Oxidative stress refers to the imbalance between the production of free radicals in the body and the capability of cells and tissues to clear them (1). Free radicals are generated from endogenous and exogenous sources by enzymatic and non-enzymatic reactions. They play crucial roles in human health. Free radicals, such as nitric oxide radical (NO^•) and superoxide radical (O2•-), are involved in the defense mechanism to fight pathogens, the syntheses of some cellular structures, and cellular signaling pathways. In addition, they control blood flow by being cell-to-cell messengers, and they are required for non-specific host defense and induction of a mitogenic response (1). Thus, regular exposure to free radicals is one of the cellular homeostasis. Despite their benefits, free radicals can also contribute to the anomaly by being pro-oxidant. Long-term and high concentrations of free radicals are undesirable phenomenon (2). Oxidative stress occurs when there are excessive and rising levels of free radicals and oxidants in the body. Uncontrolled conditions lead to health problems and eventually increase the risk of metabolic, chronic, and degenerative diseases, such as cardiovascular diseases, neurodegenerative disorders, nephropathy, inflammation and immune-related diseases, sexual maturation and fertility disorders, and cancers (1, 3–7).

Oxidative stress is caused by excessive oxidants and a lack of antioxidants. Antioxidants refer to compounds able to impede or retard the oxidation of a substrate, acting at a lower concentration compared with that of the protected substrate (8). Antioxidants can be both endogenous and exogenous substances, similar to oxidants. Endogenous antioxidants are classified as enzymatic and non-enzymatic antioxidants. Exogenous antioxidants are introduced to the body in the form of a diet, and they act as oxidative defenses through different mechanisms and in different cellular compartments (6). Antioxidants such as vitamin C and E, coenzyme Q10, zinc and selenium, and polyphenols are sometimes inadequately consumed through routine diets. They have therefore sometimes been applied in the forms of dietary supplements or additive substances in foodstuffs.

Phytochemicals that are well-known as antioxidants are polyphenols, vitamins, carotenoids, minerals, and organosulfur compounds (9). There is plenty of research looking for the sources of powerful antioxidants due to their promising benefits for health from either their preventive or treatment perspectives. Thus, many plants that contain the aforementioned phytochemicals have been examined for their antioxidant activities and developed as sources of natural exogenous antioxidants.

Tamarindus indica L. (Fabaceae, Caesalpinioideae), or tamarind, is a tropical plant native to Africa. The plant has long been used as a food and herbal medicine. Its fruit pulp is well-known as a good source of vitamins, minerals, and organic acids. Tamarind fruit possesses several pharmacological activities, such as antifungal, antiasthmatic, hepatoprotective, and wound healing activities (10–13). Moreover, other parts of this plant, such as its leaf, stem bark, root bark, and seed, have also been reported as medicaments, e.g., antibacterial, antihyperlipidemic, antiulcer, anticancer, antifungal, wound healing, hepatoprotective and immunopotentiation agents (10, 11, 13–19).

There are various chemical constituents in T. indica. The fruit pulps contain furan derivatives, carboxylic acid, phlobatannin, grape acid, apple acid, flavonoids, pectin, sugars, and the like (20, 21). The seeds contain campesterol, β-amyrin, β-sitosterol, fatty acids, tannins, sugars, mucilage and polysaccharides, cardiac glycosides, and phenolics, among others (20, 22, 23). The components of the bark include tannins, saponins, glycosides, peroxidase, and lipids (20). The leaves contain orientin, iso-orientin, vitexin, iso-vitexin, glycosides, peroxidase, vitamin B₃, and vitamin C (20, 23). Polyphenols, e.g., flavonoids and phenolics, are present in almost every part of the plant, making T. indica an up-and-coming source of antioxidative agents.

In this review article, we describe original research on the antioxidant activities of T. indica, focusing on antioxidant effect of its leaves obtained from the in vitro experiments (antioxidant capacity) (24). The description and morphology of T. indica, major chemical constituents especially phenolic compounds and flavonoids, and toxicity of T. indica leaves were also summarized and discussed. T. indica leaf extracts possessed antioxidant capacity by free radicals scavenging, heavy metal chelating and transition. Total phenolic and total flavonoids contents might relate to antioxidant capacity of T. indica leaves. The elements in T. indica leaves might also be responsible for the antioxidant capacity. No toxic was reported from the using of T. indica leaves either in vitro or in vivo experiments. The limitations of this study are lack of in vivo antioxidant activity assay, standard compounds were applied in only some included studies, no specific chemical was reported as biomarker and no quantification analysis of active compounds was conducted, maturity level of raw materials used in the included studies was reported in only one study.

This systematic review examined 21 in vitro studies of the antioxidant capacity of T. indica leaves. The samples used in antioxidant tests were prepared using water or organic solvents by different methods and with different pretreatments of the raw materials. The antioxidant capacity assays also differed between studies.

Oxidative stress is the disruption of redox signaling and control caused by the imbalance of free radicals and antioxidant defenses (8). Free radicals are found in human cells, animal cells, and other living organisms. They are generated by endogenous reactions and are caused by exogenous sources. In the human body, free radicals are produced by several biochemical processes. For example, H₂O₂ and HO^• radicals result from the reduction of molecular oxygen during aerobic respiration, and O2•- radicals and hypochlorous acid (HOCl) arise from the activation of phagocytes (67). Exogenous oxidants are caused by pollution and environmental stressors such as cigarette smoking, air pollution, radiation, and diet (8). Oxidative stress leads to a variety of health problems. It is both the primary cause of pathology and a secondary contributor to disease progression, e.g., cancers, cardiovascular disease, neurodegenerative disorders, diabetes, and metabolic syndrome (9). Thus, antioxidants are the first option to prevent and treat various health issues and anomalies, especially environmental pollution.

Antioxidants prevent oxidative stress-related damage by breaking radical chain reactions (9). Antioxidants are divided into endogenous and exogenous antioxidants. The latter class must be ingested through the diet. Endogenous antioxidants comprise enzymatic and non-enzymatic antioxidants, while exogenous antioxidants consist of water-soluble and lipid-soluble antioxidants. The well-known antioxidants obtained from natural sources are vitamin C and E, carotenoids, tannins, phenolic acids, and flavonoids (1, 8, 9).

Phenolics are strong antioxidants and are members of the “polyphenols,” which are a large class of plant secondary metabolites. Flavonoids are natural compounds that contain hydroxyl groups that are responsible for metal chelation and free radical scavenging capacity. The compounds can react with O2•- and HO^• and are also able to chelate metal ions, so they regulate both iron homeostasis and redox state (9, 68). Pizzino et al. (1) summarized the antioxidant properties of flavonoids as ROS scavengers and ROS synthesis suppressors, antioxidant defense enhancers, enzyme inhibitors, and trace element chelators (1). Several flavonoids are present in T. indica leaf extracts, e.g., luteolin and its derivatives, apigenin, orientin, vitexin, quercetin, isorhamnetin, catechin, and epicatechin (Table 4). Non-flavonoid compounds such as caffeic acid and chlorogenic acid were also detected (Table 4).

Vitexin and iso-vitexin, which are apigenin derivatives, and orientin (the luteolin glycoside) were investigated for their in vitro antioxidant capacities and in vivo antioxidant activities. Khole et al. (69) studied the mechanism of vitexin and iso-vitexin for their antioxidant effects. They found that the compounds exhibited different capacities against ROS. Iso-vitexin scavenges O2•- radicals better than vitexin, while vitexin scavenges NO^• radicals better. Both compounds were active against short-lived radicals: ABTS^•+ radical and CO3•- radical. These compounds protected HepG2 cells from H₂O₂₋induced oxidative insult by modulating antioxidant enzyme levels and reducing intracellular ROS levels (69). In 2012, An et al. (70) observed the antioxidant activities of vitexin and orientin compared with vitamin E in an age mouse model. The results showed that vitexin and orientin had the capacity to improve the antioxidative system as well as to improve the levels of ATPase in the tissue and serum in aged mice induced by D-galactose. Furthermore, the compounds at 40 mg/kg BW were comparable to vitamin E for the improvement of neuronal cell structure and function in mice (70).

Catechin, epicatechin, rutin, and quercetin are ubiquitous polyphenols in herbs and food plants. These compounds showed better DPPH^• radical scavenging capacity than the analog of vitamin E, Trolox, in the study of Iacopini et al. (71). Similarly, in the DPPH^• scavenging, ABTS^•+ radical scavenging, and FRAP assays performed by Tian et al. (72), quercetin manifested better antioxidant capacities than vitamin C and BHT in all assays. In a mechanistic study, quercetin exhibited several mechanisms against oxidative stress. It inhibited inducible nitric oxide synthase in macrophages, so oxidative damage was inhibited. The compound also directly scavenges free radicals and inhibits the formation of oxygen free radicals through the chelation of ions of transition metals such as iron. Quercetin also inhibited xanthine oxidase and suppressed TNF-α modulated by oxidative stress, resulting in the decrement of oxidative injury and the modulation of immune response (73).

Luteolin and apigenin are plant flavonoids with a broad spectrum of biological activities. Both displayed superior ABTS^•+ radical scavenging capacity to vitamin C and BHT in the studies of Tian et al. (72). In addition, luteolin showed surpassing results in DPPH^• radical scavenging and FRAP assays compared with both vitamin C and BHT (72). The in vitro mechanism of apigenin against oxidative stress includes oxidative enzyme inhibition, modulation of redox signaling pathways (NF-κB, Nrf2, MAPK, and P13/Akt), reinforcement of enzymatic and non-enzymatic antioxidants, free radical scavenging, and metal chelation (74). An in vivo experiment in arterial aging mice conducted by Clayton et al. (75) revealed that apigenin could increase NO bioavailability; normalize ROS, antioxidant expression, and oxidative stress; and abolish the inhibitory effect of ROS (75). The mechanisms of the antioxidant action of luteolin have been summarized as ROS scavenging, ROS-generating oxidase inhibition, enhancement and protection of endogenous antioxidants, direct inhibition of oxidative-catalyzed enzymes, and chelation of transition metal ions (76).

Chlorogenic acid and its major metabolite, caffeic acid, are classified as phenolic acids. Chlorogenic acid is hydrolyzed into caffeic acid in the intestine after ingestion. The antioxidative effect of caffeic acid has been evinced using different in vitro assays by Gülçin (77), i.e., the ferric thiocyanate method, total reduction capability, ABTS^•+ radical scavenging, DPPH^• radical scavenging, O2•- radical scavenging, and ferrous metal chelating capacity (77). The in vivo antioxidant assay of chlorogenic acid and caffeic acid was performed using the 2-methyl-6-p-methoxyphenylethynylimidazopyrazynone method to emit O2•- radical scavenging capacity. The IC₅₀ values of chlorogenic acid and caffeic acid were 41.0 and 10.1 μM, respectively, whereas allopurinol provided an IC₅₀ of 15.0 μM (78). Caffeic acid exerted its cytoprotective effect in ischemia/reperfusion injury in the rat small intestine caused by ROS. The compound decreased lipid peroxidation and reduced DNA damage in UV radiation-induced oxidative stress. In addition, caffeic acid showed in vivo antioxidant activity against chemical-induced toxicity (such as cisplatin-, carbon tetrachloride-, and cadmium-induced toxicity of the liver and kidney) in various animals (79).

Not only polyphenols but also other phytochemicals, vitamins and elements found in plants have also been reported to be responsible for antioxidant capacity. T. indica leaves contain the sugar acid form of ascorbic acid and some elements that possess antioxidant effects, e.g., selenium, copper, zinc, and manganese. These constituents might exhibit antioxidative effects via different mechanisms, and the overall antioxidant capacity might be caused by antagonistic, synergistic, or additional effects of these compounds and elements.

Most of the 21 studies that were reviewed executed antioxidant capacity by using more than one assay. This might be due to the differences in method principles and their strengths and limitations regarding cost and facility requirements, difficulty of operation, time spent, sensitivity and specificity, reproducibility and repeatability, correlation with phytochemical content, coverage spectrum of biological samples, and representativeness of the in vivo system. As a consequence of these factors, the results obtained from each study did not correlate with others.

Furthermore, the results still differed even when the same antioxidant assay was used. These occurrences were attributed to variations in the sources of the raw materials, their pretreatments, the extraction methods, and the solvents used for sample preparation. Furthermore, the phytochemical screening results differed between studies in that some classes of plant constituents were detected by some but not others. Other reasons for discrepancies in the screening results are that the quantity of the compounds was below the detection limit of the particular screening method employed and interference from other chemicals. Both factors might cause false-positive and false-negative results.

Considering toxicity, several studies performed the acute oral toxicity of T. indica leaf extracts, and no death was observed at the maximum single dose of 5,000 mg/kg BW and a 14-day repeated dose of 4,000 mg/kg BW. However, it was found that after exposure to T. indica leaf fluid extract in the right malar bag in Syrian hamsters, signs of mucous irritation were observed. These findings were explained by the presence of organic acids and polyphenols in T. indica leaves, which could slightly irritate the mucous membrane. Hence, the extract is considered a light irritant to the mucous membrane and could be a very mild irritant to the skin (49).

In the present study, the antioxidant capacity of T. indica leaves was reviewed. T. indica leaf extracts exhibited in vitro antioxidant capacity through free radical scavenging capacity and transition and heavy metal chelating capacity. There is a high possibility that the antioxidant capacities are responsible for the polyphenols and the elements. The polyphenols found in T. indica leaves are flavonoids and phenolic acids such as catechin, vitexin, orientin, apigenin, and luteolin. In addition, elements such as selenium, copper, manganese, and zinc are present. These chemicals and elements are well-known as strong antioxidants, which makes T. indica leaves a promising natural antioxidant mixture. The safety of T. indica leaves was investigated in erythrocytes and animals. The extracts were found to be safe after oral administration of 4,000 mg/kg BW for 14 days, and no death was observed after the ingestion of 5,000 mg/kg BW. The 50% lethal intraperitoneal dose was 566 mg/kg BW.

The limitations of this systematic review are as follows:

To apply T. indica leaf extract as a source of antioxidant, confirmed results from an in vivo study and a clinical trial should be considered. Standardization of the extract with regard to its active constituents or total phenolic and total flavonoid content should be performed, especially if the extract is prepared using a different method and solvent. The effective dose should be taken into account to avoid excessive intake and antioxidative stress. The toxicity might also be a concern. An intensive in vivo study of subacute, subchronic, and chronic toxicity should be performed.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author/s.

SS, AD, and SSa: conceptualization. SSo and SSa: data curation, formal analysis, and writing—original draft. SS, AD, and SSa: methodology. SSa, AD, and PP: supervision. AD and PP: validation. SSo, AD, SSa, and PP: writing—review and editing. All authors contributed to the article and approved the submitted version.

This work was partially supported by the Unit of Excellence on Clinical Outcomes Research and IntegratioN (UNICORN) (grant number: FF65-UoE005), School of Pharmaceutical Sciences, University of Phayao and Phayao Provincial Public Health Office. The funding sources had no role in the study design or the collection, analysis, and interpretation of the data.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.