1. Introduction
Fruits and vegetables (FVs) are important components of a healthy diet. Based on the data reported by the World Health Organization (WHO), around 3.9 million deaths were associated with inadequate FV intake in 2017 worldwide [
1]. In addition, a low intake of FVs has been linked to the development of chronic non-communicable diseases [
1,
2,
3,
4,
5]. Indeed, FV is characterized by a low content of saturated fats, adequate fiber content, and a high amount of vitamins along with a broad spectrum of non-nutrient molecules such as plant sterols and flavonoids, which exert important health benefits [
6,
7,
8,
9,
10]. Carotenoids, a group of natural fat-soluble pigments of the polyene type produced by plants and algae, are one of the most representative classes of active compounds found in FV [
11,
12,
13], influencing the color of FV as well as their beneficial actions [
14,
15,
16]. Although more than 700 carotenoids occur in nature, only 6, including β-carotene, β-cryptoxanthin, α-carotene, lycopene, lutein, and zeaxanthin, have been found in the diet and represent more than 95% of total blood carotenoids [
17]. Since humans are not able to endogenously produce carotenoids [
18], they primarily obtain these biomolecules via dietary sources, with FV serving as a primary contributor to their intake [
19]. Thus, the total serum carotenoid levels represent the best biomarker of FV intake from the diet [
20,
21]. The evaluation of carotenoids in the body is based on the collection of plasma or serum samples and subsequent analysis via high-performance liquid chromatography (HPLC) with mass spectrometry (MS) [
22,
23]. However, this serum evaluation of carotenoid levels is an invasive and expensive method. In addition, the short half-life of carotenoids reduces the accuracy of the measurements. Therefore, more suitable methods for the evaluation of carotenoids are developing.
In this scenario, the assessment of skin carotenoids using the Veggie Meter
® is becoming an interesting accurate tool to evaluate the carotenoid content in the human body. In fact, the Veggie Meter
® is an innovative portable device that uses reflection spectroscopy and pressure-mediated reflection to determine the content of skin carotenoids in the fingertip in a single reading of 15 s. The Veggie Meter
® is connected to a computer, which shows the skin carotenoid score from 0 to 800 [
24], allowing evaluation in a less expensive and complex procedure compared to other traditional analytic methods used for the skin carotenoid measurement.
Several studies have reported that the number of carotenoids in the skin is highly correlated with the serum carotenoid concentration and reflects the FV dietary intake in the previous 2–4 weeks [
25]. Thus, Veggie Meter
® represents a feasible, reliable, and potentially valid measure of FV consumption [
26,
27,
28,
29]. Although high FV consumption is recommended for healthy benefits, a reduced intake of FV has been observed in the population, including adolescents [
30,
31,
32]. This is alarming, as dietary patterns established during childhood and adolescence are likely to continue into adulthood [
33]. In this context, different initiatives have been taken to promote and evaluate FV consumption among children and adolescents in schools, including developing nutritional programs based on dietary patterns that suggest a high FV intake. A recent pilot study has been carried out in a school children population to assess the intake of FV via Veggie Meter
® measurement, which resulted in an objective evaluation of FV consumption with respect to their self-reported intake [
34]. Regarding the promotion of FV consumption, the Mediterranean diet (MD) is considered one of the healthiest dietary patterns based on the consumption of plant-origin foods [
35]. The term MD describes the eating habits and lifestyle of countries from the Mediterranean Basin, mostly Greece, Cyprus, and Southern Italy, where the food cultures of ancient civilizations developed. MD model is based on the predominant consumption of plant foods such as cereals, fruits, vegetables, legumes, nuts, and seeds; a regular intake of olive oil, as the main source of fat; a moderate consumption of white meat, fish, seafood, low-fat dairy, and red wine; and a low intake of red meat, processed meat products, and sweets [
36]. Thus, according to the MD recommendation, every meal should include two or more servings of vegetables and three or more servings of fruits. Several food frequency questionnaires are currently available to assess adherence to the MD, although the data do not always have acceptable validity and good reliability.
The Mediterranean Diet Quality Index (KIDMED) is one of the most frequently used scores to assess adherence to the MD pattern among children and adolescents in different countries from the Mediterranean and non-Mediterranean areas [
37,
38,
39,
40,
41]. In our recent investigation, we used the KIDMED test to evaluate adherence to the MD in a cohort of adolescents living in a Southern Italy region [
42]. According to other studies carried out in the Mediterranean countries [
43,
44], we found, in our population sample, a medium adherence to the MD, and, specifically, the compliance rates for the intake of more vegetables a day and a second fruit each day were definitively outside the recommendations [
42], suggesting the need to promote the beneficial effects of FV intake.
Another tool used to investigate the food frequency consumption among the young population is represented by the National Youth Physical Activity and Nutrition Study (NYPANS) questionnaire, which measured the prevalence of determinants related to physical activity and nutrition among high school students [
45].
Here, we evaluated the adherence to the MD via the administration of two online food questionnaires, and we measured the content of skin carotenoids using the Veggie Meter® in a population of Calabrian healthy school adolescents enrolled in the context of the Pre.Di.Re study.
4. Discussion
In this study, we used the Veggie Meter
® as a non-invasive tool for the detection of the skin carotenoid content in a population of healthy Calabrian adolescents, and we demonstrated for the first time, to our knowledge, that the carotenoid score was positively associated with the adherence to the MD pattern. The MD represents one of the healthiest eating and lifestyle patterns worldwide based on the high consumption of local and seasonal products of plant origin, including FV. It has been widely demonstrated that good adherence to the MD decreases the development and progression of several metabolic and chronic-degenerative diseases, including type 2 diabetes, obesity, cardiovascular and neurodegenerative diseases, metabolic syndrome, and different types of cancer [
10], and improves the quality of life [
42,
46]. To date, adherence to the MD is assessed via dietary scores based on food consumption questionnaires.
Here, using the KIDMED and the MD Pyramid tests as two different MD pattern-consistent and pattern-inconsistent food consumption questionnaires, we found a medium MD adherence in our sample population. These findings are in agreement with the results from our and other studies in which adolescents living in the Mediterranean regions of Southern Italy showed average adherence to the MD [
42,
43]. In our work, we observed a higher adherence to the MD in the male than in the female population. In contrast, Morelli et al. have reported that the medium MD adherence evaluated using the KIDMED test for the total adolescent sample population of Southern Italy was independent of sex [
42]. In the same geographical area, no gender-related differences were found in MD adherence in an adult population of Southern Italy [
48]. Similarly, Raparelli et al. have demonstrated that sex did not impact MD adherence in an adult population with ischemic heart disease [
49]. It has been demonstrated that optimal adherence to the MD has a positive effect on health, preventing the development of non-communicable diseases [
8,
46,
50,
51]. Augimeri et al. have found that adolescents with optimal adherence to the MD had higher estimated consumption of polyphenols calculated by FV intake and displayed a better serum lipid profile with respect to adolescents consuming low polyphenol amounts [
8]. Interestingly, the serum from subjects who optimally adhered to MD recommendations was able to prevent lipid accumulation in hepatic cells and to reduce the production of proinflammatory cytokines in inflammatory macrophages in vitro with respect to lower MD adherers, supporting the importance of enhancing the adherence to the MD pattern toward an optimal compliance [
8,
51]. Thus, campaigns to promote MD adherence are needed, especially in the young population, in order to prevent the development of chronic degenerative diseases in adulthood.
Although it has been widely demonstrated that dietary scores are validated tools that establish adherence to the MD, the heterogeneity of the MD adherence scores highlights the importance of developing innovative methods for a more accurate interpretation of MD adherence. Based on the MD recommendations, daily FV consumption is suggested to provide an adequate intake of micronutrients. To date, the intake of FV is evaluated by self-reported FV intake or by measuring the blood carotenoid levels via an invasive and expensive liquid chromatography/mass spectrometry-based analytic method [
23,
52]. The use of the Veggie Meter
® has been already described as a validated and non-invasive method for skin carotenoid detection, which accurately reflects the FV intake [
25,
33,
34,
53]. Therefore, it represents an important tool for an objective measurement of FV intake to overcome bias associated with self-reported questionnaires, allowing us to easily analyze the FV intake in community-based settings. According to the data reported in the literature, we found that the content of the skin carotenoids was positively associated with the FV intake evaluated using both the KIDMED and the Pyramid tests in our adolescents.
Interestingly, we found a mean carotenoid score of 364.75 in our sample population, which is within the normal range and directly related to MD adherence. However, the carotenoid score in our adolescents was higher compared to that observed in the American adolescent population, potentially due to an increased FV consumption [
54]. Furthermore, we highlighted significantly higher values of skin carotenoids in the male compared to the female population. Conflicting data are present in the literature regarding the correlation between carotenoid levels and gender. In fact, some authors did not detect a gender difference in the carotenoid score [
34,
55], whereas Obana et al. reported higher carotenoid levels in females than in males [
56]. Further studies are needed in order to understand whether gender plays a significant role in the accumulation of and variation in the skin carotenoid content and to establish a clearer relationship between gender and carotenoid status. Although the body’s carotenoid content depends on their dietary intake, some anthropometric parameters and lifestyle habits can influence their accumulation in the tissues. In agreement with data present in the literature [
55], in our study, we identified a negative correlation between the carotenoid score and BMI, which was also confirmed in a multiple regression model with age and sex. Collectively, we demonstrated, for the first time, that the skin carotenoid content measured with the Veggie Meter
® is positively associated with adherence to the MD evaluated via two MD food questionnaires, suggesting that it represents a promising tool to assess MD adherence.
This study has several limitations. All adolescents were volunteers and enrolled without exclusion criteria. We also did not formally ask about health problems, drug use, and any type of restrictive diet or digestive problems, which could impair the absorption of carotenoids because this was not part of the ethical approval. Finally, some FV do not contain carotenoids, and thus, the Veggie Meter® failed in their detection.