Indigenous leafy vegetables grow naturally in open land, back yards and dumping sites [1]. They are plant species that are either genuinely native to a region or that were introduced to that region in the past and have since evolved through natural processes [2]. In rural settlements of developing countries, indigenous leafy vegetables (ILVs) are essential in boosting household food security and enhancing the quality of diets ^[3][4][3,4]. Several studies have reported that ILVs address gaps in nutrition by providing wholesome, reasonably priced and nutrient-dense food substitutes ^[2][3][5][2,3,5] due to their high concentration of essential nutrients ^[6][7][6,7]. Such nutrients include minerals (zinc, calcium, magnesium, iron and potassium), provitamin A, vitamin C and fibre ^[1][5][8][9][1,5,8,9]; in addition, ILVs are extremely low in fat and carbohydrates [1]. However, consumption of these vegetables is very limited as they have been marginalised in favour of exotic vegetables such as carrots, lettuce, tomatoes and others that are cultivated at an expense [2]. The few ILVs consumed include African kale, African eggplant, jute mallow, pumpkin leaves, slender leaf, African nightshade, cowpea, amaranth and spider plant ^[2][10][2,10]. In addition, ILVs can also be directly or indirectly used for medicinal purposes ^[3][11][3,11] due to their high phytochemical content ^[1][2][1,2].

Phytochemicals are known to possess antioxidant properties that are important in decreasing the likelihood of chronic and other non-communicable or age-related conditions, such as cancer, diabetes and cardiovascular disease ^[1][2][12][1,2,12]. Therefore, their high levels of essential nutrients, combined with their high phytochemical content, make ILVs suitable alternative functional food ingredients. In general, leafy vegetables are minimally processed (i.e., cut, shredded, etc.) and packaged before storage at low temperatures. Other processing methods include hot-air oven drying, freeze drying, sun drying, solar drying or hydro-thermal processing such as boiling, cooking and blanching ^[13][14][15][13,14,15]. In the case of ILVs, only sun and solar drying methods are currently practiced and, as a result, there are no ILV-derived products or ingredients available on the market. Thus, the use of locally available ILVs to develop new food products and ingredients could have a significant socio-economic impact on rural settlements since ILVs ingredients could easily be incorporated into drinks, soups and bakery products. 

2. The Role of Vegetables as Food Security Crops

Indigenous leafy vegetables are an important, readily accessible resource that may be used to attain food security and alleviate poverty in rural, semi-urban and urban areas ^{[16][17][18][19]}[17,18,19,20]. This is because ILVs have a relatively higher nutritional value compared to exotic vegetables ^[18][19]. Even though ILVs have not been fully exploited in the quest to achieve food safety and alleviate malnutrition [10], many researchers agree that the vitamins, minerals, phytochemicals with strong antioxidant properties and nutritional (micronutrient) content in ILVs are their most valuable attributes ^{[9][16][17][20]}[9,17,18,21]. The capacity of a food to provide nutrients is of great importance in food security ^[21][22]. Food security does not centre only on the availability and accessibility of food—it is also concerned about feedstocks that may also contribute an income to alleviate food insecurity as well as essential nutrients in correct proportions. Thus, lack of both economic diversity and nutritious foods plays a significant role in malnutrition and food insecurity in Africa where most rural and urban households depend on carbohydrate-rich diets which may increase the availability of energy but may not necessarily improve the nutritional status of consumers ^[22][23]. In contrast, such diets may be combined with micronutrient-rich foods such as ILVs that provide a rich source of micronutrients and health-promoting compounds that can supplement the nutritional value of these staple diets ^[23][24]. In addition, ILVs are very diverse, colourful and tasty foods that can be used to play a strategic role in achieving balanced diets [6].

3. Biodiversity among Indigenous Vegetables

Indigenous leafy vegetables are diverse and include many different species ^[2][24][25][2,26,27]. Globally, over 7000 species of leafy vegetables are either cultivated or harvested from the wild for food ^[4][26][4,28]. Amongst these, ILVs that vary in shape, size, colour, taste and nutritional value are consumed in nearly all countries ^[9][27][9,25]. South Africa is home to over 100 species of wild and domesticated or cultivated ILVs including amaranth (Amaranthus spp.), cowpea (Vigna unguiculata), nightshade (Solanum spp.), spider plant (Chlorophytum comosum), lamb’s quarters (Chenopodium album Linn), purslane (Portulaca oleracea L.), blackjack (Bidens pilosa), jute mallow (Corchorus olitorius), Chinese cabbage (Brassica rapa L. ssp. Pekinensis), bitter melon (Citrullus lanatus), African cabbage (Cleome gynandra), pumpkin (Cucurbita maxima) ^{[2][28][29][30][31][32][33][34][35][36][37][38]}[2,29,30,31,32,33,34,35,36,37,38,39] and slender leaf (Crotalaria brevidens). The indigenous knowledge of a community determines the diversity of ILVs in a region and also influences the type of farming systems for ILVs, plant innovations that are practiced and the degree to which certain exotic plant species have been incorporated into the farming system ^[28][29]. Various factors, together with cultural disparities and the ancient inspirations of particular communities, have a substantial influence on the number of varieties of ILVs that are still in existence in different communities [10]. Numerous uses of ILVs have been documented, including those for food, cash crops, medicine, indigenous culture and ornamentation. Different types of ILVs exist, their definition as such being based on the utilisation of part of the plant, mostly leaves, shoot tips, fruit, seed, roots and flowers ^[18][39][19,40]. They can be prepared fresh or dried, depending on cultural preferences and the characteristics of the leaves or their immature seeds ^[40][41]. Indigenous leafy vegetables can be mixed together and eaten in various ways (e.g., as a relish, in soups and as part of a salad) ^[41][42].

4. Nutritional Composition and Health Benefits Associated with ILVs

Compared to exotic variants, ILVs offer a higher nutritional value ^[42][52]. Such vegetables would ensure an appropriate supply of those nutrients identified to be lacking if they are consumed in suitable proportions ^[43][53]. Since they contain significant amounts of vitamins, pro-vitamin A in particular ^[44][43], ILVs are highly beneficial since they maintain health and prevent diseases ^[43][53]. As an antioxidant, vitamin A plays an important role in preventing free oxygen radicals from causing damage to cells and, by so doing, reduces the incidence of some cancers, heart attacks, strokes, and maintains eyesight and the immune, skeletal, respiratory, reproductive, and integumentary (skin) systems ^[27][44][25,43]. However, due to the underutilisation of ILVs, there is little available information regarding their nutritional value ^[45][54]. Vitamin C is important for iron absorption in addition to maintaining the health of teeth and gums ^[9][27][9,25]. Folic acid decreases the likelihood of birth defects and vitamin K protects bones from osteoporosis and helps to contain inflammation ^[9][46][9,55]. Different studies have demonstrated that high folate intake from ILVs may decrease the risk of colon polyps by 30% to 40% ^[47][48][49][56,57,58]. Significant amounts of vitamin D, E, K, pantothenic acid, pyridoxine, niacin, folate, riboflavin and cyanocobalamin have been reported in ILVs ^[44][43]. Moreover, the carotenoids lutein, B-carotene, violaxanthin and neoxanthin are among those found in abundance in ILVs ^[50][51][59,60]. Additionally, ILVs are essential sources of health-providing antioxidants [6]. Compared to other food sources, ILVs contain more micronutrients and various amounts of compounds that are needed to address nutritional and health requirements ^[52][69]. Antioxidants in ILVs are a group of compounds that actively inhibit or delay the oxidation of lipids and other biomolecules, thus minimising oxidation damage to cells and that also assist in repairing cell damage ^[53][70]. They are, thus, an essential part of the defence system of the body ^[54][71].

5. Functional Components of Indigenous Leafy Vegetables and Human Health

As indicated, ILVs are rich sources of polyphenols, flavonoids, amino acids, minerals, vitamins A and C, β-carotene and dietary fibre. These bioactive components are involved in protection against various conditions including cancer, diabetes mellitus, arthritis and cardiovascular disease ^[55][79]. Epidemiological studies have demonstrated that ILVs are effective in combating such health conditions and that this ability is related to their natural bioactive components ^[56][57][58][80,81,82] such as polyphenols and α-tocopherol being synthesised by plants to protect themselves against oxidative damage resulting from environmental stresses ^[59][60][83,84].  Polyphenols are the most studied secondary metabolites in ILVs, being linked to various health benefits ^[61][85]. The composition of polyphenols may vary in different parts of the plants, so that phenolics such as catechins and quercetins may be generally present in vegetables while others may only be available in a particular species ^[62][86]. Moreover, the composition, content and biological characteristics related to polyphenols are influenced by plant phenology and changes in environmental conditions ^[63][64][87,88]. For example, ILVs such as amaranth and purslane have demonstrated changes in polyphenolic profiles and their related antioxidant activity at different stages of phenology ^[65][66][89,90]. Seven phenolic compounds including ferulic acid, p-coumaric acid, cinnamic acid, gentisic acid, caffeic acid, p-hydroxybenzoic acid and protocatechuic acid have been isolated in pumpkin leaves hydrolysed by subcritical water ^[67][91]. Of these, ferulic, p-coumaric, caffeic and gentisic acids are phenolics found at high concentration. Yields of phenolic acids, apart from gentisic acid, were high at a temperature of 160 °C, while high yields of gentisic acid required a temperature of 180 °C. However, the total phenolic content of hydrolysed pumpkin leaves was significantly reduced at temperatures above 160 °C and this was attributed to their decomposition at high temperatures. While hydrolysis temperatures above 160 °C decreased the levels of polyphenols, at the same time they increased antioxidant activity since higher temperatures result in the generation of antioxidant components from polyphenols ^[68][92].  Flavonoids are also secondary plant metabolites with a range of biological properties, including antioxidant, anti-microbial, anti-cancer, and anti-inflammatory effects ^[69][134]. Most plant tissue contains flavonoids ^[70][135] and flavonoid glycoside is abundant in leaves, flowers and fruits; aglycone is present in the woody tissue while flavonoid glycosides or aglycones are also available in the seeds ^[59][83]. According to the position of ring B, the level of oxidation as well as the cyclic condition of ring C, flavonoids can be divided into distinct categories including flavanols, flavonols, anthocyanidins, flavones, flavanones and isoflavones ^[71][136]. Myricetin, quercetin and kaempferol are the three main flavonoids found in the Brassicaceae family, which includes Chinese cabbage ^[72][137]. The total flavonoid content (TFC) of cowpea ranged from 0.95 to 0.36 mg quercetin equivalents/g, and the dark seed coat cultivars showed higher TFC levels than the white seed coat cultivars ^[73][138]. Major anthocyanins such as delphinidin-3-O-glucoside and cyanidin-3-O-glucoside were isolated and characterised in cowpea leaves. Various cowpea cultivars such as green, navy blue, black, grey and black/grey mottled have been reported to contain anthocyanins ^[74][75][139,140]. The lack of quantifiable anthocyanins in other coloured cowpea variants including red, maroon and brown demonstrates that the genetic component of cowpea’s anthocyanin production is important ^[76][141]. Nevertheless, the dominating anthocyanins in cowpea, regardless of genotype, are delphinidin-3-O-glucoside and cyanidin-3-O-glucoside ^[75][77][140,142]. Flavonoids were identified in the leaves of the spider plant, including quercetin, quercetin 3-glucoside and kaempferol 3-O-rutinoside ^[78][129]. The formation of collagen depends significantly on vitamin C (L-ascorbic acid) as well as the absorption of iron ^[79][145]. A study of leafy vegetables determined the influence of seasonal variation on the nutritional compositions of spider plants in southern Côte d’Ivoire to show that the vitamin C content of spider plants is higher in the rainy season (33.33 mg/100 g FW) than in the dry season (24.33 mg/100 g FW) ^[80][146]. The variation in vitamin C might be attributed to genetic factors, maturity level of the plant as well as the extent of exposure to the sun since high temperature destroys the vitamin ^[81][147]. The results of the seasonal study reflected higher vitamin C levels than those obtained in another study where the vitamin C content of spider plants was 13 mg/100 g FW ^[82][148]. The skin, skeleton tissues and respiratory organs all depend on carotenoid, which is a precursor of vitamin A ^[83][151]. The exposure of plants to environmental stress results in the biosynthesis of carotenoid ^[84][152]. Chinese cabbage has a total carotenoid content that ranges from 3.93 mg/100 g to 18.87 mg/100 g, which is in line with the patterns of other functional components. In S. scabrum leaves, the total carotenoid content varied from 586 to 691 µg/g on a DW basis, whereas in S. retroflexum leaves, the total carotenoid content was 0.733 µg/g on a FW basis ^[85][86][87][153,154,155]. The levels of β-carotene in the leaves of S. nigrum and S. scabrum ranged from 28.1 to 141.7 µg/g DW and from 55.1 to 96.0 µg/g DW, respectively. The highest amounts of total carotenoids were noted in S. villosum, which contained 138.11 µg/g DW, whereas S. scabrum contained 65.2 µg/g ^[87][155]. African nightshade has a higher amount of vitamin A (422 g retinol activity equivalent, RAE) than Jew’s mallow (329 g RAE) or pumpkin leaves ^[87][88][155,156]. Indigenous leafy vegetables supply more than 75% of the daily recommended amount for vitamin A. The ILVs that have high fibre content include Bidens pilosa, which has 3770 mg/100 g, and C. gynandra, which has 1800–2100 mg/100 g ^[88][89][156,157]. Significant variations were noted in the dietary fibre content of twelve amaranths with the green morph genotype ^[90][158]. Genotype GRA9 had a higher value of 9.55 g 100/g FW, followed by GRA26 (8.56 g 100/g FW) and GRA4 (8.21 g 100/g FW). Genotype GRA11 showed the lowest value of 6.02 g 100/g FW, with an average value of 7.51 g 100/g FW. A study of the soluble and insoluble fibre content of spider plants during the dry and rainy seasons showed that soluble fibre was higher in the rainy season (16%) compared with 13.33% in the dry season. Moreover, insoluble dietary fibre content was higher in the dry season (25.1%) and lower in the rainy season (23.07%) ^[80][146]. The high levels of insoluble dietary fibre in spider plants is essential considering that dietary fibre enhances the metabolism of carbohydrate and lipid ^[91][159].  Like other legumes, cowpea leaves are a rich source of proteins, containing 23–40% protein on a dry basis; they are storage proteins, and their main purpose is to provide nutrients to the germination of the seed ^[92][93][164,165]. The storage proteins of pulses, including cowpea, contain amino acid sequences that, when digested, release peptides that may have additional bioactive properties, such as acting as angiotensin I-converting enzyme (ACE) inhibitors and antioxidants, among others ^[94][95][166,167]. Peptides are typically made of three to twenty amino acid residues that are released due to the enzymatic proteolysis of different animal and plant proteins ^[95][167]. Peptides that are referred to as functional or biologically active compounds have been reported to have antimicrobial, anti-hypertensive, antioxidative, anti-dyslipidemic, anti-carcinogenic and anti-diabetic properties ^{[96][97][98][99][100]}[168,169,170,171,172].

6. Potential Toxicity of Some Traditional Leafy Vegetables

The presence of phytochemical secondary metabolites makes edible plants potentially toxic ^[101][200]. These compounds are not necessary for the survival of the plant, but they are produced to improve the plant’s ability to fend off predators or interact with its surroundings, herbivores and/or symbiotic insects ^[102][201]. For example, Solanum species are among ILVs that contain toxic alkaloids such as glycosides of solasodine and solanidine ^[86][154]. In addition to the presence of secondary metabolites, ILVs tend to acquire higher amounts of heavy metals which are known to be micronutrients in minute quantities ^[103][202]. Some of the micronutrients that are vital for the growth and development of the plant as well as productivity may become toxic when their content levels exceed permissible limits ^[104][203]. Such micronutrients include heavy metals such as copper, iron, Molybdenum and zinc ^[105][204]. Consuming contaminated ILVs can expose consumers to heavy metals which are harmful and pose a severe health risk because they can cause malnutrition, immune system fragility, mental growth retardation and gastrointestinal cancer ^[106][107][205,206]. High levels of Cu and Zn may cause oxidative stress through redox reactions ^[108][207]. Therefore, there is a need to measure their concentration in plants as their deficiency and presence in unacceptable levels cause a health risk to humans.

7. Conclusions

Indigenous leafy vegetables have been part of the human diet and recognised for their valuable attributes by contributing to household food security and consumer health, generating dietary diversity, creating employment opportunities and alleviating malnutrition and poverty. However, indigenous knowledge regarding their importance is contested and increasingly becoming scarce and inadequate due to the small quantities of these vegetables being used to augment diets. As such they do not play a significant role in food security and consequently this may enhance their disappearance as a nutritional food source. Lack of seeds has also resulted in low yields and some types of ILVs becoming extinct, which adds to a decreased consumption of ILVs. As ILVs are still neglected, unrecognised, unappreciated, undervalued, viewed as shameful to eat, as a poor man’s meal, and being out-of-date, several measures should be put in place by all sectors to ensure their continued availability. There is an urgent need to educate the population on the inherent potential of indigenous vegetables in order to make them an important part of mainstream diets which would enhance food security, prevent malnutrition and generate income. Generally, ILVs contain high amounts of nutrients such as minerals and provide an inexpensive and valuable source of nutrition. Diversification of these crops is essential if the world is to have secure food supplies. In addition, ILVs are extremely perishable, and their quality begins to deteriorate as soon as they are harvested and continues through processing until they are consumed. Therefore, post-harvest processing methods need to be developed and promoted to improve handling, minimise post-harvest losses, increase shelf-life and add value to these vegetables. Furthermore, cultivation and consumption of ILVs should be promoted and indigenous knowledge on various preparation, food processing and cooking methods should be disseminated, in order to enhance their utilisation. It is recommended that authorities promote these ILVs for household food security, dietary diversity, creating employment opportunities, malnutrition and poverty alleviation.