Cleome gynandra L. (common names: Shona cabbage, spider plant, African cabbage or cat's whiskers) is a widely consumed wild leafy vegetable, which belongs to the Cleomaceae family. It is an erect and branched herbaceous plant, which grows wild in southern Africa (van Rensburg et al., 2004), East Africa as well as South Asia (Bala et al., 2010; Cernansky, 2015). C. gynandra is traditionally harvested from the veld or semi-cultivated in sub-Saharan Africa (van Rensburg et al., 2004; Habwe et al., 2009; Gao et al., 2011). Besides its nutritional benefits, C. gynandra has anti-bacterial (Hamill et al., 2003), anti-inflammatory (Narendhirakannan et al., 2007), antioxidant (Muchuweti et al., 2007), and anticancer (Bala et al., 2010) properties. The nutritional qualities and biological activities of C. gynandra are largely attributed to its “large amounts” of vitamins and minerals (Gao et al., 2011), and its diversity of chemical constituents, which include tannins, saponins, alkaloids, steroids, glycosides, flavonoids, and phenolic compounds (Narendhirakannan et al., 2007). Based on its high visibility in scientific literature, C. gynandra is fast emerging as one of the most widely consumed “super vegetables” (Cernansky, 2015), especially in sub-Saharan Africa and Asia.

For millennia, communities throughout the African continent have relied on natural plant resources for life's necessities, most notably food and medicine. In particular, they have used indigenous leafy vegetables as a source of vitamins, dietary mineral nutrients and for general well-being maintenance (Uusiku et al., 2010). The consumption of traditional dishes is a global phenomenon: for example, wild food plants were recently reported to constitute a significant part of diets across Europe (Guarrera and Savo, 2016). Worldwide, it is estimated that 7,000 plant species are semi-cultivated or harvested from the wild for food (Schönfeldt and Pretorius, 2011). The characteristics of indigenous leafy vegetables, which make them attractive, include their ability to grow on low fertility soils, high relative drought tolerance and faster growth rates; hence, they can be harvested within a short period of time (van Jaarsveld et al., 2014). Despite being in the scientific doldrums, this important food source has gradually evolved over many generations of continuous local scale innovation. It is only recently that scientists have awakened to the promising potential of indigenous leafy vegetables as sources of nutrition in enhancing food security and maintaining healthy lifestyles. Although there are documented studies conducted on some individual vegetables, a comparison of nutrient content results from different data sources could be problematic due to variations in sample preparation and analytical conditions used in different studies (van Jaarsveld et al., 2014).

The objective of the current study was to compare the mineral composition, chemical constituents and antioxidant activity of C. gynandra against two of the most widely-consumed commercial vegetables in the world; viz Brassica oleracea var. capitata (cabbage, Brassicaceae) and Beta vulgaris L. (Swiss chard, Amaranthaceae). Cabbage (Brassica oleracea var. capitata) is regarded as one the world's leading vegetable crops that is rich in vitamins, anticarcinogenic glucosinolates, and antioxidant metabolites as well as amino acids (Rengasamy et al., 2016). On the other hand, Swiss chard (B. vulgaris L.) spread from Europe, where it has been cultivated since classical antiquity, to the rest of the world (Pyo et al., 2004). Coupled to its nutritional characteristics, B. vulgaris has been demonstrated to possess anti-acetylcholinesterase, anti-inflammatory and antioxidant properties (Ninfali et al., 2007).

The search for alternative nutritious foods may hold the key to nourishing many people on the African continent and beyond. Wild green vegetables have been found to be rich sources of vitamins, namely β-carotene, ascorbic acid, riboflavin, folate as well as minerals such as iron, calcium and phosphorous (Gupta et al., 2005). The incorporation of wild food plants into the diet can introduce variety and diversity in terms of nutrition and taste (Cernansky, 2015). It is common knowledge that a diverse range of foods including vegetables is required to provide all the necessary micronutrients and phytochemical constituents for a healthy diet. Hence, dietary diversification is critical in order to mitigate the widespread nutrient deficiencies of vitamin A, iron, and zinc (FAO/WHO, 2004). The dietary requirement for a micronutrient refers to the intake level, which meets specified criteria for adequacy; however, threshold levels remain undefined for most dietary nutrients (FAO/WHO, 2004). The nutrient content of plant foods such as green leafy vegetables may vary widely due to several factors, which include stage of maturity and post-harvest handling (van Jaarsveld et al., 2014). In the present study, the vegetables were obtained in a fresh state at the point of sale in order to determine the nutritional and phytochemical properties of the quality available to consumers. Both C. gynandra and B. vulgaris exhibited higher concentrations of all the quantified nutrient elements compared to B. oleracea. B. vulgaris had higher levels of sodium, magnesium, manganese and copper, whereas phosphorus, potassium, calcium, iron and zinc content was higher in C. gynandra. This implies that a diverse diet incorporating both vegetables may be needed to provide adequate levels of the desired nutrients. Considerably high iron content in some wild, traditional leafy green vegetables was also recently reported (Schönfeldt and Pretorius, 2011; van Jaarsveld et al., 2014). Iron plays a critical role as an oxygen carrier from lungs to body tissues, a transport medium for electrons within cells and as an integral part of important enzyme systems such as cytochromes (Wessling-Resnick, 2000). Worldwide, but particularly in developing countries, iron deficiency is probably the most widespread nutritional deficiency disorder (Hallberg, 2001). Similar to our findings, some recent studies have reported underutilized wild leafy vegetables to be good sources of calcium (Gupta et al., 2005; van Jaarsveld et al., 2014). Calcium, an essential mineral comprising 1.9% of the body weight provides rigidity to the skeleton, and is vital in neuromuscular function, enzyme-mediated processes and blood clotting (WHO, 2009). Globally, calcium intake levels per capita vary widely and increase from developing countries, particularly Asia to developed countries such as North America (FAO/WHO, 2004). In the present study, zinc content was 1.2 and 2.1 times greater in C. gynandra compared to B. vulgaris and B. oleracea, respectively. Similarly, high zinc content in C. gynandra was reported in a study comparing five wild green vegetables (Schönfeldt and Pretorius, 2011). Zinc plays an important role in more than 300 enzymes involved in synthesis and degradation of biomolecules, metabolism of other micronutrients as well as the immune system (MacDonald, 2000). Similar to iron, cooking has no adverse effect on zinc levels in green leafy vegetables (Uusiku et al., 2010). However, bioavailability of zinc may be adversely affected by anti-nutritional factors such as phytic acid (WHO, 2009). Oxalic acid is another anti-nutritional factor, which interferes with calcium absorption by forming insoluble complexes (Gupta et al., 2005). Future studies on nutritional composition of wild green leafy vegetables should evaluate the content of these anti-nutritional compounds.

Vitamin A, an essential nutrient in humans, provides a vital role in the functioning of the visual system, and maintenance of cell function for growth and epithelial cellular integrity as well as production of red blood cells (WHO, 2009). Preformed retinol (mainly as retinyl ester) and provitamin A carotenoids such as β-carotene, lutein, violaxanthin, and neoxanthin are the main sources of dietary vitamin A (Blomhoff et al., 1991). In the present study, β-carotene content in B. vulgaris and C. gynandra was 28- and 22-times greater when compared to B. oleracea, respectively. With regards to plant sources, levels of vitamin A are dependent on β-carotene content, its bioavailability and bio-efficacy (West et al., 2002). Based on WHO (2009) recommendation, dietary diversification through the consumption of provitamin A-rich foods, namely dark green leafy vegetables is one of the three strategies to curb prevalence of vitamin A deficiency. The vitamin C content of C. gynandra was 4.5- and 3-times higher in comparison to B. oleracea and B. vulgaris, respectively. In a study by Pennington and Fisher (2010), dark green leafy vegetable subgroup (which included B. vulgaris and eight other commercial leafy vegetables) provided at least 50% of dietary reference intakes (DRIs) for vitamin C. A substitution of B. vulgaris with C. gynandra or an inclusion of C. gynandra in this subgroup will certainly amplify the percentage value for vitamin C DRIs. Besides its role as an electron donor for enzymes involved in collagen hydroxylation, carnitine biosynthesis and tyrosine metabolism, vitamin C is a potent antioxidant (Prockop and Kivirikko, 1995). Thus, the significantly high antioxidant activity observed in C. gynandra may correspond to its significant high level of vitamin C in comparison to other vegetables in the present study.

Apart from vitamins, non-nutritional components of wild green vegetables, notably phenolic compounds are also known to possess powerful radical scavenging properties against reactive oxygen species (ROS) (Stangeland et al., 2009). Total phenolic and flavonoid contents as well as specific phenolic compounds (hydroxybenzoic and hydroxycinnamic acid derivatives) were quantified in the three leafy vegetables. Notably, the concentration of protocatechuic acid, p-hydroxybenzoic acid, salicylic acid, caffeic acid, and p-coumaric acid were significantly higher in C. gynandra compared to the two widely consumed commercial vegetables. Oxidative stress is generally associated with the occurrence of numerous health conditions including neurodegenerative disorders, cardiovascular diseases and, most recently cancers (FAO/WHO, 2004). In the present study, C. gynandra exhibited the best antioxidant activity using both the DPPH and ORAC model systems. In particular, the ORAC affords a robust antioxidant test system, which measures both hydrophilic and lipophilic chain-breaking antioxidant capacity (Prior et al., 2003). Compared to synthetic antioxidant supplements, natural antioxidants derived from plant products such as green leafy vegetables may be more effective in reducing ROS levels due to the synergistic actions of a wide range of constituent biomolecules such as vitamins C and E, phenolic compounds, carotenoids, terpenoids, and phytonutrients (Podsedek, 2007). Regular intake of dietary antioxidants from phenolic compound-rich vegetables can reduce the adverse risk of these lifestyle-related diseases. Perhaps the reported biological activities of C. gynandra (Hamill et al., 2003; Muchuweti et al., 2007; Bala et al., 2010) are an emergent property of its high total phenolic and flavonoid content. However, there is a lack of documentation regarding the antioxidant status of the sub-Saharan Africa leafy vegetable population (Uusiku et al., 2010). Therefore, in most countries this area requires further scientific studies to establish baseline health data that can be used for better-targeted interventions. It is a paradox that such a plant diversity-rich continent is still characterized as food insecure and undernourished (FAO, 2014) in the twenty-first century.

For a generation, Africa has been depicted as a hunger-stricken continent, stereotyped in the international media by imagery of malnourished children. The consumption of wild leafy greens or vegetables holds immense potential in nourishing many on the continent and beyond. Results of the current study on the chemical, nutritional and antioxidant properties of C. gynandra, a popular wild leafy vegetable in many parts of sub-Saharan Africa, provides compelling scientific evidence of its potential in adding diversity to the diet and contributing toward the daily nutritional requirements of millions of people. However, the realization of this “dream” depends on a number of factors, which include significant investments in the research on under-utilized foods, in particular wild leafy vegetables. Chemical constituents, nutritional and antioxidant qualities of C. gynandra provide critical insights about its potential in adding diversity to diets in sub-Saharan Africa and beyond. Thus, C. gynandra and other indigenous leafy vegetables may provide the ultimate weapon against dietary deficiencies.

MM conceived research idea and prepared draft manuscript. MM, SOA, AOA, JG, MS, MJ, PT, and KD performed analysis of bioactive compounds and edited the manuscript.