Citrus hystrix DC for Metabolic Disorders: Comparison
Please note this is a comparison between Version 1 by Yusof Kamisah and Version 2 by Camila Xu.

Metabolic disorder, which includes hypertension, diabetes mellitus, dyslipidemia, and obesity, represents a major global health concern due to increased morbidity and mortality. It occurs due to disturbance in normal metabolic process leading to redox and energy imbalance. Metabolic disorder, which includes hypertension, diabetes mellitus, dyslipidemia, and obesity, represents a major global health concern due to increased morbidity and mortality. It occurs due to disturbance in normal metabolic process leading to redox and energy imbalance.

  • kaffir lime
  • diabetes mellitus
  • hypertension
  • dyslipidemia
  • obesity

1. Introduction

Metabolic disorder, which includes hypertension, diabetes mellitus, dyslipidemia, and obesity, represents a major global health concern due to increased morbidity and mortality. It occurs due to disturbance in normal metabolic process leading to redox and energy imbalance [1]. The incidence of metabolic disorders increased during the COVID-19 lockdown due to lack of activities and physical exercise, as well as increased intake of homemade food rich in fat [2]. Many factors contribute to the development of this disorder, including an unhealthy diet, sedentary lifestyle, lack of physical exercise, and smoking [1]. Several therapeutic goals have been established to educate high-risk individuals to modify their lifestyle to slow down or prevent the progression of metabolic disorders.
Oxidative stress and inflammation play a major role in the development and progression of metabolic disorders [1][3][1,3]. Therefore, it has been theorized that plant extracts with antioxidant and anti-inflammatory properties could have beneficial effects on patients with metabolic disorders. Animal studies have shown that plant extracts such as those of Parkia speciosa Hassk., which is rich in flavonoids, confer protective effects against hypertension [4], while Ganoderma lucidum proteoglycans protect against diabetes [5], and açaí seed extract rich in proanthocyanidins protects against dyslipidemia [6]. Citrus hystrix DC extract has shown protective effects against diabetes [7], hypertension [8], and dyslipidemia [9].
Citrus hystrix (Figure 1), also known as kaffir lime or makrut lime, also goes by the following names: Citrus auraria Michel, Citrus echinata SaintLager, Citrus hyalopulpa Tanaka, and Citrus kerrii (Swingle) Tanaka [10]. It is a flowering, shrubby plant in the family Rutaceae that grows 3 to 6 m high and is indigenous to tropical Southeast Asia, southern China, and northeastern India [10][11][10,11]. It bears green, warty, and bumpy fruits. The leaves and fruits are often used as spices in Asian cooking [12]. Many bioactive compounds from the plant have been studied for their therapeutic potential in improving the symptoms of metabolic disorders in animal studies.
Figure 1.
The plant of
Citrus hystrix
DC.

2. Traditional Medicinal Uses

The fruits, leaves, and rind of C. hystrix are the most common parts traditionally used to reduce the severity of certain illnesses (Table 1). The fruits are used for the treatment of stomachache by hilly Tripura tribes in northeastern India [11], while the leaves and fruits are both used in steam-bathing for postpartum mothers, to relieve headache, rheumatism, fever, and to treat diabetes mellitus in North Sumatra, Indonesia [13]. In Malaysia, the fruits are used in hair shampoo to decrease dandruff and to promote hair growth [10]. The leaves and fruits are also used to boost sexual performance [13] and to treat hypertension, heart disease, and diarrhea [14][15][14,15].
Table 1.
Traditional medicinal uses of
C. hystrix
.

3. Phytochemical Properties

Various phytochemical compounds have been detected in the leaves, roots, fruits, and rind of C. hystrix (Table 2). Terpenoids are the major compounds identified in the leaves of the plant, while coumarins are predominantly found in the roots. The leaves also contain phytosterol and flavonoids. The rind extract, which is rich in flavonoids, possesses high antioxidant. It also demonstrates lipase-inhibiting activity which is beneficial for the treatment of obesity, angiotensin-converting enzyme-inhibiting property for the management of hypertension, moderate inhibiting activity against α-amylase and α-glucosidase which could be useful in diabetes, as well as inhibitions against acetylcholinesterase, butyrylcholinesterase, and β-secretase-1 which are favorable in the treatment of Alzheimer’s disease [19].
Table 2.
Phytochemical compounds in various parts of
C. hystrix
.
Essential oils extracted from the twigs and leaves contain monoterpenes like citronellal, citronellol, linalool, sabinene, and limonene [35][36][37][38][35,36,37,38], while the major compounds in the oil from the rind are β-pinene, sabinene, limonene, citronellal, α-pinene, and terpinen-4-ol [38]. The oil from the leaves was noted to be inactive against Staphylococcus aureus, Staphylococcus epidermidis, Bacillus subtilis, Escherichia coli, and Klebsiella pneumoniae, but was moderately effective in inhibiting the fungal growth of Candida albicans, Saccharomyces cerevisiae, and Cryptococcus neoformans [36]. The antifungal activity of the leaf oil could be attributable to the presence of oxygenated monoterpenes [40]. Figure 23 shows the chemical structure of major phytochemical compounds found in the plant.
Figure 23.
Molecular structure of major phytochemical compounds in
C. hystrix
.

4. Effects on Diabetes

Diabetes mellitus is one of the metabolic disorders that has become a global public health burden. Type 1 diabetes occurs due to pancreatic β-cell damage leading to impaired insulin release, while type 2 diabetes occurs due to insulin resistance and is commonly associated with obesity [41]. Oxidative stress and inflammation are involved in the pathogenesis of the disorder [42]. Based on this knowledge, various plant extracts with antioxidant and anti-inflammatory properties, including C. hystrix, are being studied to assess potential protective effects against diabetes, in particular, type 2 diabetes.
Abirami et al. [43] (Table 3) demonstrated in an in vitro study that the powdered rind and pulp of C. hystrix exhibited higher concentration-dependent glucose adsorption capacity than that of xanthan and guar gum, and that the capacity was augmented with increasing concentration of glucose. This finding suggests that the extract could decrease postprandial glucose levels. The extract also decreased glucose dialysis retardation index and starch digestibility [43], suggesting a delay in glucose absorption possibly due to lower starch assimilation in the gastrointestinal tract. The extract could be beneficial in decreasing postprandial glycemic and insulinemic response in type 2 diabetics, thus controlling the uptake of glucose. However, the active phytochemical compounds responsible for the activity were not identified. The functional groups responsible for the glucose absorbency were most likely hydroxyl group and possibly methyl ester group of galacturonic acid detected in the C. hystrix extract [43].
Table 3.
Studies of
C. hystrix
on diabetes mellitus.
[68], while rats that were fed a diet containing 15% extract-treated heated oil plus 2% cholesterol had lower levels of cardiac C-reactive protein, TNF-α, troponin, and lactate dehydrogenase [67]. These findings suggest that C. hystrix leaf extract conserves the integrity of the kidneys and heart in rats fed a heated oil diet, likely by decreasing oxidative stress and inflammation in the organs. Further investigation of NADPH oxidase-4/H2O2/mTOR complex 1 (NOX4/H2O2/mTORC1) and PI3K/Akt signaling pathways should be conducted to validate the protective effects of the extract on hypertension-induced renal injury. The former pathway was reported to be activated, while the latter was suppressed in such models [74][75][74,75].

6. Effects on Cholesterol Level and Obesity

Hyperlipidemia is a metabolic disorder that can lead to the development of other metabolic disorders like hypertension and obesity. It arises from an imbalance in blood cholesterol levels: high in total cholesterol (TC) and low-density lipoprotein (LDL), and low in high-density lipoprotein (HDL) levels [76]. Many factors contribute to its pathogenesis including a high-fat diet and lack of physical activity [75]. Studies on C. hystrix have been conducted to assess its effects on the blood lipid profile (Table 5). A study by Nugraheni et al. [9] reported that plant rind or peel extract at 35, 70, and 140 mg/kg produced no significant change in serum LDL level in rats fed a high-fat diet. Conversely, another study by the same group [77] demonstrated a reduction in serum TC following administration of the extract; however, there was no significant increase in the cholesterol levels of the high-fat group as compared with the control group, suggesting that the hyperlipidemia model was not properly established in this study. Moreover, the investigators used the Least Significant Difference test as the post hoc test, which tends to give a false-positive result [78]. Therefore, the findings of the study are questionable and were not confirmative.
Table 5.
Effects of
C. hystrix
and its phytochemical compounds on hyperlipidemia and organ cholesterol level.
Another in vitro study investigated the potential antidiabetic effects of different fractions of C. hystrix rind extract in ethanol [44]. In the study, the ethyl acetate fraction and water residue demonstrated a significant α-amylase-inhibiting activity comparable to metformin, a biguanide antihyperglycemic drug. The ethyl acetate fraction exhibited better α-glucosidase-inhibiting activity than the water residue (Table 3). The inhibition exhibited by the ethyl acetate fraction was similar to that of acarbose, an α-glucosidase inhibitor. However, the hexane fraction did not exhibit inhibitory activity against either enzyme. It appears that the increased polarity of certain compounds conferred the beneficial properties. α-Amylase hydrolyzes starch into disaccharides [48], while α-glucosidase converts disaccharides into monosaccharides before absorption into a portal vein [49]. In terms of antioxidant properties, the ethyl acetate fraction exhibited the highest activity followed by the water residue and hexane fraction. Both the ethyl acetate fraction and water residue contained alkaloids, saponins, tannins, phenolics, and flavonoids, while the hexane fraction contained alkaloids, phenolics, and flavonoids. It is possible that either certain phytochemical compounds in the ethyl acetate fraction and water residue exhibited the positive effects on glucose metabolism or that the compounds had interacted synergistically. Further studies should be performed to elucidate the responsible compounds. Fruit juice from the plant also demonstrated similar inhibitory effects on both enzymes (α-amylase and α-glucosidase) [45]. Ethanol rind extract exhibited a high inhibitory effect on α-amylase activity as compared with the low activity seen in the aqueous and ethyl acetate extracts [46]. This finding suggests that phytochemical compounds which have more polarity than ethyl acetate but less than the aqueous solution were responsible for the effect.
To the best of researchers' knowledge, only a study had investigated in vivo effects of C. hystrix extract. Drinking 150 or 300 mg/kg body weight of C. hystrix leaf extract in solution with water lowered fasting blood glucose in streptozotocin-induced diabetic rats [7]. The beneficial effects of the extract led to a significant reduction in the incidence of cataracts in the rats, a phenomenon which was believed to be associated with reductions in oxidative stress and inflammation, indicated by lower levels of malondialdehyde, prostaglandin E2, and tumor necrosis factor-α (TNF-α); attenuated vascular permeability was also observed due to decreased levels of vascular endothelial growth factor. In the study, bioactive compounds were identified in the extract, namely apiin, apigetrin, saponarin, rutin, diosmin, hesperidin, and xanthotoxol [7]. These compounds have been shown to possess anti-inflammatory and antioxidant activity [50][51][52][53][50,51,52,53]. Further studies should be conducted to confirm the potential antidiabetic effects of the compounds in vivo as well as their mechanisms of action. The effects of the compounds on poly (ADP-ribose) polymerase and aldose reductase activity in the lens should be investigated as these enzymes are elevated in diabetic cataracts [54][55][54,55].
In a study by Rekasih et al. [47], a functional food drink was created containing various medicinal plants: Orthosiphon aristatus (Blume) Miq., Zingiber officinale Roscoe, Caesalpinia sappan L., Curcuma xanthorriza Roxb., Citrus limon (L.) Osbeck, Citrus aurantifolia (Christm.) Swingle, and C. hystrix. The drink contained 1% C. hystrix fruit juice and was administered at a dose of 18.2 mL/kg body weight to streptozotocin-induced diabetic rats for two weeks in ready-to-drink, microencapsulated, or nanoencapsulated formulations. The exact concentration of the C. hystrix juice in the drink could not be determined as the yield percentage was not reported. All formulations significantly reduced fasting blood glucose and elevated pancreatic β-cell and Langerhans islet viability in the rats, but the improvements were more significant in the microencapsulated and nanoencapsulated formulations. Encapsulation was believed to have increased the contact surface and improved the bioavailability of the bioactive phytocompounds in the drink [56]. The study demonstrated possible synergistic interactions among the components of the drink. Possible mechanisms of the hypoglycemic effects were not elucidated.
Collectively, almost all parts of C. hystrix possess antidiabetic properties most likely attributable to the plant’s flavonoid content. These compounds may act by suppressing α-amylase and α-glucosidase activities, in addition to possessing antioxidant and anti-inflammatory properties (Figure 34). The effect of the compounds on the mitogen-activated protein kinase pathway involved in the cellular inflammatory response, apoptotic and phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathways, gene and protein expression of glucose transporters and insulin receptors should be further elucidated. The PI3K/Akt/mTOR pathway is involved in intracellular cell cycle signaling, especially apoptosis and proliferation, and may also be involved in the viability of pancreatic β-cell and Langerhans islets. However, no clinical study has been conducted so far to assess the effects of the extract on diabetes. The phytochemical compounds have the potential for development as drugs for the treatment of type 2 diabetes mellitus.
Figure 34. Possible sites of action of C. hystrix and its bioactive compounds in diabetes. PGE2, prostaglandin E2; TBARS, thiobarbituric acid reactive substance; TNF-α, tumor necrosis factor-α; VEGF, vascular endothelial growth factor; →, induces, → (with −), inhibits.

5. Effects on Hypertension

Hypertension is the most common cardiovascular disease. Several mechanisms have been proposed for its pathogenesis, including impairment of renal salt and water handling and increased formation of angiotensin II involving the renin–angiotensin–aldosterone system (RAAS), abnormalities in the sympathetic system, and elevation of vascular oxidative stress and inflammation [57][58][57,58].
The aqueous extract of C. hystrix leaves demonstrated good angiotensin-converting enzyme (ACE) inhibition activity (>90%) in vitro [59]. The enzyme metabolizes angiotensin I into angiotensin II which then promotes aldosterone release, leading to salt and water retention [4]. There was a strong correlation between ACE inhibition and total phenolic content in the extract [59]. Plants rich in polyphenols were reported to exhibit ACE inhibitory effects [4][60][4,60]. The inhibitory effect exhibited by the C. hystrix extract suggests that its bioactive compounds could be potential candidates for treating hypertension. However, the compounds were not identified in the study and the effect of the extract was not compared with a positive control to validate its activity [59].
Heated oil has been reported to raise blood pressure in rats [61][62][61,62]. Heating causes chemical changes like thermal oxidation and polymerization in the oil, leading to a configuration change in the fatty acids from cis to trans [61]. Addition of C. hystrix leaf extract at 1% into frying oils that were heated five and ten times reduced peroxide levels and increased the total phenolic content of the oils [63][64][63,64] (Table 4). Reduction in the oxidized content of the oil by the addition of the extract prevented the elevation of blood pressure in rats that were fed the heated oils for 16 weeks, starting on week 4. It is possible that the beneficial effect was conferred by the improvement in vascular response due to preserved plasma nitric oxide level. The groups that were fed the extract-treated oils also showed better organization of vascular elastic lamellae, smaller aortic tunica intima to tunica media ratio, and lower expression of vascular cell adhesion molecule-1 [63]. The findings indicated that the extract prevented oxidation in the oils upon heating, thus increasing their stability and antioxidant content. Heating was reported to reduce the antioxidant content of the oil, particularly reducing vitamin E levels [65]. Consumption of such oils prevented the development of hypertension due to preservation of vascular microstructure.
Table 4.
Effects of
C. hystrix
extract on hypertension and related end-organ damage.
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