**4. Classification of vegetables**

There are thousands of species of plants used as vegetables globally. Classification of these species can be done by taking into considerations some common features such as the part of the plant used for nutrition and the specific nutritional value. A summary of the health benefits of vegetables described below and their antioxidants content are displayed in **Table 2**.

## **4.1 Leafy vegetables**

#### *4.1.1 Lettuce*

Lettuce (Lactuca sativa L.) is from the Asteraceae (Compositae) family. It is a vegetable that is extensively cultivated globally, commonly consumed fresh and as one of the salad ingredients owing to its health-promoting effects [68]. It comes in different textures, colours, leaf shapes and in a wide variety of head formations. According to Mou [69], it is classified into six major types which are butterhead, Cos or Romaine, Crisphead, Leaf or Cutting, Stalk or Stem, and Latin. lettuce is known to be a great source of flavonoids and vitamin B9. [70, 71] reported in their studies that, its vitamin B9 and flavonoid contents were higher compared to spinach, which is another popular green leafy vegetable that is widely consumed.


*Antioxidant-Rich Vegetables: Impact on Human Health DOI: http://dx.doi.org/10.5772/intechopen.101126*

#### **Table 2.**

*Summary of the vegetables and their health functions.*

Variation with regards to quality/quantity of vitamins and phytochemicals depends on some pre-harvest factors such as agricultural practices, maturity, genotype, maturity and environmental circumstances [72].

However, genetic composition has a great influence on the determination of synthesis and bioaccumulation of carotenoids, chlorophylls, vitamin E, phenolic compounds, vitamin C and antioxidant molecules [73]. Vitamins are vital micronutrients present in lettuce and have been implicated in the reduction of certain diseases such as cardiovascular and degenerative diseases [74]. Variations in the amount of vitamins found in lettuce may depend on leaf type, colouration and butterhead. Romaine lettuces have been particularly found to be good sources of folate [70], with green lettuce having the highest vitamin C concentration [75].

Also, the content of carotenoids such as carotene and lutein and colours across the 52 genotypes of lettuce was discovered to vary as assessed by [76] and categorised in the following order: Romaine and green leaf > red leaf > butterhead > crisphead. Report given about these two carotenoids that they were remarkably and positively correlated with chlorophyll a and b as well as with total chlorophyll content was found to be quite contrary to the findings of [77] who argued that the content of these carotenoids may not correlate with leaf green pigmentation, since the contents of carotenoids seemed to be lower in green compared to red-pigmented lettuce. This contradictory opinion may suggest that the content in carotenoids may not be consistently and outrightly related to leaf pigmentation [78]. Nevertheless, the frequent consumption of carotenoids-rich lettuce has been linked with a reduction in the occurrence of chronic diseases such as certain types of cancer (lung, prostate, and colon), heart disease and vision impairment [47].

Further, many researchers also reported variation in the content of the secondary metabolites present in lettuce with respect to the genotypes and leaf colours [79]. According to [80], red lettuce contains just a single anthocyanin, called cyanidin-3-O-(6″-malonyl-\_-glucopyranoside) which is further converted to two cyanidin derivatives named as (cyanidin-3-O-(6″-malonyl-\_-glucopyranoside methyl ester) and cyanidin-3-O--glucopyranoside). These cyanidin derivatives possess antioxidant activities against lipid peroxidation and cyclooxygenase activity.

The significance of leaf pigmentation was further accentuated by [78] in their study where leaf pigmentation was found to correlate with the concentration of phenolic compounds such as flavonoids, and anthocyanins. For instance, the total phenolic content in red butterhead, red leaf and red romaine lettuces was higher than the green counterparts [81, 82]. The red colour of lettuce has been associated with a high level of total phenolics, popular for imparting a higher antioxidant activity than vitamins C and E [83]. The health benefits of red-pigmented lettuce have been highlighted in an *in vivo* study done by Lee et al. [48] on mice fed with a high-fat diet and supplemented with red lettuce. The results indicated that total cholesterol and low-density lipoprotein (LDL) were reduced, thus underscoring the prospects of red-pigmented lettuce consumption against cardiovascular disease. Similarly, a study carried out by [84] demonstrated that rats fed with red oak-leaf lettuce reduced appreciably LDL and cholesterol levels. Also, the new cultivar B-2 of red-pigmented lettuce, characterised by a high concentration of flavones, anthocyanins and phenolic acids has been reported by [80] to contribute to decline in diseases caused by oxidative stress, leading to anti-tumour activities against some cancer cell lines.

Based on the above considerations, clinical studies have validated the inherent benefits of frequent consumption of fresh lettuce, in particular the red-pigmented varieties.

#### *4.1.2 Brassica leafy vegetables*

This category of vegetable was formerly referred to as cruciferous vegetables and it includes a broad range of species with promising health-benefitting properties. These species include kale, pack Choi, mizuna, watercress, wild and salad rocket DC and Eruca vesicaria [L.] Cav. Database has shown that, about 12% of the vegetables grown globally belong to the Brassicaceae family. They are rich sources of phytochemicals [49], vitamins C, E and K, carotenoids, and phenolic compounds.

Genetic factor has been considered as a factor that influences and modulates the biosynthesis and accumulation of phytochemicals in Brassica leafy vegetables [49]. Variations in the amount of phytochemicals have been observed in a comparative study of antioxidant molecules involving four Brassica leafy vegetables [85]. Watercress showed the highest polyphenol and vitamin C content, while salad and wild rocket showed high concentrations of kaempferol and quercetin derivatives.

Mizuna displayed remarkable concentrations of isorhamnetin and sinapic acid [85]. The potential value of salad Brassica leafy vegetables as dietary sources of antioxidants has been highlighted. Its positive effects against type 2 diabetes and cardiovascular diseases have been affirmed.

Brassica leafy vegetables, in particular kale, are considered as a valuable source of carotenoids such as lutein and β-carotene as well as chlorophyll a and b. During the analysis to determine the concentrations of carotenoid of 33 kale cultivars, zeaxanthin was the most abundant carotenoid in 21 cultivars. American and hybrid cultivars were shown to have high concentrations of zeaxanthin, while German landraces, German commercial varieties, Italian, and red-coloured kale varieties exhibited high concentrations of chlorophyll a and b [86].

Coloured Brassica leafy vegetables like violet kale or pack Choi containing anthocyanins have taken the attention of nutritionists and horticulturists. The anthocyanins content could serve as a marker for differentiating between varieties/ cultivars. The phytochemical concentrations in pack Choi are dependent on the particular colour. Aiyeloja and Bello [87] observed that red pack Choi produced higher concentrations of total flavonoids, total phenolic compounds, glucosinolates, carotenoids and anthocyanins than its green counterpart. Regular intake of leafy vegetables containing anthocyanin could contribute to prevention of various liver diseases, reduction in susceptibility to cancer of the colon and oxidative stress [50].

### *4.1.3 Bitter leaf (VA)*

*Vernonia amygdalina* from *Asteraceae* family is popularly known as the bitter leaf in English. Its petiolate leaves of are about 6 mm in diameter and elliptic in shape. The green leaves have a characteristic bitter taste [88]. They are well distributed in tropical Africa and Asia. The leaves of VA serve as condiments in soup after being washed or boiled to remove the bitter taste. In folk medicine. It has a long history of being used in the treatment of malaria fever and cough [51].

In traditional medicine, many practitioners make use of the different parts of the plants for the treatment of antihelminth, antimalaria [89]. Many others use the aqueous extract got from the leaves as a tonic, an appetiser and for wound healing [90]. Traditional birth attendants from Malawia and Uganda find it useful in aiding removal of the placenta after birth, post-pertum uterine contraction, induction of breast milk production and management of postpartum haemorrhage.

The local use of VA in various parts of Africa for treatment of several ailments and general well-being has been backed up scientifically. Antidiabetic potential of the aqueous extract of VA in streptozotocin-induced diabetic rats has been reported [52]. The finding showed that VA was capable of diminishing plasma glucose, levels of triglyceride/LDL-cholesterol and malondialdehyde (an index of lipid peroxidation). This can occur via scavenging of the reactive oxygen species or by promoting the synthesis of antioxidant enzymes [91] which can subsequently lead to reduction in oxidative stress.

Reference [88] described the antidiabetic effect of VA when combined with another vegetable named *Gongronema latifolium*, on the pancreatic β – cells of rats induced with streptozotocin. The animals administered with the extracts were observed to gain body weight as compared to weight loss experienced in the diabetic group. Further, blood glucose level significantly declined after 28 days of treatment with the combined extracts. Regeneration of islets cells was believed to be the explanation as this would induce a rise in insulin production and secretion [92]. Active ingredients such as flavonoids are believed to be present in VA [44] which may be responsible for their potentials in altering pancreatic damage initiated by streptozotocin or alloxan in experimental animals. In addition, the bitter principle of VA

may also be responsible for insulin production, stimulation and release of pancreatic islets from the beta-cells [93]. Likewise, tannin, flavonoids glycosides and phytosterols of this plant could also inhibit the action of alpha-glucosidase inhibitor which may have contributed to the hypoglycemic effect being exhibited by this plant.

VA is increasingly becoming a powerful and strong challenger for cancer management as coumarins, flavonoids, sesquiterpene lactones have been implicated as the active principles in VA that may be responsible for its anticancer activity [94]. Aqueous extracts of VA was found to exhibit a cytostatic action on cultured human breast tumour cells (MCF-7) growth *in vitro* implying its tumour stabilisation and protective effects *in vivo* [94]. Its potential effects in inhibiting DNA synthesis even at physiological concentrations have been demonstrated in cancer cells [95]. Its hexane, chloroform, butanol and ethylacetate fractions were found to be capable of inhibiting the growth of human breast cancer cells even at concentrations as low as 0.1 mg/ml to 1 mg/ml, with an inhibition rate as high as 98% [53].

Other findings have established the usefulness of VA and its biopeptides (derived from the aqueous extracts of its leaves) against cancer via apoptotic mitogen-activated protein kinases and signal transduction pathways [96].

#### *4.1.4 Fluted pumpkin (Telfairia occidentalis)*

*T. occidentalis* (TO) commonly called fluted pumpkin is from the family: Cucurbitaceae. It is a popular vegetable that occurs in the forest zone of West and Central Africa. It is a perennial vine, growing to 10 m or more in length with its stems having branching tendrils and the leaves divided into 3–5 leaflets. The leaf is widely consumed due to its diverse benefits. The young succulent shoots and leaves are consumed as vegetables in the eastern part of Nigeria. Its herbal preparation has been applied in treating anaemia, malaria convulsion, gastrointestinal disorders [54]. Also, in addition to its nutritional value, this vegetable has agricultural and industrial importance [97].

Some scientific researchers have discovered its free radical scavenging and antioxidant properties. The leaves are rich in ascorbic acid and phenols [97]. Utilisation of the leaves in folk medicine in the treatment of some diseases in which the involvement of generation of free radicals have been implicated could be as a result of the antioxidative and radical scavenging ability [98].

Kim et al. [55] reported that aqueous extract of TO leaves lowers blood glucose level and elevates haematological parameters in rats. The chemical composition of TO shows vitamin A and C as part of its constituents which are well-known antioxidants and capable of scavenging free radicals [99]. They are well-established haemopoietic factors that have a direct impact on blood production in the bone marrow. Amino acids are also derived from TO and could also be useful in production of the globin component of the haemoglobin, contributing to elevation in haemoglobin concentration.

The leaf extract has also been documented to have the ability to improve sperm parameters which can assist in improving sperm quality [56]. Some of its active ingredients possess spermatogenic activities. Therefore, the leaves may be very applicable in the treatment and management of infertility especially those linked with a reduction in sperm performance.

The anti-anaemic potentials of the aqueous extract of leaves against phenyl hydrazine-induced anaemia in rabbits have been investigated [100]. The finding revealed that the leaves are notably rich in iron and play a major role in curing anaemia.

#### *4.1.5 Basil (Ocimum)*

*Ocimum basilicum* and *Ocimum gratissimum* are known for the management of different diseases in Africa. They belong to the Lamiaceae family. The leaves can be *Antioxidant-Rich Vegetables: Impact on Human Health DOI: http://dx.doi.org/10.5772/intechopen.101126*

petiolate or sessile and most times toothed at the margin. The presence of volatile oil which contain up to 75% of thymol, gives them a characteristic pleasant aroma. The leaf of *O. gratissimum* or even the whole plant is a well-known remedy for diarrhoea and other diseases [101].

In folklore medicine, *O. basilicum* (basil) is a medicinal plant used for various ailments, such as cough, diarrhoea, headaches, constipation and kidney malfunction. Its oil commonly referred to as basil oil contains camphor with antibacterial properties. The vapour of the boiling leaves can be inhaled by people with catarrh and colds while the leaves may be rubbed between the palms and sniffed for treatment of a cold.

#### *4.1.6 Jute mallow (Corchorus olitorius)*

*C. olitorius* (Linn) is a leafy vegetable belonging to the family Tiliaceae. It is commonly called jute mallow in English and "ewedu" in south western Nigeria. The plant is characterised by the viscosity of its leaves which usually forms a thick viscous soup after being cooked and can be added to stew or soup. The leaves are rich sources of vitamin and minerals.

In folklore medicine, the leaf extract is employed in the treatment of pain, fever, gonorrhoea and tumour. It is rich in minerals, vitamins B1, B2, C and E, carotenoids, [57]. In some parts of Nigeria, the leaves' decoction is used to treat shortage of folic acid and iron, as well as anaemia. The leaves are also used as a blood purifier [58] while the cold leaf extract infusion is consumed to restore appetite and strength and the leaves are used for treating fever, tumours, gonorrhoea and piles [102].

The hepatoprotective effect of the ethanolic extracts against carbontetrachloride-induced hepatotoxicity in rats has been studied [103]. The extracts produced a significant hepatoprotective effect by reducing the levels of liver function enzymes and lipid peroxidation. Some of the phenolic antioxidants in the leaves are 3, 5-dicaffeoylquinic acid, quercetin 3-galactoside, phenolic [5-caffeoylquinic acid (chlorogenic acid), quercetin 3-(6-malonylglucoside), quercetin 3-glucoside, and quercetin 3-(6-malonylgalactoside).

#### *4.1.7 Amaranth globe (Gongronema latifolium)*

*Gongronema latifolium* commonly known as the amaranth globe is from Asclepiadaceae family. It is a tropical rainforest plant in Nigeria, used as spice and vegetable in traditional folk medicine [104]. It has a sharp-bitter, sweet taste. Useful in making sauces, preparation of salads and soup. In West Africa, it is widely used for nutritional and medicinal purposes. The aerial parts can be prepared as an infusion to treat malaria, intestinal worms, cough and dysentery. It can be taken as a tonic to address the loss of appetite. The decoction made from its stem with lime juice is taken to treat stomach-ache, diabetes and high blood pressure. Senegal and Ghana believed that the leaves when rubbed on the joints of small children could assist them to walk. The latex is applied to teeth affected by caries, used in weight loss in lactating women and for general health management. A decoction of the roots, combined with other plant species, is utilised in the treatment of sickle cell anaemia. The leaves, when macerated in alcohol is used to treat bilharzia and hepatitis [105].

Screening of Gongronema latifolium vegetable revealed the presence of phytochemicals such as alkaloids, tannins, glycosides, polyphenols, saponins and flavonoids [106]. Its antidiabetic properties have been revealed in streptozotocininduced diabetic rats during the oral administration of its aqueous and ethanolic

extracts [104]. Its antibacterial activity and ability to maintain healthy blood glucose level has been documented.

Gongronema latifolium is a vegetable with pool of antioxidants with capability to prevent and treat many diseases.

#### **4.2 Root, bulb and tuber vegetables**

#### *4.2.1 Alliums*

Alliums vegetables belong to the *Alliaceae family* and they include garlic, onion, leek, chive, welsh onion, among others. Alliums are very rich in thiosulphides, which have an association with the reduction of various chronic diseases. Variations in the total thiosulphide content among alliums, even when grown under identical conditions have been reported [107]. The report revealed that, the total thiosulphide contents in green onion leaves, chive, and onion bulbs were found to be 0.2, 0.72, and 1.02 g/kg fresh weight, respectively. Even, the type of thiosulphides in these vegetables were found to also vary. For instance, onion bulbs contain 34% methiin, 5% ethiin, 6% propiin, 5% alliin, and 49% isoalliin [107], while garlic cloves contained about 92% alliin, 8% methiin, and trace amounts of ethiin, propiin, and isoalliin [108]. Flavonoids such as anthocyanins and flavonols like quercetin and kaempferol are found to be present in red onions and yellow fresh cultivars respectively.

Onion and garlic are rich and abundant sources of calcium, potassium and manganese providing close to 10% of their daily requirements in humans. Onion and garlic can acquire selenium if they are cultivated in soils rich in selenium, in the form of selenocysteine and seleno-proteins. This led to the proposal by [109] that selenium-enriched garlic and onion could provide a safe efficient delivery system of selenium into the body for cancer prevention [110]. Onions are also good source of chromium relevant in diabetes prevention. The mechanism appears to be through the potentiation of insulin receptor kinases [29]. A lot of clinical studies have demonstrated the ability of chromium to regulate fasting blood glucose levels, which can lead to improved glucose tolerance, decreased insulin levels and improved lipid profile levels in diabetic patients.

Onions are also rich source of dietary fibres like inulin with varying degrees of health benefits [111]. Its prebiotic properties are reflected in its preference fermentation by beneficial bowel bacteria like *Lactobacilli* and *Bifidobacteria*, thereby changing the bacterial microflora of the intestine to make pathogenic, or diseasecausing bacteria less abundant [59]. Fructans are abundant in onions and they are excellent supporters of the growth of beneficial bacteria [60]. In addition, fructans facilitate absorption of calcium and this could serve a useful purpose in the prevention of osteoporosis [112]. Diets high in fructans have also been associated with a decrease in the levels of lipid and blood glucose profiles glucose [113]. Antidiabetic potentials and antihyperglycemic effects of onions have been demonstrated [114].

The therapeutic merit and positive impact of onions, garlic and other *Allium* vegetables have been further confirmed in various epidemiological studies carried out where their consumption has been found to delay the growth of a broad spectrum of cancers. Consumption of onions has been linked to reduced incidence of stomach and intestine cancers [61] and reduction in mortality due to prostate cancer [115].

Routine intake of garlic has been linked with a decline in the occurrence of preneoplastic lesions in individuals infected by *Helicobacter pylori* [116]. It is also involved in reduction in the risk for colorectal and prostate cancers. Presumably, some garlic constituents can inhibit tumour initiation through deactivation and elimination of pro-carcinogens [117]. Some studies presented the ability of onion extracts to inhibit mutation [118], reduce multiplication process of cancer cells [119]


*Antioxidant-Rich Vegetables: Impact on Human Health DOI: http://dx.doi.org/10.5772/intechopen.101126*

#### **Table 3.**

*List of selected vegetables and their antioxidant-richness.*

and risks for cardiovascular diseases [120], which are effects being attributed to the presence of bioactive molecules such as quercetin (**Table 3**).

Carrot is an important root vegetable belonging to the family Apiaceae, rich in flavonoids, carotenoids, vitamin C and vitamin E. It is a coloured vegetable with a gold mine of antioxidants such as carotenoids, polyphenols and vitamins which can function as antioxidants, anticarcinogens and immune enhancers. Carotenoids especially the ones present in orange carrots, are potent antioxidants with capacity to neutralise the toxic effect of free radicals, inhibit mutagenesis to decrease the risk of some cancers. The importance of carotenoids in lowering blood glucose level has been identified in a study where high blood glucose levels were observed in some participants with low level of carotenoids. Carotenoid level can decrease in response to severity of glucose intolerance. This suggests the impact that carrot and vitamin A-rich carotenoids could have on diabetics in the management of their condition [62].

#### **4.3 Solanaceous vegetables**

#### *4.3.1 Tomato*

Tomato is a widely grown vegetable that is globally consumed. It can be consumed fresh or in its processed forms. The phytochemical constituents in tomatoes are carotenoids, lycopene, phytoene, neurosporene, and carotenes [132]. Assessment of lycopene content based on fresh weight shows that tomato (on average) contains about 35 mg/kg of lycopene, the red cultivars have an average of 90 mg/kg of lycopene while the yellow ones have just 5 mg/kg [133].

It has been revealed that tomatoes and tomato-enriched foods are the richest sources of lycopene in the world. Dietary intake of lycopene shows that, about 25 mg is taken each day and 85% is obtained from fresh and processed products of tomatoes [134]. Also, an appreciable amount of *α*-, β-, *γ*-, *δ*-carotene is found in tomatoes [135]. Tomato is also a remarkable source of ascorbic acid, potassium [67], lutein, tocopherols and flavonoids [63].

The cultivar and culture have been proved to have a great influence on the flavonoid content. For instance, cherry tomatoes notably have a flavonoid content that is higher than standard tomato cultivars while the field-grown have higher flavonoid content than the greenhouse-grown [136].

Several research investigations have been ongoing to determine the relationship between dietary intake of tomato/lycopene and the risk of having cancer. Findings on different cancers relative to lycopene and tomato intake showed a great reduction in prostate cancer risk and an increased apoptotic cell death in carcinomas [64]. People subjected to diets rich in tomato and tomato-based products with high lycopene content, were found to unlikely develop stomach and rectal cancers when compared to those who consumed a lower amount of lycopene-rich vegetables [65].

The antioxidant properties of tomatoes have been described. Its daily consumption for an average of 2–4 weeks elevates the antioxidant defence system and reduces susceptibility to lipid peroxidation [137] as oxidative modification of lowdensity lipoproteins is key to developing atherosclerosis. Comparative studies [138] in healthy individuals and people with type 2 diabetes, showed reduced vulnerability to lipid peroxidation [139] after daily intake of tomatoes or tomato juice.

Possible anti-inflammatory, anti-thrombotic and lipid-lowering effects of tomatoes and their products have also been investigated where an aqueous extract from tomatoes demonstrated antiplatelet activity *in vitro* [140]. In humans, research shows remarkable deductions in *ex vivo* platelet aggregation a few hours after supplementation with tomato extract [140].

#### *4.3.2 Sweet and hot peppers*

Peppers are always available in a beautiful array of colours and shapes. They contribute to the flavour and colourful appearances of our dishes. Fresh peppers are excellent sources of vitamins (in form of C, K), carotenoids and flavonoids (quercetin and luteolin) [141]. Vitamins A and C are involved in the prevention of cancer, age-related diseases, reduction of inflammation and they support immune function. Vitamin K improves blood clotting, bone formation, and protects the cells against oxidative damage. Red peppers are rich in lycopene, a phytochemical commonly known for preventing prostate cancer and cancers of the bladder, cervix, and pancreas.

The nutrient content of bell peppers varies with colour as studies have shown that red coloured bell peppers have significantly higher amount of nutrients than the green counterpart. Their role in prevention of blood clot formation and reduction in the risk of heart attacks and strokes could probably be due to vitamin C, capsaicin, and flavonoids content.

Hot peppers are known for their spiciness. The major phytochemicals identified in hot peppers are capsaicinoids. Capsaicin releases about 70% of the pungent flavour in hot pepper, while dihydrocapsaicin constitutes the remaining 30% [142]. The hotness or heat experienced in the taste bud from hot peppers comes from capsaicin. It relays this sensation by acting on pain receptors in the mouth.

#### *Antioxidant-Rich Vegetables: Impact on Human Health DOI: http://dx.doi.org/10.5772/intechopen.101126*

Predominantly, capsaicin is located in the white membranes of peppers, imparting its "heat" to seeds as well. It can lower blood cholesterol and triglycerides levels, boost immunity, and reduce the risk of stomach ulcer. Capsaicin also possesses analgesic, anti-bacterial, and anti- diabetic properties. Capsaicin is included in many commercial formulations for the treatment of painful diabetic neuropathy, rheumatoid-arthritis, muscle pains, aches in the tooth and gastric ulceration [143].

The levels of vitamins and minerals present in chilli hot peppers are amazingly high. 100 g of chilli hot peppers provides 240% of vitamin C, 39% of vitamin B6 (pyridoxine), 32% of vitamin A, 13% of iron, 14% of copper and 7% of potassium [144]. They are rich in vitamins such as niacin, pyridoxine (vitamin B6), riboflavin and thiamin (vitamin B1) and minerals like manganese, iron, potassium, and magnesium. It is pertinent to note that, potassium is an important component of cell and body fluids, useful in controlling heart rate and blood pressure. Manganese serves as a co-factor for superoxide dismutase.

Hot and sweet peppers contain substances that can stimulate the body's heat production and oxygen utilisation for about 20 minutes after eating. This experience can contribute to losing extra calories and weight loss.

#### *4.3.3 Eggplant*

Eggplant is a very common and popular vegetable grown in many countries. It is grown in the subtropics, tropics and Mediterranean areas because of its demand for a long season of warm weather to produce good yields. Eggplant contains phenolic compounds such as caffeic, chlorogenic acid and flavonoids. Chlorogenic acid is a notable phenolic compound found in all eggplant cultivars with potent free radical scavenging activity [145]. Some of the benefits attributed to chlorogenic acid include antimicrobial, antiviral and anticancer activities. In addition to their nutritional potentials, these phenolic acids found in eggplant are accountable for the bitter taste that comes from the flesh when cut. Breeders have already started working on the development of eggplant cultivars that will give and ensure a balance of optimal nutritional value and pleasant taste.

There are other antioxidants such as lycopene, lutein, *α*-carotene, myricetin and kaempferol present in eggplant [146]. This is also depicted in **Tables 2** and **3**.

Eggplant is a good source of dietary fibre and bone-building manganese and vitamin K that can support digestion and bone-building respectively. It is also an excellent source of molybdenum and potassium, copper, vitamin C, vitamin B6, folate, and niacin [147]. Studies have demonstrated the effectiveness of eggplant in controlling the level of high blood cholesterol [66]. This was shown in a clinical study, where volunteers were fed with eggplant powder and a significant decrease in blood-lipid profile levels was observed [148]. Its relevance in inhibition of invasion of human fibrosarcoma HT-180 cell, type 2 diabetes and hypertension has been revealed [67].

### **5. Conclusion**

Chronic diseases are the principal causes of death. Excessive production of reactive oxygen species has been identified to be responsible for the pathogenesis of many of these chronic diseases. Thus, antioxidant phytochemicals are considered as potential agents for the prevention/management of these diseases due to their biological activities and health benefits such as free radical scavenging abilities, anti-inflammatory action, anticancer and protective action against numerous diseases.

Regular intake of diet rich in vegetables has a favourable impact and indisputable positive effects on human health and offers the human body protection from different chronic diseases. This review contributed to the body of evidence that supported the biomedical importance of regular consumption of antioxidant-rich vegetables. This antioxidant property of vegetables has been linked to the presence of phytochemical compounds contained in them. Therefore, the antioxidant constituents may be responsible for the mechanism by which vegetables decrease the risk of diseases by directly quenching free radicals, altering gene expressions or indirectly participating in cellular signalling involved in redox balance. In order to get all the health benefits inherent in diet-rich vegetables, it is recommended to consume a great diversity of vegetables to ensure the delivery of a unique blend of health-promoting phytonutriceuticals.
