The skin moisturizing agents can be emollients, occlusives, and humectants.

Emollients cover the skin with a protective film to hydrate and soothe it. They contribute to decreasing flaky skin and roughness. Foods used as emollients include butter and oils such as the butter of shea, cocoa, cupuacu, mango, kombo, and murumuru butter; and the oil of almond, avocado, argan, borage, olive, babassu, broccoli, rapeseed, chia seed, castor bean, coconut, primrose, palm, passion fruit, pomegranate, raspberry, safflower, and sunflower.

Occlusives form an epidermal barrier to stop trans-epidermal water loss and regulate keratinocyte proliferation [21]. Foods used as occlusive moisturizing agents are oils and waxes such as olive, jojoba, and coconut oils; and the wax of candelilla and bees [22]. The oils of coconut and castor have both functions as emollients and occlusives.

Humectants are water-loving moisturizing agents that draw moisture from the dermis to the stratum corneum and binding water vapor from the environment [23]. Honey, hyaluronic acid, sorbitol, glycerine, and glycerol are examples of humectants’ moisturizing agents [24].

The skin barrier stops transepidermal water loss and defends against pathogens [25]. Barrier repair agents are the essential fatty acids, phenolic compounds, tocopherols, phospholipids, cholesterol, and ceramide. The ratio of the essential fatty acids is a critical point to benefit barrier repair. Higher levels of linoleic acid to oleic acid have better skin-barrier potential [26]. It enhances the permeability of the skin barrier [26,27], being an integral component of the lipid matrix of the stratum corneum [28]. Oleic acid, disrupting the skin barrier, acts as permeability enhancers for the other bioactive molecules present in plant oils [29]. The antioxidant compounds (tocopherols and phenolics) modulate skin barrier homeostasis, wound healing, and inflammation [30,31]. Phospholipids act as chemical permeability enhancers [32]. They show anti-inflammatory effects by controlling the covalently bound, ω-hydroxy ceramides and inhibiting thymic stromal lymphopoietin and chemokine [33]. Cholesterol and ceramides are other important lipid classes in the stratum corneum [34]. Cholesterol in the plasma membrane can be an essential factor for the magnitude of the oxygen gradient observed across the cell membrane [35]. Twelve ceramide subclasses are identified in the stratum corneum [36]. Ceramide influences firm and plump skin. Topical application of a ceramide cream decreases IL-31 and damages the skin barrier’s physical and function [37]. Some natural oils contain fatty acids that play critical roles in maintaining the skin barrier. Flaxseed oil, walnut oil, and chia oil contain omega-3s and grapeseed oil, safflower oil, sunflower oil, blackcurrant seed oil, evening primrose oil, and borage oil hold omega-6s [34].

Skin lightening agents decrease the concentration of melanin (skin’s pigment). The skin tone is lighter when there is less melanin. Skin whitening agents act as inhibitors of the tyrosinase (a key enzyme in melanogenesis) and/or melanosome transfer (pigment granules in the melanocytes, contained in the basal layer of skin epidermis) [38,39] or increasing the epidermal turnover and the effect of anti-inflammatory and antioxidant actives [40]. Ethnic differences, chronic inflammation, hormonal changes, and UV exposure are examples of conditions that can determine hypo- or hyper-pigmentation [41]. The commonly used active ingredients include citrus extracts, kojic acid, licorice extract, white mulberry extract, bearberry extract, Indian gooseberry, vitamin C, vitamin B3, hydroquinone, retinoids, resveratrol, and alpha- and beta-hydroxy acids [42].

Exogenous stimuli sometimes can determine wound, skin aging, inflammatory dermatoses, or skin carcinogenesis. Damages of the skin barrier determine the inflammatory response, which provides tissue repair and infection control. Initially, the keratinocytes and the innate immune cells (e.g., leukocytes, dendritic cells, and mast cells) are activated [43], and successively make cytokines (e.g., IL-1α, IL-6, and TNF-α) that draw the immune cells to the injury site. Finally, ROS, elastases, and proteinases are produced [43]. Thus, inflammation is involved in acne’s pathogenesis and determines pain, swelling, and redness in the skin. Licorice root, turmeric, oats, chamomile, and nuts are some food plants with anti-inflammatory activity [44,45].

UV radiation is divided into three main categories: UV-A (320–400 nm), UV-B (280–320 nm), and UV-C (100–280 nm), based on the wavelength. Elevated exposure to UV radiation can cause edema, erythema, hyperpigmentation, photoaging, immune suppression, and skin cancer based on the intensity and range of UV radiation [46,47]. Continuous exposure to UV radiation can cause pigmentation, lesions, sunburn, dark spots, degradation of collagen fibers, wrinkles photoaging, and cancer [48,49]. UV-A photons cause damage to fibroblasts and keratinocytes [50]. In the skin, cellular chromophores absorb them, and reactive oxygen species (e.g., superoxide, hydrogen peroxide, and hydroxyl radicals) are made [51]. Oxidative stress can cause DNA damage [52]. UV-B is known as burning rays and is considered the most active constituent of solar radiation. It can induce direct and indirect adverse effects on DNA and proteins [53], inducing immunosuppression and skin cancer [54]. The most dangerous UV wavelengths are UV-C. Fortunately, these radiations are absorbed by the atmosphere before they reach our skin [55]. They are potent mutagens and can trigger cancer and immune-mediated disease [56]. Aloe vera, green tea, coconut oil, grape seeds, and ginger contain phytochemicals that prevent photoaging and skin cancer [24].