Citrus has been proposed as an interesting ingredient in the elaboration of food products as soft drinks due to its distinctive aroma and high nutritive value. It is a rich source of nutrients that contains higher amounts of vitamin C, citric acid, minerals, and flavonoids, especially flavanones and flavones (reaching values of 400–600 mg/L) and in lesser amounts flavonols and hydroxycinnamic acids. Citrus flavonoids decrease capillary permeability and are beneficial in the treatment of vascular diseases. Scientific studies suggest that the ingestion of food products based on citrus fruits improves the blood lipid profile, reduces oxidative stress, prevents atherogenic modifications of LDL and platelet aggregation, as well as contributes to the improvement of HDL levels. Other benefits attributed to citrus are antiaging, anticancer, neuroprotective, and antidiabetic. The present revision tries to empathize the most relevant studies regarding citrus and health.
- cardiovascular diseases
There are numerous evidences supporting the crucial influence of diet in the prevention of diseases related to oxidative and inflammatory processes. Citrus are one of the most important foods included in a healthy lifestyle, due to their composition in bioactive compounds. The biological activity of citrus bioactive compounds is mainly their free radical scavenging property, increasing the antioxidant activity which closely related to disease prevention.
Healthy properties of citrus have been linked to its high vitamin content C and flavonoids, mainly attributed to its antioxidant capacity. Citrus are considered adjuvant in the prevention of cardiovascular diseases and metabolic diseases such as obesity, diabetes mellitus or dyslipidemia, as well as certain types of cancer. In citrus (particularly lemon), more than 60 individual flavonoids have been identified.
2. Citrus and their role in different pathologies
The health benefits described with the consumption of these fruits are related to their complete profile on nutrients, including simple sugars, fiber, potassium, high contents of vitamin C and phytochemicals as flavonoids, particularly flavanones that may act synergistically. They are low in fat and proteins, ranging from 0.1 to 0.3 g and from 0.69 to 0.94 g/100 g fresh weight, respectively. Citrus are particularly rich in vitamin C (ascorbic acid), providing amounts in the range of 23–83 g/100 g fresh weight. Considering that the Recommended Dietary Allowance (RDA) is set at 75–80 mg and a medium-sized orange or grapefruit contains from 50 to 70 mg ascorbic acid, it is easy to provide the necessary quantities with these fruits in a daily dietary pattern .
Micronutrients are secondary metabolites synthesized in the plant as a defense mechanism against pathogens, parasites, or to protect from UV radiation. We find two main groups in citrus fruits: terpenes and flavanones.
Terpenes are present in the essential aromatic oil produced by cells in the flavedo, and the main compounds are limonene and citral (mixture of isomers geranial and neral) (Figure 1). These volatile substances contribute to the flavor of citrics; similar to the protection effect against biotic stress in plants, they have shown antimicrobial activities interesting for food preservation and medicinal purposes [2, 3].
Besides, citrus fruits are especially rich in the flavanones hesperetin, naringenin, and eriodictyol . Flavanones have the characteristic 15-carbon backbone ring structure common in the flavonoids (C6▬C3▬C6), consisting of two aromatic rings linked by three carbon atoms in an oxygenated structure as pirane derivative . In particular, flavanones have a further degree of oxidation, with a ketone group at position C-4 in C-ring.
These compounds are mainly found glycosylated, with a disaccharide linked by glycosidic bond; common positions are the hydroxyl groups of C3 and C7. The free form (aglycone) can render different flavanones, depending on the position and type of sugar linked. In this sense, grapefruit is abundant in narirutin and naringin, which are both heterosides from the aglycone naringenin, but the glucose moiety is different (rutinoside or neohesperoside, respectively). Orange is rich in hesperidin, that is the glycoside of hesperetin, while lemon is rich in eriocitrin that contains the aglycone eriodictyol .
Flavanones are not uniformly distributed in the fruit but are more abundant in the albedo. Because this part is discarded in juice processing, the level of flavanones is lower in citrus juices than in the whole fresh fruit . In fact, levels of in orange fruit range between 35 and 147 mg/100 g of total flavanones and 44 and 106 mg/100 g of naringin and narirutin in grapefruit . By contrast, orange and grapefruit juices showed values between 10 and 80 mg/100 g of hesperidin and narirutin  and naringenin . Therefore, the pattern of consumption of these fruits greatly affects the flavanone total intake and further biological activities. Moreover, the sugar moiety modifies the in vivo pharmacokinetic properties of the compound. Aglycones are more easily absorbed than their heterosides counterparts, as glycosides are more hydrophilic and need active transport by proteins in gastrointestinal lumen and/or hydrolysis by gut microflora in order to be absorbed .
2.1. Citrus and cancer
Flavonoids are major compounds in citrus, and have been investigated since more than 20 years ago. Dietary flavonoids have showed to be able to exert chemopreventive or anticancer capacity . Anticancer capacity of citrus flavonoids takes place through three main mechanisms: defense against DNA injury, inhibition of tumor growth, and inhibition of cell proliferation .
The best anticancer compound must exert the most possible inhibition of tumor growth or to able to destroy cancer cells, but origins minimum adverse health collateral effects . Flavonoids are natural and considered innocuous and great compounds for the treatment of cancerous processes [15, 16]. The synthetic molecules that can be used for the treatment of cancer are extremely noxious, and can be able to destroy healthy cells. Due to the safe long-term consumption of flavonoids, and their innate biological activity, flavonoids can be considered as good applicants regarding cancer treatment. In fact, scientific literature has revealed cytotoxic effects of citrus flavonoids on cancer cells, with slightest adverse health effects.
That fact has led the research in order to implement flavonoid-based cancer treatments . As other polyphenols, the presence of aromatic rings in flavonoids leads to pro- and antioxidant capacity that can be useful for chemotherapies . Cancerous cells show an increment on oxidative stress, which leads to the possibility to be attacked by a substance that improves reactive oxygen species level as flavonoids do [19, 20]. As defined by Pacifico et al. , pro- or antioxidant capacity of citrus flavonoids is dependent on the concentration, type of cell, and culture condition (in vitro models).
Flavonoids exert DNA protection by their ability to absorb ultraviolet light. Some experiments on a UV-irradiated model of plasmidic DNA indicate protecting capacity of naringenin and rutin against UV-induced damage of DNA . Indeed, naringin plays an important role in regulating antioxidative capacity by increasing superoxide dismutase and catalase activities and by upregulating the gene expressions of superoxide dismutase, catalase, and glutathione peroxidase in cholesterol-rich diet-fed rabbits .
Apart from UV protection, flavonoids can also diminish tumor promotion at the beginning of carcinogenesis by the intensification of the detoxification processes. In particular, citrus flavonoids inhibit ornithine decarboxylase induction of skin tumor promotion, activating protein kinase C [24, 25]. Miller et al.  studied the inhibition of oral carcinogenesis by citrus flavonoids in hamsters and the antineoplastic activity, concluding that hesperetin, neohesperetin, tangeretin, and nobiletin were ineffective, while naringin and naringenin gave good results.
Citrus flavonoids can inhibit invasion, by rat malignant cells, in cardiac and hepatic tissue of syngenetic rats . Hydroxycinnamates, glycosylated flavonoids, and the polymethoxylated flavones have shown inhibitory activity on several tumoral cell line proliferations . Other studies showed eriocitrin and its aglycone, eriodictyol, as potent inhibitors of lipoxygenases, which are involved in the biosynthesis of various bioregulators that are closely related to the pathogenesis of several diseases such as allergy and atherosclerosis and cancer . Hesperidin in different citrus juices also showed antiproliferative activity , reporting lemon in particular potent antiproliferative activities on HepG2 human liver-cancer cell in a dose-dependent manner .
Also, the positive effect of vitamin C in reducing the incidence of stomach cancer has been studied, being most probably due to the inhibitory action in the generation of nitrous compounds by interrupting the reaction between nitrites and amine groups , although it has recently shown that this effect may be due to a cytotoxic effect of vitamin C on human gastric cancer cell line AGS . Consistent protective effect of vitamin C has also been found in lung and colorectal cancer .
One stretched revision done by Turati  reported a diminution on cancers of the digestive tract and larynx regarding high intake of citrus. That effect was found to be due to the content on vitamin C, flavanones, and other compounds with antioxidant, antimutagenic, and antiproliferative properties . Subjects consuming more than one portion of citrus fruit per week showed OR between 0·42 and 0·82 for oral cavity and pharyngeal cancer, esophageal cancer, stomach cancer, colorectal cancer, and laryngeal cancer. However, despite the good results obtained, no correlation was found for other neoplasms, including cancers of breast, ovary, endometrium, prostate, or kidney .
The most recent and huge research about citrus and cancer was published in 2016. An adaptive meta-analysis of cohort studies revealed that regular dietary intake of citrus prevents the development of gastric cancer, particularly cardia gastric cancer .
2.2. Citrus and cardiovascular diseases
Cardiovascular diseases (CVD) are one of the main causes of illness and death in Western countries, and cardiovascular drugs are the most commonly used medications. There are two types of factors involved in the development of CVD. Some factor can be modified, like life style, diet, environment, or smoking. Other cannot be modified: genetic factors, gender, history, or age. Atherosclerotic plaque formation is the most common phenomenon involved in CVD .
Consumption of citrus is inversely associated with incidence of CVD, due to the presence of bioactive compounds like flavonoids. Current research has focused on diet containing bioactive compounds, as an alternative to pharmaceutical medication. It can be concluded from the analysis of multiple studies that as the mean consumption of flavonoids increases, mortality due to CVD decreases. Epidemiological evidence of clinical and preclinical studies suggest that flavanones present in the citrus fruits positively influence cardiac and metabolic parameters, preventing CVD .
A study performed on approximately 70.000 women highlighted an inverse correlation between the intake of flavanones and the risk of suffering a cerebral ischemia, which is significantly different when contemplate women who consume high levels of flavanones . Another recent meta-analysis study of three randomized clinical trials, including 233 patients, demonstrated a correlation between flavanones intake and a reduction in blood pressure .
Another recognized cardiovascular risk factor is metabolic syndrome, characterized by altered glucose metabolism, elevated blood pressure, dyslipidemia, and obesity. In 2016, a study on 10,000 subjects demonstrated an inverse association between polyphenols and metabolic syndrome, which was particularly significant in individuals with the highest intake of polyphenols .
Several studies carried out so far support a preventive role of citrus fruits on the main risk factors of CVD, such as hypertension, dyslipidemia, overweight, and hyperglycemia. Among CVDs, the effect of flavonoids on stroke is not clear. Mursu et al.  studied the association between intake of flavonoid and risk of stroke and mortality caused by stroke and concluded that a greater intake of flavonoids decreases the chances of ischemic stroke as well as mortality caused by CVD.
Chronic inflammation is caused by the excessive production of chemokines and cytokines. Cytokines and chemokines act as regulatory proteins under normal physiological conditions, but their excessive production disrupts the gradient balance and more reactive oxygen species (ROS) are produced. It has been shown that the grape flavonoids control chronic inflammation by reducing ROS level and by modulating pathways of inflammation. As flavonoids are natural compounds, they can target multiple steps in the inflammation pathway as compared to monotargeted synthetic anti-inflammatory drugs .
Atherosclerosis, characterized by the plaque formation in arteries, is one of the major factors contributing to incidence of stroke and myocardial infarction. It is caused by high level of lipoprotein and cholesterol in plasma . High intake of citrus flavonoids reduces several risk factors for development of atherosclerosis including: high tolerance to glucose, maintaining good body mass index, and lowering blood pressure .
In another study, patients with metabolic syndrome had reduced cholesterol and ApoB due to the intake of a supplement of hesperidin for 3 weeks . Furthermore, in a 2012 clinical study performed in our laboratory on patients with metabolic syndrome diagnosed, after 4 or 6 months drinking a citrus fruit juice, the glycemic profile was unchanged but the lipid profile improved, as observed by decrease in the cholesterol, LDL-C, and C-reactive protein .
Naringenin plays an important role to overcome the metabolic problem that is connected to dyslipidemia and resistance to insulin. It was shown to prevent atherosclerosis development in mice fed a high fat diet. Naringenin treatment attenuated the adverse effects caused by hyperinsulinemia and hyperlipidemia which was induced by western style diet. In mice that were fed a western diet, hyperlipidemia led to development of atherosclerosis in the aortic sinus evidenced by the development of plaque is that increased 10 times as compared to chow-fed animals. Naringenin treatment decreased the incidence of atherosclerosis by 70% .
A clinical study with 500 mg of naringin plus 800 mg of hesperidin did not show a significant improvement in the lipid profile in patients with moderate hypercholesterolemia. This study suggests that citrus flavonoids have no effect on LDL-C in humans, at least not when consumed in a capsule format . A plausible explication of this results could be the interindividuals variability of pharmacokinetic parameters. Despite preclinical results are clearer, further clinical studies need to be performed.
2.3. Citrus and diabetes
Diabetes is a chronic disease in which metabolic alterations of multiple etiologies characterized by chronic hyperglycemia and disorders in the metabolism of carbohydrates, fats, and proteins occur. These alterations are the result of defects in the secretion of insulin, in the action itself or in both. The long-term manifestation of insulin results in damage and dysfunction of various organs like nerves, kidneys, eyes, blood vessels, and heart. People living with diabetes have a higher risk of morbidity and mortality than the general population .
Diabetes is an important public health problem, one of four priority noncommunicable diseases (NCDs) targeted for action by world leaders. Both the number of cases and the prevalence of diabetes have been steadily increasing over the past few decades .
A recent report on diabetes by the World Health Organization estimates that 422 million cases in 2014 , and an expected number of nearly 650 million subjects in 2040 was estimated . This dramatic rise is largely due to type 2 diabetes (T2D).
The treatment of diabetes consists of pharmacological, dietary, and lifestyle measures. Many trials have effectively tested different lifestyle and pharmacological intervention methods both in terms of prevention and treatment .
The use of plants with antidiabetic properties is widely known and described in the scientific literature. A lot of studies have reported that either plant parts or extracts of plant parts possess antidiabetic properties. This antidiabetic activity of plants is due to the presence of phytochemicals which are termed as flavonoids. In this way, several studies reported antidiabetic activities of flavonoids [51, 52].
Citrus fruits are one of the most consumed fruits mainly as fresh or raw materials for juices. Additionally, citrus fruits can also be used in the food, beverage, cosmetic, and pharmaceutical industries .
Citrus fruits show several bioactivities of vital importance to human health, like antioxidative and anti-inflammatory activity, cardiovascular protective effects, antidiabetic activity, among others. Citrus species contain a number of secondary metabolites, such as flavonoids, alkaloids, coumarins, limonoids, carotenoids, phenol acids, and essential oils . Of all of them, flavonoids (especially flavanone, flavanonol, and methoxylated flavones) are more active compared to other secondary metabolites in citrus for their remarkable various bioactivities. There are a lot of studies where have been widely reported on plentiful bioactivities from flavonoids.
Flavonoids, a group of natural substances with variable phenolic structures, are well known for their beneficial effects on health. Flavonoids are now considered as an indispensable component in a variety of nutraceutical, pharmaceutical, medicinal, and cosmetic applications . Flavonoids are distinct based on structural characteristics in the following six subclasses: flavonols, flavones, isoflavones, flavanones, anthocyanins, and flavanols (catechins and proanthocyanidins) . In Citrus genus, flavanones comprise approximately 95% of the total flavonoids, and these foods are the main source of flavanones .
Citrus flavanones are glycosylated in vegetables. The same aglycone can be combined with several glycosides to give different flavanones; for example, the most representative flavanones in grapefruit are narirutin and naringin, those in orange fruit are hesperidin and narirutin, and that in lemon is eriocitrin . Naringin, naringenin, nobiletin, narirutin, and hesperidin are the most important flavonoids thus far isolated from citrus fruits .
There has been a substantial body of evidence suggesting that oxidative stress is a key mechanism in pathogenesis of diabetes. Flavanones and flavanones-rich botanical extracts have been a subject of great interest for scientific research. Citrus flavanones like naringin and hesperidin exert a variety of biological activities such as antioxidant, anti-inflammatory, antihyperglycemic, antiapoptotic, etc. Naringin and hesperidin along with their respective aglycones, naringenin, and hesperetin have been shown to attenuate diabetes and its related complications . In this way, Ashafaq et al.  demonstrated that hesperidin treatment significantly attenuated the altered levels of oxidative stress and neurotoxicity biomarkers. Their results demonstrate that hesperidin exhibits potent antioxidant and neuroprotective effects on the brain tissue against the diabetic oxidative damage in STZ-induced rodent model.
Iskuender et al. observed that after administration of hesperidin and quercetin in STZ-induced diabetic rats, glucose levels increased and liver and kidney damage markers decreased significantly . In the same way, Akiyama et al.  demonstrated that hesperidin normalizes blood glucose by altering the activity of glucose-regulating enzymes, and lowering serum and liver lipid levels in STZ-induced marginal type 1 diabetic rats without any body weight loss due to STZ injection. Thus, hesperidin showed both hypoglycemic and hypolipidemic effects.
In a study, Gupta et al.  demonstrate the dipeptidyl peprtidase-4 (DPP-4) inhibition activity of citrus bioflavonoid nutraceuticals as compared to known gliptins (oral antidiabetic agents). The naringin and hesperidin compounds have the best individual activity in comparison to that of the gliptins. Natural gliptin-like alternatives may make these supplements a promising group of natural products for use in improving blood glucose levels in prediabetes and early stages of type 2 diabetes.
The hypoglycemic effect of naringin and naringenin is very well documented in animal and cell studies. So, naringin (30 mg/kg) and vitamin C (50 mg/kg) cotreatment ameliorated streptozotocin-induced diabetes in rats by improving insulin concentration and prevented oxidative stress . Naringenin supplementation (0.2 g/kg of diet) improved glucose intolerance and insulin resistance in a model of high-fat-diet-fed mice . More research is needed to determine the mechanism by which naringenin has hypoglycemic effect. So far, some authors have suggested the following: that is mediated via uptake of glucose in the skeletal muscle ; increased activities of hexokinase ; decreased production and expression of IL-1b, IL-6, and MCP-1 .
Rutin is another flavonoid present in citrus fruits to which many biological activities have been attributed, among them having antihyperglycemic properties. In 2017, Ghorbani  in a review discussed the antihyperglycemic property of rutin. Proposed mechanisms for this effect include a decrease of carbohydrates absorption from the small intestine, inhibition of tissue gluconeogenesis, an increase of tissue glucose uptake, stimulation of insulin secretion from beta cells, and protecting Langerhans islet against degeneration. Rutin also decreases the formation of sorbitol, reactive oxygen species, advanced glycation end product precursors, and inflammatory cytokines.
In conclusion, it can be affirmed that flavonoids are useful in the prevention and treatment of diabetes, especially in diabetes type 2, as Xu et al.  affirm the meta-analysis of prospective cohort studies carried out in 2018. Now, more studies are needed to elucidate the mechanism or mechanisms by which they carry out this antidiabetic activity.
2.4. Citrus and neurodegenerative diseases
Neurodegenerative disorders such as Alzheimer’s, Parkinson’s, and Huntington’s disease represent rapidly growing causes of disability and death, which have profound economic and social implications; nonetheless, only few effective disease-modifying therapies are available for these diseases [65, 66].
Citrus flavonoids exert little adverse effect and have low or no cytotoxicity to healthy, normal cells. The main citrus flavonoids can also traverse the blood-brain barrier; hence, they are promising candidates for intervention in neurodegeneration and as constituents in brain foods .
Assessment of cognitive performance in middle-aged individuals has indicated that consumption of different polyphenols such as catechins, flavonols, and hydroxybenzoic acids is strongly associated with language and verbal memory. Hydroxycinnamates, phenolic acids, and phenolic alcohol are also capable of inducing neuroprotective effects in the same way as flavonoids .
Naringenin and hesperidin are abundant polyphenols in citrus fruits and have been shown to have protective effects in Huntington’s disease due to their mechanism of nitric acid against 3-nitropropionic acid, which presents neurotoxicity in experimental models with rats .
5-Hydroxy-3,6,7,8,3′,4′-hexamethoxyflavone (HHMF) from the Citrus genus and nobiletin, the most abundant polymethoxyflavone in orange peel extract are compounds that enhance neuronal survival and exerted prosurvival action in PC12 cells .
Ushikubo et al.  demonstrated that 3,3′,4′,5,5′-pentahydroxyflavone prevents Aβ fibril formation and that lowering fibril formation decreases Aβ-induced cell death in rat hippocampal neuronal cells. In another study, ursolic acid, p-coumaric acid, and gallic acid extracted from Corni fructus plant were shown to attenuate apoptotic features such as morphological nuclear changes, DNA fragmentation, and cell blebbing induced by Aβ peptide in PC12 cells .
The citrus flavanones hesperidin, hesperetin, and neohesperidin are known to exhibit antioxidant activities and could traverse the blood-brain barrier . These authors showed that hesperetin, hesperidin, and neohesperidin inhibited the decrease of cell viability (MTT reduction), prevented membrane damage (LDH release), scavenged ROS formation, increased catalase activity, and attenuated the elevation of intracellular free Ca2+, the decrease of mitochondrial membrane potential and the increase of caspase-3 activity in H2O2-induced PC12 cells. Meanwhile, hesperidin and hesperetin attenuated decreases of glutathione peroxidase and glutathione reductase activities and decreased DNA damage in H2O2-induced PC12 cells. These results first demonstrate that the citrus flavanones, such as hesperidin, hesperetin, and neohesperidin, even at physiological concentrations, have neuroprotective effects against H2O2-induced cytotoxicity in PC12 cells. These dietary antioxidants are potential candidates for use in the intervention for neurodegenerative diseases.
Antunes et al.  demonstrated that hesperidin (50 mg/kg) treatment was effective in preventing memory impairment in the Morris water maze test, as well as depressive-like behavior in the tail suspension test. Hesperidin attenuated the 6-OHDA-induced reduction in glutathione peroxidase and catalase activity, total reactive antioxidant potential, and the dopamine and its metabolite levels in the striatum of aged mice. This study demonstrated a protective effect of hesperidin on the neurotoxicity induced by 6-OHDA in aged mice, indicating that it could be useful as a therapy for the treatment of PD.
Chakraborty et al.  showed that hesperidin completely inhibits the amyloid fibril formation which is further supported by atomic force microscopy. Hesperidin exhibited moderate ABTS(+) radical scavenging assay but strong hydroxyl radical scavenging ability, as evident from DNA nicking assay.
The Mediterranean diet, considered a good example of a prudent and healthy diet, has undergone important changes in recent years. Factors such as urbanization, pollution, economic development, excessive working hours, and the adoption of inadequate lifestyles cause the population to be exposed to environmental and nutritional factors associated with the onset and progression of diseases related to aging. In this sense, citrus fruits are an important source of bioactive compounds, powerful antioxidants whose health benefits have been scientifically demonstrated in several studies for their protective role against oxidative damage. For this reason, the regular consumption of citrus fruits should be promoted as part of a varied and balanced diet. The absence of sufficient scientific evidence and validated tests to reliably measure the antioxidant activity in vivo of the bioactive compounds present in citrus justifies the need of interventional studies in humans for the correct determination of bioactive properties of citrus and their bioactive compounds.
Conflict of interest
Authors declare that they do not have conflict of interest.