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Globally, communities experience food insecurity, highlighting a need for access to food sources that are readily available with nutritional benefits. African leafy vegetables represent a plant-based food source that is rich in nutritional content and health benefits. These vegetables can grow unattended in the wild with minimal agricultural inputs which may negatively affect the environment, highlighting the advantages of their use. However, there is still a need to investigate the nutritional and functional value of these vegetables, focusing on their advantages and limitations before they can be recommended as an alternative food source. The chapter will focus on evaluating peer-reviewed journal articles, book chapters, and other publications to conduct a qualitative review.
Central University of Technology, Free State, Bloemfontein, South Africa
*Address all correspondence to: nmalebo@cut.ac.za
1. Introduction
The literature demonstrates that the Southern African Development Community (SADC) region is warming up faster than other regions in the global North prompting action towards adaptation strategies. The SADC region has observed adverse changes in rain patterns resulting in floods or multiple droughts. Reports by the Food Agriculture Organization [1] indicate the doubling of areas on the planet affected by droughts over a 40-year period [2]. The observed threat is a concern as the most vulnerable communities affected by high levels of unemployment, poverty, malnutrition, and inequality can be found in the SADC region. Studies indicate that climate change threatens health, ecology, and food security in sub-Saharan Africa [3, 4]. For ecology, an increase in woody vegetation has been observed which continues to replace indigenous habitats in certain regions [3]. The rise in temperatures leads to increased levels of evapotranspiration [5] which may affect food production and highlights the need to increase the propagation of climate-resilient crops such as African leafy vegetables (ALVs). Furthermore, because ALVs are considered climate-resilient crops, they can support climate adaptation strategies that are needed to address food insecurity [2]. Researchers [4] further argue that science interventions should be linked with local indigenous knowledge for adaptation and food security to address challenges such as climate change.
Maseko et al. [6] contend that the addition of ALVs in cropping systems can contribute to climate change adaptation strategies. This argument is supported by Nyathi et al. [7] who advocate for the diversification of current food production systems because vegetables such as Swiss chard which are available in commercial markets require high cultivation input whereas ALVs grow easily in the wild with minimal input from fertilizers and water. It has also been demonstrated that as climate changes, the presence of additional carbon dioxide in the air enhances the drought adaptation mechanisms of ALVs such as stomatal conductance which enables their growth during times of drought. This ensures the availability of highly nutritious vegetables which can support a plant-based diet.
Although commercially available C3 vegetables such as Swiss chard increase biomass when carbon dioxide levels increase, their propagation requires high water input and fertilizers when compared to ALVs [8]. This indicates that such commercially available vegetables would not support climate adaptation strategies. Furthermore, an increase in carbon dioxide reportedly has a direct effect on nutrition as it reduces iron, zinc, and protein content in grains [4, 8], this implies that an increase in biomass may not result in high nutritional content. ALVs are recognized as a rich source of minerals and vitamins which are currently not available in commercially accessible vegetables [9, 10, 11]. Despite the known nutritional benefits of ALVs, urbanization, access to disposable income, and availability of commercial vegetables continue to influence changes in diet patterns in favor of commercial vegetables over ALVs [2]. In most African communities, ALVs are still regarded as weeds or food for the poor. Researchers argue that ALVs can be used in what is referred to as “hidden hunger” which is defined as micronutrient deficiencies [10, 12] observed globally due to a preference for ready-to-eat foods with limited nutritional value.
A review of existing databases (Google Scholar, Science Direct, Web of Science, Scopus) was conducted for data published on ALVs from 2010 to 2023. Keywords such as “African leafy vegetables”, “indigenous leafy vegetables”, “nutritional value”, and “functional value” were used to search for open access journal papers and book chapters. Inclusion criteria: Reviews and experimental papers published in peer-reviewed journals, book chapters, and conference proceedings between 2010 and 2023 were selected. Exclusion criteria: papers published prior to 2010 and not in peer-reviewed publications from sub-Saharan Africa. A total of 80 articles were retrieved and relevant papers were selected and used for the current review. Study limitations: The current study was limited to publications in peer-reviewed journals and book chapters, other studies published in non-peer-reviewed papers and also in other languages may have been excluded. The study also focused on ALVs mainly consumed in countries within sub-Sahara Africa and may have excluded leafy vegetables consumed in other parts of the African continent including the studied areas.
3. Type of African leafy vegetables (ALVs) consumed by communities
Different types of African leafy vegetables (ALVs) are consumed in various countries in sub-Saharan Africa. In Zimbabwe, ALVs such as Cleome gynandra, Amaranthus thunbergii Moq, Vigna unguiculata, Corchorus tridens, Corchorus olitorius, Bidens pilosa, and Amaranthus hybridus are consumed [13]. Cleome gynandra also known as African cabbage belongs to the family Capparaceae, an erect herb with palmately compound leaves (Figure 1A). Amaranthus species (Figure 1B), from the Amaranthaceae family, leaves are described as succulent and simple in some taxa. Vigna unguiculata is classified in the Leguminosae family and distinguished from other ALVs by tri-foliate leaves. Corchorus (Figure 1C) species belongs to the Tiliaceae family, with oblong to lanceolate leaves that have serrated margins and distinct hair-like teeth at the base. B. pilosa belongs to the family Asteraceae, differentiated by lobed, serrate, and opposite green leaves, and yellow or white flowers (Figure 1D).
Figure 1.
An image showing selected ALVs. A – Cleome gynandra; B – Amaranthus thunbergii; C – Corchorus olitorius and D – Bidens pilosa.
According to [13], C. gynandra (Cleomaceae family) and Vigna unguiculata (Fabaceae family) are considered amongst the top five most important traditional vegetables in Zimbabwe. Although these vegetables are important traditional food sources, other species such as B. pilosa (Asteraceae family) are used not only as a food product but as medicine [14] based on the indigenous use of the plants. B. pilosa L. [15] is reportedly used to treat malaria, dysentery, diarrhea, and infected wounds or burns [16]. Most of these ALVs grow in the wild with minimal production input but [13] that vegetable species such as C. gynadra have been reportedly domesticated in home gardens and are purposely protected during activities such as digging, weeding, and land clearing due to the known benefits they provide.
Although ALVs grow in the wild, a study by [17] indicated that species such as Amaranthus hybridus and C. gynandra occur in all soil types, however, C. gynandratends to be associated with sandy soils. Although B. pilosa reportedly thrives in any environment [14], it can be negatively affected by frost. Nightshade (S. retroflexum Dun.; Solanaceae family) leaves and tender shoots are consumed in most African countries [18]. African leafy vegetables mainly consumed in South Africa include Amaranthus species [6] such as Amaranthus thunbergii Moq., Amaranthus spinosus (L.) followed by Corchorus (C. asplenifolius, Corchorus trilocularis, C. tridens and C. olitorius), Cleome monophylla L. (C. monophyla, C. hirta) [19], Vigna unguiculata, and Bidens pilosa [20]. Cleome gynandra, Curcubita maxima, Vigna unguiculata, Vigna unguiculata subsp. dekindtiana var. dekindtiana, V. unguiculate subsp. dekindtiana, var. huillensis, V. unguiculata subsp. rotracta, V. unguiculata subsp. stenophylla, V. unguiculate subsp. tenuis var. ovata, V. unguiculata subsp. unguiculata, Vigna unguiculata subsp. unguiculata, Solanum nigram, Urtica urens, Ribes uva crispa, Taraxacum officianale, and Beta vulgaris [21] which are rich in nutrients, are also consumed in South Africa.
Solanum macrocarpon, Talinum fruticosum, Corchorus (Figure 2), and Amaranthus are consumed in Ghana [22]. In Côte d’Ivoire, although there are various ALVs consumed, the main reported vegetables include Acalypha ciliate, Celosia trygina, Cleome gynandra, Solanum nigrum, and Sesammum radiatum. In Southern Angola, Bidens pilosa and Amaranth are reported as the main ALVs consumed amongst the various vegetables reported [23]. The literature demonstrates that certain species from genera Amaranth, Cleome, Bidens, Vigna, Solanum, and Corchorus are readily available in most African communities. In most of these communities, although consumption continues to decrease, the nutritional benefits of the ALVs are recognized.
Figure 2.
An image of ALVs showing A – Solanum spp.; B – Talinum; C – Acalypha; D – Celosia; E – Sesammum; F – Urtica.
African leafy vegetables are generally characterized by an abundance of carbohydrates, low protein, and fat content (Table 1). When consumed with high-calorie vegetables such as maize, supplementation for protein is needed to fully address malnutrition. Other studies have shown that adding peanut butter during cooking of ALVs enhances the presence of proteins and oils [26]. Generally, ALVs contain high amounts of flavonoids (Table 1) which are associated with health benefits such as protection against cardiovascular diseases, stroke, and cancer [22]. Additionally, ALVs contain moisture levels which are similar to commercially available leafy vegetables that are generally accepted by consumers [26]. The ash contents of C. gynandra and Amaranth [24] indicate that both vegetables may be good sources of minerals. The ALVs are considered sources of crude fiber which assists in the absorption of excess cholesterol.
Doue et al. [26] reported the presence of carotenoids in Acalypha ciliate, Celosia trygina, Cleome gynandra, Solanum nigrum, and Sesammum radiatum (Table 1). Although cooking methods may affect carotenoid content, the carotenoid content identified in these ALVs remains higher than what is reported for commercially available vegetables. Carotenoid pigments which include beta-carotene have high antioxidant properties which play an important role in reducing the occurrence of cancer. Additionally, vegetables with antioxidant properties can be used for the prevention of degenerative diseases [22]. African nightshade is one of the ALVs which reportedly serves as a rich source of minerals such as potassium, manganese, and vitamins in the form of Vitamin A and folate [18]. Other studies indicate that African nightshade is a source of protein, minerals, and beta-carotene which is reportedly higher than commonly consumed vegetables [10, 12, 29]. Other vitamins identified in African nightshade include thiamine, riboflavin, and folate [12, 28].
Amaranth is recognized as a good source of pro-vitamin A [25] which are carotenoids that the body convert to vitamin A. Potassium, calcium, magnesium, phosphorus, iron, manganese, and zinc were identified in high amounts in Cleome leaf tissue [27]. Vitamins such as beta-carotene, vitamin A, vitamin C and medium in vitamin E and protein have also been reported in blackjack [14]. Minerals identified in blackjack include calcium, phosphorus, sodium, manganese, copper, zinc, magnesium, and iron [14]. The minerals are beneficial for human health with iron playing an important role in blood production [18]. The presence of high carbohydrate content in leafy vegetables indicates a high caloric content, a study conducted by [22] showed a high carbohydrate content in dried Solanum macrocarpon, Talinum fruticosum, Corchorus olitorius and Amaranthus spp. when compared to other studies. Crude fiber was observed by [22] in dried Solanum macrocarpon, Talinum fruticosum, Corchorus olitorius, and Amaranthus; the study further showed that these dried ALVs are good sources of protein. However, other studies observed minimal amounts [26] of protein in ALVs.
Various studies have demonstrated that traditional leafy vegetables possess phenols (gallic acid) and flavonoids which grant these vegetables various functional properties. Obeng [22] showed that Solanum macrocarpon, Talinum fruticosum, Corchorus olitorius, and Amaranthus spp. possess phenols and flavonoids. Obeng [22] reported antioxidant properties when dried Solanum macrocarpon, Talinum fruticosum, Corchorus olitorius, and Amaranthus spp. were assessed. Although antioxidant properties were identified, the study reported that high quantities of ALVs and frequent consumption are recommended to provide consumers with the reported health benefits. Nightshade possesses antioxidant properties due to the presence of phenolic metabolites [18, 30]. Jiménez-Aguilar and Grusak [31] demonstrated that some Amaranth species possess antioxidant properties which are higher than properties reported in spider flower (Table 2), African nightshade, and spinach. Although additional studies are needed, [14] have reported that blackjack possesses antioxidant properties. The health benefits of vegetables with antioxidant properties have been demonstrated.
Functional properties of selected African leafy vegetables.
Blackjack possesses various chemical compounds such as astragalin, a flavonoid found in various plants, reportedly has anticandidal activity. The compound was shown to inhibit fungous biofilm development. Kissanga et al. [23] indicate that A. hybridus is used for medicinal properties in Angola. Antibacterial properties of blackjack against gram-negative E. coli as well as antifungal properties have been reported [14]. In addition to the identified antimicrobial properties, blackjack has been used to treat various diseases such as indigestion, diarrhea, dysentery, wounds, and respiratory infections [14]. One of the bioactive compounds, astragalin, isolated from blackjack reportedly has anti-parasite properties against Trigonoscuta cruzi [14]; it affects the growth of the parasite by changing the morphology of the cell membrane.
Mtenga and Ripanda [14] also report that blackjack possesses anti-inflammatory properties due to the presence of squalene. Mokganya and Tshisikhawe [15] further indicate that liquid extracted from blackjack has been used to treat inflammation and wounds as it contains antimicrobial compounds such as Iso-Vanillin and Daucene. Similar properties were reported in nightshade due to the presence of phenolic metabolites such as chlorogenic acids [18, 32]. C. gynandra, which is commonly used by various African communities, is characterized by compounds such as quercetin glycosides that possess anti-inflammatory properties, and claims by communities indicate that decoctions are consumed orally to reduce blood pressure [15]. Other commonly used ALVs such as S. retroflexum L. are reportedly used to treat earache in young children [15] because the ALV contains hydroxycinnamic acid derivatives with antimicrobial and anti-inflammatory properties (Table 2). V. unguiculata L. is used to treat stomach problems [15] because it possesses compounds (Coumaroyl derivatives) with anti-inflammatory and anti-microbial properties. Researchers reported antidiabetic, anti-obesity, and antihypertensive properties of B. pilosa worldwide, indicating the possibility of its application in the mitigation of diabetes, hypertension, and obesity.
It is generally accepted that ALV is resistant to drought, pests, and diseases when compared to commercially available vegetables [6, 10]. Some communities are consuming ALVs as an important resource for climate adaptation strategies [12, 33] and for food security. Kissanga and others [23] reported that vegetables are used by communities during droughts because ALVs are resistant to environmental changes. The morphology of some of the ALVs such as leaves with a waxy cuticle observed in Amaranthus spp. protects the vegetable against rapid moisture loss. African leafy vegetables such as Amaranthus, Brassica nigra, and Cleome gynandra are also drought hardy because of their excellent stomatal conductance. Some ALV species such as Bidens pilosa have an extraordinary recovery rate after experiencing prolonged drought periods [20].
ALVs grow in the wild with low input from pesticides and fertilizers. Additionally, bioactive compounds such as alkaloids from Bidens pilosa can be used as organic pesticides which are environmentally friendly and biodegradable [14]; compounds from blackjack can be used to control fungal pathogens and weeds which affect plant growth. Doue et al. [26] have reported that ALVs can serve as fertilizers as they improve the growth of other plants by improving soil fertility. Economic opportunities exist to include ALVs such as Amaranth in bread products, the advantages of this type of postharvest processing not only provides communities with a source of nutritious bread but increases the need and value of these vegetables [2] and also provides an opportunity to sell a highly nutritious product.
Despite the added advantages of ALVs, there are some limitations that affect their use. African leafy vegetables are highly perishable, and preparation and preservation approaches affect their long-term nutritional quality [6]. Preparation methods at high temperatures can affect compounds such as vitamins and minerals. This can have a negative effect on some of the reported health benefits of ALVs. Other studies indicate that alternative methods for preparation are needed and supplementation with other sources of protein and fat may be necessary to enhance the nutritional benefits of ALVs with low fat and protein content. Doue et al. [26] argue that preparation of ALV with red palm oil may enhance the availability of vitamin A which is needed to address deficiencies that are reported in the sub-Saharan region. Although steaming reduces the nutritional properties in other ALVs, [18] demonstrated that this process can significantly reduce anti-nutritive compounds in nightshade, making it the most suitable processing method. Similar studies have shown that post-harvest processes can also be beneficial and assist with the release of complex minerals, enhancing bioavailability and bioactivity [12, 34].
Lack of post-harvest processing of vegetables was reported to result in loss of quantitative, nutritive, and economic value of nightshade along the supply chain [35]. Similar losses were observed in other ALVs such as Cleome gynandra L., African nightshade, and Amaranthus spp. Gogo and others [35] demonstrated in their study that loss due to pests and diseases occurred mainly postharvest and this is influenced by various factors which include transport of vegetables in suitable conditions, packaging, and poor handling. African leafy vegetables are highly perishable [10] and to date, different methods to prolong their shelf life are explored. Lactic acid fermentation [36] is a method commonly used by African communities which prevents the growth of spoilage microbes and increases the sensory and shelf life properties of vegetables. Although postharvest value addition opportunities exist for ALVs, their availability in small quantities affects the possibility of establishing their availability in the food production chain [2].
Poor seed quality limits propagation and affects yield which results in a focus on exotic plants rather than African leafy vegetables [20]. Stoll and others [10] have also reported poor seed quality as one of the factors which influences the availability and consumption of ALVs. The challenge continues to exist due to limited investment in research and development [12, 21, 37]. These challenges are intensified by the presence of heavy metals in ALVs due to contamination of soils. This is a concern that studies indicate may affect the advocacy and use of these vegetables [23]. The presence of antinutritional compounds such as cyanogenic glycosides, oxalates, phytates, nitrates, and tannins is one of the challenges identified that discourages the use of ALVs. However, the implementation of suitable agro-processing techniques to facilitate the elimination of antinutritive compounds should be further explored [12] if the use of ALVs is going to be promoted. Addressing the presence of antinutritional properties of ALVs is important because their presence influences the absorption of vitamins and minerals present in vegetables, reducing their value. The biggest challenge which affects the use of ALVs is the unsustainable harvesting because the vegetables are not formally cultivated [6].
Consumer acceptability and perceptions about ALVs have generally influenced their limited consumption and propagation. The availability of mainstream vegetables influences the consumption and propagation of ALVs [38]. Blackjack (Biden pilosa) is underutilized in sub-Saharan Africa due to its classification as a weed or a wild plant, which creates a negative perception in the community concerning the consumption of wild or weed plants [14]. The use of blackjack despite its identified benefits is hindered by its categorization as inversive species (weeds). The perceptions that exist in communities affect the marketing of ALVs as an alternative source of nutrition. There is a general agreement that ALVs are poorly marketed [10] despite the nutritional and medicinal properties known by indigenous people. According to [21], information about the economic value of ALV is lacking. ALVs are perceived as foods with low social status and only meant for the poor by certain communities [25, 39]. Young consumers still prefer Western-based diets which are widely promoted on media platforms and represent a higher-class status within their communities. But [36] have reported that in Kenya, consumer awareness about the nutritional benefits of ALVs has resulted in an increase in their use.
The current review has demonstrated the advantages of using ALVs as part of a plant-based diet, however, awareness about the nutritional and medicinal benefits of ALVs is still needed to change current consumer perceptions and preferences. Most of the ALVs discussed have micronutrients which can be used to address the challenge of hunger. Their sale by communities can contribute to socio-economic development [18] and address the goal of ending poverty in the sub-Saharan region and hidden hunger globally. Research is still needed to assess the bioavailability of nutritional compounds and their benefits following digestion to recommend their use in plant-based diets. The ability of the vegetables to withstand adverse climatic conditions and growth with minimal water and pesticide input was also demonstrated. This highlights the continuous availability of ALVs despite climate change challenges. The way forward would be the development of policies that advocate for and promote the use of ALVs as proposed elsewhere [12]. Although fermentation has been used as an effective method to prolong the shelf life of ALVs, sensory evaluation studies indicate consumer acceptance of fermented vegetables is minimal. This highlights the need to continuously investigate various postharvest preservation methods to protect this valuable nutritional resource for plant-based diets.
The author would like to thank the Central University of Technology, Free State, the South African National Research Foundation (NRF), and the Southern African Science Service Centre for Climate Change and Adaptive Land Management (SASSCAL) for supporting the work conducted in this study.
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Written By
Ntsoaki Joyce Malebo
Submitted: 09 May 2023Reviewed: 23 August 2023Published: 14 November 2023
Open access peer-reviewed chapter
