Okra (Abelmoschus Esculentus) as a Potential Dietary Medicine with Nutraceutical Importance for Sustainable Health Applications

Abstract

Recently, there has been a paradigm shift from conventional therapies to relatively safer phytotherapies. This divergence is crucial for the management of various chronic diseases. Okra (Abelmoschus esculentus L.) is a popular vegetable crop with good nutritional significance, along with certain therapeutic values, which makes it a potential candidate in the use of a variety of nutraceuticals. Different parts of the okra fruit (mucilage, seed, and pods) contain certain important bioactive components, which confer its medicinal properties. The phytochemicals of okra have been studied for their potential therapeutic activities on various chronic diseases, such as type-2 diabetes, cardiovascular, and digestive diseases, as well as the antifatigue effect, liver detoxification, antibacterial, and chemo-preventive activities. Moreover, okra mucilage has been widely used in medicinal applications such as a plasma replacement or blood volume expanders. Overall, okra is considered to be an easily available, low-cost vegetable crop with various nutritional values and potential health benefits. Despite several reports about its therapeutic benefits and potential nutraceutical significance, there is a dearth of research on the pharmacokinetics and bioavailability of okra, which has hampered its widespread use in the nutraceutical industry. This review summarizes the available literature on the bioactive composition of okra and its potential nutraceutical significance. It will also provide a platform for further research on the pharmacokinetics and bioavailability of okra for its possible commercial production as a therapeutic agent against various chronic diseases.

1. Introduction

Okra (Abelmoschus esculentus L.), belonging to the family Malvaceae, is commonly known as Lady’s finger, as well as by several vernacular names, including okra, bhindi, okura, quimgombo, bamia, gombo, and lai long ma, in the different geographical regions of its cultivation [1]. Okra is believed to have originated near Ethiopia, where it was frequently cultivated by the Egyptians during the 12th century, and thereafter spread throughout the Middle East and North Africa [2,3]. Okra is an annual shrub that is cultivated mostly within tropical and subtropical regions across the globe and represents a popular garden crop, as well as a farm crop. It is a widely cultivated food crop and is globally known for its palatability. The immature green pods of okra are usually consumed as vegetables, while the extract of the pods also serves as a thickening agent in numerous recipes for soups, as well as sauces, to augment their viscosity [4,5]. Another noteworthy application of okra fruit is their wide use in the pickle industry. The polysaccharides present in okra are used in sweetened frozen foods such as icecreams, as well as bakery products, due to their health benefits and longer shelf-lives [6,7,8]. Anatomically, the fruits, stem, and leaves of okra are covered with minute soft, hairy structures. Although the flowering of the okra plant is perennial, it is highly dependent on various biotic and abiotic factors. The leaves of okra are polymorphous, characterized by hairy upper and lower surfaces, whereas the petioles are around 15 cmlong. The flowers of okra can be easily recognized due to their slight yellowish color with a crimson center. The edible part of okra or its capsule (pod) measures approximately 15–20 cm in length and has a pyramidal-oblong, pentagonal, hispid appearance. Historically, okra pods were utilized for various purposes, such as in food, appetite boosters, astringents, and as an aphrodisiac. Furthermore, okra pods have also been recommended to cure dysentery, gonorrhea, and urinary complications [9]. Extracts of young okra pods have also been reported to display moisturizing and diuretic properties, whereas the seeds of this plant have been reported to possess anticancer and fungicidal properties [10].

Recently, okra has been used not only for its nutritional values but, also, for its nutraceutical and therapeutic properties, owing to the presence of various important bioactive compounds and their associated bioactivities. This review presents a summary of the nutritional significance of okra, as well as the possible pharmacological applications of okra bioactive components, and to explore the possible characteristics forthe development and formulation of nutraceuticals andfunctional food. In addition, this review also focuses on the nutraceutical potential of Abelmoschus esculentus for various therapeutic purposes, as well as to demonstrate the benefit of okra-based nutraceuticals and their consumption.

2. Nutritional and Bioactive Constituents in Okra

Okra is probably a proficient dietary constituent rather than a staple food crop. Small industries (Surajbala Exports Private Limited, New Delhi, India and Hunan Qiyi** resistances against the currently available drugs, like vinca alkaloids and taxanes [56,57].

The term cancer represents a broad group of malignancies showing the key characteristic of uncontrolled proliferation, aided by various regulatory and functional changes, which, in turn, ensure a systemic spread throughout the body [58]. Although scientific cancer research communities have made remarkable progress in understanding the mechanisms responsible for such a debilitating disease, an effective treatment strategy without accompanying toxic effects has yet to be made available. The therapeutic management of such life-threatening modalities should focus on the increased exploration of a chemo-preventive approach through plant-based natural agents from different sources. To this end, plant products have attracted several researchers across the world because of their selective toxicity against malignant cells [59,60,61]. It is also reported that the flowers of okra contain substantial amounts of flavonoids and phenols, as compared to the pods, peel, leaves, and seeds [62,63]. A recent report elucidated that purified fractions of flavonoids from the flowers of okra plants had a significant antitumor effect on colorectal malignancy both in-vitro, as well as in vivo, exerting a strong antioxidant potency concomitantly with substantial antiproliferative effects on tumor growth. The antiproliferative effect of flavonoids within okra flowers induced the activation of p53, culminating into the ceasing of mitochondrial functions within colorectal tumor cells, ultimately resulting in apoptosis and restraining the autophagy [64].

Subsequently, the anticancerous effect of okra seed extracts have also been documentedin vitrothrough several other cell lines. The flavonoid constituents of seed extracts showed enhanced cytotoxic effects on human-derived breast cancer cells (MCF-7) in comparison with hepatoma cells of human origin (HepG2) and human cervical cancer (HeLa) cells in a dose-dependent manner. These observations affirmed that the flavonoid isoquercitrin, in a synergistic association with other flavonoids, inhibited vascular endothelial growth factor (VEGF), resulting in the apoptosis of cancerous cells [65]. Additionally, Hyperin—also known as quercetin-3-O-β-d-galactoside—is an important flavonoid constituent of okra. Hyperin has also been reported for its anticancerous potential in gastric cancer cells (CHI) by establishing an antiproliferative, antimigratory, and anti-invasive environment resulting in apoptosis by blocking the Wnt/β-catenin signal pathway [66].

Carbohydrate-binding proteins such as lectins have been widely investigated for their anticancer effects [67,68]. Lectins from okra have been documented to activate caspase-mediated downstream signaling in MCF7 cells and normal fibroblasts (CCD-1059 SK) [69]. Furthermore, the extract of okra pulp has also been attached to gold nanoparticles and observed to exhibit a significant induction of oxidative stress, followed by the depolarization of mitochondrial membrane potential and apoptosis in Jurkat cells, thereby indicating its anticancer potential [70]. However, studies focusing on the capability of okra and its parts to reduce cancer progression and its associated effects are rare.

3.4. Immunomodulatory Potential

The immune system acts as a well-defined protective shield against noxious external or internal intervening agents. The immune system plays a critical role in protecting the human body from infectious diseases. Its two main contributors include innate and acquired immunity responses. The most important feature of innate immunity is its lack of specific recognition. This type of immune system responds to all pathogens, regardless of their nature. Innate immunity is composed of immune and nonimmune components, whilst acquired immunity has only immune elements. Phytochemicals are the naturally occurring secondary metabolites present in abundance in fruits and vegetables. They do not have any nutritional importance, but they are essential for the growth and maintenance of plants, and with evolution, humans have learnt the ways to harvest and manipulate these phytochemicals for their own benefits [71,72].

The biologically potent constituents of A. esculentus have also been reported to modulate the complex immune system. The administration of lectin from okra in mice at low concentrations (0.01, 0.1, and 1 mg/kg) has demonstrated significant inflammatory effects [45]. Recently, the administration of the ethanolic extract of okra lowered inflammation in Wistar rats subjected to acute gastric mucosal injury [73]. The polysaccharide constituents of an aqueous okra extract were demonstrated to augment the hemoglobin content and expression of major histocompatibility complex (MHC) II and CD80/89 within the bone marrow hematopoietic cells derived from rats, as well as reduce endocytosis. Furthermore, the aqueous extracts also upregulate the expression of interleukin-12 and interferon-γ, along with the simultaneous reduction of the anti-inflammatory cytokine interleukin-10 [74]. Macrophages are key components of innate immunity and act as a prerequisite for the effective functioning of the innate immune systems. The immunomodulatory effects of polysaccharides from okra have also been evaluated on macrophage cell lines by Chen et al. (2016), who reported an increase in nitric oxide (NO) synthesis, inducible NO synthase (iNOS) expression, and the levels of tumor necrosis factor-α and cytokines in RAW264.7 cells following treatment [75].

3.5. Microbicidal Action

The lipid content—namely, palmitic and stearic acids of lyophilized extracts from okra pods—along with its aqueous counterpart, have earlier been reported to inhibit Rhodococcus opacus, Rhodococcus erythropolis, Mycobacterium aurum, Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Xanthobacter Py2 [76]. Moreover, the gold nanoparticles synthesized from the okra extract (pulp) also displayed substantial microbicidal efficacy against Bacillus cereus, Bacillus subtilis, E. coli, P. aeruginosa, and M. luteus [77]. Furthermore, fractionsof the pods from okra that are rich in carbohydrates were also documented for their activity against Helicobacter pylori [78]. Apart from the above-mentioned therapeutic effects of okra fruits, some therapeutic effects and their mechanisms of action are presented below in

Table 3. Different bioactive components derived from okra showing their therapeutic benefits on human health, along with their mechanisms of action.

Bioactive Components	Therapeutic Benefits	Mechanisms of Action	Reference
Polysaccharide	Antidiabetic	It helps to lower body weight and glucose levels, improve glucose tolerance, and decrease the total serum cholesterol levels in high-fat diet-fed C57BL/6 mice.	[79]
Rhamnogalacturonan	Antidiabetic	Hypoglycemic effect,	[80]
Lectins	Anticancer	Arrest the cell cycle and activate the caspase cascades.	[81]
		Inhibit cellular proliferation in human breast cancer in vitro.	[70]
Pectin	Anticancer	Involved in cell adhesion, growth, and survival, as well as tumor development and cancer prevention therapy.	[82,83,84]
	Antiproliferative and proapoptotic	Induce apoptosis and inhibit cellular proliferation.	[85]
Pectin	Lower bad cholesterol	Okra promotes cholesterol degradation and inhibits the production of fat in the body. It lowers bad cholesterol by altering the bile production in the intestines. This helps in eliminating the clots and deposited cholesterol.	[86]
Polyphenolic compounds	Antioxidant	Extract exhibits a strong DPPH radical scavenging activity and reducing power.	[87]
Quercetin 3-O-glucosyl (1→6) glucoside (QDG) and quercetin 3-O-glucoside (QG)	Antioxidant	Excellent reducing power and free radical scavenging capabilities, including DPPH, superoxide anions, and hydroxyl radicals.	[88]
Vitamin C, calcium, iron, manganese, and magnesium	Antioxidant	Eliminating free radicals.	[86]
Quercetin derivatives and epigallocatechin	Antioxidant	Inhibitory effects on the generation of reactive oxygen species (ROS).	[54]
Polysaccharide	Metabolic disorders	Inhibition of LXR and PPAR signaling.	[79]
Polyphenolic compounds,	Antioxidant	Perform the function of capturing free radicals and stop** the chain reactions.	[89]
Vitamin A; B vitamins (B1, B2, B6); and vitamin C and traces of zinc, calcium, folic acid, and fiber	Pregnancy benefits	Folates prevent miscarriages. They are also beneficial in the formation of the neural tube of the fetus and protect these tubes, preventing defects. This helps prevent birth defects like spina bifida and can even stop constipation during pregnancy.	[86]
Polyphenols like catechin and flavonoids like quercetin possess	Antifatigue effects	Decreased the levels of blood lactic acid (BLA) and BUN in the blood; MDA in the liver; and increased the levels of HG, SOD, and GSH in the liver during fatigue recovery, which proved that OSD could alleviate physical fatigue and promote recovery.	[51]
Probiotics	Gut bacteria-friendly	Biosynthesis of the vitamin B complex.	[86]
Glutathione	Detoxify liver, antioxidant	The slimy substance in okra contains substances that bind bile acid and cholesterol to detoxify the liver.	[86]
Mucilaginous	Ulcer treatment	The slimy stuff in okra is alkaline. This helps in neutralizing the acid. Additionally, it provides a protective coating within the digestive tract, which speeds up the healing process of peptic ulcers.	[86]
Mucilaginous with fiber	Relieves and prevents constipation	Bind toxins and lubricates the large intestines. This ensures effortless and normal bowel movement due to its natural laxative property.	[86]
Vitamin K and C	Bone health and essential for the blood-clotting process. It also helps restore bone density and prevent osteoporosis.	Several mechanisms are suggested by which vitamin K can modulate bone metabolism. Besides the gamma-carboxylation of osteocalcin, a protein believed to be involved in bone mineralization, there is increasing evidence that vitamin K also positively affects the calcium balance, a key mineral in bone metabolism.	[86,90]
Vitamin A, along with antioxidant contents such as lutein, xanthein, and carotenes	Improves vision	Okra contains beta-carotenes (precursor of vitamin A), xanthin, and lutein, all with antioxidant properties preventing eye problems like cataract and glaucoma.	[86]
Glycosylated compounds	Antibacterial activity	Inhibit the adhesion of Helicobacter pylori to the human gastric mucosa.	[78]
Rhamnogalacturonan Polysaccharides	Antiadhesive properties	Interrupt the adhesion of H. pylori to human stomach tissues via interfering with the outer membrane proteins.	[78,91,92,93]
Polyphenols and flavonoids (okra seeds)	Antifatigue	Reduce the levels of BLA and BUN, enhancing hepatic glycogen storage and the promoting antioxidant ability by lowering the MDA level and increasing the SOD and GSH-PX levels.	[51]

, alongwith the presentation of various bioactive components present in okra and their chemical structures (Figure 1).