Keywords

Introduction

Consumption of fruits and vegetables are predicted to prevent many deadly diseases and disorders; though no direct evidence is available (UNESCO 2008), these can be due to the presence of various antioxidants, phenolic compounds, minerals, vitamins, and other pigments in it. Still consumption of these fruits and vegetables are found to be very less. Among these green leafy vegetables (GLV) are considered best instant sources of fibers, essential amino acids, vitamins, and minerals (Sharma and Kumar 2013; Adenipekun and Oyetunji 2010) producing various biological effects likes antimicrobial effect (Kubo et al. 2004; Hedges and Lister 2009; Dhiman et al. 2012), antihistaminic (Kesari et al. 2005), hypolipidemic (Khanna et al. 2002), anticarcinogenic (Rajeshkumar et al. 2002), and antidiabetic properties (Yamamura et al. 1998) to cure and prevent many lifestyle and harmful diseases such as insomnia, hypertension, diabetes, age-related ailments, oxidative strains, hypertension, and cardiovascular diseases (Iyer Shanti et al. 2012; Vishwakarma and Dubey 2011; Patro et al. 2011). However, these bioactive compounds are present in low amount; higher consumption of these GLVs along with physical exercise can prevent age-related disorders.

GLVs are also used as medicine to produce physiological actions on the human health because of the presence of their bioactive compounds, which produce immune system repair, antibacterial, antioxidant, antiviral, detoxification, and anti-inflammatory activities (Lampe 2003; Raju et al. 2007). Vitamins and secondary metabolites may be commonly termed as “phytochemicals,” considering certain vitamins as primary metabolites whereas provitamins such as β-cryptoxanthin or β-carotene, etc. as a part of secondary metabolites (Poiroux-Gonord et al. 2010). These provitamins are converted to vitamins in the animal body producing the essential affect. GLVs are also rich source of antinutrient, reducing many diseases like CVD, high blood pressure, stroke, etc. (Aletor and Adeogun 1995).

Plant Metabolites

The level of plant metabolites are greatly influenced by genetic and environmental factors as well as storage and transportation conditions. Growth factors including temperature, humidity, light, type of soil, damage by microorganisms and insects, stress induced by UV radiation, application of fertilizers, pesticides, and heavy metals alter the metabolite composition of plants (Orcutt and Nilsen 2000).

Primary Metabolites

The primary metabolites such as amino acids, carbohydrates, fatty acids, and organic acids are commonly found in all species across broad range of phylogenetic groups. These compounds are directly related to the growth and development, hormone and protein synthesis, respiration, and photosynthesis. The biochemical pathways used for modifying and synthesizing these primary metabolites, which are found essentially same in all organisms, apart from minor variations (Hounsome et al. 2008). Classification of primary metabolites is given in Fig. 1.

Fig. 1
figure 1

Classification of primary metabolites, Boyle 2005; Belitz et al. 2009

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Protein

Proteins are complex molecules having various compositions of amino acids. They play a vital role in regulating body metabolism, cellular function, and structure. Hence, they add value to daily diet of consumers. Green leafy vegetables are rich and inexpensive sources of proteins because of their synthesizing ability of amino acids (Aletor et al. 2002). Ribulose-1, 5-bisphosphate carboxylase/oxygenase (RUBISCO) is a major leaf cell protein (accounts to about 50%) which plays a critical role in carbon fixation during photosynthesis (Kawashima Nobumaro and Wildman 1970). This is a similar protein found in leaf chloroplasts of all green leafy vegetables with minor changes in amino acid base for different species. Recent research reported that the green leafy vegetables such as broccoli (Brassica oleracea var. italica), duckweed (Lemna perpusilla), and spinach (Spinacia oleracea) render all the essential amino acids that meet the FAO nutrition standards (Edelman and Colt 2016). Studies have also proven that cassava (Manihot esculenta) leaves have amino acid profile balanced with pulse and dairy products (Fasuyi 2005). The protein content in African leafy vegetables such as green leaves of septic weed (Senna occidentalis) and cassava (7 g/100 g of fresh weight) are greater than exotic leafy vegetables such as Brassica oleracea subsp. capitata (1 g/100 g of fresh weight) (Uusiku et al. 2010). However, African leafy vegetables have relatively less protein content than legume proteins (white lupine (Lupinus albus) with 11.5 g protein/100 g of fresh weight) (Kalogeropoulos et al. 2010).

According to prevailing environmental conditions and farming practices, the amount of protein in leafy vegetables may vary (Odhav et al. 2007). Thermal processing inactivates heat-labile anti-nutritional factors such as lectins, goitrogens, thiaminases, and protease inhibitors.

It improves digestibility of proteins and starch but leads to protein denaturation and hence effects the bioavailability of proteins in leafy vegetables (Gibson et al. 2006).

Dietary Fiber

Dietary fiber is considered a class of compound, which comprises a mixture of plant carbohydrate polymer, noncarbohydrate component, polysaccharides, non-starch polysaccharides, and oligosaccharides (Elluech et al. 2010). In these mixtures, non-starch polysaccharides and oligosaccharides are considered a major component of dietary fiber. Based on their solubility, the dietary fiber can be classified into insoluble fibers (cellulose, lignin, hemicellulose) and soluble fibers (β-glucan, pectin, gums) (Natesh et al. 2017). Not all these components can be digested by human enzyme, and it is passed through the gastrointestinal tract as bulk fiber. Later this bulk fiber undergoes digestion and modification by the enzymes of colon microbes (Blaut 2002). The consumption of dietary fiber may reduce the cardiovascular diseases, colon cancer, diabetes, diverticulosis, obesity, and constipation (Jenkins et al. 2001; Spiller 2001). According to the Food and Nutritional Board, Institute of Medicine (2001), the daily requirement of dietary fiber is 30 g/day for men older than age 50 and 38 g/day for men younger than age 50. The daily requirement for women is 25 g/day (age below 50) and 21 g/day (age above 50).

The green leafy vegetables are considered as good source of dietary fiber. For example, the soluble fiber content of curry leaf is 4.4% and insoluble fiber is 55.6% (Bako et al. 2002). But the amount of dietary fiber may vary based on species, climate condition, geographical condition, maturity stages, and fertilizers used (Natesh et al. 2017). According to the literature, those Indian green leafy vegetables such as hibiscus (Hibiscus cannabinus), cabbage (Brassica oleracea), spinach (Spinacia oleracea), coriander (Coriandrum sativum), fenugreek (Trigonella foenum-graecum), and basella (Basella rubra) are rich sources of soluble dietary fiber (Jenkins et al. 2001). African green leafy vegetables such as Portulaca oleracea, Galinsoga parviflora, Justicia flava, Adansonia digitata, Amaranthus sp., Arachis hypogaea, Bidens pilosa, Brassica sp., Ceratotheca triloba, Vigna unguiculata, Chenopodium album, Emex australis, Cleome sp., Cucurbita pepo, Senna occidentalis, Bidens pilosa, Manihot esculenta, Solanum sp., and Chenopodium album are good sources of dietary fiber. The fiber content of African leafy vegetables may range from 1 g/100 g to 8 g/100 g (Uusiku et al. 2010). Leafy vegetables such as water leaf (Talinum triangulare), Telfairia occidentalis, and Amaranthus hybridus are found in different parts of Nigeria. Literature reveals that the African green leafy vegetables lose their dietary fiber content when cooking at high temperature, because it breaks the weak bonds between polysaccharides and cleavage of glycosidic linkages which leads to solubilization of the dietary fiber (Svanberg et al. 1997). The cooking effect is also examined in Indian leafy vegetables, in that there is a minute increase in total dietary content which may be due to the polymerization or hydration of total dietary fiber fractions (Kala and Prakash 2004). Puupponen Pimiä et al. (2003) reported that there is no consequential change in insoluble, soluble, and total dietary fiber of freezer and blanched stored spinach. Determination of dietary fiber in green leafy vegetables can be estimated by enzymatic or gravimetric methods given by AOAC.

Vitamins

Vitamins are available in the form of precursors in vegetables and are essential for proper functioning of bones, vision, hairs, teeth, skin, and the mucous membranes. Calcium and phosphorous help in the growth and maintenance of bones. Vitamins play a key role for the normal functioning of nervous system, the endocrine glands, and clotting of blood. They are also essential for macromolecule metabolism (Randhawa et al. 2015). The WHO (2009) reported 15 million pregnant women and 190 million young children from developing countries are suffering from vitamin A deficiency. This deficiency can be reduced by consumption of vegetables and green leafy vegetables instead of antioxidant supplements.

Raw green leafy vegetables have considerable amount of various vitamins such as fat-soluble vitamins (vitamin E and vitamin A) and water-soluble vitamins (vitamins C, B1, B2, B3, B5, B7, B9). Green leafy vegetables have high vitamin K contents when compared to fruits and vegetables because of their direct involvement in the photosynthetic process. Leafy vegetables like spinach have sufficient vitamin C content to prevent and cure scurvy (Randhawa et al. 2015). Green leafy vegetables such as spinach, asparagus, Brussels sprouts, turnip, lettuce, and cauliflower are reasonably good sources of B vitamins (USDA 2005).

Vitamin A

In leaves, vitamin A is in the form of provitamin A carotenoid such as α-carotene, β-carotene (abundant source), γ-carotene, β-cryptoxanthin, and non-provitamin A carotenoids such as neoxanthin, lutein, and violaxanthin. Beta-carotene is one of the most important provitamin carotenoids with respect to its quantitative contribution to the diet and relative provitamin A activity (SACN 2005). The level of vitamin A in the diet relies on the amount of beta-carotene of the African green leafy vegetables (ALVs), amount consumed, bioavailability, and bioefficacy (West et al. 2002). Vitamin A content is expressed in terms of retinol equivalents (RE), where 1 RE is equivalent to 6 μg of β-carotene and 12 μg of the other provitamin carotenoids such as α-carotene, β-cryptoxanthin, and γ-carotene. The US Institute of Medicine has recently replaced retinol equivalents (RE) with “retinol activity equivalent” (IOM 2000). According to the Institute of Medicine dietary reference, intake of vitamin A content is recommended to be 700 and 900 μg RAE for an adult female and male, respectively.

The β-carotene content of African leafy vegetables varies according to the species from 99 mg RE in Vigna unguiculata to 1970 mg RE (per 100 g edible portion) for M. esculenta. Considering the most recent edition of Recommended Nutrient Intakes (RNI), 1 RE is equivalent to 6 mg β-carotene (FAO/WHO 2001). According to the RNI, 300 g fresh African leafy vegetables intake would satisfy the dietary requirements of vitamin A for children. But for adults, 300 g of fresh Cucurbita pepo would provide 97% of male RNI and 116% of female RNI, while 300 g of fresh V. unguiculata would provide only 50 and 59% of male and female daily requirements, respectively. The stability and retention of carotenoids in various foods are affected by their food particle size, chemical nature, storage time and storage conditions, and the method and severity of processing (Cheynier 2005). Cooking of leafy vegetables enhances the bioavailability of α-carotene and β-carotene (Khachik et al. 1992). Heat treatment such as steaming and boiling enables to release carotenoids which are bound by protein and helps them to be extracted readily (Howard et al. 1999). Higher retention of β-carotene in the vegetables was achieved by microwave steaming and stir-frying with oil compared to stir-frying with water or boiled (Masrizal et al. 1997). Addition of oil to Ceiba sp. and Manihot sp. has shown to improve serum retinol in Ghanaian preschool children (Takyi 1999). When compared to the raw vegetables, cooked and pureed spinach provides higher plasma total β-carotene concentrations (Rock et al. 1998). While cooking, extractability increases because of enzyme destruction which could otherwise lead to carotene degradation (Kala and Prakash 2004).

Riboflavin

Riboflavin characterized as vitamin B2 which may have neuroprotective effects in neurological disorders like migraine, multiple sclerosis, and Parkinson disease. It is also having an antioxidant property which can prevent lipid peroxidation and reperfusion oxidative injury. Riboflavin is found in various plants and animal foods such as milk, eggs, and green leafy vegetables (Saedisomeolia and Ashoori 2018). African green leafy vegetables (ALVs) contain reasonably good concentrations of riboflavin. The riboflavin content in ALVs ranges from 0.04 mg/100 g (Brassica species) (Kruger et al. 1998) to 0.6 mg/100 g (M. esculenta) (FAO 1990). Intake of 300 g of fresh Cleome sp. would give 33–60% RNI of riboflavin for children, 30% for adult women, and 23% for adult men. Riboflavin deficiency commonly arises with lactose intolerance (FAO/WHO 2001), and this condition is majorly found in African populations. The deficiency of this vitamin is related to the increased risk of cardiovascular diseases (Powers 2003).

Folic Acid

Folic acid characterized as vitamin B9 also known as pteroylmonoglutamic acid or simply as “folates.” It is also an essential part of B complex (Cossins 2000). GLV contains abundant amount of folic acid and related compounds. Folic acid contains two to eight glutamic acids (essential amino acids) in the primary structure of pteroylmonoglutamic acids (Herbert et al. 1999). It is an important part of transport of amino acids to specific location in the protein synthesis pathway (Kelly 1998) and for the methylation of RNA, DNA, and amino acids (Lucock et al. 1996; Ma et al. 1997). Methionine is regenerated from homocysteine by folic acid to maintain the cardiovascular working (Leclerc et al. 1998). Being an essential part of cell proliferation and DNA synthesis, it is also essential for the cell division to regulate sleep, mood, appetite, and working of central nervous system (Bottiglieri et al. 2000). For the same reason, this is very important for the growth of fetus and during gestation (FDA 1997; Antony 2007). RDA suggests 400 μg/day of folate for the nonpregnant women, whereas 4 mg/day of folate is suggested for women having history of delivering babies with neural tube defects. RDA also suggests 600 μg/day for the development of maternal tissues, the placenta, and the fetus. Deficiency of folates also leads to many serious issues like neural tube defects in babies such as spina anencephaly and bifida and neurocristopathies. Studies have shown that daily intake of folic acid in women belonging to low-economic status of rural and urban areas of India is just 75–165 μg (Misra et al. 2002), which is very much less than that of FDA recommendations. Reports are not available on toxicity of folic acids with no to consumption of folic acid-supplemented food or folic acid-fortified foods. This might be due to the fact that folic acids are solubilized in water and are excreted from the body in the form of urine (Hathcock 1997). Scientists are continuously working for increasing such health-promoting compounds in the fruits and GLV by new methods of genetics and plant breeding. They proved that use of good quality soil for growing fruits and GLVs can abundantly improve the concentration of health-promoting compounds (Lester and Eischen 1996; Lester and Crosby 2002).

Vitamin C

The ascorbic acid content in some of the unprocessed ALVs varies from 2 to 311 mg (per 100 g edible portion) in Solanum nigrum and M. esculenta, respectively. It is evident from the literature that this vitamin is immensely affected by processing. Ascorbic acid reduced by 19, 61, and almost 100% in cooked amaranth, dried Vernonia amygdalina, and dried Adansonia digitata respectively (FAO 1990). In order to minimize the degradation of ascorbic acid and browning, it is advisable to store the leafy vegetables at low temperature (Negi and Roy 2001).

The ascorbic acid intake from green leafy vegetables in sub-Saharan Africa is often determined by various seasonal factors, time and temperature of storage, as well as postharvest storage, chlorination of water, and cooking practices (FAO 2001). When vegetables are blanched, the loss of ascorbic acid is relatively higher than those under frozen storage. These losses are primarily due to the leaching effect occurring during thermal processing rather than the chemical degradation (Sreeramulu et al. 1983; Wallace et al. 1998; Howard et al. 1999; Mepba et al. 2007; Rawson et al. 2012; Rawson et al. 2013) and reported a significant reduction in ascorbic acid content of several African green leafy vegetables after thermal treatment. The most effective preservation methods reported in retaining the ascorbic acid content is steam blanching followed by dehydration (Schippers 2000). In comparison with sun drying, shade drying, and vacuum drying, freeze-drying is found to be the most suitable method for retaining ascorbic acid content of African leafy vegetables (Shitanda and Wanjala 2006).

Minerals

Green leafy vegetables are good sources of calcium, magnesium, iron, zinc, sodium, and potassium. The absorption of these minerals is influenced by the presence of various inhibitors such as phytate and oxalates (Kumari et al. 2004). During processing, minerals have higher stability as compared to vitamins and proteins (Kala and Prakash 2004). But mineral contents in green leafy vegetables are affected by several factors including climatic conditions, water availability, soil type and pH, plant variety and age, and especially the application of fertilizers.

For instance, mineral profile of spinach has higher calcium content (1036 mg/100 g), followed by magnesium (827 mg/100 g), sodium (827 mg/100 g), and iron (28.4 mg/100 g), while duck weed is rich in zinc (15 mg/100 g). However, soy seed also has notable calcium content (195 mg/100 g), magnesium (407 mg/100 g), iron (6 mg/100 g), phosphorus (469 mg/100 g), sodium (12.3 mg/100 g), potassium (2387 mg/100 g), and zinc (3.7 mg/100 g) when compared to all the seeds but relatively higher in the green leafy vegetables (Natesh et al. 2017).

Iron

Iron deficiency in women and children results in the development of anemia (Galloway 2003). The iron content of African leafy vegetables ranges between 0.2 and 12.8 mg/100 g edible portion for Solanum nigrum and Lesianthera africana, respectively. Intake of 300 g of fresh L. africana and S. nigrum would give 7 and 431% of RNI for children and 2 and 131% of RNI for adult women, respectively (Uusiku et al. 2010).

Zinc

Zinc deficiency leads to impaired gastrointestinal and immune functions. It varies within the same species and ranges from 0.03 to 3.1 mg/100 g edible portion for Ipomoea batatas, 1.4 to 18.5 mg/100 g for C. album, and 0.02 to 8.4 mg/100 g for Amaranthus species. Zinc bioavailability is adversely influenced by phytate concentration (Turnlund et al. 1984).

Calcium and Magnesium

The calcium content in African leafy vegetables is 668 mg/100 g in U. urens and 15 mg/100 g edible portion in some Chenopodium species, while the magnesium content is 225 mg/100 g in J. flava and 13 mg/100 g in Brassica species. The bioavailability of these minerals is affected by the presence of anti-nutritional factors, age and sex of an individual, and also the fat content in the diet (Uusiku et al. 2010) (Table 1).

Table 1 Primary metabolite composition of selected green leafy vegetables (Fresh weight)
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Secondary Metabolites

Biologically active substances like secondary metabolites are majorly found in fruits and vegetables (Tables 2 and 3). These metabolites do not directly participate in the growth and development of the human body but are important for all the biochemical processes and producing positive effects in human life. British Nutrition Foundation divided these secondary metabolites into four categories – alkaloids (12,000 compounds), terpenoids (25,000 compounds), phenolic compounds (8000 compounds), and sulfur-containing compounds (Goldberg 2008). Plant samples tested for secondary metabolites were also found to be used for body building, cell growth, and repair (Kubmarawa et al. 2008). Classification of secondary metabolites is shown in Fig. 2.

Fig. 2
figure 2

Classification of secondary metabolites, Randhawa et al. (2015); Goldberg (2008); Croteau et al. (2000); Naczk and Shahidi (2004)

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Table 2 Secondary metabolite composition of selected green leafy vegetables (fresh weight)
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Table 3 Anti-nutritional content of selected green leafy vegetables (fresh weight)
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Phenolic Compounds

Polyphenols are considered as main antioxidant having in vitro effect on the human health including vitamins and carotenoids (Gardner et al. 2000). Correlation between the polyphenol content and antioxidant activity was found for the African GLVs (Mai et al. 2007), whereas no such correlation was found in Indian (Dasgupta and De 2007) and Malasian GLVs (Ismail et al. 2004). This correlation depends on the methodology and type of the vegetable, which is due to contribution of antioxidant component with different antioxidant activity (Ismail et al. 2004). Similar work done by (Modi 2007) on boiled Amaranthus proves the correlation between the growth temperature and plant age with respect to antioxidant activity. He found that Amaranthus harvested 60 days after sowing had higher antioxidant capacity than that harvested after 20 or 40 days of sowing. Correlation was also found between the food processing or cooking method on the phenolic content and thus affecting the antioxidant activity. Increase and decrease in flavonoids were observed with the cooking method. While increase in flavonoids may be due to inhibition of oxidative enzymes and increased release of flavonoids from the breakdown of cell wall, which is binding site flavonoids and decrease was observed due to heat lability or leaching of specific flavonoids (Yamaguchi et al. 2001). Neugart et al. (2015) in his study found indigenous African leafy vegetables as high source of flavonoid glycosides and hydroxycinnamic acid derivatives, which is evident from the fragmentation patterns, retention time, maximum absorption wavelength, and molecular masses. He also observed high concentration of phenolic compounds in Ethiopian kale (17,206 and12,228 μg/g DW), African nightshade (16,677 and 16,387 μg/g DW), and cowpea (16,185 and 17,408 μg/g DW), whereas medium concentration was observed in common kale (6690 and 12,143 μg/g DW) and amaranth (14,221 and 9229 μg/g DW) and low concentrations in spider plant (5297 and 6295 μg/g DW). Vietnamese plants used for making drinks showed highest antioxidant activity and amount of polyphenol compared to that of edible wild vegetables, herbs, and dark green vegetables (Mai et al. 2007).

Leaves of sub-Saharan Africa plants of P. oleracea, Momordica balsamina, and J. flava had high antioxidant activity and 96, 94, and 96% scavenging capacity, respectively (Odhav et al. 2007). Amaranth, a famous and highly nutritious leaf, also showed good content of polyphenolics including anthocyanin in amaranth grains also called as Rama’s grain (Rajgira) (Paśko et al. 2009). These high contents of bioactive compounds are responsible for the antioxidant potential of flowers and leaves of Amaranthus spp. and their extracts. This might be due to the presence of free radical scavenger rutin (Kraujalis et al. 2013). A comparison study proved that A. tricolor (red stem) had higher phenolic content and antioxidant capacity than A. viridis (green stem) due to the presence of anthocyanin, a red pigment (Routrey et al. 2013). Higher and lower scavenging activities were also found in Xanthosoma mafafa, Celosia argentea, Manihot utilissima, Ocimum gratissimum sp., and Stractium sp. as 99, 90, 90, 11, and 22%, respectively (Akindahunsi and Salawu 2005).

Tannins, a group of phenolic compounds, are anti-nutritional elements which react with digestive enzymes, proteins, and starches, reducing their nutritional value (Chung et al. 1998; Serrano et al. 2009). Tannins are also observed to hinder the absorption of protein and availability of iron (Bravo 1998). These were found to be high in African leafy vegetables, and their concentration was found to range between 655 mg/100 g in Xanthosoma sp. and 1222 mg/100 g in Euphorbia hirta (Wallace et al. 1998).

Alkaloids

Lower acceptability of leafy vegetables is due to the bitterness which is due to the presence of alkaloids (Wallace et al. 1998). Effect on transit time in the small intestine of human proves its microbial property, being used for medicinal purpose (Cowan 1999). Two major groups of alkaloids often studied are quinolizidine and pyrrolizidine, where the former is mostly found in genus Lupinus and the latter is mostly found in the family of Asteraceae and in the Boraginaceae (Croteau et al. 2000). Positive test for the presence of alkaloids was found in Sida acuta, Asystasia mysorensis, Amaranthus sp., Portulaca quadrifida, and Crotalaria ochroleuca spp. (Orech et al. 2005).

Carotenoids

Plants impart their yellow, red, or orange color from the carotenoids a subclass of terpenoids. Absorption of this compound requires little of fat in their cooking method after being pureed and chopped. This class of pigment is widely accepted to prevent heart disease, cancer, and eye disease. Vitamin A is mainly produced by these carotenoids. Some other examples of these pigments are zeaxanthin, lycopene, lutein, and β-carotene, where lutein and zeaxanthin are obtained from dark green and leafy vegetables that are important for preventing CVD, oxidative damage to eyes, and age-related macular degeneration. Lycopene is obtained from the processed and cooked tomato for producing red color and preventing cancer and heart diseases. Spider plants contained high amount of β-carotene a provitamin of vitamin A (up to 64.7 μg/g DW). High concentration of carotenoids was also observed in amaranth (up to101.7 μg/g DW), whereas A. cruentus was found to be potentially a good source of carotenoid, a provitamin of vitamin A. It was found that the highest amount of β-carotene is present in the leaves of this species followed by seeds, stem, and roots. Highest content of antitumor agent, canthaxanthin, was found followed by a retardant for age-related eye problems – b-carotene and lutein. This species has 28.5 mg/100 g of β-carotene which is sevenfold higher than tomato and thus can be used to treat anemia reported in the African countries (Dlamini et al. 2010).

Flavonoids

Flavonoids are produced by plants and belong to the group of phenolic compounds or phytochemical compounds. This compound is proved to reduce the risk of CVD (Ali et al. 2000). There is inverse relation between the incident of CVD and consumption of fruits and vegetables (Ness and Powles 1997). Consuming F&V also reduces the blood pressure in the human body (Alonso et al. 2004). He et al. (2006) observed that consumption of 600 g/day of fruits and vegetables can decrease the threat of CVD by 31% and the risk of stroke by 19%.

Anti-nutritional Components

Oxalic Acid

The presence of oxalic acid in various GLVs like Vernonia sp., E. hirta, Xanthosoma sp., Celosia argentea, Ipomoea involucrata, M. esculenta, Amaranthus sp., and Telfairia occidentalis was reported (Aletor and Adeogun 1995; Isong and Idiong 1997; Wallace et al. 1998; Ejoh et al. 2007). This is present in various species in the soluble or insoluble form, while the former is present as potassium or sodium salt, being excreted from the body, and the latter is present as salts of magnesium, calcium, or iron or combination of any two (Noonan and Savage 1999). Soluble oxalate forms a strong chelating bond with calcium, making it unavailable for absorption and assimilation (Gupta et al. 2005; Radek and Savage 2008), but its high intake can lead to kidney stone (Radek and Savage 2008). Dietary supplement of divalent minerals or addition of source of calcium can lead to unavailability of intestinal oxalate from such foods (Ponka et al. 2006; Radek and Savage 2008).

Phytic Acid

Phytic acid also known as myoinositol 1,2,3,4,5,6 hexakis-dihydrogen phosphate is an antioxidant and source of phosphorous in GLVs. It inhibits the absorption of multivalent metal ions such as calcium, zinc, and iron, inhibiting its free radicals (Schlemmer et al. 2009) and producing adverse effect in digestion of starch and protein (Reddy and Pierson 1994). Phytic acid also has beneficial effect by enhancing immunity by increasing natural killer cell’s function and activity, inhibiting iron-mediated oxidative reactions, serving as antioxidant, and stimulating bacterial killing by neutrophils (Bohn et al. 2008). This also lowers the phosphate donor/acceptor capabilities and reducing inositol’s involvement in the signaling mechanism (Bohn et al. 2008; Schlemmer et al. 2009). Phytic acid ranges between 9 mg/100 g and 655 mg/100 g in Launaea sp. and E. hirta, respectively (Wallace et al. 1998).

Glucosinolates

Glucosinolate precursors of isothiocyanate are used for its therapeutic and prophylactic properties (Fahey et al. 2001).Majorly glucosinolate are found in Capparaceae, Brassicaceae, and Caricaceae family and in L. Africana, which belong to Icacineae family (Isong and Idiong 1997). Since it is thermal and stable, they can only be enzymatically hydrolyzed or more specifically driven by myrosinase (Verkerk et al. 2009). After cell rapture, contact of myrosinase and glucosinolates lead to hydrolysis of the thioglucosidic bond and formation of various ranges of bioactive compounds (Baghurst et al. 1999; Verkerk et al. 2009) having chemopreventive and carcinogenic properties, depending on dosage (Fahey et al. 2001; Verkerk et al. 2009).

Saponin

Saponins are poorly absorbed, but it has cholesterol-lowering (Van Duyn and Pivonka 2000), antiparasitic, antifungal/antiyeast, antiviral, antibacterial/antimicrobial, hemolytic, anti-inflammatory, molluscicidal, antitumor, cytotoxicity, and other biological activities (Sparg et al. 2004). The effect of saponin mainly depends on the hydrophobic/hydrophilic asymmetry which in turn affects their capacity to reduce interfacial tension (Champ 2002). Higher saponin concentration of 481 mg/100 g, 424 mg/100 g, and 384 mg/100 g was found in Xanthosoma sp., Launaea sp., and Ipomoea involucrata, respectively, while low concentration of 6.7 mg/100 g and 0.1–0.3 mg/100 g (Ejoh et al. 2007) was found in Euphorbia hirta and Vernonia sp., respectively (Wallace et al. 1998).

Protease Inhibitor

Trypsin and chymotrypsin inhibitors bind the proteolytic enzymes and inhibit the activity of these enzymes, thus reducing the availability and absorption of proteins and amino acids present (Mosha and Gaga 1999; Glew et al. 2005). Temperature, amount of water, and rate of heating period influence the activity of these inhibitors (Mosha and Gaga 1999). Various works were done to reduce the activity of these inhibitors. Mosha and Gaga (1999) in his study proved conventional blanching to be better than microwave blanching to inactivate these inhibitors. Heat stability after 5 min boiling was found for these trypsin and chymotrypsin inhibitors, which result in poor protein utilization in human (Vanderjagt et al. 2000) (Tables 2 and 3).

Therapeutic Values and Health Benefits of GLVs

Antidiabetic Properties

Diabetes mellitus (DM) is most prevalent and noncommunicable disease found in every country in the world. The number of affected person is expected to reach 300 million people by 2025 (King et al. 1998). The two types of DM are type I and type II. Type I DM is also known as insulin-dependent diabetes or juvenile-onset disease, and type II is known as non-insulin-dependent diabetes or adult-onset diabetes. The former category is identified as autoimmune-mediated slow or faster destruction of beta-cells produced by the pancreas (Zimmet et al. 1994), while the latter category is due to deficiency of insulin in individual, and they are also known to be resistant to actions of insulin (Defronzo et al. 2015; Lillioja et al. 1993). Type II DM is most familiar, and it is expected to increase the effect at rate of 7.7% by 2030 (Shaw et al. 2010).

This disease has two conditions like hyperglycemia and hypoglycemia. Many oral medicines like alpha-amylase and beta-amylase are given to patients to prevent digestion of complex carbohydrate (Nishikawa et al. 2000). The body frequently produces free oxygen radicals, and reactive oxygen species leads to oxidative damage (Giugliano et al. 1996) in all biochemical process, which in turn leads to increase in lipid peroxidation and adverse effects, specifically in type II DM patients. Prevention of such complications and their effects is the most important target for type II DM patient (Stanely Mainzen Prince and Menon 2001). Almost all GLVs have minerals, vitamins, antioxidants, phytochemicals, and carbohydrates which prevent CVDs and diabetes. GLVs also have low energy intake, low glycemic loads, and high fiber content along with high concentration of proteins, phytochemicals, antioxidant vitamins, magnesium, and potassium, which play vital role in reducing the risk of type II DM. Also, dark yellow and green leafy vegetables prevent this risk in the overweight women (Liu et al. 2004). Polyphenolic antioxidants including ascorbic acids, b-carotene, phenolics, and alpha-tocopherol of GLVs (Kwon et al. 2006) have high therapeutic properties and good amount of antioxidant, anti-diabetic, and antihypertensive activity (Chu et al. 2002; Oboh and Rocha 2007; Oboh et al. 2008). Wild basil and jute contains higher quantity of phenolic content than chaya; fluted pumpkin, bitter leaf, and waterleaf contain the lowest. Ferric-reducing property of GLV leads to accumulation of ferric in acinar cells and islets of Langerhans of the pancreas; this in turn destructs the beta cells and prevents type II DM (Pulido et al. 2000). Reddish leaves, lettuce leaves, and omum leaves are the cheaper and richer source of many micronutrients and phytochemicals having good antioxidant capacity (Tarwadi and Agte 2003). This property makes GLV a cheaper alternative for diabetes mellitus disease and hypertension than synthetic drugs (Saliu and Oboh 2013).

In vivo clinical studies were carried out to prove their antidiabetic effect. Streptozotocin (STZ)-induced diabetic rats showed a significant reduction in the blood glucose level and cholesterol level with methanolic extracts of A. viridis, A. spinosus, and A. caudatus leaves, proving it to be good antidiabetic and anti-cholesterolemic agent (Girija et al. 2011). Similarly, 50% ethanolic extract of A. spinosus showed similar reduction in blood glucose level on the STZ-induced albino mice, while, in STZ-induced diabetic rats, significant reduction in degeneration of pancreatic cells was observed. This extract also increased the activity of nonenzymatic and enzymatic oxidant (Mishra et al. 2012).

Antimicrobial and Anti-inflammatory Activity

Human consumption of fresh and processed GLVs are generally recognized as safe (GRAS). The bioactive components of GLVs possess many biological activities such as antimicrobial and anti-inflammatory activity (Faller and Fialho 2009). These are famous among the population due to their esthetic features like flavor, color, and therapeutic values (Gutierrez et al. 2008). Due to their GRAS status, food industry makes use of these GLVs as antimicrobial especially antibacterial agent. These vegetables also exhibit many pharmacological effects like enhancing fertility in females, reducing blood pressure, and using antibiotics in the human body (Mensah et al. 2008). Important GLVs such as Lactuca sativa, Coriandrum sativum, Mentha piperita, Raphanus sativus, and Portulaca oleracea in methanolic extract have high antibacterial activity (Bhat and Al-Daihan 2014). Freeze-dried cilantro (C. sativum) and parsley (Petroselinum crispum) were used on Escherichia Coli and Bacillus subtilis with effect cell damaged and growth inhibition. These GLVs having high antibacterial activity and being the future of food system need to be studied for their potential role against specific microbes (García-Lafuente et al. 2009).

Phenolics and flavonoids of GLVs are studied from many decades since they function like plant hormone regulators, which is natural protectant against many microbes in the plant cell. Phenolic compounds of GLVs like spinach and the mustard plant are also studied for their anti-inflammatory potential (García-Lafuente et al. 2009). Activation of NF-6B is very important for many inflammatory processes which can be inhibited by the use of extract of Urtica dioica (Guil-Guerrero et al. 2003). Saponins present in Euphorbia hirta and Ipomoea involucrata have antifungal/antiyeast, anti-inflammatory, antibacterial, cytotoxicity, antiparasitic, antiviral, antitumor, and many other biological functions (Sparg et al. 2004). Hydroalcoholic leaf extract of A. tricolor showed an antinociceptive and anti-inflammatory activity in the clinical studies on the induced rats (Bihani et al. 2013). Seed and leaf extract was obtained from A. viridis, and it showed significant inhibition of targeted bacterial and fungal strains, thus proving to be an effective antimicrobial agent (Ahmed et al. 2013).

Antioxidant Property

Cells pretreated with A. lividus and A. tricolor extracts showed decrease in toxicity and suppression in the production of oxidative genes like RAGE, HMOX-1, and RelA/NF-jB in SH-SY5Y cells. This property is helpful in preventing neurodegenerative disorders and age-related diseases (Amornrit and Santiyanont 2016). Partially purified alkaloids (PPA) from A. viridis were used on human erythrocytes and were observed to reduce the lipid peroxidase activity while maintaining the antioxidant concentration from decline. This proves that alkaloid can be effectively used to prevent age-related problems and other free radical-mediated oxidative damages in the body (Sasikumar et al. 2015).

Cardiovascular Disease and Leafy Vegetables

In various countries of America and South Africa as well as non-African countries, the extent of obesity among children and adults are increasing in the tremendous rate. This might be due to less consumption of fruits and vegetables, implementing western lifestyle and lack of physical exercise (Mauriello et al. 2006). Thus, addition of GLVs in their diet along with physical activity and change of lifestyle has proved to reduce the risk of CVD. Higher intake of fiber from GLVs and weight of human have shown an inverse relation (Tohill et al. 2004). Similar relation is found between the evidence of coronary heart disease (CHD) as well as stroke and intake of GLVs (He et al. 2006). It was proved by meta-analysis that incidents of CVD can be reduced by 4% with the addition of GLV (Dauchet et al. 2006). Soares et al. (2015) in his study used three peptides from A. cruentus showing a significant reduction of cholesterol enzyme like HMG-CoA reductase, proving to be an effective anti-cholesterolemic agent.

Hypertension and Leafy Vegetables

Israili et al. (2007) reported that the major reasons for visit of patients in hospital are hypertension and their rate of increase in many countries are exponential. Direct relationship between the conditions such as stroke, CHD, cerebrovascular accident (CVA), end-stage renal disease, myocardial infarction, and congestive heart failure and the rate of morbidity and mortality is observed. Patients with diabetes and kidney disease have added problems with low blood pressure; thus its control is more harmful. The main reason for this disorder is improper diet, and thus it can be treated with diet high in magnesium and potassium, while the intake of sodium should be reduced. Direct relation is observed between the occurrence of CVD, stroke, and BP with sodium, whereas inverse relation is observed when diet with potassium is consumed.

Risk of hypertension is one in every three persons in the highly industrialized countries where people consume diet high in refined carbohydrates, trans fats, saturated fats, processed foods, low fiber, reduced dietary magnesium, potassium, and large amounts of dietary sodium (Adrogué and Madias 2007). Epidemiological studies prove that people consuming vegetarian diet/primeval diet have low risk of CVD and blood pressure in the industrial area (Fujiwara et al. 2000).

Homeostatic balance maintained by low level of sodium and high level of potassium leads to vasodilatation of the blood vessels for sufficient flow rate of blood to heart. When such conditions are not maintained, it leads to vasoconstriction and low supply of blood to heart. Calcium also plays an important role in maintaining such balance for the healthy life. Increasing vasodilation by altering potassium is possible with many mechanisms such as reduced vasoconstrictive sensitivity to norepinephrine, intracellular sodium and tonicity, and natriuresis, angiotensin II, increased sodium/potassium ATPase activity, and urinary kallikrein, increased serum, proliferation in vascular smooth muscle, improved insulin sensitivity, alteration in DNA synthesis, sympathetic nervous system cells, reduction in cardiac diastolic dysfunction, decrease in vascular intracellular sodium and tonicity, reduction in transforming growth factor (TGF)-beta, decrease in NADPH oxidase, decrease in neointimal formation, oxidative stress and inflammation (Randhawa et al. 2015).

Fertility and LV

Dark colored GLVs are rich in chlorophyll, antioxidants, vitamins, fibers, phytochemicals, and minerals. These vegetables can be eaten raw, semi-cooked, or cooked. But it is recommended to eat raw because it prevents the loss of vitamins and certain important minerals. Minerals which are important for fertility in women are folate, vitamin C, and zinc, which depletes on cooking. The presence of diindolylmethane in certain GLV like Brussels sprouts, broccoli, and cabbage not only retains nutritional factors in the body but also helps the body to get rid of “bad” estrogen, which is very important for fertility. Thus, it is recommended to consume these vegetables at least twice in a week. Folic acid is very important for the development of fetus even before the women gets the positive pregnancy test. Spinach and asparagus are rich source of iron and are proved to reduce the condition of anovulation and possibly poor egg health during pregnancy. Inhibition of pregnancy-related issues is 60% reduced with consumption of iron-rich food than those with insufficient iron in their blood. Vitamin C is very important for absorption of iron, and being an antioxidant, it is also important to protect cells from the damage caused by the free radicals. GLVs also provide humans with sufficient amount of vitamins, minerals, and other nutrients while maintaining acidic/alkaline balance. Spinach being rich in vitamins, nutrients, iron, and folic acid is a very good source for good health of fetus, fertility, and reproduction. Where iron promotes oxygen levels in fetus, organs, and cells, folic acid is important for preventing neural tube defects in the growing fetus. Healthy alkaline environment is important for the survival of sperm and its journey to egg. Minerals serve as a medium for activating whipping of sperm tail in the vaginal fluid, which is important for mating and creating healthy new born.

Anticancerous Properties

Antiproliferative and chemopreventive effects of polyphenols are available with in vitro studies. A. cruentus is suggested to be cheap, commercial, and biocompatible alternative on peripheral lymphocytes than already available antiproliferative therapeutics (Gandhi and Niraj 2011). Ethyl acetate, hexane, and methanolic extracts of A. tristis Roxb. were also found to be effective against colon adenocarcinoma cell line with minimum side effects (Baskar et al. 2012). A. hybridus and A. caudatus reduce micronuclei formation and safeguard the detoxifying enzymes like alkaline phosphatase (ALP) and GGT in sodium arsenite-treated albino Wistar rats, thus proving it to be an affective carcinogen (Adewale and Olorunju 2013). Stem extract of A. lividus and seed extract of A. hybridus inhibited the growth of EAC cells by 43 and 45%, respectively. This also showed downregulation of Bcl-2 mRNA in treated mice and upregulation of Bax, p53, and caspase-3, which clearly proves the mitochondrial apoptosis of EAC cells when compared with control sample (Al-Mamun et al. 2016). A. viridis Linn. leaves extract was prepared by 50% ethanolic solution which had better antiproliferative effect against CEM, Jurkat, and HL-603 (human leukemic cell lines) when compared with the stem. It was observed in the study that standard curcumin showed antiproliferative effect on both normal cells and leukemic cells, while both stem and leaves extracts had positive effect by enhancing the growth of normal cells (Larbie et al. 2015).

Hepatoprotective

Hepatotoxicity can be caused due to excess of alcohol consumption, autoimmune response, toxic effect of curative drugs, or any other infections. This can be prevented by giving a hepatoprotective medicine, allergy medicine, or natural extracts. The hepatoprotective effect was shown by the extract of A. tricolor roots against the biochemical, physical, functional, and histological changes produced by paracetamol in Wistar albino rats. This effect is evident by reduced serum enzyme activities such as SGOT, SGPT, TB, and ALP, and this was compared with standard drug silymarin which is hepatotoxic drug against paracetamol. This was also supported by histopathological studies of the liver (Aneja et al. 2013)

Gastroprotective

Gastroprotectors of plant origin have positive effect on the mucosa of gastrointestinal tract and are used clinically. Gastric ulcers were healed by using poly-herbal formulation having A. tricolor as one of the ingredient (Devaraj and Krishna 2013). Similar work with hydroalcoholic extract of piperine and A. roxburghianus roots showed hemorrhage, minimal ulceration, necrosis, and decreased levels of myeloperoxidase, leucocyte infiltration in histopathological observation, and MDA and increased glutathione levels in colon and blood tissue in rats having ulcerative colitis (Nirmal et al. 2013). Cysteamine-induced duodenal ulcers and ethanol-induced gastric ulcers can be significantly treated by A. spinosus leaves powder (Mitra et al. 2013).

Antimalarial

The development of resistance by the exposed species, nontarget specificity of chemical insecticides, and eco-hazardous nature of chemical pesticides provoke us to find an eco-friendly alternative in terms of usage of herbs, their extract, or powders. Utilizing ethanolic extract of A. spinosus leads to increased hemoglobin content, gain of BW, and blood schizonticidal activity in Plasmodium berghei-infected mice. This antimalarial effect was significantly comparable to synthetic drug, chloroquine (Susantiningsih et al. 2012). Another study was carried out on A. hybridus L. for treating malaria. This species is used from ancient times by Msambweni community of Kenyan South Coast (Nguta et al. 2010).

Other Effects

Extracts from various species of Amaranthus like A. spinosus were found to have spasmolytic, laxative, and bronchodilator properties (Koffuor et al. 2017), where ethanolic extract of up to 240 mg/kg had no adverse effect proving to be a better alternative against ischuria. While the higher dosage was found to show certain physiological changes when compared with that of escitalopram and imipramine effects (Kumar et al. 2014).

Different Methods of Processing Green Leafy Vegetables

Green leafy vegetables generally grow amply during the monsoon season; however, they are greatly perishable which affects its shelf life, and sometimes raw leafy vegetables are unpalatable (Chinyere and Obasi 2011).Various processing treatments such as sun drying, shade drying, freeze-drying, blanching, and vacuum cooling are employed to improve the palatability and bioavailability and extend the shelf life of the green leaves. The quality of green leafy vegetables can be improved by suitable processing treatment which prevents the loss of nutrient content and processing should be done after immediate harvest. The safety of green leafy vegetables can be achieved by proper handling during harvesting and appropriate refrigeration condition. For example, cooling the crops at 4 °C prevents the growth of microbes such as E. coli O157:H7, L. monocytogenes, and Salmonella (Solomon et al. 2009). In freeze-drying, dehydration takes place by the sublimation of water from frozen product. It improves the quality of vegetables and increases the shelf life of the product. Because at low temperature and in the absence of liquid water, microbial reaction and deterioration are arrested this improves the final quality of the product. It also prevents the loss of flavor and bioactive compound and protects the primary structure and shape of the product (Sadikoglu and Liapis 1997; Tambunan et al. 2001; Rawson et al. 2011).

Oboh and Akindahunsi (2004) examined the effect of sun drying on ascorbic acid, total phenol, and antioxidant activity of common green leaves in Nigeria such as Telfairia occidentalis (Ugu), Amaranthus cruentus (Atetedaye), Corchorus olitorius (Ewedu), Solanum macrocarpon (gbagba), Structium sparejanophora (Ewuro-odo), Ocimum gratissimum (Efinrin), Baselia allia (Amunu tutu), and Vernonia amygdalina (Ewuro). The results revealed that sun drying of green leafy vegetables reduces the ascorbic acid content (16.67–64.68% loss), but contrarily it increases the total phenol content (6.45–223.08% gain). It also increases the antioxidant activity of green leafy vegetables (126.00–5757.00% gain). Even though the ascorbic acid content decreases, the antioxidant activity of the green leafy vegetables won’t be reduced by the sun drying, because the phenol content of leaves contributes more to the antioxidant properties than ascorbic acid of green leafy vegetables. Suffo et al. 2016 reported that the shade drying and sun drying improve the palatability of green leafy vegetables by reducing their anti-nutritional content. And it also extends their shelf life. They also examined the impact of processing method on chemical composition and antioxidant activity of two Amaranthus sp. (A. hybridus and A. cruentus). The results of shade drying showed significant increase in phenol content and antioxidant activity and also caused significant decrease in ascorbic acid of green leafy vegetables.

Blanching of vegetables at high temperature may inactivate some microorganism and destroy the enzymes in the tissue. It also inhibits the enzyme action, changes the color, eliminates the anti-nutritional factor and acid component, and reduces the dehydration and drying time (Akindahunsi and Oboh 1999). Green leafy vegetables are good source of minerals (iron, magnesium, zinc, and calcium) and vitamins (A, B complex, and C). The processing treatment like blanching has an adverse effect on minerals and vitamin availability. The iron availability of green leafy vegetables is reduced by processing due to the loss of ascorbic acid content (iron enhancer) because vitamin C is light and temperature sensitive. Latunde-Dada 1990 also reported that blanching and squeeze washing of vegetables result in reduction of dialyzable iron. Effects of processing on phytochemicals of green leafy vegetables are mentioned in Table 4 (Kuriakose and Rawson 2015).

Table 4 Effect of processing on phytochemicals (Adopted from Kuriakose and Rawson 2015)
Full size table

Conclusions

Green leafy vegetables are significant source of primary and secondary metabolites. And their consumption in human diet could increase the chances of various health benefits and may play a role in preventing human disease in which free radicals are involved, such as cancer, cardiovascular diseases, and aging. Processing also affects the composition of primary and secondary metabolites in leafy vegetables, and a low-temperature treatment may be able to retain the metabolites in leafy vegetables.