BITTER HERBS OF EASTERN NIGERIA (GONGRONEMA LATIFOLIUM, VERNONIA AMYGDALINA AND VITEX DONIANA): A REVIEW

Background: The article is a review of three bitter vegetables consumed in South-eastern Nigeria (Gongronema latifolium, Vernonia amygdalina and Vitex doniana). These vegetables are not very palatable to taste but are highly famed because of the myriad of illnesses that they are reputed to cure and manage. The different aspects of these vegetables reviewed are their ethnobotany, zoo pharmacology, bioactivity, phytochemistry and toxicity. Materials and method: The literatures consulted were searched using electronic search engines (Google, Google scholar and Pubmed). Substantial amount of literature was consulted but only those directly related to the main review were selected. Result: The review revealed that the three bitter vegetables possessed phytochemicals like saponins, flavonoids, alkaloids among others that justify the claims of curing and managing many of the illnesses that traditional folks attribute to them. From literature, antimicrobial, antinflammatory, antidiabetic, antioxidant and anti cancer activities were attributed to these three bitter herbs. Antinutritional substances like phytate and oxalate reported in these herbs were not regarded as harmful for human consumption because of the traditional method of processing the vegetables by squeeze washing in several changes of water. Conclusion: The three bitter herbs are regarded as beneficial for human consumption and the information should be disseminated to a large audience.


Introduction
The usefulness of vegetables globally has caused an increase in the demand of knowledge of their nutrients and chemical composition. Green leafy vegetables provide vitamins, mineral elements and chemicals that are necessary for growth and maintenance of good health.
Bitter vegetables although not palatable to taste are nevertheless consumed because of their ability to prevent and cure ailments. They are consumed alone or in combination with other vegetables. They contain numerous chemical substances that possess a lot of health benefits. The ones found in south eastern Nigeria include Gongronema latifolium, Vernonia amygdalina and Vitex doniana. Some of the chemicals present in vegetables, which are responsible for their bitter taste are alkaloids, saponins and tannins.
The leaves of G. latifolium are used by asthmatic patients to relieve wheezing (Mosango, 2011). Maceration of leaves of G. latifolium is reported by Akuodor et al. (2010) as a treatment for and management of diabetes. The aqueous extracts of the leaves of V. amygdalina is reported to be used as a digestive tonic, appetizer and for wound treatment. Ijeh and Ejike (2011) reported that the leaves of V. amygdalina are used during child delivery to increase uterine contraction and hasten childbirth. They also stated that the leaf decoction is drunk to increase breast milk production in nursing mothers, cure cough or whooping cough and treat diabetes. Egharevba et al. (2010) reported that the leaves of V. doniana are used in the management of diabetes, high blood pressure, in treating ulcers, swellings and oedema.
This article aims at reviewing the ethnobotany, phytochemistry, bioactivity and toxicity of the three bitter vegetables consumed in south eastern Nigeria and to highlight the scientific reasons why the consumption of these vegetables should be encouraged.

Materials and Methods
Literature was collected by using electronic search engines (Google Scholar and Pubmed) with the words Gongronema latifolium, Vernonia amygdalina and Vitex doniana, ethnobotany, phytochemistry, bioactivity and toxicity of the three species. The information presented in this review was selected after being considered to be directly related to the main thrust of this article.

Result and Discussion
Gongronema latifolium Benth.

Botany of Gongronema latifolium
Gongronema latifolium belongs to the family Asclepiadaceae and genus Gongronema with up to 70 species. Gongronema latifolium is commonly called 'utazi' in south eastern Nigeria and 'arokeke' in south western Nigeria (Eleyinmi, 2008). It is found mainly in the tropics and subtropical regions of Africa, Asia and Oceania (Dutta, 2005). It is a climbing and twining shrub and the mode of propagation is mainly by stem cuttings and by seeds. Leaf blade is broadly ovate to almost circular. Leaves are simple, opposite, decussate, occasionally whorled, peltate and usually have no stipules. Leaf margin is entire, venation is palmate and the leaf base is deeply cordate. Leaf is glabrous with acute to acuminate apex. Length is between 7.0-8.5cm and width is between 4.0-6.5cm (Osuagwu, et al., 2013). The adventitious roots arise from the nodes when the stem makes contact with the soil.
Inflorescence is a cymose panicle up to 13 cm long. Flowers are bisexual, actinomorphic, pale yellow and fragrant, pedicel is 2-4 mm long. Calyx has five basal glands with elliptical to rounded lobes. Corolla is tubular, about 5 mm long with five triangular to ovate lobes and five scale-like corona inserted at the base of gynostegium. There are five stamens and the filaments form a tube. Anthers are erect with membranous apical appendages. Ovary is superior, the styles are short and the stigma heads are conical in shape. Fruit is a follicle that is oblonglanceolate and splits open lengthwise along the seam to release seeds. The fruit is green initially, turning to brown and finally black at maturity. Fruit development takes several months from April to November and dehisces between (December-February). New flowers start to appear between January and March and often meet old fruits.
Seeds are flat and attached to a white silky tuft to aid dispersal. The seeds are coma shaped and measure about 0.5 cm in length (Mosango 2011;Osuagwu et al., 2013). Mature seeds planted within two weeks after harvest take seven to fourteen days at temperature of about 27 o C and have germination rate of 67%.

Ethnobotanical uses
Gongronema latifolium is used by folk people in the treatment of various diseases. The twigs and branches are used for cleaning the teeth (chewing stick), in the treatment of cough, diabetes, hypertension and malaria (Etetim et al., 2008). They reported that the extract or cold infusion of the pounded leafy branches together with lime juice, expels intestinal worms, dispels stomach upset and serves as blood tonic. The leaf juice mixed with pineapple and lime juice is used in the treatment of typhoid fever (Etetim et al., 2008). In Imo state of Nigeria, G. latifolium leaves are used as vegetables for preparation of pepper soup, yam stew and used with other spices to prepare soup usually given to a woman after child delivery and to treat stomach ache (Uzodimma 2013). An infusion of the aerial parts is taken to treat cough, intestinal worms, dysentery, dyspepsia and malaria (Akuodor et al., 2010; Mosango 2011). It is also taken to restore loss of appetite. In Sierra Leone, an infusion or decoction of the stem with lime juice is taken as a purgative to treat colic and stomach-ache (Mosango, 2011). The latex is applied to teeth affected by caries. Asthmatic patients use fresh leaves to relieve wheezing (Mosango 2011). G. latifolium is reputed as a remedy for inflammation, bacteria infection, ulcer, malaria, diabetes and as analgesic (Etetim et al., 2008;Edet et al., 2009 andAkuodor et al., 2010). It therefore appears that G. latifolium is a reservoir of many antioxidants capable of preventing and treating different diseases.

Bioactivity
Antimicrobial activity Eleyinmi (2008) confirmed that the methanolic extract of the leaves of G. latifolium exhibited inhibitory activity against Anteridia cholerasius and Listeria monocytogenes. The aqueous extract showed inhibitory activity against E. coli and P. aeruginosa only. Nwinyi et al. (2008) demonstrated that the ethanolic leaf extract showed more inhibitory effect against E. coli and Staphylococcus aereus than the aqueous extract. The diameter of the zone of inhibition was between 6.0 and 10.0 cm. Evaluation of inhibitory activity of the aqueous and ethanolic extracts together with the essential oil from leaves of G. latifolium against bacteria isolated from HIV patients in Lagos, Nigeria, showed moderate inhibitory activity against Staphylococcus species, E. coli, Shigella species, Salmonella species, Klebsiella pneumonia Pseudomonas aeruginosa and Onchrobactrum anthropi (Adeleye et al., 2011). The inhibitory effects they observed in the study were comparable to those of Ampicillin but less than those of Ciprofloxacin and Chloramphenicol.
The antidiabetic activities of aqueous and methanolic extracts of G. latifolium were demonstrated by administering the extracts to alloxan-induced diabetic rats through intraperitonal injection (Akah et al., 2011). Udo et al. (2013) recorded a dose-and time-dependent decrease in the blood glucose level of rats treated with ethanolic and aqueous leaf extracts of G. latifolium compared with the control. Sylvester et al. (2015) treated experimental rats that were subjected to streptozotocin-induced diabetes mellitus and reported a significant (p<0.05) lowering of blood glucose by 66.34%. In their study, the diabetic induction caused increase in total cholesterol (TC) and LDL cholesterol (54.42% and 55.4%) respectively. Treatments with the plant extract decreased (TC) by 58.70% and LDL by 71.70%. Nwanjo et al. (2006) reported that the aqueous extract of leaves of G. latifolium exhibited anti-lipid peroxidase property. In their study, the extract significantly (p <0.05) increased the activity of superoxide dismutase and lowered the level of plasma lipid peroxidation product, malondialdehyde. Investigation of the antioxidant activity of tannin extracts from the leaves of G. latifolium on partially purified lipoxygenase from seeds of Cucumeropsis monii was reported by Eze and Nwanguma (2013). The inhibition of the lipoxygenase by the tannin fraction was comparable to the activity of two known antioxidants, ascorbic acid and propyl gallate. Usoh and Akpan (2015) also reported that when the leaf of G. latifolium is used together with the leaf of Ocimum gratissimum, the antioxidant effect is higher. Iweala et al. (2015) demonstrated that the leaf extract of G. latifolium produced strong inhibitory activity against human lung carcinoma and human breast adenocarcinoma in-vitro. They also demonstrated the free radical scavenging activity of the extract against 1, 1-Diphenyl-2-picryhydrazyl (DPPH) in-vitro. Atangwho et al. (2009) suggested that the free radical scavenging activity of phytochemicals could prevent cancer by their antioxidant properties.

Phytochemistry
Eze and Nwanguma (2013) reported the occurrence of tannins in the leaves of the plant. βsitosterol, lupenyl esters, pregnane esters and essential oils were detected in the body parts.

Toxicity
On intraperitoneal administration of the leaf extract of G. latifolium to mice at 1000 mg/kg body weight, Sylvester et al. (2015) observed 0% lethality, but 100% mortality when administered at 2000 mg/kg. They suggested that the plant is not toxic at the doses consumed by humans. Abu et al., (2014) reported the presence of low amounts of phytate and oxalate which they claimed is not harmful considering that the quantity detected is small. They noted that antinutrients like phytate, when consumed at low levels offer some benefits to an organism. Sylvester et al. (2015) suggested the need for further research on the toxicity of leaf of G. latifolium.

Vernonia amygdalina Del. Botany of Vernonia amygdalina
Vernonia amygdalina Del. belongs to the family Asteraceae or Compositae. It is a common shrub or small tree that grows in tropical Africa and in Asia. It is known as etidot (Efik), onugbu (Ibo), chusar duki (Hausa), ndole (Cameroon), tuntwano (Tanzania) and mululuza (Uganda) (Egedigwe, 2010). It is found along drainage lines, natural forests, and plantation or around homestead. Its common name in Africa is bitter leaf.
They are shrubs or trees and grow up to 7 m in height. Bark: Light grey or brown, rough and flakes longitudinally. The branches are brittle (Orwa et al., 2009). Leaves are leathery, lanceolate to oblong and usually measure 10-15x4-5 cm but can get up to 28x10 cm. They are medium to dark green in colour, the adaxial (upper) surface may or may not have sparse hairs but on the abaxial surface, there are fine soft and pale hairs. Apex is acute, base is tapering and margin is entire or finely toothed. Petiole is usually very short or 1-2 cm long. Flower: Axillary and terminal with small, creamy-white heads about 10 cm long, flowers are in clusters of about 15 cm in diameter and exude sweet odour in the evening (Orwa et al., 2009).
Vernonia was named after a 17 th Century English botanist and plant collector in North American, William Vernon. The specific name means "like an almond". It is not clear why it was so named (Orwa et al., 2009).

Ethnobotanical Uses
V. amygdalina is used in the treatment of ailments in Africa. It is used for facilitating childbirth due to its ability to increase uterine contraction (Ijeh and Ejike, 2011). The leaf juice treats ringworm and other skin infections and when drunk, can treat diabetes (Ijeh and Ejike, 2011). They stated that the leaf decoction is taken to treat pneumonia, increase breast milk production in nursing mothers, cure cough and whooping cough. They also reported that chewing of the root leads to regaining of lost appetite and treats recalcitrant cough, while the root decoction is drunk to neutralize poison.

Ethnoveterinary and Zoopharmacognostic uses of V. amygdalina
Many indigenous tribes attribute the rudiments of their ethnobotany to observing the plants that sick animals consume (Jain et al., 2008). They listed V. amygdalina as one of the plants used by animals to treat amoebic dysentery, other intestinal parasites and stomach disorders. Mohammed and Zakariya'u (2012) cited the case of sick chimpanzees sucking the pith and juice of the unsavoury V. amygdalina stalk with empty stomach. They commented that this practice was not their common diet but was for self deparatization, enhanced body fitness, increased strength or appetite and reduced constipation or diarrhea, particularly in rainy season. Mohammed and Zakariya'u (2012) recorded reduced mortality in broiler birds that were fed with 600 g of V. amygdalina, whereas birds that were not fed with V. amygdalina had higher mortality. The low mortality in birds fed with V. amygdalina was attributed to the ability to acquire resistance to some pathogens conferred on the birds by V amygdalina extract. The leaf extract of V. amygdalina has been reported to be of use in treating bacilliary white diarrhoea and bronchitis (Gbolade 2009). He reported that birds fed with V. amygdalina leaf encountered lower activity of pathogens.

Bioactivities of V. amygdalina Anticancer and cytotoxic activity of V amygdalina:
Anticancer activity of V. amygdalina has been reported in literature. Opata and Izevbigie (2006)

Antioxidant activity
The aqueous and ethanolic extracts of V. amygdalina have been reported to posses potent antioxidant properties as demonstrated by the ability to inhibit the bleaching of β-carotene, oxidation of linoleic acid and lipid peroxidation induced by Fe 2+ ascorbate in a rat liver microsomal preparation. The study demonstrated that the antioxidant activity of the ethanolic extract was higher than that of aqueous extracts but compared favourably with synthetic antioxidants like butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) (Owolabi et al, .2008).

Phytochemistry
The leaves of V. amygdalina contain several active secondary metabolites that play major role in the array of bioactivity attributed to it. These secondary metabolites include alkaloids, anthraquinone, cardiac glycosides, coumarins, polyphenolics, reducing sugars, saponins, sesquiterpene lactones, steroids, steroid glucoside compounds and terpenoids (Njan et al., 2008). Ijeh  Toxicity Agomuo et al. (2016) reported the presence of oxalate in V. amygdalina that is slightly above the toxic level of 2.5 mg. Oxalates form insoluble compounds with Ca, Mg and Fe, thereby affecting the utilization of these mineral elements. However, Agomuo et al., (2016) opined that since the leaves are consumed by humans after it has been squeeze washed in several changes of water, there seems to be no danger of toxicity from oxalate.

Vitex doniana Sweet
The genus Vitex consists of over 270 species, predominantly made up of trees and shrubs. They grow mainly in tropical and subtropical areas, though a few are found in temperate zones. Vitex doniana is found in West Africa (Nigeria, Ghana, Cameroon and Burkina Faso), in East Africa and in South Africa (Egharevba et al., 2010)
Inflorescence is in cymes, the flowers are small and petals are white with the largest lobe being purple (Egharevba et al., 2010). Fruit is green when young, turning to purplish-black fleshy drupe when ripe with a woody conical seed. Seed is 1.5-2.0 cm long, 1.0-1.2 cm wide.

Ethnobotanical uses
Pare et al. (2016) reported that the raw fruits of V. doniana are used as appetite suppressant by hunters and farmers who spend many days in the wood without regular food. According to Pare et al. (2016), other uses of V. doniana include as mulch, to improve land and for nitrogen fixation , as food, as medicine, timber, in boat making, as a tree for housing bees in honey making, also to produce charcoal and dyes. Egharevba et al. (2010) claimed that the twig of V. doniana is used for teeth cleaning while the boiled leaves, stem bark, root and fruits are used as ink and dye for cloths. The leaves of V. doniana are used in the treatment of swelling, oedema, diabetes, ulcer and as diuretic in the management of high blood pressure. The decoction of the leaves is given during labour, just before childbirth to induce strong uterine contraction and shorten delivery time.

Ethnoveterinary uses
The use of the healing properties of plants by humans as well as animals is well known. In Kenya, the bark,
Antioxidant activity observed in V. amygdalina and G. latifolium can be attributed to the large quantity of secondary metabolites they contain. Flavonoids and sesquiterpene lactones are implicated in V. amygdalina for the antioxidant activity (Farombi and Owoeye, 2011). Muanda et al., (2009) opined that phenolic compounds present in the leaves of V. doniana may be responsible for its antioxidant acticity. It can thus be deduced that the presence of phytochemicals in the leaves are the active ingredients behind the antioxidant activities of these bitter herbs. Nwachukwu and Uzueto, (2010) 2015), opined that tannins, glycosides, saponins, flavonoids and alkaloids present in G. latifolium are responsible for the inhibitory activity against human breast adenocarcinoma in-vitro. There is therefore, need to determine which of the phytochemicals are actually active against cancer cell lines.
Saponins have antifungal, antiviral, antimicrobial, antibacterial, anti-inflammatory, antihelmintic, antidermatophytic, anticancer and anticytotoxic activities (Chen et al., 2010). They impact the immune system and possess cholesterol lowering potential that has been demonstrated in animal and human trials (Gūçlū and Mazza, 2007). According to Gūçlū and Mazza (2007), plant extracts containing saponins have been patented for the prevention and treatment of pre-and post-menopausal symptoms, heart disease, hypertension, dementia, gastric and duodenal ulcers.
Tannins have astringent and bitter taste and this is presumed to be used by plants as a feeding deterrent. They also have biological antioxidant activity and act as defense against oxidative damage. The presence of tannins in the three vegetables consumed in south eastern Nigeria confers on these vegetables the potency of managing the numerous illnesses attributed to them.
Alkaloids are another group of phytochemicals found in these vegetables and are known for having bitter taste. According to Ali (2012), alkaloids are reported to have antitumor, antiviral, antihypertensive, antidepressant, antimicrobial and anti-inflammatory activities. Their presence in these leaves can attest to their use in the management of diseases like high blood pressure (V. amygdalina), frequent stooling, diarrhea and dysentery (V. doniana).
Some of the phytochemicals have antinutrient properties. Phytate or phytic acid binds with Ca, Zn and other minerals and in so doing limit their bioavailability. They however have some health benefits like anticancer, antioxidant, hypochelosterol and hypolipidemic effects. Ali, (2012) reported that the level of phytate can be lowered by processing which is traditionally carried out for V. amygdalina and V. doniana by washing in several changes of water.
Oxalate is another phytochemical found in the leaves of V. amygdalina and G. latifolium. It can bind to calcium to form calcium oxalate which could precipitate conditions like rickets and osteomalacia. The quantity of oxalate in these two vegetables are either not high or the processing which involves washing in several changes of water causes reduction in the amount available since the incidence of rickets is not significant in south eastern Nigeria.
These three vegetables under discussion belong to three different families that have no close phylogenetic relationship: Gongronema latifolium (Asclepiadaceae), V. amygdalina (Asteraceae) and V. doniana (Verbenaceae). The possession of bitter taste and similar phytochemical contents in them can be attributed to congruent evolution.

Suggestions
Isolation of the phytochemical fraction responsible for different bioactivities attributed to each vegetable is recommended. Such fractions could be used as templates in the formulation of drugs in pharmaceutical industry.
Further research should be done to verify if the variety of V. amygdalina that is consumed without squeeze washing has all the bioactive compounds contained in the bitter variety.
It is proposed that further research to investigate relationship between consumption of these vegetables (in combination or singly) and lowering of risk of cancer be carried out. It is proposed that a dissemination of the information contained in this review to ordinary citizens to promote the consumption of these vegetables and acquire defense against oxidative damage to the body.

Conclusion
In conclusion, the three plants consumed as vegetables in south eastern Nigeria and as medicines have proven bioactivities and secondary metabolites to support the claims attributed to them by traditionally folks.

Statement on Conflict of Interest:
There is no conflict of interest.