Evaluation of biological properties and clinical effectiveness of Aloe vera: A systematic review

Abstract

Aloe vera (蘆薈 lú huì) is well known for its considerable medicinal properties. This plant is one of the richest natural sources of health for human beings coming. The chemistry of the plant has revealed the presence of more than 200 different biologically active substances. Many biological properties associated with Aloe species are contributed by inner gel of the leaves. Most research has been centralized on the biological activities of the various species of Aloe, which include antibacterial and antimicrobial activities of the nonvolatile constituents of the leaf gel. Aloe species are widely distributed in the African and the eastern European continents, and are spread almost throughout the world. The genus Aloe has more than 400 species but few, such as A. vera, Aloe ferox, and Aloe arborescens, are globally used for trade. A. vera has various medicinal properties such as antitumor, antiarthritic, antirheumatoid, anticancer, and antidiabetic properties. In addition, A. vera has also been promoted for constipation, gastrointestinal disorders, and for immune system deficiencies. However, not much convincing information is available on properties of the gel. The present review focuses on the detailed composition of Aloe gel, its various phytocomponents having various biological properties that help to improve health and prevent disease conditions.

Graphical abstract

1. Introduction

Aloe barbadensis Miller (蘆薈 lú huì), commonly referred to as Aloe vera, is one of more than 400 species of Aloe belonging to family Liliaceae that originated in South Africa, but have been indigenous to dry subtropical and tropical climates, including the southern USA.¹ Recently, only a few species of Aloe have been considered for commercial importance, of which A. vera is considered the most potent and, thereby, the most popular plant in the research field.² A. vera has been used in folk medicine for over 2000 years, and has remained an important component in the traditional medicine of many contemporary cultures, such as China, India, the West Indies, and Japan.³

A. vera is a succulent plant. Succulents are xerophytes, which are adapted to living in areas of low water availability and are characterized by possessing a large water storage tissue. The main feature of the A. vera plant is its high water content, ranging from 99–99.5%.⁴ The remaining 0.5–1.0% solid material is reported to contain over 75 different potentially active compounds including water- and fat-soluble vitamins, minerals, enzymes, simple/complex polysaccharides, phenolic compounds, and organic acids. In compositional studies on the structural components of the A. vera plant leaf portions, the rind was found to be 20–30% and the pulp 70–80% of the whole leaf weight. On a dry weight basis, the percentages of the rind and pulp represented as lipids (2.7% and 4.2%) and that as proteins (6.3% and 7.3%) only accounted for a minor fraction.⁵ However, the nonstarch polysaccharides and lignin represented the bulk of each leaf fraction and were found to be 62.3% and 57.6% of the dry weight of the rind and pulp, respectively. A. vera gel polysaccharides consist of linear chains of glucose and mannose molecules, of which mannose is more concentrated than glucose, thereby the molecules are referred to as polymannans.⁶ These are linear chains ranging in size from a few to several thousand molecules.⁷ The major polysaccharide, acemannan, is composed of one or more polymers of various chain lengths with molecular weights ranging from 30 kDa to 40 kDa or greater, and consisting of repeating units of glucose and mannose in a 1:3 ratio.^5,8

In western societies, especially in the USA, A. vera has been grown mainly to supply the latex component of the leaf to the pharmaceutical industry.⁹ However, over the last decade, various Aloe species have gained popularity as therapeutic botanicals and consequently a large industry has developed utilizing biological properties of A. vera.¹ Many investigators have endeavored to establish the active principles in A. vera gel. It has been used for many centuries for its curative and therapeutic properties and although over 75 active ingredients from the inner gel have been identified, therapeutic effects have not been correlated well with each individual component.¹⁰ Many of the medicinal effects of Aloe leaf extracts have been attributed to the polysaccharides found in the inner leaf parenchymatous tissue.⁶ However, it is believed that these biological activities should be assigned to a synergistic action of the compounds contained therein rather than a single chemical substance.¹¹ The Aloe parenchyma tissue or pulp has been shown to contain proteins, lipids, amino acids, vitamins, enzymes, inorganic compounds, and small organic compounds in addition to the different carbohydrates.⁴

The species of Aloe selected for commercial exploitation or selected by the traditional healer, would be based on its local availability and distribution. In South Africa, the most widely distributed Aloe species are Aloe greatheadii var. davyana (Asphodelaceae) and Aloe ferox Mill. (Asphodelaceae). A. greatheadii grows wild in the northern parts of South Africa, whereas A. ferox grows wild primarily in the Eastern and Western Cape provinces. Monosaccharides of both A. ferox and A. vera released after hydrolysis showed a potential for gel fingerprinting and allow for a distinct property. A. ferox contains various combinations of glucose and galactose as main monosaccharides, while A. vera yields only mannose.¹² Various extracts of these Aloe species are traditionally used and their application used to cure arthritis, skin cancer, burns, eczema, psoriasis, digestive problems, high blood pressure, and diabetes.¹³ As different Aloe species would have varying phytochemical contents due to interspecies variation and varying climate and soil conditions, direct correlation of biological activity would be inaccurate.

Many beneficial effects of this plant have been attributed to the polysaccharides present in the pulp. The clear pulp which is also known as gel is widely used in various medical, cosmetic, and neutraceutical applications.⁴ Studies have noted higher antioxidative activities present in its rind.¹⁴ A. vera has been used externally to treat various skin conditions such as cuts, burns and eczema.¹⁵ These Aloe species are currently listed in the pharmacopoeia of many countries in form of main Aloe, extract and powder.¹⁶

A. vera is also known for useful secondary metabolites.^1,17 Anthraquinones, tricyclic aromatic quinines are the major secondary metabolites that are abundantly present. Among the naturally occurring anthraquinone derivatives, Aloe emodin and chrysophanol are the major compounds.¹⁸ The tricyclic aromatic quinines of Aloe have been proposed to be synthesized via the type III polyketide biosynthesis pathway. Recently, novel plant-specific type III polyketide synthases (PKS), octaketide synthase, PKS4, and PKS5 were isolated from Aloe arborescens and their functions examined in E. coli.¹⁹ These novel plant enzymes might potentially be associated with biosynthesis of natural tricyclic aromatic quinines in Aloe, but it remains unclear whether these enzymes produce end products such as Aloe-emodin and chrysophanol in vivo.²⁰ Aloesin, aloin and Aloe-emodin (oxidative product of aloin) are the most important secondary metabolites found in A. vera gel. Many secondary metabolites in plants have reported potent anti-inflammatory, lipid lowering, and antioxidant activities.²¹ However, no reports have elucidated complete entire secondary metabolites present in the plant species.

2. Clinical efficacy and mechanism of action

2.1. Burn wound healing effect

Aloe is known as the healing plant. A. vera has been used for traditional medical purposes in several cultures.²² In vitro extracts of A. vera stimulate the proliferation of several cell types. Many studies have shown that treatment with whole A. vera gel extracts resulted in faster healing of wounds.^23,24 A. vera may have a direct effect on the wound healing process as a whole, which is manifested by increase in rate of contraction of wound area²⁵ and has confirmed the effect of A. vera on increasing wound contraction and collagen synthesis. This property is attributed to the mannose-6-phosphate known to be present in A. vera gel.²⁶ Polysaccharides from Aloe promote both the proliferation of fibroblasts and the production of hyaluronic acid and hydroxyproline in fibroblasts, which play important roles in extracellular matrix remodeling during wound healing.²⁷ Acemannan, significantly increases periodontal ligament cell proliferation, upregulation of growth/differentiation factor 5, type I collagen and alkaline phosphatase activity in primary human periodontal ligament cells.²⁷ In a clinical study, to check the efficacy of A. vera gel compared with 1% silver sulfadiazine cream as a burn dressing for the treatment of superficial and partial thickness burns, healing of burn wounds were remarkably early in A. vera treated patients than those patients treated with 1% silver sulfadiazine.²⁸ Polysaccharides isolated from A. vera induce matrix mellatopeptidase (MMP)-3 and metallopeptidase inhibitor-2 gene expression during the skin wound repair of rat, which directly helps to regulate the wound healing activity of A. vera gel.²⁹

2.2. Immunomodulatory effect

A. vera gel has strong immunomodulatory activity wherein it downregulates lipopolysaccharide-induced inflammatory cytokine production and expression of NLRP3 (NACHT, LRR, and PYD domain-containing protein 3) inflammasome in human macrophages.³⁰ A. vera could inhibit the inflammatory process following burn injury, as characterized by the reduction of leukocyte adhesion, as well as proinflammatory cytokines.³¹ Liu et al have shown that Aloe polysaccharides pretreatment can attenuate the cerebral ischemia and reperfusion injury in severe traumatic–hemorrhagic rats by first entering high altitude through inhibiting systemic inflammatory response and leukocyte aggregation and lipid peroxidation in the brain.³² Administration of A. vera has been universally demonstrated to result in marked increase in phagocytic and proliferative activity of the reticuloendothelial system.³³ A. vera directly inhibits the cyclooxygenase pathway and reduces prostaglandin E2 production,³⁵ which plays an important role in inflammation. Aloe also contains anthraquinones and chromone in the inner gel, which possess strong anti-inflammatory effects as shown in murine macrophages.^34,35 This report suggests that Aloe as a whole has anthraquinones (aloin) and chromone (aloesin) components, and Aloe gel has pharmacological activity to alleviate inflammatory responses in inflammatory bowel disease.³⁶ A recent report of a clinical study evaluated the therapeutic effect of A. vera gel wherein 2% oral gel is not only effective in decreasing the pain score and wound size in recurrent aphthous stomatitis patients but also decreasing the aphthous wound healing period.³⁷

2.3. Intestinal absorption

Aloe material has been used for drug absorption enhancement for drugs with low bioavailability due to extensive efflux.³⁸ Lactobacillus brevis strains were isolated from naturally fermented A. vera gel which inhibited the growth of many harmful enteropathogens without restraining most normal commensals in the gut and hence were named POAL (probiotics originating from Aloe leaf) strains; these and exhibit discriminative resistance to a wide range of antibiotics.³⁹ Aloin, present in the gel, is metabolized by the colonic flora to reactive Aloe-emodin, which is responsible for the purgative activity. Aloe-emodin isolated from A. vera inhibits colon cancer cell migration by downregulating MMP-2/9 and also inhibits ras a homolog family member B and vascular endothelial growth factor (VEGF) via reducing DNA binding activity of nuclear factor κ-light-chain-enhancer of activated B cells.⁴⁰ A. vera gel has been shown to contain five phytosterols, which are able to reduce visceral fat accumulation, and influences the metabolism of glucose and lipids in animal model experiments, where they reduced large-sized intestinal polyps and ameliorated reduction in plasma. High molecular weight adiponectin levels in adenomatous polyposis coli gene-deficient multiple intestinal neoplasia mice fed high-fat diet. Further, an in vitro study has shown that A. vera gel and whole leaf extract were able to reduce significantly the transepithelial electrical resistance of the Caco-2 cell monolayers and thereby showed the ability to open tight junctions between adjacent cells. Hence, A. vera gel and whole leaf extract solutions significantly enhanced the transport of insulin across the Caco-2 cell monolayers.⁴¹

2.4. Antidiabetic effect

Clinical studies have suggested that A. vera gel may act as a safe antihyperglycemic and antihypercholesterolemic agent for type 2 diabetic patients without any significant effects on other normal blood lipid levels or liver/kidney function.⁴² In vivo and in vitro studies strongly demonstrate that the water soluble fraction of Aloe spp. possesses glucose-lowering activities and some of its components modulate glucose transporter-4 mRNA expression.⁴³ In a randomized controlled trial, A. vera gel complex reduced body weight, body fat mass, and insulin resistance in obese prediabetes and early nontreated diabetic patients.⁴⁵ Further, in a pilot study, two Aloe products in patients with prediabetes over an 8-week period, tended to revert the impaired fasting glucose and impaired glucose tolerance observed in conditions of prediabetes/metabolic syndrome.⁴⁵ One study discussed the efficacy of aloe-emodin-8-O-glycoside isolated from A. vera gel in enhancing glucose transport by modulating the proximal and distal markers involved in glucose uptake and its transformation into glycogen.⁴⁶ Tanaka et al reported reductions in both fasting and random blood glucose levels of db/db diabetic mice chronically treated with the same phytosterols from A. vera gel.⁴⁷ Jain et al found that A. vera gel has significant antidiabetic and cardioprotective activity as it significantly reduced oxidative stress in streptozocin induced diabetic rats and improved antioxidant status.⁴⁸ A. vera gel also helps to improve the carbohydrate metabolism, with a recent report suggesting that it helps to improve metabolic condition in obese prediabetes and early nontreated diabetic patients by reducing body weight, body fat mass, fasting blood glucose, and fasting serum insulin in obese individuals.⁴⁴ Shin et al shown that dietary Aloe formula also reduces obesity-induced glucose tolerance not only by suppressing inflammatory responses but also by inducing anti-inflammatory cytokines in the white adipose tissue and liver, both of which are important peripheral tissues affected by insulin resistance.⁴⁹ A. vera also has shown improvement in the function of isolated rat pancreatic islets wherein it increased survival of the islet cells, their mitochondrial activity, and insulin levels at the same time as reducing production of reactive oxygen species.⁵⁰

2.5. Antioxidant effect

A. vera contains substantial amounts of antioxidants including α-tocopherol (vitamin E), carotenoids, ascorbic acid (vitamin C), flavonoids, and tannins,⁴ and it has been suggested that antioxidant action may be an important property of plant medicines used in treatment of various diseases. Topical A. saponaria treatment has shown antinociceptive and anti-inflammatory effects in ultraviolet B-induced sunburn model via its antioxidant components present in gel.⁵¹ Aloe gel is able to scavenge the free radicals 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)^+•, and nitric oxide in a concentration-dependent manner, as seen in an in vitro study of the radioprotective efficacy of A. vera gel.⁵² Administration of ethanolic extract of A. vera gel on tissue antioxidants led to reduction in blood glucose level in diabetic rats, which helps to prevent excessive formation of free radicals through various biochemical pathways and also reduces the potential glycation of the enzymes.^53,54 In vitro and in vivo antioxidant potentials of a polysaccharide isolated from A. vera gel were investigated. Enzymatic extracts were prepared from A. vera gel using 10 digestive enzymes including five carbohydrases and five proteases. Results suggested that Aloe polysaccharides exhibited a protective effect against 2,2′-azobis(2-amidinopropane) dihydrochloride-induced oxidative stress and cell death in kidney epithelial cells (Vero cells) as well as in an in vivo zebrafish model.³⁹ One study determined the total phenolic content of A. vera leaf skin extracts and a significant correlation was established between the total phenolic content and the antioxidant capacity.⁵⁴ The methanol extracts of leaf skins and flowers of A. vera were also screened for their antioxidant and antimycoplasmic activities, and in vitro antioxidant activities of both extracts exhibited antioxidant activity, with the leaf skin extract being the most active.⁵⁵

2.6. Hepatoprotective effect

Isolated phytosterols, namely lophenol and cycloartanol, have the ability to induce the downregulation of fatty acid synthesis and a tendency for upregulation of fatty acid oxidation in the liver, which favors the reduction in intra-abdominal fat and improvement of hyperlipidemia. Further, addition to sterol regulatory element-binding transcription factor 1/peroxisome proliferator-activated receptor (PPAR)-α ratio was decreased; metabolic syndrome-related disorders were improved and liver steatosis in Aloe-sterol-treated Zucker diabetic fatty rats.⁵⁶ Aloe formulas also suppress obesity-induced inflammatory responses by reducing levels of the proinflammatory cytokines, PPARγ/liver X receptor α, and 11β-hydroxysteroid dehydrogenase 1, and enhance anti-inflammatory cytokines in white adipose tissue and liver. The beneficial effects of Aloe formula with respect to obesity-induced insulin resistance and hepatic steatosis have been associated with its action on PPARγ/liver X receptor α.⁵⁰ Saito et al showed that A. vera gel extract prevents ethanol-induced fatty liver by suppressing mRNA expression of lipogenic genes in the liver. The combination of probiotic Lactobacillus rhamnosus GG and A. vera gel have a therapeutic potential to decrease cholesterol levels and the risk of cardiovascular diseases.⁵⁷

2.7. Anticancer activity

Aloin, an anthraquinone being a natural compound and the main ingredient of Aloe, has been documented for its remarkable potential therapeutic options in cancer, wherein it showed chemoprotective effects against 1,2-dimethylhydrazine-induced preneoplastic lesions in the colon of Wistar rats.⁵⁸ Aloin treatment could inhibit the secretion of VEGF in cancer cells. VEGF is one of the most important proangiogenic cytokines known and well characterized as an inducer of tumor neovascularization. Aloin treatment significantly inhibited in vitro VEGF-induced angiogenic response of human endothelial cells, causing an inhibition of proliferation and migration of endothelial cells.⁵⁹ Aloe-emodin (AE), is also a subtype of anthraquinone, a natural compound that has traditionally been found to have diverse biological activities including anticancer functions.^60,61 AE (1,8-dihydroxy-3-hydroxymethyl-9,10-anthracenedione) is an herbal anthracenedione derivative from A. vera leaves. Recent reports have shown that AE possesses antiproliferation effects on some types of cancer cells, such as lung, squamous, glioma, and neuroectodermal cancer cells.^62,63 The inhibitory effect of AE on the activity and gene expression of N-acetyl transferase, which plays an initial role in the metabolism of aryl amine carcinogens, was found in human malignant melanoma cells.^64,65 Recently, Lin et al demonstrated that AE-induced apoptosis in T24 human bladder. Aloin, derived from A. vera leaves, has been shown to possess anticancer potential activities,⁶⁵ as it inhibits tumor angiogenesis and growth via blocking signal transducer and activator of transcription 3 activation, with the potential of a drug candidate for cancer therapy.⁶⁶ Anthraquinone derivatives such as emodin-like natural (emodin, rhein, and aloin) and synthetic (anthraquinone-2-sulfonic acid) anthraquinones have recently been shown to protect in models of amyloid β and τ aggregation-induced cell death through antiaggregation properties, and/or enhancing the phosphatidylinositol-3-kinase/protein kinase B survival mechanism, which suggests that anthraquinone-2-sulfonic acid could be a new neuroprotective compound and a novel caspase inhibitor.⁶⁷

2.8. Antimicrobial activity

A. vera has been described as an antibacterial agent. The Aloe protein of 14 kDa from the A. vera leaf gel was isolated and the purified Aloe protein exhibited a potent antifungal activity against Candida paraprilosis, Candida krusei, and Candida albicans.⁶⁸ A. vera has anthraquinones as an active compound, which is structural analogue of tetracycline. The anthraquinones acts like tetracycline that inhibits bacterial protein synthesis by blocking the ribosomal A site (where the aminoacylated tRNA enters). Therefore, the bacteria cannot grow in the media containing A. vera extract. Pandey and Mishra established the susceptibility of Gram-positive and Gram-negative bacteria to an extract of the inner gel of A. vera.^10,69,70 Polysaccharides of A. vera gel have been attributed direct bacterial activity through the stimulation of phagocytic leucocytes to destroy bacteria.⁷¹ A. vera contains pyrocatechol a hydroxylated phenol, known to have toxic effect on microorganisms.^72,73 A recent study demonstrated that the A. vera inner gel expresses antibacterial properties against both susceptible and resistant Helicobacter pylori strains and impact on the antimicrobial resistance phenomenon of H. pylori, proposing the A. vera inner gel as a novel effective natural agent for combination with antibiotics for the treatment of H. pylori gastric infection.¹⁸

2.9. Antiviral activity

Many reports have suggested that A. vera gel has antiviral activity that prevent virus adsorption, attachment, or entry to the host cell. An in vitro study has shown that crude extract of A. vera gel has antiviral activity against herpes simplex virus type 2 strain.⁷⁴ Anthraquinone derivatives, such as Aloe-emodin, emodin, and chrysophanol, reportedly exhibit antiviral activity wherein their inhibitory mechanism and effect against influenza A virus with reducing virus-induced cytopathic effect and inhibiting replication of influenza A.⁷⁵ Preliminary trials have suggested that A. vera consumption may be of help to HIV-infected individuals as it improves the immune system by increasing the CD4 count.⁷⁶ Many methods have been developed for the successful transformation and regeneration of A. vera wherein Aloe able to produced IFNα2. This experiment was assessed using an antiviral assay with A549 cells wherein these cells treated with extracts from both the rind and pulp fractions of the shoot and subsequently infected with the lytic Encephalomyocarditis virus. This experiment demonstrated that A. vera was capable of expressing a human protein with its biological activity namely interferon alpha 2 (IFNα2).⁷⁷

2.10. Effect on estrogen status

Isolated emodin and aloe-emodin from A. vera gel specifically suppress breast cancer cell proliferation by targeting estrogen receptor-α protein stability through distinct mechanisms, which suggests a possible application of anthraquinones in preventing breast cancer cell proliferation through estrogen receptor-α inhibition.⁷⁸ A. vera gel also helps to maintain ovarian steroid status in polycystic ovary-like condition wherein steroidogenesis altered and disturbed estrogen:testosterone ratio.⁷⁹

2.11. Antihyperlipidemic activity

A. vera is known for its antihyperlipidemic property wherein it has beneficial effects on the prevention of fatty streak development and may help to reduce the development of atherosclerosis through modification of risk factors.⁴² A. vera leaf gel efficacy has been checked in hyperlipidemic type 2 diabetic patients: a randomized double-blind placebo-controlled clinical trial wherein it reduced total cholesterol and LDL levels significantly.⁴² A recent study also demonstrated that administration of phytosterols isolated from A. vera gel reduces visceral fat mass and improves hyperglycemia in Zucker diabetic fatty rats.⁸⁰ Dried pulp of Aloe succotrina leaves produced significant antihyperlipidemic effect in high-fat diet- and fructose-induced hyperlipidemic rats, where it significant decreased serum levels of total cholesterol, total triglycerides, low-density lipoprotein–cholesterol, very low-density lipoprotein, and high-density lipoprotein–cholesterol.⁸¹ Previous reports also suggested that A. vera gel-treated polycystic ovarian syndrome (PCOS) rats exhibited significant reduction in plasma triglyceride and LDL cholesterol levels, with an increase in high-density lipoprotein–cholesterol PCOS condition wherein hyperlipidemia is one of main consequences. The gel treatment also caused reversion of abnormal estrous cyclicity, glucose intolerance, and lipid metabolizing enzyme activities, bringing them to normal. It has phytocomponents with antihyperlipidemic effects and has shown efficacy also in management of PCOS but also the associated metabolic complications.^79,82

2.12. Antiulcer activity

A. vera is an herbal remedy widely used for a variety of illnesses; A. vera leaf extracts have been promoted for digestion and are used in the treatment of peptic ulcer for cytoprotective action whereby A. vera gel expresses antibacterial properties against both susceptible and resistant H. pylori strains and acts as a novel effective natural agent for combination with antibiotics for the treatment of H. pylori gastric infection.³⁷ One study demonstrated that newly formulated aloe- and myrrh-based gels proved to be effective in topical management of minor recurrent aphthous stomatitis and was superior in decreasing ulcer size, erythema, and exudation; myrrh resulted in more pain reduction in a randomized, double-blind, vehicle-controlled study.⁸³

3. Discussion

Since ages, Aloe species have been exploited for various medicinal efficacies because of their phyto-chemical constituents.¹³ Having therapeutic, rejuvenating and health enhancing properties, Aloe vera gel is widely used in food, healthcare and medicinal industries. It is seen from the literature that A. vera is a very important plant for its large number of medicinal properties as well as medicinally important chemicals such as amino acids, anthraquinones, enzymes, hormones, sterols, and vitamins.⁴

Reports have shown that polysaccharides isolated from A. vera demonstrate various pharmacological effects, such as anti-inflammatory, wound healing, antihepatitis, antigastric ulcer, and antitumorigenicity in animals, although some side effects of Aloe have been found in humans. Hence, some experimental studies have been performed to confirm the upper dose of A. vera gel without any side effects to establish the maximal allowable daily intake of active Aloe. They were based on 4-week oral toxicity investigation in imprinting control region mice, which did not induce any remarkable subacute toxic effects, but decreased male kidney weights.⁸⁴ Several studies have attempted to determine whether or not A. vera causes toxicity in animals or humans. During a chronic 90-day study, A. vera leaf pulp at a dose of 100 mg/kg in the drinking water caused decreased body and vital organ weights, decreased red cell counts, a significant spermatogenic dysfunction, and a 30% lethality compared with control animals.⁸⁵ Extracts from the leaves of A. vera have long been used as herbal remedies and are also now promoted as a dietary supplement, in liquid tonics, powders or tablets, as a laxative, and to prevent a variety of illnesses. However, some studies have demonstrated effects of A. vera extract on rats and mice to identify potential toxic or cancer-related hazards.¹⁷ Dosed water studies in mice revealed no acute toxicity of the leaf pulp at 500 mg/kg.¹⁷ At higher doses, however, a decrease of central nervous system activity was observed. During subchronic 90-day studies, increased mortality, decreased red blood cell count, and significant sperm damage were noted, in addition to decreased central nervous system activity.⁸⁶ Hence, an upper limit of dose of A. vera gel plays a crucial role for the treatment of various diseases, which helps to manage disease condition without side effects.

From the whole plant, A. vera latex is a laxative that is regulated as a drug by the Food and Drug Administration and used as a bitter flavoring additive by the food industry; A. vera gel is primarily a topical agent for skin wounds and irritations but is also taken internally for the treatment of gastric ulcers and diabetes; and the whole leaf extract, which combines both the gel and latex, is popular as a dietary supplement for various systemic ailments and is promoted as a potential anticancer, anti-AIDS, and antidiabetic agent.

4. Conclusion

The plant exhibits many pharmacological activities such antioxidant, antimicrobial, immune boosting, antitumor, hypoglycemic, hypolipidemic, wound healing, and antidiabetic.⁸⁶ Many traditional uses are also reported such as burn injury, eczema, cosmetics, inflammation, and fever, which continue to be studied, and further research still has to be done. Thus, it is quite promising as a multipurpose medicinal agent so further experiments are needed to isolate and to find out the mechanism of the bioactive chemicals using modern instruments, such as high-performance liquid chromatography, high-performance thin layer chromatography, and nuclear magnetic resonance, and extended clinical trials on the road to generate novel drugs. The US Food and Drug Administration has already approved the developmental study of A. vera in the treatment of cancer and AIDS. In future, controlled studies are required to prove the effectiveness of A. vera under various conditions.

Conflicts of interest

All contributing authors declare no conflicts of interest.