Traditional medicinal plants used for the treatment of diabetes in the Sudan : A review

The prevalence of metabolic disorders such as diabetes among population is of increasing concern worldwide. Sudan is a developing country, where several areas frequently depend on folk medicine. Several herbal preparations have been used in folklore practice in Sudan for the management of diabetes with claims asserting their hypoglycemic effect. Basic research relating to these plants are reviewed in this paper with the intention to highlight their therapeutic potential for the treatment of diabetes and promote their regular use in Sudan. Ethnobotanical information was obtained by an assessment of the available literature in electronic data bases with publications describing the medicinal plants used for the treatment of diabetes. In this review paper, different parts of 38 plant species, are described that are used in the Sudanese traditional medicine and belong to 35 genera and 23 families. Thirty three plants have been documented in scientific literature to possess in vivo antidiabetic activity and only one was ineffective in lowering blood glucose level, namely Striga hermonthica. Many of the plants in the study review have been studied in in vitro models (such as αamylase or α-glucosidase inhibition) in an effort to explain some of their biomedical interaction. The role of isolated bioactive compounds like trigonelline and 3, 5-dicaffeoylquinic acid in diabetes management is also evaluated in the present review. Ten plants original from Sudan have been already used in clinical trials for the treatment of type 2 diabetes. This review provides useful information on the characterization of such herbal medicines that are utilized in the Sudanese traditional medicine for the control of metabolic syndromes such as diabetes.


INTRODUCTION
Diabetes is a metabolic disorder characterized by chronic hyperglycemia which results from insulin deficient secretion or impaired cellular action of the hormone.
Insufficient insulin secretion caused by immune destruction of pancreatic β-cells are vital for insulin secretion and consequent development of insulin-*Corresponding author.E-mail: sakinayagi@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License dependent diabetes (Type 1).In case of abnormal insulin cellular action, type-2 diabetes is developed due to a gradual development of insulin resistance and pancreatic β-cell dysfunction.Symptoms are worsen due to obesity and lack of physical activity by the sedentary lifestyle (Gutch et al., 2014).
Diabetes prevalence is on the rise worldwide as a result of accumulating risk factors well pronounced in economically growing nations.An estimated 69% rise is observed for the prevalence of the disease in adults in developing countries versus 20% for adults in developed countries (Shaw et al., 2010).The prevalence rate of diabetes in Sudan ranks the disease high among others that are of raising great concern for the community and for the medical service (Ahmad et al., 2008).
The total population in Sudan is about 34 million, with 70% of it in the Northern parts.Prevalence of type 1 diabetes is estimated at 0.1% among school-age children 7 to 14 years old while type 2 diabetes is estimated at 10.4% among adult population (over 25 year of age) in Northern Sudan (Elamin et al., 1989;Motala et al., 2003).
A complex mechanism involving enzymes and other factors influences the action of insulin in the management of hyperglycemia.Mainstream drugs that are used to control diabetes fall into three main categories.The first category of drugs aims to enhance endogenous insulin availability and includes agents that act on the sulfonyl urea receptors in the pancreas to promote insulin secretion and others that have an impact on the small intestinal mucosal epithelium.
Medicines categorized as group two are directed to potentiate the response to insulin, among them being thiazoline.This group of drugs seems to act as initiator of peroxisomal receptors responsible for regulation of metabolism of carbohydrates, lipids and proteins.The drugs categorized as group three are represented by αglucosdiase inhibitors and are targeted at reducing the metabolism of complex sugars (Sheehan, 2003;Thompson and Davis, 2017).
Current glycemic medications burden patients due to variable contraindications and interactions with other drugs as well as limited efficacy, limited tolerability and significant side effects arising from their complex mechanisms of action (Moller, 2001;Rotenstein et al., 2012).Existing pharmacotherapy is still far from achieving optimal blood sugar control in such patients, as an effect of a dysfunction in insulin secretion, action, or both (Gupta et al., 2016).
A number of reviews from different countries have highlighted the significance of medicinal plants application for the control of diabetes (Ezuruike and Prieto, 2014).Despite the wide-spread traditional use of these plants for diabetes management in Sudan, scientific support for the safe and effective use of such plants does not exist.The aim of the present work was to collect data relevant to the traditional control of diabetes with plants wellknown for their medicinal effects in Sudan.

Traditional medicinal plants for the control of diabetes
Data about the use of plants in folklore medicine for the control of diabetes relates either with previously conducted ethnobotanical studies or published papers demonstrating the antidiabetic impact of certain plants in Sudan.The experimental evidence of the antidiabetic activity is highlighted in this review, along with biochemical analysis (where available).Considering comprehensiveness and increasing interest in antioxidant activity in relation to antidiabetic effect of plants, the antioxidant activity of these plants are also reviewed (Ezuruike and Prieto, 2014).A literature search of electronic databases on these plants was carried out and for each of the identified plants, synonyms (for plants which were not identified with their accepted names in the original publication), family name, local name, and plant part used are compiled in Table 1.Available pharmacological evidence of these plants for their therapeutic use is also summarized, and most toxicological relevant studies are presented in Table 2.

Acacia nilotica
A dose of 400 mg/kg body weight (b.w) of an aqueous methanol extract of A. nilotica pods significantly reduced the levels of blood glucose, the plasma total cholesterol (TC), total triglyceride (TTG), low-density lipids (LDL), the activity of serum glutamate oxaloacetate (GOT) and pyruvate transaminase (GPT) after one month of treatment in diabetic rabbits compared to the untreated diabetic ones.Furthermore, the same dose also significantly increased the plasma high density lipids (HDL) levels of the treated rabbits but not significant effect on creatinine clearance was observed (Ahmad et al., 2008).Hot water extract of A. nilotica pods decreased significantly the plasma glucose level of alloxan-induced Albino mice after 1 to 2 h of administration (Abd el-aziz et al., 2013).A similar observation was obtained in Wistar albino rats treated with 400 to 800 mg/kg of aqueous extract of A. nilotica pods and 800 mg/kg of ethyl-acetate and n-butanol fractionated from aqueous extract after 12 to 18 h of administration (Auwal et al., 2013).Moreover, Tanko et al. (2013) demonstrated that ethyl acetate fraction obtained from the methanolic extract of A. nilotica leaves had a remarkable hypoglycemic effect in alloxaninduced diabetic rats after treatment with 50 and 100 mg/kg for 7 to 12 days.Modified lignin extracted from the hardwood of A. nilotica exhibited increased glucose binding efficiency as demonstrated by the decreased glucose diffusion and enhanced α-amylase inhibition in

Kigelia africana
Fruits of the plant given to the experimental rats at doses 100, 200 and 400 mg/kg/day orally were toxic but not fatal.

Mitragyna inermis
Alkaloid rich extract derived from M. inermis induced a strong inhibition of protein synthesis in mammalian cells but did not exhibit mutagenic or genotoxic activity Traore et al. (2000) Momordica balsamina In vitro toxicity results raise concern for chronic use van de Venter et al. ( 2008)

Nauclea latifolia
An alkaloid rich extract derived from N. latifolia could interact in vitro with DNA of bacteria and mammalian cells, leading to G2-M cell cycle arrest and heritable DNA-damage, as well as inducing in vivo single-strand breaks in liver, kidney and blood cells

Rhynchosia minima
The plant is reported as toxic to fish.The seeds are bitter and poisonous and seed extract shows specific agglutinating action on human RBC Mali and Mahale (2008)

Senna obtusifolia
The leaves can cause marked toxic effects on rats, and the processing of the leaves by fermentation to produce kawal did not alter the toxic activity of the ingredients in the leaves

Sesamum indicum
Sesame oil or its lignans, due specifically to their methylenedioxyphenyl group, could interact with the P450 isozymes and affect the drug metabolisms or dispositions in human Gokbulut ( 2010)

Striga hermonthica
Aqueous extract of whole plant was safe up to a dose of 2000 mg/kg body weight Alamin et al. (2015) Tinospora bakis Aqueous extract of seeds was safe up to a dose of 2000 mg/kg body weight.

Alamin et al. (2015)
Trigonella foenumgraecum Short-term (90 days) and long term (24 week) feeding of fenugreek seeds to rats at levels equivalent to 2 and 4 times the therapeutic dose recommended for humans (25 g/day) produced no toxic effects.Udayasekhara et al. (1996) and Sharma et al. (1996) comparison to the controls (Barapatre et al., 2015).

Acacia senegal
Administration of 200 and 400 mg/kg b.w of ethyl acetate extract from the stem bark of A. senegal significantly lowered the levels of blood glucose, serum TC, serum TTG, serum LDL, serum urea and creatinine, and increased the serum HDL level in alloxan-induced diabetic albino rats on day 16 after the administration (Batra et al., 2013).Treatment of CCl4-induced acute hepatotoxicity in albino Wistar rats with 400 and 800 mg/kg/day of the hydroalcoholic (70% ethanol) extract of A. senegal pods, orally for 7 days, significantly reduced the liver damage and the symptoms of liver injury by restoration of architecture of liver as indicated by lower levels of serum bilirubin and prevention of hepatic damage (Pal et al., 2014).The components extracted by ethanol from the leaves of A. Senegal decreased the activity of sucrose enzyme and appeared to support the control of carbohydrate hydrolysis, and consequently reduces the rise of postprandial blood glucose in diabetics (Abdelhady and Youns, 2014).

Aloe sinkatana
The effects of aqueous extracts of A. sinkatana leaves on blood glucose and lipid profile in type 2 diabetic patients was evaluated by Gaber et al. (2013).The volunteers of the experimental group received the aqueous extract (500 g/l) at a dose of 5 mL/day.A significant reduction in levels of fasting blood sugar, TTG, TC and LDL and a significant increase in HDL levels was observed.

Allium cepa
A detailed review on the positive antidiabetic activity effect of A. cepa in different animal models, and its antioxidant activity as well as clinical studies on diabetic patients was presented by Akash et al. (2014).

Ambrosia maritima
Administration of water, 50% ethanolic, ether or petroleum ether extracts of A. maritima whole plant to albino rats significantly reduced blood glucose after 1.5 and 2 h, however without significant changes in insulin levels (Ammar et al., 1993).Alloxan-induced diabetic albino rats treated orally with 28.5 mg/ kg b.w. of aqueous extract of A. maritima aerial parts twice/ day showed significant improvement in most of biochemical parameters (levels of fasting blood glucose, serum insulin, total proteins, albumin, globulin, HDL, aspartate aminotransferase (AST), alanine aminotransferase (ALT), urea, creatinine, uric acid, serum TC, TTG and LDL) (Helal et al., 2014).

Ammi visnaga
Aqueous extract of A. visnaga at the dose of 20 mg/kg b.w significantly reduced blood glucose in inducedstreptozotocin diabetic rats after repeated oral administration for nine days (Jouad et al., 2002).

Balanites aegyptiaca
The addition of l0 % whole or extracted pulp of B. aegyptiaca fruits instead of starch in the basal diet of alloxan-induced albino rats, for 20 days, caused a significant decrease in serum glucose level and inhibited the activities of serum GOT and GPT (El-Saadany et al., 1986).Aqueous extract of mesocarps of the fruits exhibited a prominent antidiabetic activity when offered orally in streptozotocin-induced diabetic mice (Kamel et al., 1991).Administration of fruits aqueous extract (1.5 g/kg b.w daily for 45 days) in streptozotocin-induced Wistar albino diabetic rats significantly reduced the mean plasma glucose and malondialdehyde levels, and significantly increased the mean plasma insulin, liverpyruvate kinase, and total antioxidant capacity levels.An obvious increase in the weight of the pancreas and the size of the islets of Langerhans, and improvement in the histoarchitecture were also evident in the treated groups compared to untreated ones (Khalil et al., 2016).The antidiabetic activities of different fruit extracts and fractions of B. aegyptiaca were tested in cultured C2C12 skeletal muscle cells and 3T3-L1 adipocytes.An 18 h treatment with 200 µg/mL of the sugars fraction, dichloromethane (E) and ethyl acetate (F) successive extracts increased basal glucose uptake in muscle cells.Only E and F extracts accelerated the triglyceride accumulation in pre-adipocytes undergoing differentiation (Motaal et al., 2012).Dichloromethane and ethyl acetate extracts of the fruit were standardized by highperformance liquid chromatography to contain 0.031 and 0.239% of rutin, and 0.007 and 0.004% of isorhamnetin, respectively (Abdel Motaal et al., 2012).Trigonelline (3carboxy-1-methyl pyridinium) was identified in the fruits (8 and 13 mg in the peel and pulp respectively) in addition to the flavonoids quercetin, isorhamnetin flavonol and epicatechin (Farag et al., 2015).Saponins, 26-O-beta-Dglucopyranosyl-(25R)-furost-5-ene-3 beta, 22, 26-triol 3 (1--4)]-beta-Dglucopyranoside and its 22-methyl ether, 26-O-beta-Dglucopyranosyl-(25R)-furost-5-ene-3 beta,22,26-triol 3-O-(2,4-di-O-alpha-L-rhamnopyranosyl)-beta-D glucopyranoside and its methyl ether were also isolated and identified.It was revealed that the individual saponins did not show antidiabetic activity, while their combination resulted in significant activity.

Bauhinia rufescens
The oral administration of 200, 300, and 400 mg/kg b.w methanol extract from the leaves of B. rufescens (once a day, for four weeks) significantly lowered the blood glucose levels in alloxan-induced diabetic rats in a dose dependent manner (Aguh et al., 2013).

Catunaregam nilotica
Acute and chronic treatment of streptozotocin-induced diabetes rats with aqueous extracts of C. nilotica (Syn.Randia nilotica) fruit at 400 mg/kg significantly lowered blood glucose, serum lipid and creatinine levels, and brought back the activity of AST enzyme to normal level.Histopathological studies showed that the aqueous extracts of the plant reinforced the protection of liver (Alamin et al., 2015).Methanolic extracts of leaves, bark and seedcake of C. nilotica possess good antioxidant activity and high phenolic content (Mariod et al., 2012).

Capparis decidua
Fruits of C. decidua decreased the lipid peroxidation and altered free radical scavenging enzymes such as superoxide dismutase and catalase in erythrocytes, liver, kidney and heart in alloxan induced diabetic rats (Agarwal and Chavan, 1988;Yadav et al., 1997).Moreover, the fruit extract showed satisfactory inhibitory effect on α-amylase and α-glucosidase enzymes, followed by flowers and leaves extracts (Zia-Ul- Haq et al., 2011).

Cicer arietinum
Adminstration of petroleum ether extract (400 mg/kg) of the seed to alloxan-induced diabetic mice reduced significantly the serum glucose level in both acute and subacute studies (Yadav et al., 2009).The seed showed significant diphenylpicrylhydrazyl (DPPH), nitric oxide and hydrogen peroxide activity (Vadnere et al., 2013).

Cinnamomum verum
Administration of 200 mg/kg b.w of cinnamon aqueous extract to alloxan-induced diabetic rats lowered significantly the levels of fasting blood glucose, TC, HDL, LDL and TG (El-Desoky et al., 2012).Moreover, administration of bark aqueous extract of cinnamon containing 45 and 75% gallic acid equivalents of polyphenol to streptozotocin-induced diabetic rats at 200 mg per kg b.w. for 30 days displayed hypoglycemic and hypolipidimic effects (IM et al., 2014).The bark is rich in volatile oil and polyphenols including rutin, catechin, quercetin, kaempferol and isorhamnetin have been isolated (Yang et al., 2012).

Citrullus colocynthis
Administration of roots aqueous extract (2000 mg/kg) to alloxan-induced diabetic rats showed hypoglycemic effect and improved serum levels of urea and lipid (Agarwal et al., 2012).Moreover, hydroethanol extract (300 mg/kg bw) of the seed reduced significantly the blood glucose level in alloxan-induced diabetic rats (Oryan et al., 2014).Petroleum ether extract (300 and 500 mg/kg bw) of fruit pulp showed significant hypoglycemic effect in streptozotocin-induced diabetes albino rats (Jayaraman et al., 2009).

Cyperus rotundus
The ethanolic extract of C. rotundus rhizomes at dose of 250 and 500 mg/kg b.w, for 3 weeks, revealed significant antidiabetic activity and resulted in improvement of body weight and reduction in the levels of biochemical parameters such as GPT, GOT, TC and TTG in streptozotocin-induced diabetic mice (Singh et al., 2015).

Eucalyptus globulus
Administration of leaf aqueous extract of E. globulus at a dose of 150 mg/kg b. w decreased the blood glucose and lipid levels in alloxan induced diabetic rats (Patra et al., 2009).Aqueous ethanolic leaf extract at a dose of 400 mg/kg b.w reduced also the blood glucose level in glucose loaded rats (Houacine et al., 2012).Incorporation of E. globulus leaf in diet (20 g/kg) and drinking water (2.5 g/L) had hypoglycemic effect and reduced oxidative stress in streptozotocin-induced diabetic rats (Nakhaee et al., 2009).

Faidherbia albida
The administration of an aqueous extract from the stem bark of F. albida at dose 125 to 500 mg/kg b.w to alloxaninduced diabetic rats decreased significantly the fasting blood glucose level in a dose dependent manner and ameliorated the serum markers of the liver, feed and fluid Yagi and Yagi 33 intake, body weight and packed cell volume (Umar et al.,2014).

Geigeria alata
Diabetic rats orally treated with 250 mg/kg of G. alata root aqueous methanolic extract for 2 h (acute) appeared to have significantly lower blood glucose levels after 120 min.Constant treatment for 14 days of diabetic rats with 250 mg/kg of G. alata extract resulted in a significant decrease in blood glucose level (7.34±0.33 mmol/l) closer to that of nondiabetic rats.At the same time, it significantly decreased serum TTG levels, increased serum insulin levels, improved β-cell function, and the antioxidant status.G. alata also showed strong antioxidant and α-glucosidase inhibitory activities in in vitro assays (Hafizur et al., 2012).

Guiera senegalensis
A dose-dependent significant reduction in blood glucose levels which was more remarkable at the dose of 400 mg/kg was observed after the application of G. senegalensis leaves ethanolic extract (Houacine et al., 2012).

Hyphaene thebaica
Oral administration of aqueous extract of H. thebaica mesocarp experimentally caused a significant decrease in blood glucose level in Wistar albino rats, at 12 to 18 h post administration (Auwal et al., 2012).Aqueous extract improved glucose and insulin tolerance, and significantly lowered blood glycosylated hemoglobin levels.Chrysoeriol and 7-O-β-D-galactopyranosyl(1→2)-α-Larabinofuranoside, which were isolated in the aqueous extract reduced significantly AST and ALT levels of liver and improved the kidney function (Salib et al., 2013).

Khaya senegalensis
The antidiabetic activity of K. senegalensis butanol fraction of the root ethanolic extract in type 2 diabetes model of rats was examined by Ibrahim and Islam (2014).The orally administered extract, at 300 mg/kg b.w, significantly reduced blood glucose level, improved oral glucose tolerance ability and β-cell function (HOMA-β), decreased insulin resistance (HOMA-IR), stimulated hepatic glycogen synthesis, ameliorated serum lipids alterations and prevented hepatic and renal damages compared to untreated diabetic rats.Additionally, the fraction tended to improve weight gain, decrease feed and fluid intake, stimulate insulin secretion and lower serum fructosamine concentrations.Polyphenolic compounds such as catechin, rutin and procyanidins with significant antioxidant activities were also identified in different parts of the plant (Atawodi et al., 2009).

Kigelia africana
In streptozotocin-induced diabetic rats, daily administration of the defatted methanolic extract of K. africana flower at the doses of 250 and 500 mg/kg b.w for 21 days reduced significantly the blood glucose and the TC and TTG levels as well (Kumar et al., 2012).Similarly, methanolic extract from the leaves was found to significantly decrease (P<0.01)serum glucose level in alloxan-induced diabetic rats after the 21 days of oral treatment (Priya et al., 2014).The ethanolic extract, together with compounds catalpol, specioside and minecoside (10 μM) isolated from the n-butanol fraction exhibited significant stimulation of GLUT4 translocation to cell surface from intracellular compartments (Khan et al., 2012).Acetone, ethanol, chloroform, and water extracts of the leaves caused a significant α-amylase inhibitory effect (Dhriti et al., 2014).The root, stem bark, fruit and leaves were found to possess antioxidant activity (Atolani et al., 2011;Sikder et al., 2011;Agyare et al., 2013;Akanni et al., 2014).

Lupinus termis
A dose (75 mg/100 g b.w) of aqueous suspension from L. albus orally administered daily to alloxan-diabetic rats restore the changes in the levels of glucose, urea, creatinine and bilirubin and the enzymic activities of AST, ALT and lactate dehydrogenase (LDH) to their normal levels after 4 weeks of treatment (Mansour et al., 2002).In contrast, Sewani-Rusike et al. (2015) reported that the use of L. albus may not be effective in treating hyperglycaemia in type 1 diabetes but effective for treating diabetes induced dyslipidemia.They found that L. albus demonstrated significant hypoglycaemic effects in normal rats but not in diabetic rats after acute and long term treatment.Normal treated rats showed higher insulin levels compared to normal controls but insulin remained very low in diabetic rats.However, L. albus was effective in reducing atherogenic lipid levels.

Mitragyna inremis
Oral administration of aqueous extracts from M. inremis fruits at the level of 400 mg/kg to streptozotocin-induced diabetes rats, for 14 days, resulted in a significant antihyperglycemic effect and have the capacity to correct the metabolic disturbances associated with diabetes.Histopathological studies showed that the aqueous extracts of the plant reinforced the protection of liver (Alamin et al., 2015).

Momordica balsamina
Aqueous extract of M. balsamina seeds at the level of 500 mg/kg b.w dose caused a significant increase in the blood glucose levels of streptozotocin-induced diabetic rats.Furthermore, after three weeks of treatment of the diabetic animals with the aqueous extract (500 mg/kg b.w) blood sugar level was significantly higher compared to untreated diabetic rats; at the same time, lipid profile and body weight were improved (Bhardwaj et al., 2010).Moreover, aqueous and organic extracts of M. balsamina was screened against chang liver, C2C12 muscle and 3T3-L1 adipose cells using a glucose utilization assay.Results showed that M. balsamina extracts were active in myocytes and stimulated glucose utilisation in hepatocytes (van de Venter et al., 2008).

Nauclea latifolia
Aqueous leaves extracts of N. latifolia at the level of 200 mg/kg b.w significantly lowered glucose levels of the alloxan-induced diabetic rats within 4 h (Gidado et al., 2005).Moreover, the aqueous and ethanolic extracts significantly lowered the fasting blood glucose levels of the streptozotocin-diabetic Wistar rats in a dosedependent manner after 1-6 h of administration (Gidado et al., 2008).The same results were observed when ethanolic extracts (100, 200 and 400mg/kg b.w) of the leaves were provided orally for 45 days to streptozotocininduced diabetic rats (Abubakar et al., 2009).Significant reduction was found in the fasting blood glucose, lipid profile (TG and LDL) levels in diabetic rats administered 150 and 300 mg/kg b. w. of n-hexane and methanolic leaves fractions of N. latifolia (Effiong et al., 2014).Treatment of Swiss albino mice with 200 mg/kg b.w of ethanolic extract of leaves, twice a day for 21 days, decreased significantly blood glucose in diabetic animals and caused significant decrease (p<0.05) in TC, LDL level and ALT and AST activities (Sylvester and Dan, 2015).

Nigella sativa
Several studies demonstrated the hypoglycemic effect of N. sativa seed (Benhaddou-Andaloussi et al., 2011;Sathiavelu et al., 2013;Ikram and Hussain, 2014;El Rabey et al., 2017).The seed was shown to ameliorate biochemical and histopathological changes caused by diabetes, decrease oxidative stress, elevate level of insulin, reduce resistance of insulin and hepatic gluconeogenesis, enhance renewal of ß-cells of islets of Langerhans and create direct insulin-like effects at the cellular and molecular levels in various organs.Seed volatile oil and thymoquinone were found to possess the highest antidiabetic activity (Bamosa, 2015).

Salvia officinalis
The hypoglycemic effect of the aqueous ethanolic extract of S. officinalis leaves at the dose of 200 to 400 mg/kg is revealed as a dose-dependent significant reduction of blood glucose levels (Houacine et al., 2012).

Sclerocarya birrea
Following acute treatment, relatively moderate to high doses of S. birrea stem-bark aqueous extract (25 to 800 mg/kg b.w) induced a dose-dependent, significant reduction in the blood glucose concentrations of fasted streptozotocin-treated diabetic rats (Ojewole, 2003).Results from male Wistar rats subjected to oral load of glucose (4g/kg) after receiving a dose of 35 mg/kg of aqueous extracts of fresh leaves and barks of S. birrea showed that the extracts caused significant antihypergliycemic effects after 2 and 4 h (François et al., 2014).Aqueous and methanolic extracts of the stem bark inhibited the activities of α-amylase and α-glucosidase in a concentration dependent manner.Both extracts possess antioxidant activity, with the methanolic extracts displaying the strongest free radical scavenging capacity.Extracts also significantly increased glucose uptake in C2C12 myotubes, 3T3-L1 adipocytes and HepG2 hepatocarcinoma cells.However, insulin secretion from RIN-m5F cells was not affected (Mousinho et al., 2013).Crude S. birrea stem bark methanolic and acetone extracts inhibited human urinary α-amylase more potently than acarbose.Crude hexane extract displayed a strong inhibition of α-glucosidase and weak inhibition of α-amylase.Furthermore, the hexane extract significantly suppressed the rise in postprandial glucose level after oral administration of sucrose but failed to induce similar effects after oral administration of starch and glucose in both normal and diabetic rats (Mogale et al., 2011).

Sesamum indicum
Treatment of streptozotocin-induced diabetic rats with 500 mg/kg b.w ethanolic extract of S. indicum seeds for 8 weeks increased significantly the blood glucose and glycosylated hemoglobin levels but decreased significantly the serum insulin and hemoglobin levels.The liver glycogen level was significantly decreased in diabetic rats closer to normal revealing its potential effect to control hyperglycemia (Bhuvaneswari and Yagi and Yagi 35 Krishnakumari, 2012).Alloxan-induced diabetic rats provided with 10% and 20% seeds either raw or roasted as supplemented diet had significantly (p<0.05)lower levels of blood glucose, lipids and some serum enzymes (Akanya et al., 2015).Takeuchi et al. (2001) found that hot-water extract from defatted sesame seed and its methanolic fraction had a reductive effect on the plasma glucose concentration of KK-Ay mice, and this effect is suggested to have been caused by the delayed glucose absorption.Amutha and Godavari (2016) demonstrated that S. indicum can be used to reduce the postprandial hyperglycemia by inhibiting carbohydrates metabolizing enzymes α-amylase and αglucosidase, and also to combat the free radicals due to its antioxidant activity.It has been reported that sesame seeds can improve oxidative status due to the activities of their contents including sesamin, sesamolin, sesamol, and sesame (Wichitsranoi et al., 2011).

Striga hermonthica
Daily oral administration of S. hermonthica whole plant aqueous extract (400 mg/kg b.w) to streptozotocininduced diabetic rats for 14 days appeared to increase the blood glucose level, and did not improve the levels of TC, LDL, HDL, urea and blood urea nitrogen indicating that it has no antihyperglycemic effect (Alamin et al., 2015).Kiendrebeogo et al. (2005) found that the aqueous extract of S. hermonthica whole plant possessed antioxidant activity and they suggested that the isolated luteolin could be responsible for this activity.

Tinospora bakis
Acute and chronic treatment of streptozotocin-induced diabetes rats with aqueous extracts of T. bakis seeds at 400 mg/kg significantly lowered blood glucose levels, and had the capacity to correct the metabolic disturbances associated with diabetes.Histopathological studies showed that the aqueous extracts of the plant reinforced the healing of liver (Alamin et al., 2015).

Trigonella foenum-graecum
Animal's standard diet supplemented with seeds of T. foenum-graecum (5%) for 30 days to alloxan induced diabetic rats significantly decreased the levels of glucose, TG, TC and LDL-CH and increased the level of HDL-CH.Also it reduced the oxidative stress by improving the superoxide dismutase, catalase and glutathione peroxidase activities both in serum and in pancreas homogenate (Beji et al. 2016).Aministration of T. foenum-graecum seeds (2.5 and 5 g) for 4 weeks to Tunisian type 2 diabetic patients, improved blood glucose level in dose-dependent and the dose of 5 g reduced significantly TC and TG levels and serum α-amylase activity (Khlifi et al., 2016).

DISCUSSION
Sudan is a developing country that frequently depends on folk medicine in all areas of the country.Several herbal preparations have been used in folklore practice for the management of diabetes with claims asserting their hypoglycemic effect.In this paper, an effort was made to refer to the different parts of 38 plant species that are used in the Sudanese traditional medicine (Table 1).Interestingly, some of these plants have already been reported in previous studies originated from other countries like Algeria (Houacine et al., 2012), Iran (Mikaili et al., 2013), Egypt (Helal et al., 2014), India (Singh et al., 2015), Nigeria (Auwal et al., 2012) and Saudi Arabia (Bamosa, 2010).The reviewed plants have been evaluated, in in vivo experiments with diabetic animals that were induced either by alloxan or streptozotocin (Fröde and Medeiros, 2008) in addition to genetically mutated in vivo models such as KK-Ay mice.Ten of the characterized plants (Acacia nilotica, Catunaregam nilotica, Cicer arietinum, Cinnamomum verum, Geigeria alata, Guiera senegalensis, Khaya senegalensis, Mitragyna inremis, Momordica balsamina and Tinospora bakis) tested effective in animal models for their antidiabetic potential from samples collected from Sudan.

In vitro pharmacological evidence
From the 38 plants reviewed in this paper, only four of them were not tested for hypoglycaemic activity, either in vivo or in vitro.Only one from the 34 plant species was ineffective in lowering blood glucose level, namely Striga hermonthica (Alamin et al., 2015), suggesting lack of antidiabetic effect.Ezuruike and Prieto (2014) reported that the absence of an in vivo antihyperglycemic effect of some plants would not be a reason to stop their use as antidiabetics, since they may used in multicomponent preparations because of their benefits in co-morbid conditions or possibly be the foundation for comprehensive control of the disease and consequent complications.In fact, components aqueously extracted from S. hermonthica whole plant reduced the TG level, improved several liver parameters (reduced ALT activity) and possessed high antioxidant activity (Alamin et al., 2015;Kiendrebeogo et al., 2005).Many of the plants described in the present review have been studied in in vitro models that could possibly explain some of their mechanisms of action.Information on the mechanism of action would be an important element in implementing a therapeutic plan for diabetes, considering the likely benefit of the synergy of medicinal plants (Ezuruike and Prieto, 2014).Four plants (Acacia nilotica, Capparis decidua, Geigeria alata and Sclerocarya birrea) have inhibitory effects against either α-amylase or αglucosidase enzymes; Seven plants (Ambrosia maritima.Balanites aegyptiaca, Geigeria alata, Hyphaene thebaica, Khaya senegalensis, Sclerocarya birrea and Sesamum indicum) induce secretion of insulin from ß-cells of the pancrease; five plants (Balanites aegyptiaca, Cinnamomun verum, Kigelia africana, Momordica balsamina and Trigonella foenum-graecum enhance glucose absorption in muscles or liver or increase GLUT4 gene expression leading to enhanced glucose absorption by muscle and fat tissue and one plant decrease the activity of sucrose enzyme and offer a support to control carbohydrate hydrolysis in diabetic disease.

Bioactive compounds
A number of active compounds have been identified from the plants in this review paper but their role in diabetes management was not proved for most of them.However, trigonelline (3-carboxy-1-methyl pyridinium) was identified in Balanites aegyptiaca fruits (8 and 13 mg in the peel and pulp respectively) by Farag et al. (2015).Its discovery provides novel insight into the balanite fruits antidiabetic properties as the compound is known for a pronounced hypoglycemic effect (Farag et al., 2015).More recently, 3, 5-dicaffeoylquinic acid was found to be the dominant acylquinic acid in Geigeria alata roots (25.96±2.08 mg/g dry weight) and ameliorated significantly (P < 0.05) the blood glucose and liver biochemical parameters in streptozotocin-induced (40 mg/kg, i.p.) diabetic normotensive Wistar rats and spontaneously hypertensive rats (Simeonova et al., 2016).

Clinical studies
Clinical evaluation, involving human subjects, of biologically active plants is necessary towards the progress of incorporation of medicinal plant products in the health service system (Ezuruike and Prieto, 2014).In this review, 10 plants sourced from Sudan were subjected to clinical trials in Type 2 diabetic patients and results showed that the order of effectiveness of the aqueous extracts of the studied plants to lower fasting blood sugar level was Lupinus albus > Balanites aegyptiaca > Allium Sativum > Allium cepa > Guiera senegalensis > Aloe sinkatana > Hyphaene thebaica > Trigonella foenum-graecum (Gaber et al., 2013).Capsules containing Nigella sativa seeds were administered orally to human volunteers, in Saudi Arabia, in a dose of 1, 2 and 3 g/day for three months.The dose of 2 g/day caused significant reduction in fasting blood glucose levels, while β-cell function was increased after 12 weeks of treatment (Bamosa, 2010).

Toxicological evidence
Assessment of the safety and toxicity profile of herbal medicine is essential to ensure its therapeutic potential.A summary of the studies that describe the toxicological effects of medicinal plants is presented in Table 2.It has been noted that the majority of the investigations corresponded mainly to the determination of acute toxicity and safe dose and included very limited information concerning toxicological and herb-drug interactions.However, some of the plants listed in Table 2, like Allium cepa, A. sativum, Cinnamomun verum and Trigonella foenum-graecum are actually consumed frequently in Sudan and other countries, and are usually perceived as safe.However, Zaid et al. (2010) reported that garlic and onion bulbs share many similar active compounds (for example, allyl propyl and diallyl sulfide) and decrease blood glucose levels also by normalizing liver hexokinase and glucose-6-phosphatase activities and increase insulin secretion from the pancreas but excessive consumption of these two bulbous plants might lead to harmful effects.T. foenum-graecum and C. verum exhibited cytototoxic effects at concentrations higher than 500 µg/mL (Kadan et al., 2013).Moreover, consumers Yagi and Yagi 37 are usually aware of possible health hazards occurring after consumption of certain plants and the necessity of their proper process to remove toxicants before utilization.For example, the toxic lupinine found in Lupins (Lupinus termis) is removed through debittering process, including soaking in water and daily replacement of water until bitterness disappears before the seeds could be safely consumed.Although, people in Sudan and other African countries consume kawal (fermented fresh leaves of Senna obtusifolia), studies have shown that fermentation has not altered the toxic activity of the ingredients in the leaves (Yagi et al., 1998).Thus, toxicological evaluation of medicinal plants is equally significant as their evaluation for efficacy and there is an urgent need for a vibrant pharmacovigilance system to ensure their use in therapeutic management (Shaw et al., 2012).

Conclusion
The quest for control of diabetes has led to an increasing research at different fronts, among which is medicinal plants.Given the observation of an increasing use of medicinal plants for diabetes in Sudan, this necessitates validation of efficacy and safety.In vitro experiments are carried out to ascertain the mechanism of action of medicinal plants.The hypoglycemic effect of certain plants arises as a side effect of their in vivo toxicity (Marles and Farnsworth, 1995).However, a risk may be posed by the fact that such hypoglycemic effect is probably partially exhibited via an unfavourable physical mechanism overriding a physiological one.As for the validation of experiments, ethical considerations concerning animal use are increased (Festing and Wilkinson, 2007), and therefore the use of non-animal models should be seriously considered.A number of standardization measures, such as reference pharmacopoeial monographs, are necessary to assert the medicinal value of these herbal medicines as reliable and therapeutically effective.Case studies involving standardized medicinal plant products should be carried out in order to validate the usefulness of plant preparations in diabetes management, which will give support to the pre-clinical results.

Table 1 .
Plants used in Sudanese traditional medicine for treatment of diabetes.

Table 2 .
Toxicological studies on plants used in Sudanese traditional medicine for treatment of diabetes.
Dehghani and Shahin (2006)r cells which may induce hepatocyte necrosis and liver fibrosisDehghani and Shahin (2006)Cyperus rotundusEthanol extract of rhizomes was safe up to the dose 2000 mg/kg to Wistar rats Jebasingh et al. (2012) Eucalyptus globulus Ethanolic leaf extract have shown moderate brine shrimp lethality with LC50 value of 55.95 μg/mL.Houacine et al. (2012) Faidherbia albida Ethanolic stem bark extract of F. albida is relatively safe when used sub-acutely in rats.Oluwakanyinsola et al. (2010) Guiera senegalensis Ethanolic leaf extract have shown moderate brine shrimp lethality with LC50 value of 26.94 μg/mL Houacine et al. (2012)