Anti-Diabetic Potential of Polyphenol-Rich Fruits from the Maleae Tribe—A Review of In Vitro and In Vivo Animal and Human Trials

The Maleae tribe consists of over one thousand species, including many well-known polyphenol-containing fruit crops with wide-ranging biological properties, e.g., apples (Malus), chokeberries (Aronia), pears (Pyrus), quinces (Cydonia, Chaenomeles), saskatoon (Amelanchier), loquats (Eriobotrya), medlars (Mespilus), rowans (Sorbus), and hawthorns (Crataegus). Considering the current interest in the concept of functional foods and the still-insufficient methods of diabetes management, the anti-diabetic potential of fruits has been studied intensively, including those of the Maleae tribe. This paper is the first comprehensive overview of this selected topic, covering articles published from 2000 to 2023 (131 articles in total). The first part of this review focuses on the potential mechanisms of action of fruits investigated so far (46 species), including their effects on tissue-specific glucose transport and the expression or activity of proteins in the insulin signalling pathway. The second part covers the phytocompounds responsible for particular fruits’ activity—primarily polyphenols (e.g., flavonols, dihydrochalcones, proanthocyanidins, anthocyanins, phenolic acids), but also polysaccharides, triterpenes, and their additive and synergistic effects. In summary, fruits from the Maleae tribe seem promising as functional foods and anti-diabetic agents; however, their prospects for more expansive pro-health application require further research, especially more profound in vivo trials.


Introduction
Diabetes is a chronic, progressive disorder characterised by raised blood glucose levels due to insufficient production of the hormone insulin or decreased effectiveness of the insulin that the body produces [1,2]. Long-term hyperglycaemia has become a significant healthcare burden worldwide, leading to life-threatening damage to the blood vessels, heart, nerves, and kidneys [2]. It was estimated that 353 million patients suffered from diabetes in 2021, and it is projected that by 2030 there will be about 643 million people diagnosed with this disorder [1]. Therefore, diabetes has been recognised as one of four non-communicable diseases targeted as a priority by the world's public health leaders [2].
The pharmacotherapy of diabetes includes insulin (when insulin deficiency is seen) or oral hypoglycaemic drugs that exert anti-diabetic effects through different mechanisms. These mechanisms comprise, i.a., stimulation of endogenous insulin secretion from pancreatic β-cells by sulfonylureas, glucagon-like peptide 1 (GLP-1) analogues, or dipeptidyl peptidase-4 (DPP IV) inhibitors; the increase in the insulin sensitivity, the boost of peripheral absorption of glucose, and the reduction in hepatic gluconeogenesis by peroxisome proliferator-activated receptor γ (PPARγ) activators or biguanides; the delay in the absorption of carbohydrates from the intestine by α-glucosidase inhibitors; or the increase in glucose elimination via the kidneys by sodium-glucose cotransporter-2 (SGLT2) inhibitors [3][4][5]. However, the conventional treatment of diabetes often fails due to drug

Materials and Methods
The literature selection was performed based on the Scopus, Web of Science, and Google Scholar databases, searching for original articles written in English and published (at least electronically) between January 2000 and June 2023. The search was conducted using the following keyword combination pattern: (1) the genus Latin or common nomenclature (i.e., "Amelanchier" or "Amelasorbus" or "Aronia" or "Chaenomeles" or "Chamaemeles" or "Cotoneaster" or "Crataegus" or "Crataemespilus" or "Cydonia" or "Dichotomanthes" or "Docynia" or "Eriobotrya" or "Eriolobus" or "Hesperomeles" or "Heteromeles" or "Kageneckia" or "Lindleya" or "Malacomeles" or "Malus" or "Mespilus" or "Osteomeles" or "Peraphyllum" or "Photinia" or "Pseudocydonia" or "Pyracantha" or "Pyrus" or "Rhaphiolepis" or "Sorbus" or "Sorbaronia" or "Sorbocotoneaster" or "Stranvaesia" or "Vauquelinia" or "saskatoon" or "chokeberry" or "hawthorn" or "quince" or "apple" or "crabapple" or "medlar" or "pear" or "loquat" or "rowan" or "service tree" or "whitebeam" or "toyon"); (2) description of the plant part/product (i.e., "fruit/-s" or "berry/-ies" or "juice/-s" or "extract/-s"); (3) activity descriptor (i.e., "diabetes" or "diabetic" or "anti-diabetic" or "glucose" or "insulin" or "glycaemia/glycemia"). Only articles covering the topic of fruits' effects on carbohydrate bioavailability/metabolism and direct toxic effects of hyperglycaemia (e.g., AGE formation) were included. Consequently, the studies on the conditions accompanying or resulting from diabetes as an outcome of complex mechanisms (e.g., inflammation, neurodegenerative diseases or cardiovascular complications of diabetes) were not reviewed. Moreover, if the paper included the analysis of both diabetic and diabetes comorbid-disorder-related parameters (e.g., lipid profiles, cytokine levels, etc.), only the first part was included in this review. The further exclusion criteria were as follows: papers covering only ethnobotanical research on plants used in diabetes (without activity studies), the effects of a complex diet or combination of plants/drugs (making it impossible to indicate which component determines the activity), and studies of single, isolated compounds of plant origin, not covering the activity or health impact of whole fruits. The inclusion or exclusion of the articles was validated manually by reading the entire item. The binomial names of the reviewed species were checked and revised according to World Flora Online [11].

Results and Discussion
As a result of an in-depth analysis of the literature data covering articles published from 2000 to 2023, 131 studies were included in the present review. The majority of them covered only in vitro tests (67 papers); then, there were in vivo studies on animal models (51 items, including some mixed in vitro/in vivo tests) and human in vivo trials (14 papers, including one in vivo animal and human study) ( Figure 1). The selected documents were based on the activity testing of fruits from 46 species belonging to the genera Amelanchier, Aronia, Chaenomeles, Cotoneaster, Crataegus, Cydonia, Malus, Mespilus, Pyracantha, Pyrus, Sorbus, and Vauquelinia. The most widely studied taxa among this group were Aronia melanocarpa (26 studies, including 13 animal and 5 human trials) and Malus domestica (26 studies, including 6 animal and 5 human trials) ( Figure 1).
The second most frequent type of in vitro research into Maleae fruits involves cellular studies on glucose transport and metabolism (Table 1). Stimulating glucose uptake through skeletal muscle or hepatic cells is one of the mechanisms that may lead to the enhancement of glucose metabolism and reduction in blood sugar levels. This effect, associated with the increased expression of insulin-dependent glucose transporter 4 (GLUT-4), has been observed for Aronia melanocarpa [28], Chaenomeles japonica [29], Malus pumila [30], and Pyrus pyrifolia [31]. Moreover, the inhibition of intestinal glucose absorption (i.e., transport from the intestinal lumen into enterocytes) via, e.g., the modulation of SGLT1 (sodium-glucose transport protein-1) and GLUT-2 (glucose transporter 2) levels or activity, has been proposed for Malus domestica [32][33][34]. As for the metabolic part, the effects on the expression or activity of the PI3K/Akt pathway proteins ( Figure 2a) were observed for Aronia melanocarpa [28,35], Chaenomeles japonica [29,36], Crataegus pinnatifida [17], Malus domestica [37,38], Malus pumila [30], and Pyrus pyrifolia [31,39]. Considering the complexity of the processes involved in insulin and glucose regulation, the results of the studies mentioned above may indicate the ability of the tested extracts to stimulate glycolysis and glycogen synthesis, inhibit gluconeogenesis and glycogenolysis, and lower blood glucose levels. Moreover, fruits from Amelanchier alnifolia [40], Crataegus pinnatifida [17], Malus domestica [41], Malus sieversii [42], Sorbus aucuparia [43], and Sorbus domestica [44] have been suggested to inhibit enzymes involved in the polyol pathway of glucose metabolism, such as aldose reductase (ALR) and sorbitol dehydrogenase (SDH), and to impair the production of advanced glycation end products (AGEs). Thus, they may prevent diabetic complications, primarily oxidative-stress-related damage to the microvascular systems, caused by pathological glucose metabolism (Figure 2b). Furthermore, one of the investigated species, i.e., Chaenomeles japonica [36], was proven to have cytoprotective effects on βTC3 pancreatic β-cells (model of induced toxicity), enabling their viability and normal proliferation to be preserved. Detailed information on the accumulated research can be found in Table 1.
There were several mechanisms behind the activity observed in vivo that were tested for Maleae fruits (Table 2), including the following: (1) The effects on intestinal absorption of glucose; (2) The effects on skeletal, hepatic, or adipose transport of glucose; (3) The changes in the expression of proteins involved in the insulin signalling pathway; (4) The modulation of the activity of enzymes involved in glucose metabolism; (5) The inhibition of glucose-derived protein damage.
(1) The impairment of intestinal glucose transporter (SGLT-1), which may decrease the absorption of sugars consumed with foods, was proven for Malus domestica [75]. Moreover, modification of the gut microbiota's function was also reported to be involved in reducing intestinal sugar absorption. This effect was observed for fruits from Amelanchier alnifolia, which were able to, i.a., alter the α-diversity and β-diversity of gut microbiota and reduce the ratio of Firmicutes/Bacteroidetes, which was negatively correlated with carbohydrate digestion [104][105][106]. Finally, the inhibition of mucosal enzymes' activity, i.e., sucrase and maltase (α-glucosidases that catalyse the hydrolysis of sucrose and maltose O-glycosidic bonds), was observed for the Aronia melanocarpa juice and extract and proposed as another mechanism of lowering the blood/plasma glucose levels by inhibiting the digestion and absorption of sugars [114,115].
(3) The homeostasis of glucose metabolism depends on several simultaneous ongoing processes under the control of insulin. The binding of insulin to its receptor leads to a cascade of reactions that promote glucose usage and storage by different tissues (liver, muscle, or adipose), e.g., the stimulation of glycogen synthesis (i.e., the transformation of glucose to glycogen) and glycolysis (i.e., the conversion of glucose to pyruvate and production of ATP energy). At the same time, de novo synthesis of glucose (gluconeogenesis) and glycogenolysis (metabolism of glycogen to glucose) are suppressed [50]. However, insulin secretion is only the beginning of the chain reactions, and these processes can be controlled at multiple stages (see Figure 2). As mentioned earlier, the results of the hypoglycaemic activity studies of Maleae fruits suggested that the reduction in blood/plasma glucose levels was driven by the improvement of insulin sensitivity rather than the stimulation of insulin secretion. Thus, the effects on the expression of different proteins in the insulin signalling pathway were tested for some species. Depending on the studies, the Aronia melanocarpa juices or extracts were able to increase the expression of p-PI3K (phosphorylated phosphoinositide 3-kinase), p-Akt (phosphorylated protein kinase B), GYS (glycogen synthase), and GLP-1 (glucagon-like peptide 1), as well as the ratios of p-IRS-1(2)/IRS-1(2) (phosphorylated insulin receptor substrate 1(2)/ insulin receptor substrate 1(2)) and p-GSK-3β/GSK-3β (phosphorylated glycogen synthase kinase 3 beta/ glycogen synthase kinase 3 beta); they were also documented to decrease the levels of PTEN (phosphatase and tensin homolog) and SOCS3 (suppressor of cytokine signalling 3), which may result in the enhancement of glycolysis and glycogen synthesis, as well as the inhibition of gluconeogenesis and glycogenolysis [100,[115][116][117]119]. Similar effects were observed for Crataegus pinnatifida-by increasing p-Akt, p-AMPK (phosphorylated AMP-activated protein kinase), p-IRS-1, and p-PI3K, and decreasing PEPCK (phosphoenolpyruvate carboxykinase) levels [102,118]; various Crataegus spp.-by increasing the GPC-4 (glypican-4) level [120]; Cydonia oblonga-by increasing p-AMPK and decreasing PPARγ levels [108]; and Malus pumila-by increasing the p-Akt level [113]. On the other hand, the results of gene expression studies in Amelanchier alnifolia berry powder suggested the enhancement of opposing processes of glycolysis and gluconeogenesis, but the final effect on glucose metabolism was still positive, i.e., oral glucose tolerance test parameters were improved in comparison to diabetic controls [121]. In this case, the answer may be not in the levels of particular enzymes, but rather in their activity, which needs further investigation.
(4) Therefore, in addition to protein expression studies, some of the reviewed papers include alternative enzyme activity tests. According to these examinations, the juice or acidified 60% ethanol extracts from Aronia melanocarpa [115,116,119] were able to increase the activity of glucokinase (GK) and pyruvate kinase (PK) (which may enhance the glycolysis process), inhibit the activity of enzymes involved in gluconeogenesis (e.g., PEPCK, phosphoenolpyruvate carboxykinase; G6Pase, glucose-6-phosphatase), and regulate glucose homeostasis through enzymatic termination of incretin action (DPP IV inhibition). The inhibition of G6Pase was also observed for 70% methanol macerate from Malus sp. [122].
(5) Finally, some mechanisms that may prevent the impairment of the function of various proteins caused by high glucose levels were also revealed. For example, it was observed that the fruit extract from Chaenomeles sinensis [123] decreased the levels of intermediate products of glycation, i.e., glyoxal (GO) and methylglyoxal (MG). Moreover, the extract from Crataegus orientalis [112] was able to inhibit the activity of the ALR enzyme, which may prevent the development of diabetic complications caused by pathological glucose metabolism (polyol pathway).
80% acetone extract; detected compounds (NMR): chlorogenic and neochlorogenic acids, carbohydrates C57BL/6J male mice (n = 6-10/group); STZ-induced diabetes; duration: acute consumption (3.5 h); tested: mice fed starch (2 g/kg) and berry extract (600, 900, or 1250 mg/kg) or mice fed glucose (2 g/kg) and berry extract (1250 mg/kg); controls: mice fed starch and acarbose (25 mg/kg, positive control) or mice fed starch/glucose (negative control) oral starch TT: ↓ maximal blood glucose compared to negative controls; the activity of berry extract at 900-1250 mg/kg was comparable to that of acarbose; for extract at 1250 mg/kg, ↓ iAUC; OGTT: for extract at 1250 mg/kg, ↓ in blood glucose after 30 min compared to the negative control [22] * Suggestions of what compounds were responsible for the observed effects were taken directly from the cited papers; for a critical discussion, see

Human Studies
The in vivo human studies on the anti-diabetic potential of fruits from the Maleae tribe referred to five species, i.e., Aronia melanocarpa, Malus domestica, Malus sylvestris, Malus pumila, and Pyrus pyrifolia ( Table 3).
The 1-3-month Aronia melanocarpa juice or extract supplementation resulted in reduced fasting blood glucose and glycated haemoglobin (HbA1c) levels in diabetic patients [143][144][145]. Moreover, the study of Simeonov et al. [143] showed that 60 min after ingestion of Aronia juice, the blood glucose levels in diabetic patients were reduced; however, this effect was statistically insignificant when the Aronia supplementation was combined with the meal. On the other hand, it was suggested that one-time Aronia juice supplementation decreased the postprandial blood glucose excursion in healthy people [146].
The anti-diabetic potential of Malus domestica fruits was tested only on healthy adults during one-time consumption of the apple powder or commercial apple extracts with the carbohydrate meal (postprandial response tests) or glucose (oral glucose tolerance test (OGTT)) [32,34,73,75,147]. These trials indicated the delay time (T max ) and the lower or unchanged maximal glucose and insulin levels (C max ), as well as the reduction in the total glucose concentration during the time of the study (iAUC, incremental area under the curve of glycaemic excursion). Moreover, a significant decrease in the glucose-dependent insulinotropic peptide (GIP) and C-peptide (an indicator of pancreatic β-cell function) was observed. The urinary glucose excretion was increased or unchanged.
The positive effects on the OGTT parameters (decreased iAUC and glucose levels after 30 min) [148,149] or postprandial response to carbohydrate meals (decreased iAUC and C max ) [148,149] were also observed for Malus pumila and Pyrus pyrifolia. In the first study, the effect was significant in the patients with high-normal or borderline glucose levels (100-125 mg/dL) who were regularly supplemented with the apple extract for 12 weeks. In the second study, the participants with normal glucose levels received a single apple or pear preload combined with specialised carbohydrate meals. Finally, the fasting blood glucose levels were significantly reduced in diabetes mellitus type II patients who consumed Malus sylvestris fruits for 14 days [150].    * Suggestions of what compounds were responsible for the observed effects were taken directly from the cited papers; for a critical discussion, see Section 3.4. ↑, increase; ↓, decrease; C max , maximal level; GAE, gallic acid equivalents; GIP, glucose-dependent insulinotropic polypeptide; GLP-1, glucagon-like peptide 1; HbA1c, glycated haemoglobin; HOMA-IR, the homeostasis model assessment-estimated insulin resistance; HPLC, high-performance liquid chromatography; iAUC, incremental area under the curve of tested parameter excursion; OGTT, oral glucose tolerance test; T max , time to reach maximal level.

Polyphenols and Other Chemical Contributors to the Anti-Diabetic Activity of Maleae Fruits
The Maleae fruits contain different bioactive substances, primarily polyphenols (e.g., anthocyanins, flavonoids, proanthocyanidins, phenolic acids), but also terpenoids, proteins, carbohydrates, vitamins, and minerals. In this review, we do not present the detailed chemical composition of all species, since their profiles are highly complex and described in detail in other reviews [153][154][155], but we discuss the suggestions from the literature on the chemical constituents that might be responsible for the observed anti-diabetic effects of fruits. To this end, we focused not on the hypothetical indication of active markers based only on fruit composition (i.e., the presence of particular components), but on activity confirmation by testing pure compounds or their purified fractions (issued in 26 papers). The details, i.a., the chemical structures in question and the brief chemical profiles of the fruits (if covered in the reviewed articles), are shown in Tables 1-3. In addition, the impacts of various compounds on the anti-diabetic potential of Maleae fruits are discussed below, and summarised in the form of Figures 3 and 4. All in all, as the leading phytochemical constituents of Maleae fruits, polyphenols were most often tested for biological effects and confirmed as being responsible for the anti-diabetic activity of different species through various mechanisms. In addition, some papers pointed out the contribution of polysaccharides or triterpenes to the biological effects of, e.g., Chaenomeles or Sorbus species, as well as their synergy with polyphenols.    [12,14,15,17,20,22,24,27

Anthocyanins' Contribution to the Amelanchier and Aronia Fruits' Activity
Anthocyanins were studied as active constituents of Amelanchier alnifolia [104][105][106][107] and Aronia melanocarpa [12,58,119]. Cyanidin 3-O-glucoside (7.2 mg/kg/day) was revealed to reduce the fasting plasma glucose levels in the mice fed a high-fat, high-sucrose diet, reaching levels similar to those observed for Amelanchier berry powder containing an equal amount of cyanidin glucoside [104]. Cyanidin monoglycosides isolated from Aronia melanocarpa fruits (986.48 mg of cyanidin galactoside, glucoside, arabinoside, and xyloside per g of fraction; 150-300 mg/kg/day) reduced blood glucose and HbA1c serum levels, increased the glycogen levels in the liver, and modulated hepatic protein expression (↑ p-GSK-3β, ↑ GLUT-4, ↓ SOCS3) in the diabetic mice [100]. Moreover, the in vitro studies of the α-glucosidaseinhibitory potential of individual anthocyanins from Aronia fruits suggested the higher anti-diabetic potential of cyanidin arabinoside and glucoside (IC 50 = 0.37-0.87 µg/mL) compared to cyanidin galactoside and xyloside (IC 50 = 1.54-5.5 µg/mL). Still, all cyanidin monoglycosides were considered to be co-responsible for the anti-glucosidase activity of the chokeberry 50% ethanol extract (IC 50 = 3.5 µg/mL) [12]. The in vivo (animal model) anti-diabetic potential was also tested for cyanidin 3,5-diglucoside (10 µg/mL solution), but its effectiveness was weaker than that of Aronia juice [119]. The contribution of anthocyanins to the anti-diabetic potential of fruits may explain the higher activity observed for extracts from peel than from the flesh of Amelanchier, as well as that of acidified alcoholic extracts from Aronia fruits compared with non-acidified extracts (higher extraction potential and content of anthocyanins) [13,55].

Dihydrochalcone's Contribution to the Malus Fruits' Activity
Phlorizin intake (1.96 mg/kg) significantly reduced the iAUC (in vivo animal studies, OGTT) to levels comparable to those achieved by Malus domestica commercial extract (12.24 mg/kg) containing the same amount of phlorizin [75]. This observation may be due to the inhibition of intestinal glucose absorption that was reported for phlorizin, e.g., it was calculated that phlorizin contributed to 52% of the glucose transport reduction (Caco-2 cells) noted for Malus domestica extract [33]. Phlorizin was also able to inhibit α-glucosidase activity (IC 50 = 0.01 mg/mL, with significantly stronger effectiveness than extracts from different Malus sp. cultivars IC 50 = 7-256 mg/mL) [42]. Moreover, phlorizin stimulated the glucose uptake by hepatic cells and modified the protein levels in the insulin signalling pathway in vitro, with effects comparable to those achieved with Malus domestica 80% ethanolic extract [37].
3.4.6. Contribution of Non-Phenolic Compounds to the Crataegus, Chaenomeles, and Sorbus Fruits' Activity, and Their Synergy with Polyphenols Apart from polyphenols, some triterpenes were also suggested as anti-diabetic agents. For instance, 3-epicorosolic acid isolated from Crataegus pinnatifida inhibited protein tyrosine phosphatase 1B (PTP1B) and α-glucosidase to comparable or higher extents than the respective crude methanol extract and its organic fractions [17]. In addition, α-glucosidaseinhibitory activity was observed for oleanolic acid from Chaenomeles sp. [59,63].
Last, but not least, the anti-diabetic potential has been confirmed for polysaccharides. The α-glucosidase inhibition observed for the polysaccharide fraction from Chaenomeles speciosa fruits was potent (100% inhibition at 0.5 mg/mL) and significantly higher compared to all other compounds isolated from the fruits, including various polyphenols and triterpenes. However, there was no clear relationship between activity parameters and polyphenol/polysaccharide/triterpene concentrations in different Chaenomeles extracts/fractions. Consequently, statistical analysis performed using composition data of the tested extracts/fractions and activity of pure compounds suggested that α-glucosidase inhibition of the fruits seems additive or synergic and depends on various chemical constituents and proportions between them [59]. The synergy of polysaccharides with polyphenols in the context of anti-diabetic potential was also confirmed for Sorbus norvegica. In this case, the inhibition of α-amylase activity was significantly higher for the whole-fruit extract (IC 50 = 2.5 µg/mL) than for its two fractions, i.e., polysaccharides (IC 50 = 48 µg/mL) and polyphenols (IC 50 = 20 µg/mL) [22]. The structures of both tested polymers were not analysed. Still, another study on the carbohydrates from Chaenomeles and Sorbus species suggested that galacturonic acid, arabinose, and galactose may be the primary components of these active polysaccharides [156,157].

Impact of Monosaccharides on Fruits' Anti-Diabetic Potential
Considering the possibility of using fruits as potential herbal drug candidates for the prevention or treatment of diabetes, it is worth paying attention to the presence of simple sugars and the related glycaemic index (GI). The GI expresses the capacity of the organism to deal with carbohydrates in foods as a percentage of the response to an equal weight of glucose. Since all fruits from the Maleae tribe are classified as having a low GI (<55), their consumption by diabetic patients is considered to be safe [158][159][160]. This is especially important for ingesting the whole, fresh fruits or juices, while supplementation of fruit extracts with concentrated contents of only selected compounds is all the more secure.

The Anti-Diabetic Potential of the Most Promising Maleae Fruits-Concluding Thoughts
Considering all studies on the anti-diabetic effects of Maleae fruits, their number, and the results presented above, the most promising species for more expansive pro-health applications today seem to be Aronia melanocarpa and Malus domestica, which are among the few that have been tested in human trials. Therefore, in this chapter, we sum up the currently available data on these two species and discuss the potential outcomes for future diabetes management using fruit products. At the same time, we do not forget about other species, research on which is less advanced but still promising and worthy of further attention, e.g., Amelanchier sp., Chaenomeles sp., Crataegus sp., Pyrus sp., Sorbus sp., and other Aronia sp., and Malus sp.

The Anti-Diabetic Potential of Aronia melanocarpa Fruits
The anti-diabetic potential of Aronia melanocarpa seems to be mainly due to the increase in insulin sensitivity and the boost in the peripheral absorption of glucose (likewise, e.g., metformin), as well as the delay in the absorption of carbohydrates from the intestine (likewise, e.g., acarbose).
According to Chen and Meng [100], the five-week supplementation of acidified 80% ethanol Aronia melanocarpa fruit extract, at 300 mg/kg/day, resulted in about 1.6-fold lower glucose levels, about 2-fold lower insulin levels, and about 1.8-fold lower HbA1c levels in comparison to diabetic mice, while the effect of metformin (200 mg/kg/day) was about 2-fold lower glucose levels, about 2.2-fold lower insulin levels, and about 2.3-fold lower HbA1c levels. The effectiveness of Aronia extract relative to the synthetic anti-diabetic drug was also observed in the enhancement of GLUT-4 and p-GSK-3β expression, resulting in higher glucose uptake by hepatic cells and stimulating glycogen synthesis [100]. The increased tissue-specific glucose uptake and glycogen levels, as well as the modulation of different proteins' expression in the insulin signalling pathway, which led to a decrease in glucose levels, was confirmed by many in vitro and in vivo studies (Tables 1-3). What is essential in this metformin-like mechanism of action is that the sensitisation of tissues to the action of insulin occurs without increasing the insulin level, which reduces the risk of hypoglycaemia as the main disadvantage of sulfonylurea drugs (e.g., glipizide).
The delay in the absorption of carbohydrates from the intestine observed for Aronia melanocarpa fruit products relies on glucosidase inhibition, which is therefore another verified mechanism of action of the fruit. According to different studies [114,115], Aronia juices and extracts significantly inhibited α-glucosidase, maltase, and sucrase activity in the intestine (in vivo animal models). The effectiveness of Aronia extracts towards αglucosidase compared to acarbose (in vitro studies) was measured as 37-186 times higher, depending on the fruit cultivar, the origin of the sample, and the type of extract (and, thus, chemical composition). It is also advantageous that synthetic glucosidase inhibitors are associated with some slight gastrointestinal side effects, of which fruits seem to be devoid [143,144].
Finally, as mentioned above, the activity of fruit products is closely related to their composition. From the whole chemical pool of Aronia melanocarpa fruits (over one hundred phenolic compounds identified), only a few have been tested as contributors to the antidiabetic potential of fruit. These were mainly anthocyanins (i.e., cyanidin 3-arabinoside, cyanidin 3-glucoside, cyanidin 3-galactoside, cyanidin 3-xyloside, cyanidin 3,5-diglucoside), which accounted for about 10-25% (or 98% in case of the purified fraction) of extracts/juice dry mass (Tables 1-3). Like Aronia fruits, these compounds were able to lower blood glucose, HbA1c, and GLP-1 levels, inhibit DPP IV and α-glucosidase activity, increase glycogen levels in the liver, and modulate hepatic protein expression (in vitro and in vivo models) [12,58,100,119]. The contribution to the α-glucosidase-inhibitory activity was also noticed for procyanidin dimers, trimers, and some phenolic acids (i.e., chlorogenic and caffeic acids) [12,14,58]. Moreover, considering the contents of individual compounds and their activity, as well as the activity of relevant plant samples, the synergistic and additive effects of different constituents were suggested [119,131]. Synergy with synthetic anti-diabetic drugs is also possible, but this matter has not been studied so far.

The Anti-Diabetic Potential of Malus domestica Fruits
In the case of Malus domestica fruits, their effectiveness is suggested to be mainly due to the inhibition of both sodium-dependent and sodium-independent glucose transporters in the intestine (SGLT-1 and GLUT-2), which results in a delay in the absorption of carbohydrates [32,34]. SGLT-1 inhibitors are currently an eagerly studied group of anti-diabetic drugs (synthetic and plant-derived) whose potential is due not only to their glucoselowering ability but also to their cardioprotective effects. One of the leading natural SGLT-1 inhibitors is phlorizin-a dihydrochalcone isolated from apples [147]. Indeed, the phlorizincontaining apple products were also confirmed to reduce glucose-derived protein damage by inhibiting the activity of ALR and SDH enzymes and the formation of AGEs (polyol pathway); thus, they may prevent the development of diabetic complications [41,74,80].
The second type of Malus domestica fruit activity related to the content of phlorizin (5-16%) was suggested to be the increased insulin sensitivity resulting from the enhanced hepatic glucose uptake and expression of proteins involved in glycogen synthesis and glycolysis (e.g., ↑ p-GSK3β/GSK3β, ↑ p-FOXO1/FOXO1) [147]. Therefore, Malus domestica is another species with a metformin-like mechanism of action, i.e., the sensitisation of tissues to the action of insulin without increasing the insulin level. Indeed, the in vivo studies confirmed the Malus extracts'/juices' ability to lower both glucose and insulin in diabetic animals to levels comparable to those observed in healthy subjects, as well as to normalise the postprandial and OGTT parameters in healthy people [32,34,73,75,109,139,140,143].
As for anti-diabetic bioactive compounds, aside from phlorizin, there has also been research on the contributions of chlorogenic acid, procyanidins, and quercetin derivatives to hepatic and intestinal glucose transport, as well as AGEs and α-glucosidase inhibition [33,37,42,79]. Interestingly, while the inhibitory potential of apples towards α-glucosidase seems to be less evident (i.e., only extracts with increased contents of polyphenols-about 40-80%-were able to inhibit glucosidase more strongly than acarbose), the synergistic anti-glucosidase activity with acarbose was confirmed for apple juice. Thus, this is another beneficial aspect of using fruit products in combinatory therapy for diabetes.

Conclusions
The data reported in this review show that many Maleae fruits indeed have antidiabetic potential and may be recommended for the prevention and treatment of diabetes. However, from over 1000 species belonging to the Maleae tribe, only 46 have been investigated so far in the context of diabetes. The majority of the conducted studies covered only in vitro tests (67 papers); then, there are in vivo studies on animal models (51 reports, including some mixed in vitro/in vivo tests), as well as in vivo human studies (14 trials on Aronia melanocarpa, Malus sp., and Pyrus pyrifolia). The species most thoroughly stud-ied in terms of anti-diabetic effects and mechanisms were Amelanchier alnifolia, Aronia melanocarpa, Chaenomeles japonica, Crataegus pinnatifida, Malus domestica, Malus pumila, and Pyrus pyrifolia. The reviewed papers indicated the ability of Maleae fruits, e.g., to modulate the expression and activity of the proteins in the insulin-mediated PI3K/Akt pathway, induce incretin-based effects (e.g., GLP-1 and GIP agonism, DPP IV inhibition), regulate the intestinal glucose absorption and tissue-specific glucose uptake by affecting the glucose transporters (GLUTs and SGLT1), and inhibit enzymes involved in carbohydrate digestion (α-glucosidase) or in the polyol pathway of glucose metabolism (ALR, SDH). As for phytochemicals responsible for the anti-diabetic effectiveness of Maleae fruits, some reviewed papers suggested contributions of various compounds to the observed effects-primarily polyphenols (e.g., flavonols, dihydrochalcones, proanthocyanidins, anthocyanins, phenolic acids), but also triterpenes and polysaccharides. Thanks to the additive and synergistic actions of individual phytochemicals, the biological effects of fruits/juices/extracts are significantly higher than those of pure compounds. Therefore, various fruits from the Maleae tribe seem to be advantageous anti-diabetic agents. Still, their potential for functional application depends on various factors, in addition to the obvious plant species/variety, all of affect the composition of fruit products and, thus, may explain some discrepancies in the results of different studies. These include variations in climatic growth conditions, maturity stage, type of sample (i.e., the part of the fruit, like peel or flesh), processing conditions, or ingested form, e.g., if consumed as fresh fruits, juices, or specific extracts with high concentrations of selected active compounds. The latter form seems especially promising, as it reduces excessive intake of ballast constituents, primarily diabetes-promoting free monosaccharides. Finally, the prospects of different fruits and their extracts for more expansive pro-health applications require further research, especially more profound in vivo trials with the establishment of effective doses, and formulation and toxicity studies on the fruit extracts as potential herbal drug candidates.