showed that the extracts have ROS-inhibiting potential and the ability to inhibit enzymes involved in the loss of skin elasticity and hyperpigmentation and could act as anti-aging agents. The main polyphenolic compounds found in nutricosmetics are resveratrol, quercetin (synonym quercetin), curcumin and catechins / epigallocatechin 3-O-gallate (EGCG). Numerous studies have established that the benefits of these compounds are based on antioxidant and anti-inflammatory mechanisms. The inclusion of polyphenols in nutricosmetic products can raise formulation issues regarding their stability and bioavailability. The polyphenol content of a product is very easily influenced by a series of reactions caused by acidity, heat, light and the presence of certain enzymes that can occur during the processing of fresh fruits and vegetables. The relatively low solubility in water is a disadvantage that influences bioavailability. Therefore, the formulation of polyphenols requires some methods to protect them such as inclusion in liposomal forms or the formation of complexes with higher solubility [19].

Carotenoids. Carotenoids are isoprenoid metabolites synthesized by all organisms capable of photosynthesis, and for humans they are essential nutrients, the exogenous intake being necessary for the production of retinoids such as vitamin A. These compounds represent the most efficient singlet oxygen (1O2) neutralizing agents in biological systems with a protective role against oxidative damage caused by UV light in the tissues in which they accumulate: β-carotene in the skin and lutein and zeaxanthin in the macula lutea. Several studies have demonstrated that carotenoid supplements protect the skin from UV-induced erythema. In addition to the photoprotective effect, they have also proven beneficial effects in preventing and improving skin aging (improved skin elasticity and hydration, skin texture, wrinkles and age spots). Therefore, the inclusion of carotenoids as antioxidant ingredients in nutricosmetics is justified by their effects and their accumulation in the target tissue, the skin [20].

The aging process and, implicitly, the general state of health are influenced by three main factors: genetics and family history; lifestyle and physical exercise and, last but not least, diet and nutrition. Nutrition is probably the most accessible solution in preventing aging and its associated morbidity, and the multitude of nutricosmetic products is proof of this. Collagen is the main structural element of the skin that undergoes changes throughout life, determining the general appearance of aging skin. Maintaining collagen levels from youth to old age is a challenge because interventions can only be carried out on extrinsic factors. This is also the reason why collagen is the main ingredient in nutricosmetic products, with research focusing both on finding sustainable and efficient sources and on optimizing extraction methods and discovering the perfect qualitative and quantitative composition. Supplementing the diet with collagen peptides has been shown to be effective in slowing down the aging process, thus contributing to an increase in the quality of life. As has been observed, not all products containing collagen are equivalent, on the contrary, the differences between them being very large. Choosing a product that contains a clinically tested form of collagen is always an advantage. Even though hyaluronic acid produces rapid effects when administered by an invasive method, oral intake has shown that it can be a promising ingredient in nutricosmetics.