1 Background

Flavonoids are phytochemicals responsible for the various colors in the seeds, flowers, fruits, leaves, and bark [1]. Flavonoids are a large class of natural aromatic compounds as there are reported to be the most common plants’ phenolics [2, 3]. Over the years, flavonoids have represented a vast percentage of phytochemicals from natural sources. It has been reported that more than 10,000 different classes of flavonoids have been found in kingdom Plantae [4,5,6]. Flavonoids are secondary metabolites found in organs of these plants with different functions [7, 8]. Also, they have been reported from sources such as vegetables, wine, fruits, and beverages (tea) [9].

The chemical structures of flavonoids consist of C6-C3-C6 [10] rings which correspond to two aromatic rings A and B linked by three carbon atoms, which may lead to the formation of a third ring (C). Variations in this basic structure give the various subclasses of flavonoidal compounds. These are flavanones, isoflavones, flavones, flavanols (catechins), chalcones, flavonols, and anthocyanins [11,12,13,14]. The flavonoids present in the diet help in the prevention of cardiovascular disease [15]. The biological and oxidative properties of flavonoids are responsible for their anti-allergic, cardioprotective, anti-diabetic, anti-inflammatory, anti-oxidative activity, and free radical scavenging capacity [15, 16]. Also, flavonoids have been reported to exhibit anti-cancer activity [5]. Studies of flavonoids revealed that they are free radical scavengers and reducing agents [17]. Recent researches have focused on the health benefits of these secondary metabolites because of their preventive activity against diseases and anti-oxidative activity, anti-cancer activities, anti-viral activities, and anti-inflammatory [18, 19]. Aside from the anti-oxidant activity of flavonoids, chelating properties [20], their usage as anti-aging substances [21], capillary permeability, and inflammatory response [22], anti-bacterial and therapeutic [23], gastro-protective, and anti-diabetic activity [3] of these phenolics have been reported.

The protective effects such as the anti-inflammatory, anti-oxidant, anti-viral, and anti-tumor activity of flavonoids from natural sources are well documented [24]. The anti-carcinogenic activity of flavonoids has been linked to their anti-oxidant properties [25] which is due to the hydroxyl groups on the structure of the flavonoids [26]. Because of the importance of these phytochemicals, this review summarizes the isolation, characterization, and health benefits of these flavonoids taking into consideration those phytochemicals responsible for these activities. The health benefits reviewed were anti-cancer, anti-microbial, anti-oxidant, anti-inflammatory, anti-fungal, anti-ulcer, and anti-edematogenic activities.

2 Main text

2.1 Methods

The search was done by using keywords such as flavonoids on “science direct,” “google scholar,” “Scopus” database, and many journal sites. Journals employed in the search are Elsevier and Springer. Other search engines used as well as papers published between 2006 and 2019.

3 Results

3.1 Flavonoids biosynthetic pathway

Flavonoids or stilbenes biosynthetic pathway (Fig. 1) follow the extension of 4-hydroxycinnamoyl-CoA with three malonyl-CoA units, in which the poly-β-keto chain folded in different ways, via Aldo or Claisen reactions. Stilbene synthase and chalcone synthase (enzymes) couple the three malonyl-CoA with cinnamoyl-CoA unit to give chalcones or stilbene. Chalcones are precursors for the vast range of flavonoids and their derivatives found in plants. The nucleophilic attack (Michael type) of a phenolic group on α, β-unsaturated ketone forms a six-membered heterocyclic ring such as naringenin. This isomerization reaction in an acid condition favors the flavanone while in basic condition, the chalcone [27]. Flavanones then give rise to variants of flavonoids such as flavonols, flavones, anthocyanidins, and catechins as shown in Fig. 1

Fig. 1
figure 1

Biosynthetic pathway of flavonoid

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3.2 Sources and classification of flavonoids

The various sources of flavonoids have been reported [28] and are given in Table 1 while their classification [29] is shown in Table 2 showing the subclasses. Similarly, Fig. 2 showed the basic skeleton of flavonoids and their various classes.

Table 1 Sources of flavonoids [28]
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Table 2 Classification of flavonoids [29]
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Fig. 2
figure 2

The basic skeleton of flavonoids and their various classes

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3.3 Isolation of flavonoids

The isolation of flavonoids involves various techniques available to natural products researchers that have simplified their isolation from crude extracts. These techniques are column chromatography (CC), high-performance liquid chromatography (HPLC), high-speed counter-current chromatography (HSCCC), counter-current chromatography (CCC), open centrifugal preparative thin layer chromatography (CPTLC), preparative thin-layer chromatography (PTLC), medium pressure liquid chromatography (MPLC), and high-pressure preparative liquid chromatography (HPLC) [29, 30]. The details of the extraction, steps, and the solvents used for the structural elucidation and characterization of the flavonoids summarized in this review are shown in Table 5.

3.4 Techniques used to elucidate the structure of flavonoids

Natural products researchers use the following spectroscopic techniques to elucidate the structure of flavonoids. These are infrared spectroscopy (IR), nuclear magnetic resonance (NMR), ultra-violet spectrophotometry (UV), mass spectrometry (MS), and physical properties as electronic circular dichroism (ECD), melting point (m.pt), and specific rotation power \( \Big({\left[\alpha \right]}_D^T \)) for flavonoids with a stereocenter for ECD and [α]D. Flavonoids have unique chemical shifts. These characteristic chemical shifts make it easier to characterize them. The characteristic chemical shift values reported [29] for some flavonoid classes are given (Table 3) and the UV absorption ranges for these flavonoids are shown (Table 4).

Table 3 Characteristic chemical shifts for flavonoids [29]
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Table 4 UV absorption ranges for flavonoids [29]
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The isolation, characterization, and health benefits of these flavonoids are represented (Scheme 1).

Scheme 1
scheme 1

Flavonoids: isolation, characterization, and health benefits

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4 Discussion

4.1 Health benefits of flavonoids

Extensive biological studies of flavonoids have revealed their health benefits including disease prevention [5, 72]. They have exhibited anti-oxidant, anti-inflammatory, anti-bacterial, and anti-viral activities [73], anti-oxidant [74], anti-allergic anti-carcinogenic properties [75]. The protective effects of flavonoids have been reported as they help to reduce oxidative stress in the body. The cholesterol-lowering activity, anti-cancer, anti-oxidant of myricetin, tricin, apigenin, luteolin, quercetin, and isorhamnetin has been reported [24]. The anti-viral, anti-bacterial, anti-cancer, cardioprotective, and anti-inflammatory activity [76], as chelating agents and, are strong topoisomerase inhibitors [24] anti-aggregational, anti-atherosclerotic, and detoxification activities [77] of various flavonoids have been reported. These biological activities depend to a larger extent on the hydroxyl group in the flavonoids [78]. Quercetin has reduced the risk of cancer, eye diseases, arthritis, and allergic disorders [9]. The decreased risk of cardiovascular disease by proanthocyanins and flavone-3-ols has been reported [23]. The techniques used for the isolation, characterization, and the health benefits of these flavonoids are as shown in Table 5.

Table 5 Isolation, characterization, and health benefits of flavonoids
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The pharmacological activities of the phytochemical constituents from 22 plant families reported in the literature as reviewed in Fig. 3 showed the percentages of these activities. Out of the references cited, 30% of the flavonoids showed anti-oxidant activity. Because of this vast anti-oxidant activity, flavonoids reduce aging by protecting the body against free radicals oxidation [26, 79].

Fig. 3
figure 3

Representation of the activities of the flavonoids

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4.2 Anti-oxidant activity

Anti-oxidants are compounds that slow or prevent oxidation in living cells. They act against the effects of free radicals. Flavonoids protect the body against reactive oxygen species. Chemically, flavonoids have hydroxyl groups and a highly conjugated π-electron system, which allows them to act as free radical scavengers [80]. Anti-oxidant activity of flavonoids [16, 20, 33, 76, 81], chelating properties [20], makes them acts as protective agents against free radicals [26, 79]. In the body, anti-oxidants protect the human body from free radicals oxidation [17] thereby retarding the progress of many chronic diseases. Epicatechin, epigallocatechin, and gallocatechin have exhibited anti-oxidant activity [31].

Naringenin has shown anti-oxidant, anti-diabetic, anti-atherogenic, anti-depressant, immunomodulatory, antitumor, antiinflammatory, and hypolipidaemic, activity [12]. Catechin, epicatechin, rutin, quercetin, and naringin have been reported for anti-oxidant activity against free radicals [32]. The anti-oxidant activity of spectaflovoside A, kaempferol-3-O-(2′′,3′′-di-O-acetyl)-α-L-rhamnopyranoside, kaempferol-3-O(3′′,4′′-di-O-acetyl)-α-L-rhamnopyranoside, kaempferol-3-O- (2′′,4′′-di-O-acetyl)-α-L-rhamnopyranoside, kaempferol, and kaempferol-3-O-(4′′-Oacetyl)-α-L-rhamnopyranoside were documented [36]. The anti-oxidant activity of flavonoids extracted from Phlomis bovei De Noé [82], quercetin, taxifolin, catechin, and galangin, anthocyanidin, kaempferol, catechins, and catechin gallate esters have been reported [83]. Quercetin, anthocyanidin and kaempferol, catechins, and catechin gallate esters are effective anti-oxidants against free radicals. Quercetin had showed excellent in vitro anti-oxidant capacity [59]. Quercimeritrin, scutellarein, and rutin isolated from C. angustifolia showed strong anti-oxidant activity against oxidative stress [54]. The phytochemical investigation of the ethanol extract of Ximenia parviflora Benth. Var led to the isolation of quercetin, kaempferol, and apigenin with anti-oxidant activity. Similarly, the naringenin, quercetin, and kaempferol isolated from the ethanolic extract of Viscum album L showed anti-oxidant activity [62]. Catechins have been reported for its protection against oxidative stress, cancer, and cardiovascular disorder [84]. In Fig. 4, the chemical structures of flavonoids reported for their benefits are as shown.

Fig. 4
figure 4

Chemical structures of flavonoids with health benefits

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4.3 Anti-microbial activity

The broth microdilution assay of spectaflovoside A, kaempferol-3-O-(2′′,3′′-di-O-acetyl)-α-L-rhamnopyranoside, kaempferol-3-O-(3′′,4′′-di-O-acetyl)-α-L-rhamnopyranoside, kaempferol-3-O-(4′′-Oacetyl)-α-L-rhamnopyranoside, kaempferol, and kaempferol-3-O-(2′′,4′′-di-O-acetyl)-α-L-rhamnopyranoside showed that these flavonoids exhibited remarkable anti-bacterial activities with a MIC values between 62.50 μg/mL and 500 μg/mL against E. coli, K. pneumoniae, S. aureus, and B. cereus [36]. In another study, luteolin, 3,5-dihydroxy-6,7,8,4′tetramethoxyflavone, apigenin, 3,5-dihydroxy-6,7,8-trimethoxyflavone, apigenin 7-O-glucoside, apigenin 4′-O-glucoside, kaempferol 3-O-glucoside, luteolin 4′-O-glucoside, luteolin 4′,7-O-diglucoside, kaempferol, kaempferol 7-O-glucoside, and quercetin 3-O-glucoside were reported for their vast anti-microbial activities against P. aeruginosa [38]. Apigenin and isoflavones exhibited anti-bacterial activity [16]. Genistein, kaempferol, naringenin, and catechin isolated from Brassica oleracea var. Capitata L. possessed anti-bacterial activity against E. coli and S. aureus [52].

The flavonoids, 3-hydroxy flavone derivatives, and 3-methyl flavanone showed activity against Gram +ve bacteria [75]. Scandenone, kaempferol-3,7-O-α-L-dirhamnoside tiliroside, quercetin-3,7-O-α-L-dirhamnoside showed anti-microbial activity [43]. 7-Methoxy-3, 3′,4′,6-tetrahydroxyflavone, 3,3′,4′,7-tetrahydroxyflavone (fisetin ), naringenin, 2′,7-dihydroxy-4′,5′-dimethoxyisoflavon, 3′-hydroxydaidzein and xenognosin B exhibited the anti-bacterial activities against B. subtili ATCC 6633, S. aureus ATCC25932, and B. cereus ATCC7064 [48]. The 2′,4′-dihydroxy-4methoxy-3′-prenyldihydrochalcone, 4-hydroxyonchocarpin, isobavachalcone, 2′,4′-dihydroxy-3,4-(2′′,2′′dimethylchromeno)-3′-prenyldihydrochalcone, 5,7-dihydroxy4′-methoxy-6-prenylflavanone, 5-hydroxy-6,7-(2,2dimethylchromano)-4′-methoxyflavanone, 4′,5-dihydroxy-6,7(2,2-dimethylchromeno)-2′-methoxy-8-γ,γ-dimethylallylflavone, artocarpin, pyranocycloartobiloxanthone A, and cycloheterophyllin isolated from Artocarpus lowii King and Artocarpus anisophyllus Miq showed activity against S. aureus, P. putida, B. cereus, E. coli, C. albicans, and C. glabrata [49].

4.4 Anti-cancer activity

The term cancer refers to a disease in which cells of a tissue undergo uncontrolled and often rapid proliferation [85]. This is also the loss of control of growth [86]. Alternative medicine has been used to treat cancer [87] and flavonoids, especially from plant sources, have not been left out. The pharmacological properties of flavonoids have made them useful alternatives to inhibit cell damage [73]. Flavonoids have been reported to be good anti-cancer therapeutics [88]. Quercetin particularly has been reported to be effective in the treatment of stomach, lung, prostate, and breast cancers [29]. Pradhan et al. 2015 reported the anti-tumor activity of quercetin without toxicity on the breast cancer cell, MCF-7 [89]. The anti-cancer activity of quercetin has been linked to the inhibition of the enzyme (DNA gyrase) [90]. Luteolin isolated from the leaves of Struchium sparganophora has caused cell death of melanoma and ovarian cancer cell lines [51]. Luteolin-7-O-β-glucopyranoside, formononetin-7-O-β-D-glucoside, and quercetin-3-O-β-d-glucuronide isolated from the leaves of Cassia tora linn were active against breast cancer (MCF7) [53]. Luteolin from the Vitex negundo Lin was an active anti-tumor agent [55]. In another study, the 2′, 5-dihydroxy-7-methoxyflavanone and 2′, 5-dihydroxy-7-methoxyflavone isolated from Andrographis glandulosa were active against HeLa, MIA PaCa, and U-8 [56]. Quercetin has been reported to induce cytotoxicity in cancer cells [91]. Kaempferol isolated from Ageratum conyzoides L. exhibited activity against lung cancer, gastric cancer, colon cancer, and glioma cancer [57]. Pinostrobin isolated from Cajanus cojan exhibited anti-leukemia activity [58]. The cynaroside isolated from H. chillensis was active against OVACAR-8, HCT-116, and SF-295 [59]. Nobiletin at 20 μM inhibited human ovarian cancer [76].

Quercetin, kaempferol, myricetin, luteolin, and apigenin isolated from the aqueous and/or alcohol extracts of Khlu leaves exhibited anti-cancer activities [34]. Quercetin has shown anti-tumor activity [25]. Myricetin and quercetin inhibited mammalian TrxRs with IC50 values of 0.62 and 0.97 Mmol/L, respectively [92]. Apigenin has been reported to arrest HT29 colon cancer [50]. Oncamex showed a strong anti-tumor effect against breast cancer while hesperetin was active against lung and carcinoma cancer cells [93]. Hesperidin and naringin isolated from the alcoholic extract of Colvillea racemosa were active against colon carcinoma cell lines (HCT-116) [40]. Quercitin, morin, and myricetin have shown protective effects in the prevention of liver, cardiovascular diseases, and cancer [16].

4.5 Anti-inflammatory

Inflammation is a normal biological process in response to pathogen infection, tissue injury, or chemical irritation [16]. The anti-inflammatory properties of anthocyanins have been reported [81]. Previous in vitro studies of flavonoids showed inhibition against LPS-induced TNF-α production [94]. The report of the phytochemical investigation of Vicia sativa led to the isolation of apigenin 4′-Oβ-D-glucopyranoside, isoquercetin, nicotiflorin, and apigenin 6-Cα-L-arabinopyranoside-8-Cβ-D-glucopyranoside that showed significant anti-edematogenic activity (P < 0.05) [66]. Isorhamnetin 3-O-b-D-glucoside, cirsiliol, chrysosplenol D, and artemetin isolated from Chrysanthemum morifolim Ramat inhibited the NO production in LPS-induced RAW 264.7 cells [67]. Daidzein, quercetin, genistein, and kaempferol have inhibited the production of both STAT-1 and NF-κB [95]. Similarly, the 3-methoxy quercetin isolated from Garcinia kola Heckel at concentrations (25 and 125 μM) inhibited the production of TNF-α [69]. The quercetin 3-O-훼-L-arabinopyranosyl(1→2)-훼-L-rhamnopyranoside isolated from Kalanchoe pinnata (Lamarck) Persoon showed anti-inflammatory activity [70]. At higher doses, flavonoids have shown a decrease in proliferation, CD14 surface marker, and NO production [96]. The flavonoids, artocarpanone A, artocarpanone, and heteroflavanones B and C have shown a remarkable inhibitory effect on iNOS protein expression and NO production in RAW 264.7 cells [97].

4.6 Anti-fungal activity

The flavonoids, diglycosylflavones, flavonol-3-O-glycosides, and proanthocyanidins isolated from Ephedra alata showed anti-fungal activity [22], nobiletin and langeritin, and hesperidin exhibited strong and weak anti-fungal activities, respectively [2]. The flavonoid, baicalein, showed anti-candidal activity against C. tropicalis 170.06, C. albicans ATCC 64550, and C. parapsilosis 153.07 with the MIC50 of 2.6, 26, and 13 μg ml−1, respectively [98]. The 3,5-dihydroxy-7-methoxy anthocynidines isolated from Monanthotaxis littoralis was active against mycotoxigenic [71]. (-)-epicatechin-3-Ob-glucopyranoside, 6-p-hydroxybenzyltaxifolin-7-O-b-D-glucoside, 5-hydroxy-3-(4-hydroxylphenyl) pyrano[3,2-g]chromene-4(8H)-one (-)-epicatechin(2(3,4-dihydroxyphenyl)-3,4-dihydro-2H-chromene-3,5,7-triol, and quercetin-3-O-a-glucopyranosyl-(1 2)-b-D-glucopyranoside isolated from Mangifera indica L exhibited anti-fungal activity [99].

4.7 Anti-ulcer activity

An ulcer is a disease of the alimentary tract caused by an inflamed break in the mucus lining membrane [100]. The anti-ulcer activity of quercetin in animals has been reported [72]. The phytochemical investigation of the aqueous extract of Ximenia americana led to the isolation of procyanidins B and C as well as catechin/epicatechin which were active against acute gastric ulcer [60]. The spinacetin 3-O-[α-L-rhamnopyranosyl-(1→6)-βD-glucopyranoside]-7-O-[α-L-rhamnopyranoside], kaempferol-3-Orutinoside, kaempferol-7-O-β-D-glucopyranoside, and quercetin-7-O-β-D-glucopyranoside isolated from the ethanol extracts of Anvillea garcinii showed a powerful anti-ulcer [64]. The hesperidin isolated from Citrus sinensis showed anti-ulcer activity [65].

4.8 Anti-edematogenic activity

Catechin, rutin, quercitrin, quercetin, and kaempferol isolated from the H2O 100% EtOH (1:1) of Ximenia americana L showed anti-edematogenic activity against ear edema in rat [63]. Artemetin isolated from Cordia curassavica DC exhibited remarkable anti-edematogenic activity [68].

5 Conclusions

Twenty-two members of the different families containing flavonoids studied for their health benefits confirmed the medicinal importance of these phytochemicals from these sources. The pharmacological pieces of evidence reported in the literature has proven that these flavonoids have shown anti-cancer, anti-microbial, anti-oxidant, anti-inflammatory, anti-fungal, anti-ulcer, and anti-edematogenic activity. Out of the references cited, 30% focused on the anti-oxidant activity of flavonoids, key in protecting the body against free radicals and oxidative stress. Also, 18% of the references showed anti-microbial and anti-cancer activities. Further literature reports indicated that flavonoids from these families exhibited anti-inflammatory and anti-edematogenic (9%), anti-viral and anti-ulcer (5%), anti-fungal, anti-nociceptive, and anti-histaminice (2%). The pharmacological activities of flavonoids from the various sources reviewed in this study show that the secondary metabolites could provide a scaffold for the development of potent anti-cancer drugs in the future.