Nutritional Approaches of the Changing Consumer after the Pandemic: Sustainable Potential of Phytoene and Phytofluene for Photoprotection and Skin Health

Abstract

Ascertaining sustainable endogenous nutrients for long-term healthy skin from the nutritional perspective of changing consumers, is an interesting method to prevent photoaging symptoms which are the main cause of skin aging. This narrative literature review aims to focus on the sustainable potential of phytoene (PT) and phytofluene (PTF) to engage in biological actions with photoprotection, dietary intervention, nutrition, stability, safety, and distinct structure of PT and PTF for skin health. As a result, this review demonstrates that dietary interventions of colourless carotenoids, PT and PTF, according to their distinct structural and biological actions, maximize the absorption of damaging light in the ultraviolet range unlike coloured carotenoids that have maximum absorption in the visible range. Furthermore, in addition to skin health, their distinct chemical structures and biological actions are attributed to antioxidant, anti-inflammatory, and anticancer activities and suggest the sustainable potential of PT and PTF to provide various additional health benefits, such as improving the immune system and cardiovascular health. “Healthy and beautiful skin starts from within”. It is hoped that dietary interventions of PT and PTF, along with topical photoprotection by sunscreen, will become better known to consumers as a nutritional approach to long-term basal protection/defense and complementary photoprotection and skin health strategies.

1. Introduction

Beauty and health go hand in hand today, and consumers consider nutrition as an important pillar of skin care [1]. The role of skin health nutrition is recently receiving more awareness, and interest in nutrition, cosmetics, and safety plays a pivotal role in driving change for consumers’ skin health. More attention has been paid to healthier nutrition during the pandemic, including mass quarantines and stay-at-home orders in many countries [2], and people started wanting to have healthier and clearer skin [3] and to be younger and more beautiful [4]. Many innovative efforts are currently underway in the industries of cosmetics, cosmeceuticals, functional foods, etc. [5]. Healthy skin can indicate overall health and can be affected by the intake of antioxidant substances [6]. Therefore, the use of dietary and oral supplements may provide cosmetic benefits [6]. These trends have been reflected in recent years, and the global consumption of “Inner Beauty” is accelerating. Non-artificial foods that can improve health and beauty, in the long run, are called “Inner Beauty Foods” [7]. After the pandemic, consumers now take inner beauty as an important strategy for healthy skin, and interest in inner beauty through proper nutrition, extrinsic protection, and various methods is being adopted [7]. The body’s largest organ is the skin which shows one of the main signs of aging [8]. It can be separated into heat aging, photoaging, and natural aging [9]. Photoaging is the most important factor which causes skin aging, and excessive exposure to extrinsic factors such as ultraviolet (UV) rays can also cause premature aging. Photoaging of the skin is caused by long-term exposure to UV rays [10]. Exposure to these exogenous environmental factors may cause premature skin aging. Skin damage through UV radiation includes erythema (also known as sunburn), pigmentation (tanning), photoaging, photocarcinogenesis, and oxidative stress which leads to inflammatory reactions such as acute erythema and chronic damage [11,12,13]. The most problematic chronic damage results from premature skin aging and skin cancer [14].

Consuming dietary compounds with photoprotective properties can protect the skin from photoaging [15,16,17]. Nutritional habits affect the risk for cutaneous structure, skin diseases, premature skin aging, and thus overall skin texture and appearance [18,19]. In this context, the structure, physicochemical properties, and activities of dietary carotenoids currently play important roles in nutritional and health processes [20], and they are known for their role in promoting general health in reducing the risk or progression of diseases related to oxidative stress [21,22]. The human body cannot produce carotenoids internally but can accumulate them from the diet [23]. These carotenoids are found in many fruits and vegetables. They are also a natural dietary product that is consumed through other products such as food, additives, or supplements [24,25]. Several carotenoids can be invariably found in breast milk, human plasma, and various tissues including the skin, with significant carotenoids usually being α-carotene, β-carotene, β-cryptoxanthin, lutein, lycopene (LYC), zeaxanthin, phytoene (PT), and phytofluene (PTF) [26]. Among these, PT and PTF have properties that can protect against the UV radiation that causes skin damage [5,15,27,28,29,30]. The body can absorb PT and PTF well and they are found in the blood and several tissues [31,32]. However, because they are colourless they have been called rare carotenoids, and research in the context of innovative product development, agro-food, nutrition, and health has been largely overlooked [33]. PT and PTF can all be absorbed from the diet, and plasma concentrations increase after consuming tomato products [34]. LYC is found in most tomatoes along with PT and PTF, this raises the question of whether the colourless carotenoid is being overlooked in studies of associations between the consumption of LYC and the health benefits of tomatoes [5,27]. PT and PTF should also be evaluated in studies of the relationship between health outcomes and the consumption of tomato products [30]. There is now evidence that PT and PTF may be more stable compared to other carotenoids in tissues [35,36], and no health risks have been reported in association with their oral intake [27]. Today, consumers are interested in sustainable safety, and the stability of food consumed in a processed form should be considered very important.

Conjugated double bond (c.d.b.) systems, i.e., alternating single and double bonds, are a major structural feature of carotenoids, and PTs and PTFs have far fewer c.d.b.s than almost all other carotenoids [5,27,28,30]. Because of this, they are expected to show significant differences in some of the physiochemical properties when compared to other carotenoids [27,30]. They are absorbed maximally in the UV range with plasma and tissue uptake levels comparable to those of other major dietary carotenoids, and they were found to be involved in the biological action of health and beauty benefits [15,27,28]. The major dietary carotenoids are readily bioavailable and can affect skin health [5,37]. PT and PTF as dietary carotenoids have recently appeared as compelling alternative strategies for preventing and alleviating the symptoms of photoaging. Virtually all carotenoids have maximum absorption of visible light, while PT and PTF provide photoprotection by maximally absorbing light within the UV range, which protects against damage from UVA and UVB [5,15,27,28,29,30]. These things considered, it is not surprising that biological actions through dietary intervention and the presence of colourless carotenoids in the skin are expected to protect the skin from UV exposure and provide various skin health benefits. In addition to simple content, several aspects including daily intake and bioavailability should also be considered to estimate possible health benefits and gain insight into the action of carotenoids in the human body [30]. Like other carotenoids, PT and PTF exhibit antioxidant effects that help defend against oxidative stress in humans [15,32,38,39,40,41,42,43,44,45,46]. They are also the precursors of most carotenoids, which are important from a biosynthetic point of view and require extensive studies [5,27]. It may be involved, alone or in combination with other compounds, in a variety of health-promoting biological actions [5,37,38,39,47,48,49,50,51,52].

Methods to prevent or treat photoaging of the skin in everyday life include physical photoprotective means (such as sunglasses), sunscreens, medical cosmetology, and topical treatment of active ingredients and the like [53]. However, dietary interventions of PT and PTF along with their photoprotective effects [5,15,27,28,29,30] and antioxidant capacity for intrinsically healthy skin can be an important long-term defense mechanism against environmental damage [54]. Recently, PT and PTF have been discussed from dietary sources as components of products promoting “Beauty from Within” to their usefulness in industries such as functional foods [28]. True beauty starts from within [18,19,55]. Most lifetime UV photoaging occurs when the skin is not protected, and it is the most important factor relating to skin aging. That is, sustainable endogenous nutrients are an interesting aspect to consider for long-term healthy skin aging. In this context, this review aims to focus on the sustainable potential of PT and PTF to engage in biological actions with photoprotection, dietary interventions, nutrition, stability, safety, and the distinct structure of PT and PTF for skin health. Therefore, we present important implications and strategies to promote internal beauty from the nutritional point of view of changing consumers.

2. Materials and Methods

2.1. Search Strategy

This narrative review, with the intent to consolidate and critically appraise the literature, was undertaken to explore the sustainable potential of PT and PTF for photoprotection and skin health from a changing consumer nutritional point of view after the COVID-19 pandemic. Although this review was a narrative literature review, we searched PubMed, Medline, Scopus, RISS, ResearchGate, and Google Scholar according to the PRISMA guidelines. A literature search was conducted using a broad search strategy with the keywords: COVID-19; consumer; healthy skin; promote skin health; skin health; skin beauty; skin aging; inner beauty; beauty from within; photoaging; sunscreen; ultraviolet; UV; UV radiation; UVA; UVB; UVC; ultraviolet skin photoprotection; skin photoprotection; photoprotection strategy; phytoene; phytofluene; phytoene safety; phytofluene safety; phytoene stability; phytofluene stability; carotenoids; colourless carotenoids; coloured carotenoids; dietary carotenoids; dietary photoprotection; dietary interventions; bioavailability; antioxidant; antioxidant activity; antioxidant capacity; dietary antioxidants; foods; supplements; nutrition; nutrient; nutritional protection; nutritional sources. The literature search strategy and review process according to the PRISMA 2009 flow rules can be seen in Figure 1 [56].

2.2. Eligibility Criteria

Articles included in this review had to meet the eligibility criteria as follows. With respect to photoprotection and skin health, PT and PTF, COVID-19, nutritional point of view of the consumer, and dietary intervention, studies on the following were considered eligible for inclusion: negative effects of UV exposure, the importance of UV Protection, strategies for skin health, photoprotection strategies, effective strategies to prevent premature skin aging and promote skin health, long-term defense strategies against skin aging and disorders caused by UV radiation, benefits of PT and PTF, benefits of colourless carotenoid dietary interventions, the safety and stability of PT and PTF, the sustainable potential of PT and PTF for photoprotection and skin health, the sustainable role of colourless carotenoids in photoprotection and skin health.

2.3. Screening and Data Extraction

Consideration of various article types such as original research and articles, review articles, short communications, viewpoints, and internet articles were included in the criteria. There were no restrictions on the date of publication or language. The exclusion criteria were the following: full texts with no raw data; full texts with no accessibility; dissertations; inappropriate topics; not pertinent to the main focus of this review.

2.4. Risk of Bias Assessment

The methodological quality of the included studies was independently evaluated by all authors.

2.5. Study Selection and Data Extraction

Among the citations of the papers examined in the first search, additional references were identified through a manual search, and titles and abstracts were reviewed to assess eligibility. Articles that did not match the inclusion/exclusion criteria were then excluded from this review. Ultimately, the full-text articles were reviewed to determine the inclusion of the remaining papers, and articles not aligned with the criteria were excluded. The draft results were discussed among the co-authors and the final version was approved by all. The results from a total of 117 papers included in this review are proposed under the following headings: “Distinct chemical Structure and Potential of Colourless Carotenoids, PT and PTF”, “Colourless Carotenoids Involved in Biological Actions: Potential for High Bioavailability”, “Sustainable Potential of PT and PTF to Maximize UV Absorption”, “Photoprotection of the Skin; “Inner Beauty Strategy: Nutritionally Important Colourless Carotenoids as Consumer Products”, “A Major Source of the Common Dietary Carotenoids, PT and PTF”, “Stability of Dietary Carotenoids, PT and PTF”, “Limitations of the Present Review and Future Studies”. A total of 117 articles were selected in the final stage, and publication dates ranged from 1975 to 2022. Figure 1 shows the PRISMA flowchart [56]. The model diagram of this study was organized as shown in Figure 2.

3. Results

3.1. Distinct Chemical Structure and Potential of Colourless Carotenoids PT and PTF

Along with bacteria, several known invertebrate animals, and fungi, carotenoids are ancient and widespread isoprenoids biosynthesized by photosynthetic organisms (algae, plants, cyanobacteria) [20]. The distinct structures and properties of PT and PTF have the potential of colourless carotenoids compared to other dietary ones. PT and PTF have the molecular formulas C40H64 (MW = 544 Da) and C40H62 (MW = 542 Da), respectively, of linear hydrocarbons [27]. The characteristic commit stage in carotenoids biosynthesis is the condensation of two molecules of the C20 compound geranylgeranyl pyrophosphate, and most carotene-producing organisms form the major PT isomer, (15Z)-PT [57]. Condensation forms PT which is the precursor of almost all coloured carotenoids [30]. This reaction is catalyzed by the pathway’s key regulatory enzyme, PT synthase [58,59], and PTF is formed with PT desaturase action [30]. The insertion of a double bond that extends the c.d.b. system from three to five is catalyzed by this enzyme. [20]. There are 9 and 10 double bonds in PT and PTF, respectively, but the number of c.d.b. are much smaller than that of coloured carotenoids, which have at least 7 [5,28]. These two molecules are the first biosynthetic path compounds of the coloured carotenoids [60]. Significantly fewer c.d.b. are in PT and PTF than other major dietary carotenoids [28]. Coloured carotenoids have at least seven c.d.b. [28,61], and three of the c.d.b. in PT, whereas there are five c.d.b. in PTF (Figure 3) [62].

The main structural characteristic of carotenoids is the c.d.b. system which is largely accountable for critical physicochemical attributes such as shape or reactivity, and radiation absorption [30]. As mentioned above, PT and PTF are rare among carotenoids due to the low number of c.d.b., and consequently, PT and PTF are colourless and immersed maximally in the UV range due to the shorter polyene chain [5,15,27,28,29,30]. Therefore, coloured carotenoids have the maximum absorption in the visible light range, but PT and PTF absorb the maximum in the UV range, showing important differences in some physicochemical properties [5,15,27,28,29,30]. PT and PTF also showed higher bioavailability than other carotenoids present in food matrices [63]. Bioavailability is the proportion of a consumed compound (nutrient or bioactive compound) with physiological benefits or can be stored in a nutritional context [28]. One of the key factors in explaining the bioavailability of carotenoids is bioaccessibility [5]. Currently, evidence suggests that the bioaccessibility of colourless carotenoids is significantly higher than that of other carotenoids this is suggested to be partly due to its less rigid shape (due to the geometric configuration resulting from the lower number of c.d.b.) [5].

3.2. Colourless Carotenoids Involved in Biological Actions: Potential for High Bioavailability

PT and PTF are bioavailable, making them important for involvement in several health-promoting activities [5]. Plasma carotenoid concentrations increase when foods rich in PT and PTF are consumed [64,65] and have been reported in plasma at concentrations in the 0–1 μM range [31]. The impacts of food components such as carotenoids are related to plasma and tissue levels rather than food content; therefore, bioavailability needs to be kept under consideration [28]. The use of standardized simulated digestive bioaccessibility to assess in vitro has become commonplace since carotenoid bioavailability is influenced by many factors and cannot be assessed [66]. Bioavailability is one of the key explanatory factors for carotenoid bioavailability in this context as carotenoids must be incorporated into these structures to be absorbed by enterocytes before reaching the bloodstream [26,67]. Generally, the term bioaccessibility means the percentage of carotenoids that can be incorporated into mixed micelles and thereon taken up by enterocytes [30]. Another recent concept of the actual amount of carotenoids that can potentially be absorbed from a given portion of food is “Carotenoid bioaccessible content”, which means the absolute carotenoid level of the micellar fractions of the portion [21,68,69]. For the carotenoids to reach the bloodstream after being released from the food matrix during digestion, carotenoids must be absorbed by the membranes of the intestine enterocytes [28]. Bioaccessibility is usually incorporated into the mixed micelles and taken up by enterocytes after a proportion of carotenoids [66]. Carotenoids must foremost be released from the food matrix and incorporated into mixed micelles before flowing into the bloodstream and reaching the target tissue [67,70]. Once inside the mixed micelles, PT and PTF are presumed to be incorporated into chylomicrons before being taken up by enterocytes, transported to the basolateral aspect, and secreted into the lymph [67,70,71]. The elevated micellization efficiency of PT and PTF can partially explain their high bioaccessibility [48]. Several studies have shown that PT and PTF have higher bioavailability, i.e., better integration into mixed micelles, compared to β-carotene, lutein, and LYC [37,47,48,49]. There is sufficient evidence to postulate an involvement in effective intestinal absorption of PT and PTF. This process of absorption and transport is very efficient for colourless carotenoids and is exemplified by the fact that the major carotenoid in various tissues is PT which has nearly three times higher bioavailability than that of LYC [50]. Moreover, regarding the absorption of colourless carotenoids in the small intestine, colourless carotenoids were mostly seen in the distal parts, while LYC similarly accumulated along the small intestine [49]. Interestingly, in studies on the absorption of PT and PTF by intestinal cells, it is reported that the absorption efficiency is much higher than that of LYC [48]. PT and PTF were filtered from tomato extract and their micellization and cellular uptake efficiencies were compared with those of commonly consumed pure carotenoids.

The number of carotenoids was measured for inclusion in the mixed micelle fraction at three target carotenoid concentrations of 0.5, 2, and 10 μM [48]. PT and lutein showed the highest blending efficiency linearly across three concentration tests, and the blending efficiency of PTF was similar to that of PT and lutein up to about 2 µM, i.e., more heightened dietary concentrations [48]. It appears to be starting to stabilize. LYC displayed the lowest incorporation efficiency of 0.06 µM maximum micelle concentration at all three concentration tests [48]. The results of this study provided the reason for the unexpectedly high concentration, compared to the other major dietary carotenoids (i.e., LYC), as the bioaccessibility of PT and PTF. There was little evidence in reviews about the health benefits of LYC alone, since most tomato extracts also contain other carotenoids [72,73]. As a potential mediator of biological actions related to tomato consumption, the colourless carotenoid is presented together with LYC [50]. In fact, the intake of colourless carotenoids in general may be higher than that of LYC [27]. According to Biehler et al. [74] the PT and PTF intake was estimated to be 16% (2.7 mg, specifically 2.0 mg for PT and 0.7 mg for PTF) of the total carotenoid consumption. Figure 4 shows the significant contribution of PT and PTF to the daily intake of bioavailable carotenoids from commonly consumed foods in a representative sample of the Luxembourg population [74].

Daily intake highlighted the importance of colourless carotenoids in relation to dietary intake for estimating health benefits [37]. In addition, it is necessary to know the dietary intake as well as the bioaccessibility truly available to the human body to support bioavailability and estimate the possible health benefits of colourless carotenoids [37]. Juice has previously been pointed out as having a great influence on the bioavailability of carotenoids [37,75]. Recent studies have shown that tomato juice, which provides the highest amount of potentially absorbable PT and PTF, has better bioaccessibility than other carotenoids [37]. Interestingly, PT and PTF showed higher bioaccessibility in all matrices (considering all isomers). PT was up to four times more bioaccessible than LYC [37]. Figure 5 shows the bioaccessibility of each carotenoid by considering the all isomers in tomato juice after gastro-intestinal digestion [37]. Bioaccessibility of each carotenoid is expressed as percentage.

Regarding bioaccessibility, the greatest difference was found in tomato juice, with 15% for LYC and 62% for PT, indicating that some degree of difference in c.d.b. could be attributed [37]. Thus, colourless carotenoids may also be involved in health-promoting biological actions, either alone or in combination with other compounds [5,37,38,39,47,48,49,50,51,52], and suggests the potential for higher bioaccessibility, and bioavailability that can benefit health compared to other coloured carotenoids. In this sense, the potential contributions of these components should be considered, with synergistic and equal interaction potentials [76].

3.3. Sustainable Potential of PT and PTF to Maximize UV Absorption: Photoprotection of the Skin

More than 80% of observable facial skin aging is found to be caused by exposure to UV rays [77,78,79]. The biggest factor in skin aging is unconscious sun exposure within daily life [80]. Excessive exposure to exogenous environmental factors such as UV rays may cause premature aging, and this process is primarily associated with increased exposure to UVA rays, but UVB also initiates photoaging [81]. Clinical signs of photoaging include mottled pigmentation, loss of skin tone, wrinkles, sallowness, melanoma, deep furrows, dryness, and skin cancer [82,83]. These indications are important because they can lead to various benign or malignant tumors and are important and ever-present risk factors for skin health [84]. UV radiation can be differentiated into three types, namely UVA (320–400 nm), UVB (280–320 nm), and UVC (200–280 nm), and photoaging is mainly caused by UVA and UVB [85]. The ozone layer mainly blocks UVC and has a negligible effect on the skin [86]. UVA does not directly affect DNA damage but can indirectly cause DNA oxidative stress by generating reactive oxygen species and causing DNA damage [87], whereas UVA does not directly affect DNA damage, and erythema is caused by overexposure to UVB [28]. Excessive UVB radiation can potentially cause erythema [27]. An individual’s sensitivity to erythema-inducing UVB radiation varies. The minimal erythema dose (MED) is the lowest dose of UV radiation that produces detectable erythema 24 h after UV exposure [88]. A possible mechanism for photoprotection from UV exposure can be seen as absorption in the damaging wavelength range [40]. The profundity of UV transmission to the skin relies on the wavelength. In terms of skin photoprotection, evidence is accumulating regarding the beneficial effects of colourless carotenoids [5]. Radiation protection by dietary means is of particular interest because it differs from PT and PTF which maximally absorbs in the spectrum of UV range compared to other coloured carotenoids [89]. Unlike almost all other carotenoids, PT is rare among carotenoids because it is maximally absorbed in the UVB (280–320 nm) range and PTF is maximally absorbed in the UVA (320–400 nm) range (see Figure 6) [5,15,27,28,29,30]. In other words, PT and PTF exhibit absorption maxima at wavelengths of UV light.

The beneficial role of colourless carotenoids concerning photoprotection was first pointed out by a study with the conclusion that injected doses of PT protect against guinea pigs that were given UV radiation-induced erythema [90]. Pioneer Mathews-Roth [91] explains that long-term PT administration to mice reduces the formation and diversity of skin tumors induced/generated by UVB light. In human studies, proprietary oral food supplements in the form of PT- and PTF-enriched tomato powder (Israeli Biotechnology Research Ltd., Yavne, Israel) given to women increased the MED by an average of 10% and increased by 20% by the end of the 12-week study for two-thirds of the volunteers [42]. Aust et al.’s study [29] showed that a beverage (8.2 mg LYC, 4.6 mg PT, and 3.2 mg PTF/d) containing tomato extract reduced erythema in response to UV irradiation, and these interventions resulted in significant increases in plasma and skin carotenoid levels and prevented erythema-induced formation in all groups. Another study showed that supplementation with tomato products rich in PT and PTF attenuated UV radiation-induced damage in both male and female mice [92]. A recent study showed that SKH-1 hairless, immune-competent male mice fed a diet containing LYC, PT, and PTF tomato components developed fewer UVB-induced skin tumors compared to controls, and long-term consumption of tomatoes can alleviate erythema caused by UV light [93]. In addition, photoprotective effects can be provided by inhibiting UV radiation-induced upregulation of intercellular adhesion molecule 1, matrix metallopeptidase 1 gene, and heme oxygenase 1 [94]. It is evident that PT and PTF accumulate in the skin better than LYC [23,38,95]. In the aforementioned study, consumption of tomato extract and a beverage containing tomato extract resulted in considerably less severe erythema after 12 weeks compared to baseline, whereas no such effect was found in LYC treatment alone [29]. Volunteers given synthetic LYC, tomato extract, or tomato-based beverages (all supply the same virtual amount of LYC) also showed decreased levels of UV radiation-induced erythema formation in all groups, even though there were greater effects in volunteers given the tomato-based product than in the group given synthetic LYC alone [29]. This suggests that PT and PTF made more contributions. A previous study deduced that PT and PTF in the skin were more than 15 times higher than the ratio of the same LYC in standard red tomatoes [23], and that consuming (daily 5 mg of PT and PTF) tomato powder (PhytoflORAL^®) increased the skin’s resistance to UV damage by increasing the MED by 20% compared to baseline for 3 months [42]. Interestingly, the protective effect was shown to be more pronounced with tomato-based products, which indicates that a greater contribution or synergistic effect comes from PT and PTF [29]. PT and PTF have maximum absorption in the UV range unlike those of coloured carotenoids in the visible light range [5,15,27,28,29,30]. Therefore, it is reasonable to expect the possibility of contributing to photoprotection by maximally absorbing harmful light [5,15,27,28,29,30]. Taking this into account, interventions through a dietary supply of colourless carotenoids suggest a sustainable potential for the prevention and control of skin aging, photoprotection, and erythema formation in humans against accidental exposure to UV rays.

Table 1. The main results of this study on the photoprotection of PT and PTF.

Type of Study	Title	Key Findings	Reference
Review	Skin Carotenoids in Public Health and Nutricosmetics: The Emerging Roles and Applications of the UV Radiation-Absorbing Colourless Carotenoids Phytoene and Phytofluene	Unlike almost all other carotenoids, PT is rare among carotenoids because it is maximally absorbed in the UVB range and PTF is maximally absorbed in the UVA range, and maximum absorption of coloured carotenoids in the visible light range, PT and PTF have maximum absorption in the UV range.	[5]
Review	Nutritional Aspects of Phytoene and Phytofluene, Carotenoid Precursors to Lycopene		[15]
Research Article	Supplementation with tomato-based products increases lycopene, phytofluene, and phytoene levels in human serum and protects against UV-light-induced erythema		[29]
Review	A comprehensive review on the colorless carotenoids phytoene and phytofluene		[27]
Research Article	The colourless carotenoids phytoene and phytofluene: From dietary sources to their usefulness for the functional foods and nutricosmetics industries		[28]
Review	The undercover colorless carotenoids phytoene and phytofluene: Importance in agro-food and health in the Green Deal era and possibilities for innovation		[30]
Research Article	Antitumor activity of β-carotene, canthaxanthin and phytoene	Point out the beneficial role of colourless carotenoids concerning photoprotection was the conclusion that injected doses of PT protect against UV radiation-induced erythema in guinea pigs.	[91]
Research Article	The photoprotective effects of a food supplement tomato powder rich in phytoene and phytofluene, the colorless carotenoids, a preliminary study	Consuming (5 mg of PT and PTF per day) tomato powder (PhytoflORAL®) increased the skin’s resistance to UV damage by increasing the MED by 20% compared to baseline for 3 months. That is, daily intake of colourless carotenoids can protect the skin from UV radiation with increased MED and the natural resistance of the skin to UV radiation-induced erythema.	[42]
Research Article	Supplementation with Tomato-Based Products Increases Lycopene, Phytofluene, and Phytoene Levels in Human Serum and Protects Against UV-light-induced Erythema	Volunteers given synthetic LYC, tomato extract, or tomato-based beverages (all of them supplying virtually the same LYC amount) also shows that all groups were prevented from UV radiation-induced erythema formation with greater effect in volunteers with tomato-based product than synthetic LYC.	[29]
Research Article	Sex differences in skin carotenoid deposition and acute UVB-induced skin damage in SKH-1 hairless mice after consumption of tangerine tomatoes	Supplementing PT- and PTF-rich tomato products mediates acute UV radiation-induced skin damage in SKH-1 mice with reduced DNA damage in both sexes, and less inflammation in the male group.	[92]
Research Article	Tomatoes protect against development of UV-induced keratinocyte carcinoma via metabolomic alterations	PT and PTF tomato components developed fewer UVB-induced skin tumors compared to controls, and long-term consumption of tomatoes can alleviate erythema caused by UV light.	[93]
Research Article	Molecular evidence that oral supplementation with lycopene or lutein protects human skin against ultraviolet radiation: results from a double-blinded, placebo-controlled, crossover study	Show that it can provide photoprotective effects by inhibiting induced UV radiation upregulation of intercellular cohesion of molecule 1, matrix metallopeptidase 1 gene, and heme oxygenase 1.	[94]

summarizes the main results of this study on the photoprotection of PT and PTF.

3.4. Inner Beauty Strategy: Nutritionally Important Colourless Carotenoids as Consumer Products

For centuries, nutrition has been used to enhance youth and beauty, and the association between nutrition and skin aging yet dwells as an area of interest [5,96]. Beauty and health go hand in hand today, and consumers consider nutrition as an important pillar of skin beauty and aging [1]. UV exposure is a lifelong problem and a major cause of skin aging and diseases, such as erythema and photodamage, that occurs when the skin is not protected. Applications such as sunscreen are found to only deliver the effect externally, from the outer surface of the skin. In this context, nutrition is important because it can have a positive effect on the risk of skin diseases, premature skin aging, i.e., overall skin appearance [18,19], and can protect the skin from damage caused by UV exposure [5,15,27,28,29,30]. UV radiation that reaches the skin can be protected by biological tissue compounds from nutritional sources via the bloodstream [28,54]. Interestingly, as mentioned earlier, colourless carotenoids are readily bioavailable and may provide aesthetic benefits related to skin aging and health. Consumption of products containing colourless carotenoids can make the skin appear more beautiful, healthier, and younger [94]. In particular, they protect the skin from UV damage and provide aesthetic benefits [41,42]. Photoprotection from dietary colourless carotenoids, which are nutritionally important as consumer products, can be an effective strategy to prevent premature skin aging and promote skin health.

These colourless carotenoids are highly biologically active and have anti-inflammatory, antioxidant, and anticancer activities in vitro and in vivo [39]. Endogenous and exogenous antioxidants play an important role in reducing skin cell problems and oxidative stress. Among all dietary components, the one that exhibits the most antioxidant activities is itself a synergistic combination that improves the structural properties of the skin and protects it from oxidative stress [97]. PT and PTF have also been shown to improve the immune system and increase the antioxidant defense system [28,46,98]. Consistent with what was observed in in vitro studies, 26 healthy subjects who consumed 250 mL of a tomato drink (approximately 4 mg of PT and 3 mg of PTF along with other carotenoids) showed a reduction in DNA damage (approximately 42%) in oxidative stress lymphocytes on day 26 in a double-blind crossover study, thus showing that the carotenoids have antioxidant effects [46]. Lymphocytes are specialized cells of the immune system that have a rapid reaction to signals of negative health effects such as the existence of pathogens or inflammatory stimuli, among others [99]. This suggests that the intake of PT and PTF tomato beverages can affect the immune defense response due to the reduction of DNA damage in lymphocytes under oxidative stress and enhance cell antioxidant protection. That is, it exhibits these properties in vitro and in vivo because it is very biologically active. Concerning antioxidant activity, PT-rich tomato fraction (including PTF) showed an antioxidant response in vitro [44] and the possible health-promoting biological actions of colourless carotenoids of other properties that may be involved in antioxidant activities [15,28,32,38,39,40,41,42,43,44,45,46]. A significant amount of PT and PTF have been detected in both human plasma and tissues such as the liver, lung, skin, prostate, and breast, and when accumulated in the skin can provide nutritional benefits to the skin due to their antioxidant activities [15,32,39,46]. Therefore, dietary antioxidants are a good natural strategy for giving the skin a basic defense against photodamage as well as further attacks that can affect its appearance [54]. It can probably protect against UV radiation-induced damage and provide anti-antioxidant activity, etc., due to its distinct light absorption spectrum with absorption maximization in the UV range [28]. von Oppen-Bezalel et al.’s [42] study showed that women who were given oral administration of tomato powder rich in colourless carotenoids, exhibited a marked improvement in overall skin beauty, including visible skin health, radiance, and so on, at the end of the 12-week study period. It has been shown that the skin can be protected from UV radiation-induced erythema and MED with increased natural resistance through the daily intake of these colourless carotenoids [42].

To improve skin for health and/or beauty purposes, ingestion of PT and PTF-rich products has the supplementary benefit of improving the circulating and internal tissue levels of these compounds that promote health [28]. That being said, it should be noted that compounds such as PT and PTF present in tomato-based foods may be involved in several other health benefits besides skin health. Since LYC commonly transpires with PT and PTF in tomato products, one can question whether these colourless carotenoids could be involved in studies searching for associations between LYC intake in tomatoes and health benefits [27]. High amounts of LYC and certain amounts of PT and PTF are usually incorporated [30]. Previous research has shown that the consumption of tomato-based foods provides better results than LYC supplementation, helping to achieve optimal cardiovascular health, which was related to the fact that tomatoes also contained other bioactive constituents such as PT and PTF in addition to LYC [27]. Interventions such as tomato consumption can greatly lower cholesterol, plasma triglycerides, and low-density lipoprotein cholesterol while increasing levels of high-density lipoprotein cholesterol [100]. PT and PTF have been portrayed as having advantageous impacts on human health with the probability of lowering the risk of cardiovascular diseases as well as certain types of cancer, such as endometrial or breast cancer [101]. A concentration of 4–6 μM in the medium of both PT and PTF is indicated to inhibit the cell proliferation generated by the tested estrogens [101]. Interestingly, studies using prostate cancer models have demonstrated a remarkably lower threat of death from a malignancy, as observed in tomato-powder fed rats [51]. This study provides evidence that compounds exhibiting anti-cancer activity can be found in tomatoes [51]. Additionally, an earlier study based on a considerable multi-ethnic US population-based sample of adults indicated a connection between tomato and LYC intake and cancer mortality; specifically, they found that cancer mortality was much lower (42.5% vs. 45.9%) in the group with the highest tomato and LYC intake [102]. High dietary intake of tomatoes and LYC suggests potential beneficial effects on cancer death. Therefore, these PTs and PTFs in commonly studied coloured carotenoids and tomato-based foods are involved in structural properties and biological actions, suggesting that they can provide various health effects in addition to the effects on the skin.

Currently, inner beauty is taking the lead in the modern health-functional food market [7]. Previous studies have shown that with the consumption of carotenoid-containing foods or supplements, the deposition of carotenoids in the skin can be achieved [29]. The terms “nutritional protection” [103] or “oral photoprotection” [104] are widely used and show prevention of skin cancer [105] and “increase in skin photoprotection” [104], by consuming products and/or dietary supplements containing antioxidants for anti-aging. Extracts and powders used as supplements are already being developed and marketed [5]. Therefore, in the development of products, including nutraceuticals, PT and PTF appear to be promising for innovation, with a high potential for facilitating consumers pursuing “Inner Beauty” in the context of the nutritional supplement industry, such as nutritionally related functional foods in dietary sources. In this context, the importance and interest in PT and PTF cannot be denied, from nutritional sources, to usefulness, to the functional food and nutritional supplement industry concerning skin aging [28]. After the pandemic, consumers’ purchase intentions and actual purchase of healthy food has increased [106]. Most people do not have easy access to fruits and vegetables in their daily diet [107]. Photoprotection as a consumer product and limitation of negative environmental factors is suggested by combining diet with oral intake of products such as juices [37,75,108] or nutritional supplements such as health foods [96]. Products such as juice could be an efficient way to include fruits and vegetables in the daily diet [37].

With respect to nutritional approaches, it should also be noted that the absorption of ingestible, colourless carotenoids is slow in nature compared to the topical application of sunscreen, and it takes several weeks to reach steady-state levels of carotenoids in plasma and skin. In other words, while protection by topical sunscreens is practically instantaneous, one key aspect related to dietary photoprotection compared to topical protection is the time frame [109]. Therefore, skin photoprotection by PT and PTF and topical photoprotection with sunscreen are complementary strategies that are not competing. Instead, PT and PTF are an additional photoprotection aid that delivers effects from within the body to the skin’s outer surface [5,42]. Dietary protection can be significantly lower than using sunscreen; however, that is because enhancing basal protection can play an important role in long-term defense against UV radiation-induced skin aging and disorders [54]. Therefore, according to the distinct structural characteristics and biological actions of PT and PTF, it is suggested that overall photoprotection with nutritional approaches can make a significant contribution to skin aging and health over a lifetime and obtain additional health benefits due to various properties (Figure 7).

Table 2. The main results of studies on colourless carotenoids address their nutritional importance as consumer products.

Type of Study	Title	Key Findings	Reference
Research Article	Molecular evidence that oral supplementation with lycopene or lutein protects human skin against ultraviolet radiation: results from a double-blinded, placebo-controlled, crossover study	Consumption of products containing colourless carotenoids can make the skin appear more beautiful, healthier, and younger.	[83]
Research Article	A carotenoid algal preparation containing phytoene and phytofluene inhibited LDL oxidation in vitro	Endogenous and exogenous antioxidants are an important part of reducing problems with skin cells and oxidative stress, and colourless carotenoids are highly biologically active and have antioxidant properties in vitro and in vivo.	[45]
Research Article	Anti-inflammatory effects of CoQ10 and colorless carotenoids		[39]
Research Article	Preliminary Data on the Safety of Phytoene- and Phytofluene-Rich Products for Human Use including Topical Application		[40]
Research Article	Phytoene and Phytofluene for (Photo) Protection, Anti Aging, Lightening and Evening of Skin Tone		[41]
Research Article	The photoprotective effects of a food supplement tomato powder rich in phytoene and phytofluene, the colorless carotenoids, a preliminary study		[42]
Research Article	Effect of tomato extract supplementation against high-fat diet-induced hepatic lesions		[33]
Research Article	Serum Testosterone Is Reduced Following Short-Term Phytofluene, Lycopene, or Tomato Powder Consumption in F344 Rats		[38]
Research Article	Phytoene, phytofluene, and lycopene from tomato powder differentially accumulate in tissues of male Fisher 344 rats		[32]
Research Article	Carotenoids activate the antioxidant response element transcription system		[44]
Research Article	Daily intake of a formulated tomato drink affects carotenoid plasma and lymphocyte concentrations and improves cellular antioxidant protection		[46]
Review	Nutritional Aspects of Phytoene and Phytofluene, Carotenoid Precursors to Lycopene		[15]
Review	The colourless carotenoids phytoene and phytofluene: From dietary sources to their usefulness for the functional foods and nutricosmetics industries		[28]
Research Article	Daily intake of a formulated tomato drink affects carotenoid plasma and lymphocyte concentrations and improves cellular antioxidant protection	26 healthy subjects that consumed 250 mL of tomato beverage (providing approximately 4 mg PT, 3 mg PTF, and other carotenoids) showed a reduction in DNA damage (approximately 42%) in oxidative stress lymphocytes on day 26 in a double-blind crossover study. This suggests that the intake of PT and PTF tomato beverages can affect the immune defense response due to the reduction of DNA damage in lymphocytes under oxidative stress and enhance cell antioxidant protection.	[46]
Research Article	The photoprotective effects of a food supplement tomato powder rich in phytoene and phytofluene, the colorless carotenoids, a preliminary study	Women were given oral administration of tomato powder rich in colourless carotenoids, and at the end of the 12-week study there was a marked improvement in overall skin beauty, including visible skin health, radiance, and so on, and daily intake of these colourless carotenoids protects the skin from UV rays. That is, PT and PTF can be an additional photoprotection aid that gives effects from within the body to the skin’s outer surface.	[42]
Research Article	Anti-inflammatory effects of CoQ10 and colorless carotenoids	Ingested PT and PTF have been detected in significant amounts in both human plasma and tissues including lung, liver, skin, prostate, and breast, and when accumulated in the skin can provide nutritional benefits to the skin due to their antioxidant activities.	[39]
Research Article	Phytoene, phytofluene, and lycopene from tomato powder differentially accumulate in tissues of male Fisher 344 rats		[32]
Research Article	Daily intake of a formulated tomato drink affects carotenoid plasma and lymphocyte concentrations and improves cellular antioxidant protection		[46]
Review	Nutritional Aspects of Phytoene and Phytofluene, Carotenoid Precursors to Lycopene		[15]
Research Article	The colourless carotenoids phytoene and phytofluene: From dietary sources to their usefulness for the functional foods and nutricosmetics industries	In order to improve skin for health and/or beauty purposes, ingestion of PT- and PTF-rich products has additional benefits of improving the circulating and internal tissue levels of these health enhancing compounds.	[28]
Review	Effect of tomato consumption on fasting blood glucose and lipid profiles: A systematic review and meta-analysis of randomized controlled trials	Interventions show that the consumption of tomatoes increases high-density lipoprotein cholesterol (HDL) levels and significantly lowers cholesterol, plasma triglycerides, and low-density lipoprotein cholesterol (LDL).	[100]
Research Article	Lycopene and other carotenoids inhibit estrogenic activity of 17b-estradiol and genistein in cancer cells	PT and PTF have been portrayed as having advantageous impacts on health since they may lower the risk of certain types of cancer, such as breast and endometrial cancer, and cardiovascular diseases.	[101]
Research Article	Prostate Carcinogenesis in N-methyl-N-nitrosourea (NMU)-Testosterone-Treated Rats Fed Tomato Powder, Lycopene, or Energy-Restricted Diets	Consumption of tomato powder inhibited prostate carcinogenesis, suggesting that tomato products that contain compounds besides lycopene can modify prostate carcinogenesis. This study evidently show that tomatoes hold other compounds that can exhibition anti-cancer activity.	[51]
Research Article	A high consumption of tomato and lycopene is associated with a lower risk of cancer mortality: results from a multi-ethnic cohort	High dietary intake of tomatoes and LYC suggests potential beneficial effects on cancer death.	[102]
Research Article	Bioaccessibility of phytoene and phytofluene is superior to other carotenoids from selected fruit and vegetable juices	The dietary source of tomato juice (5 mg/250 mL juice) provided the highest amount of potentially absorbable PT and PTF. Therefore, juice can be an efficient product for intake of fruits and vegetables daily.	[37]
Research Article	Supplementation with Tomato-Based Products Increases Lycopene, Phytofluene, and Phytoene Levels in Human Serum and Protects Against UV-light-induced Erythema	Improved skin has been shown to have been achieved through the consumption of colourless carotenoid-containing foods or supplements.	[29]
Review	β-Carotene and other carotenoids in protection from sunlight	Although dietary protection in terms of SPF can be significantly lower than that achieved using sunscreen, strengthening basal protection can play an important role in long-term defense against UV radiation-induced skin aging and disorders.	[54]

summarizes the main results of studies on colourless carotenoids that address their nutritional importance as consumer products.

3.5. A Major Source of the Common Dietary Carotenoids, PT and PTF

The colourless carotenoids are present in widely consumed foods, various fruits, and vegetables. In general, tomatoes and their derivatives (pastes, canned food, ketchup, soups, juices, purees, and sauces) are popular sources of colourless carotenoids ranging in concentrations of moderate to very high [27], and PT and PTF molecule concentrations are shown to be high (0.5–2 mg/100 g of fresh weight) or very high (>2 mg/100 g of fresh weight) [24]. According to these classification-specific criteria, high or very high levels of colourless carotenoids could be found in products of tomatoes and derivatives, and carrots or apricots [27]. Diets rich in fruits and vegetables are known to be effective in increasing carotenoid concentrations [110]. However, when foods rich in PT and PTF are restricted or consumed in low amounts, the plasma concentration decreases [111]. Previous studies have shown that tomato juice is the dietary source that provides the most elevated portion of potentially absorbable PT and PTF. Tomato juice supplies the most in vitro bioavailability of PT and PTF, followed by carrot, apricot, and sanguinello juices [37]. They are found in tomatoes, carrots, apricots, and sanguinellos (at concentrations of around 2.2 and 0.9; 0.9 and 0.6; 0.4 and 0.2; and 0.1 and 0.01 mg/100 g edible portion, for PT and PTF, respectively) [37]. The highest concentration was found in tomato juice. Previous studies, conducted in Luxembourg, have made significant contributions to colourless carotenoid contents in food [74]. A recent representative sample of the Spanish adult population also reported contents of colourless carotenoids in foods commonly consumed in the country [89]. The concentrations of PT and PTF edible portions of major fruits, vegetables, and processed products sampled from both studies are shown in Figure 8.

Foods such as apricots, carrots, red peppers, tomatoes, tomato ketchup, and tomato juice have been shown to be effective in increasing concentrations of colourless carotenoids. It is not surprising that highly variable concentrations of colourless carotenoids have been found in the literature, similar to the levels of secondary metabolite content in vegetables and fruits, as both are dependent on many factors such as genotype, agronomic practices, climate, technological and/or culinary practices, etc. [28]. However, these study results emphasize the need to consider the role of colourless carotenoids with high bioaccessibility in terms of dietary nutrition and skin health [74,89]. In conclusion, PT and PTF are common dietary carotenoids [27].

3.6. Safety and Stability of Dietary Carotenoids, PT and PTF

Consumers today are interested in the sustainability of safety. Studies about the oral intake of colourless carotenoids have not currently reported any health risks [27]. More specifically, there is sufficient toxicological evidence of safety, and several tomato products, including tomato pulp powder or concentrated tomato LYC extract, have been classified as GRAS by the FDA [112]. The EFSA similarly considered the colourless carotenoid-containing “tomato oleoresin” to be safe for human consumption [113]. The EFSA and the FDA have both given positive opinions on the safety of tomato products and, based on the classification of Britton and Khachik that the content of PT and PTF is lower than that of LYC but still high, support the fact that PT and PTF are safe as food ingredients [40]. PT and PTF have also been reported in plasma at concentrations in the 0–1 μM range [31]. These facts assume carotenoids’ safety at the dietary level and that these compounds have various health benefits [40]. Therefore, we strongly support the use of rich and safe products of PT and PTF with multiple human applications with preliminary data for safety studies [40]. Additionally, today’s consumers must consider the importance of stability in foods consumed in processed form. There are three important processes to consider in the processing and storage of food: the cause of chemical degradation or physical loss of carotenoids; cell wall disruption of carotenoids resulting in dissociation of plant tissue and nutrient-substrate complexes and/or transformation into more active molecular structures; increased bioaccessibility and bioavailability [114]. Interestingly, recent studies on the process stability of PT and PTF were performed with tomato matrices of high-water content, such as tomato juice and tomato pulp. Tangerine tomato juices (5.1 Brix, pH 4.3) were prepared with 0%, 1%, 2%, or 3% olive oil (w/w), heated (88 °C), and stowed for 0, 30, 60, or 120 min [36]. Sauces were extracted from the freeze-dried juices reconstituted with fluid to 12.7° Brix and thereafter re-processed with olive oil (w/w) of 0%, 1%, 5%, 15%, or 30% at 100 °C for 0, 30, 60, 120 or 180 min, and followed to show that boiling the sauces for 180 min did not display significant changes in the levels of PT and PTF [36]. In the tomato matrix, the total amount of carotenoids and lycopene decreased with increasing heating time, whereas PT and PTF remained stable and unchanged [36]. In the tomato matrix, the total amount of carotenoids and LYC decreased with increasing heating time, whereas PT and PTF remained stable and unchanged [36]. Thus, they show favorable stability under more extreme conditions [35,36,66,115,116] and may offer clear advantages, including presumable safety, as they are consumed continuously in the diet of people worldwide [117].

3.7. Limitations of the Present Review and Future Studies

Evidence is accumulating and well-established regarding the advantageous effects of colourless carotenoids for skin photoprotection, including their presence in plasma, skin, and other tissues, and their involvement in several health benefits. However, unlike the other major carotenoids found in humans, abundant data on their presence in foods and tissues is significantly lacking [5]. In addition, data on the content of carotenoids in foods are still scarce even though biological activities and intakes of PT and PTF are sometimes higher and comparable to those of other common carotenoids in human health studies [27,50]. It is necessary to consider including daily intake or bioavailability, to gain insight into the content of colourless carotenoids. Therefore, more studies providing empirical data on these colourless carotenoids from a food science and nutritional point of view, and studies concerning daily intake, bioavailability, and effects on systemic metabolism, should be further encouraged. In our daily life, sunscreen is generally the method of choice [5], but dietary carotenoids complement topical photoprotective agents, and photoprotection through this dietary intervention suggests a sustainable potential as an effective alternative strategy for skin aging and health promotion. Dietary photoprotection in terms of SPF can be significantly lower than that achieved using sunscreen; however, that is because enhancing basal protection can play an important role in long-term defense against UV radiation-induced skin aging and disorders [5,19]. Therefore, it is necessary to study the mechanism of the beneficial effects of colourless carotenoids on skin aging and health from a nutritional point of view [5]. Future clinical studies are much needed to further address whether and to what extent it can alleviate or prevent outcomes such as photoaging beyond UV radiation-induced erythema and pigmentation. Relative examination of the absorption of PT and PTF in comparison/contrast with coloured carotenoids would also be valuable for future studies.

4. Conclusions

Traditionally neglected, PT and PTF are the major dietary carotenoids present in widely consumed foods and an area that is currently expanding. This review demonstrates that dietary interventions of colourless carotenoids, PT and PTF, according to their distinct structural and biological actions, maximize the absorption of damaging light in the UV range unlike coloured carotenoids which have a maximum absorption in the visible range. Therefore, it was confirmed that they can contribute to the photoprotection of human skin. Furthermore, in addition to skin health, their distinct chemical structures and biological actions are attributed to antioxidant, anti-inflammatory, and anticancer activities and suggest the sustainable potential of PT and PTF to provide various additional health benefits, such as improving the immune system, cardiovascular health, and some cancers. “Healthy and beautiful skin starts from within”. Reflecting endogenous nutrients is an interesting aspect to prevent photoaging symptoms which is the main cause of skin aging. It is hoped that dietary interventions of PT and PTF, along with topical photoprotection by sunscreen, will become better known to consumers as a nutritional approach to long-term basal protection/defense and complementary photoprotection and skin health strategies. PT and PTF which are advantageous to safety and stability are also components of products that promote “Inner Beauty”, suggesting potential for innovative product development. This review emphasizes the sustainable role and the distinct structure and biological action of the dietary carotenoids PT and PTF in photoprotection and skin health.