Phytochemicals are specialized metabolites with biological properties stored in plant tissues, such as roots, stems, leaves, flowers, and fruits [1]. Phytochemicals include a wide range of compounds such as polyphenols, carotenoids, flavonoids, coumarins, terpenoids, glucosinolates, saponins, and capsaicinoids, which are often associated with the vibrant colors of fruits and vegetables [2]. Although phytochemicals are not essential nutrients in plants, they are responsible for many health benefits associated with a plant-based diet ^[3][4][3,4]. Potential phytochemical health benefits include antioxidant, anti-inflammatory, anti-cancer, and anti-microbial properties [5]. Currently, numerous phytochemicals are being studied for their possible use in developing novel drugs and dietary supplements [6].

Phytochemicals exhibit their beneficial and harmful effects by interaction with multiple cell signaling molecules [7]. Nevertheless, the molecular mechanisms underlying the effectiveness of phytochemicals continue to grow. In this context, employing cost-effective, rapid, reliable, and efficient in vitro and in vivo assays will facilitate analyzing these compounds’ metabolic processes, dosage response, and pharmacological and toxicological profiles ^[8][9][10][8,9,10]. D. melanogaster, known as the fruit fly, has emerged as an alternate animal model. Due to its short lifespan, small size, and well-understood genome, it has been widely used to examine the efficacy and safety of phytochemicals on various physiological processes, including metabolism, aging, and immunity ^[11][12][11,12].

2. Phytochemicals and Their Potential Therapeutic Benefits

Regular consumption of fruits, vegetables, and grains has been associated with a reduced risk of certain chronic diseases due to phytochemicals with antioxidant and anti-inflammatory properties ^[13][19]. Phytochemicals regulate oxidative stress, which has been recognized as a significant factor in the pathogenesis of metabolic disorders and cancer ^[14][20]. Phytochemical therapeutic benefits are divided mainly into five categories (i) enhancers of the body’s immune system; (ii) preventers of diabetes and heart diseases; (iii) hypocholesterolemic agents; (iv) promoters of digestion and absorption; and (v) retardants of aging ^[15][16][21,22]. The major classes of phytochemicals with disease-preventing functions are dietary fiber, antioxidants, detoxifying agents, immunity-potentiating, and neuropharmacological agents ^[17][23]. For instance, polyphenols such as resveratrol have been associated with decreased risk of myocardial infarction, stroke, and diabetes ^[18][19][24,25]. Polyphenols in diet also help to improve lipid profiles, blood pressure, insulin resistance, and systemic inflammation ^[20][26]. Furthermore, vitamin C and carotenoids may benefit immune function, thereby reducing cancer risk by enhancing the tumor surveillance of the immune system ^[21][27]. Capsaicinoids, including capsaicin and dihydrocapsaicin found in peppers, have been found to have beneficial roles in humans, including managing pain inflammation during rheumatoid arthritis, anti-cancer agent by generating reactive oxygen species, and in the prevention or treatment of neurodegenerative diseases such as Alzheimer’s disease due to its antioxidant activity ^[22][23][24][28,29,30].  Other spices, such as turmeric obtained from the roots of Curcuma domestica, contain a yellow coloring principle, curcumin, a powerful antioxidant that can offer protection against cancer, inhibiting lipid peroxide-induced DNA damage ^[25][35]. Flavonoid consumption, such as quercetin and kaemferol, through vegetables and fruits, reduces the risk of death from coronary heart disease ^[26][36]. 

3. Advantages of Using Drosophila as a Translational Model for Testing Phytochemicals

For decades, D. melanogaster has been widely used as an excellent animal model to study genetics, evolution, and developmental biology ^[27][39]. It is a cost-effective option due to its high reproductive rate (30–50 eggs/day), short generation time (approximately ten days at 25 °C), and low maintenance cost. In addition, Drosophila short lifespan (average three months at 25 °C) and easy generation of large populations allow for performing longevity and lifespan assays in only a few months ^[28][40]. Furthermore, it offers powerful molecular and genetic tools that permit gene overexpression or knock-down studies ^[29][41]. Although Drosophila is evolutionarily distant from humans, fly development, physiological, biological, and metabolic processes are equivalent to many of those found in mammals. Recently, similarities between humans and fruit flies in terms of metabolic regulation, including the role of insulin signaling, nutrient sensing, and energy homeostasis in metabolic disorders, such as diabetes and obesity ^{[30][31][32][33]}[42,43,44,45]. Moreover, ingesting complex foods rich in phytochemicals by an organism leads to the degradation of nutrients that directly affect the gastrointestinal microbiome because the host and microbiome share the same food source ^[34][46]. These microbiome changes influence the organism’s phenotype and behavior by altering the genome, transcriptome, epigenome, proteome, and metabolome ^[35][47].

4. Methods for Testing the Efficacy of Phytochemicals in Drosophila

Phytochemical ingestion effects are observed in general physiology, including metabolism, behavior, stress resistance, reproductive capacity, nervous system, and immune capacity in both Drosophila and humans, and aspects of these physiological changes can be used as parameters to determine the phytochemical effects and toxicity ^[36][50]. Among them, lifespan and survival rate are simple and efficient longitudinal assays to determine the effects after administration of the candidate plant-based compounds ^[37][53]. In addition, various strains with different lifespan characteristics and transgenic flies with symptoms similar to human diseases are available to evaluate plant compounds’ effects on mortality rate. Oxidative stress mitigation in Drosophila is also an essential parameter in assessing the efficacy of phytochemicals. Levels of reactive oxygen species (ROS), activities of antioxidant enzymes, such as superoxide dismutase and catalase, and lipid peroxidation in treated fly cells provide insight into the potential antioxidative activities of phytochemicals (Figure 1) ^[38][56]. Moreover, Drosophila has a relatively simple nervous system, making it an ideal model to study neurodevelopment and neurodegeneration using automated tracking systems, locomotor activity, ring assay, gustatory, social, and circadian rhythm patterns, providing insight into potential impacts of phytochemicals on the nervous system function and behavior ^[39][57].