Bioactive compounds of corn silk and their role in management of glycaemic response

Abstract

Management of glycaemic response is perhaps the most critical part of antidiabetic therapy. Hypoglycaemia is an avoidable complication caused by conventional drugs used in the treatment of diabetes. It triggers commonly during the intensification of anti-hyperglycemic therapy used to render glycemic control in diabetic patients. The commercial oral hypoglycaemic drugs, insulin, herbal medicines and plant extracts are therefore used as a part of the treatment of diabetes. The demand for treating diabetes, through herbal and plant resources is due to their lesser adverse reactions and better phytochemical benefits. Corn silk has been shown to have anti-allergic, anti-inflammatory, and anti-hypertensive effects when extracted in various solvents. Corn silk has medicinal characteristics and has long been used as a traditional medicine in many nations, although the mechanism of action is unknown. The hypoglycaemic effects of corn silk are investigated in this review. The phytochemical components present in corn silk-like flavonoids, phenolics, terpenoids, tannins, sterols, and alkaloids are phytochemical components that have hypoglycemic activity and a mechanism for lowering blood glucose levels. There is a lack of a homogenized database on the hypoglycemic properties of corn silk thus the present review attempts to critically analyse it and provide specific recommendations of its doses.

Introduction

Diabetes is a chronic disease that requires continuous medical care and patient self-management education to prevent acute complications and reduce the risk of long term complications (Association AD 2011). According to the International Diabetes Federation (IDF) Diabetes Atlas of 2019, Approximately 463 million adults (20–79 years) have diabetes and is expected to rise to 700 million by the year 2045. The frightening data of 4.2 million diabetic deaths, more than 20 million live diabetes affected births during pregnancy and 760 billion dollars in health expenditure in 2019 demands a war footing remedial measures to control/prevent this epidemic (IDF 2019).

Presently, clinically available insulin and oral anti-diabetic agents like sulfonylureas, glucosidase inhibitors, insulin sensitizers, etc. are used as therapies for diabetes in clinical practices (Cryer et al. 2009). Besides the principle approaches of prevention of hyperglycemia like diabetes self-management; continuous glucose sensing; appropriate insulin or other drug regimens; individualized glycemic goals; professional support and guidance, there are functional foods for its treatment, which have been used for human consumption for centuries and their safety in human has been well documented (Zhang et al. 2016).

Since, the populations around the globe are getting aware slowly about the economic, nutritional and health benefits of herbal products. The latest addition in the category of the herbal functional food which has been claimed to have benefits against hypoglycaemia is corn silk (Stigma maydis). Corn silk is primarily considered as an agro-waste from corn cultivation, however, it is also an inexpensive medical byproduct of the plant. Corn silk is a long, silky, and colored (yellowish to reddish) hair-like structure that covers the edible part of corn (Ho et al. 2016). It is predominantly discarded as a waste due to lack of effective utilization. The collective yield of corn silk varieties as reported in the literature vary somewhere from 123 to 283 kg per hectare (Sarepoua et al. 2013).

According to the Persistence Market Research report, the global corn silk extract market in 2017 was segmented into seven key regions depending on the geographic regions: North America, Latin America, Western Europe, Eastern Europe, Middle East & Africa (MEA), Asia Pacific Excluding Japan (APEJ) and Japan. North America and Latin America holds the top position for the production of both corn and corn silk, followed by European countries. Among European countries, Belarus holds the first position for the highest production of corn silk. China is emerging to hold a good position as a producer and consumer of corn silk extract. Other countries like India, Vietnam are also large producers of corn silk (Persistence Market Research 2021).

Corn silk was recorded with functions of diuresis, detumescence, as well as benefits on liver and gallbladder, and used to treat cystitis, jaundice, edema, prostate disorders, urinary infections, hyperglycaemia and obesity (Liu et al. 2011; Zhao et al. 2012). Recent studies have reported the bioactivities of corn silk, such as anti-oxidant, anti-diabetic (Zhang et al. 2016; Chang et al. 2016) anti-obesity (Chaiittianan et al. 2016), neuroprotective and anti-depressant (Choi et al. 2014), anticancer (Lee et al. 2014) etc. As a folk herbal medicine, corn silk has been used to treat hypoglycaemia since ancient times, but there has been very few reviewed reports on its anti-diabetic potential and its associated complications. Based on the above facts of corn silk, the current review focuses on the evaluation of the anti-diabetic capacity and hypoglycaemic activity of its various phytochemical compounds, their mode of action to furnish recommendations for corn silk as a hypoglycaemic agent. Corn silk possesses a strong hypoglycaemic activity that could be exploited for commercial use by the food and pharma industry.

Chemical composition and phytochemical potential of corn silk

Corn silk varieties obtained worldwide have shown prominent difference in its nutritive and phytochemical composition. Tables 1 and 2 represent the varieties grown in different countries. The Indonesian corn silk varieties are rich in protein whereas the Mexican corn silk varieties are rich in dietary fiber. Corn silk consists of proteins, vitamins, polysaccharides, Ca, K, Mg and Na salts and bioactive compounds which include alkaloids and tannins, saponins as well as steroids (Guo et al. 2009).

Table 1.

Chemical composition of different varieties of corn silk grown in different regions worldwide

S. No.	Country/Region Name	Varieties	Moisture (%)	Protein (%)	Fat (%)	Ash (%)	Total carbohydrate (%)		Total fiber (%)	Reference
1	Mexico	Gordo with white kernels (RG-w) Cónico with red kernels (RC-r) dark red kernels (RC-dr) white-blue kernels (RC-wb) Cristalino with yellow kernels (RCr-Y)	83.64 (fw) 82.37 (fw) 85.50 (fw) 88.38 (fw) 85.93 (fw)	16.7 (dw) 16.6 (dw) 14.3 (dw) 14.1 (dw) 19.1 (dw)	0.9 (dw) 1.1 (dw) 0.9 (dw) 1.8 (dw) 1.6 (dw)	5.0 5.9 5.7 5.8 6.1	70.9 68.9 69.3 68.1 65.1		53.3 44.9 39.5 39.4 43.9	Mendoza-Lopez et al. (2018)
2	Malaysia	Baby corn (Immature silks) Sweet corn (Mature silks)	89.31 (fw) 84.42 (dw)	12.96 8.95	1.27 0.66	5.28 5.51		27.80 29.74	48.50 51.24	Rahman and Rosli (2014)
3	Indonesia	Bisma Arjuna Srikandi Putih	11.58 (db) 14.66 (db) 8.09 (db)	17.70 (db) 12.89 (db) 14.87 (db)	0.30 (db) 0.13 (db) 0.21 (db)	3.29 (db) 2.66 (db) 3.33 (db)		67.13 (db) 69.54 (db) 73.53 (db)	–	Haslina et al. (2017)

fw = fresh weight; dw = dried weight;

db = dry basis

Table 2.

Phytochemical composition of different varieties of corn silk grown in different regions worldwide

Country/Region Name	Varieties	Total polyphenolic content (µg GAE/g)* (CGAE/100 g)** (mg GAE/g)*** (µg.g−1 dw−1)****	Total flavonoid content (µg RE/g)# (mg CE/100 g)## (mg CAE/g)###	Reference
Thailand	PWC1 (R1, R4, R6 stage) PWC2 (R1, R4, R6 stage) PWC3 (R1, R4, R6 stage) PWC4 (R1, R4, R6 stage) PWC5 (R1, R4, R6 stage) WWC1 (R1, R4, R6 stage) WWC2 (R1, R4, R6 stage) WWC3 (R1, R4, R6 stage) SSC1 (R1, R4, R6 stage) SSC2 (R1, R4, R6 stage)	123.8, 179.6, 149.1* 112.8, 169.2- 114.7* 127, 185.3, 147.4* 115.6, 147.7, 134.6* 173.4, 206.8, 187.1* 99.8, 57.3, 26.5* 89.6, 56.8, 47.6* 85.9, 56.7, 36.1* 85.5, 75.2, 65.5* 69.4, 78.3, 69.4*	83.6, 104.6, 100.0# 74.1, 99.8, 61.8# 74.4, 94.5, 86.8# 99.0, 129.2, 105.3# 121.5, 136, 101.4# 57.6, 35, 33.2# 50.9, 35.7, 33.6# 87.4, 35.7, 33.8# 117.6, 90.3, 65.5# 119.1, 92.2, 69.2#	Sarepoua et al. (2013)
Serbia	ZP Exp (5 days of emergence) ZP Exp (25 days of emergence) ZP 555 (5 days of emergence) ZP 555 (25 days of emergence) ZP 341(5 days of emergence) ZP 341(25 days of emergence) ZP 366 (5 days of emergence) ZP 366 (25 days of emergence)	8101.6** 3958.9** 8382** 2093.9** 10160.8** 4347.2** 8372** 3674.7**	5565.3## 3594.2## 5608.7## 1840.1## 6478.3## 3644.9## 5514.5## 2985.5##	Zilic et al. (2016)
Malaysia	Baby corn (Immature silks) Water Etanol Ethyacetate Sweet corn (Mature silks) Water Etanol Ethyacetate	35.35*** 92.21*** 6.70*** 64.22*** 49.88*** 4.26***	8.40### 7.55### 0.66### 2.31### 1.96### 2.10###
Portugal	–	Chlorogenic acid- 42.4**** Caffeic acid- 13.7**** Ferulic acid- 48.1**** Apigenin- 7.9**** Pelargonidin- 2.6****		Aires and Carvallo (2016)
Malaysia	Big Fruit (young corn ears) Big Fruit (Corn silk) Supersweet (young corn ears) Supersweet (Corn silk) Bi-color (young corn ears) Bi-color (Corn silk)	79.61*** 86.26*** 82.85*** 136.32*** 92.64*** 143.58***	9.31### 14.66### 10.65### 26.63### 14.41### 18.14###	Ho et al. (2016)
Indonesia	Bisma Arjuna Srikandi Putih	8262.93* 6331.15* 3367.10*	236.03* 178.33* 136.36*	Haslina et al. (2017)

PWC = Purple waxy corn; WWC = White waxy corn; SSC = Super sweet corn

R1 = Baby corn stage; R4 = Immature stage; R6- Physiologically matured stage

* = µg GAE/g; ** = CGAE/100 g

# = (µg RE/g); ## = (mg CE/100 g)

*** = mg GAE/1 g, **** = µg/g dw

Proximate composition

The difference in proximate composition in mature and immature Malaysian corn silk was analysed by Rahman and Rosli (2014) and the result showed that immature corn silk has a high moisture content (89.31 ± 0.74%), crude lipid content (1.27 ± 0.16%), crude protein (12.96 ± 0.38%) than matured corn silk whereas high values for ash content (5.51 ± 0.24%), carbohydrate content (29.74 ± 1.26%) and total dietary fiber (51.24 ± 1.50 g/100 g) were observed in matured corn silk in comparison to immatured corn silk. Three different Mexican maize varieties (Gordo with white kernels, Conico with red kernels, dark red kernels and white blue kernels and Cristalino with yellow kernels) were evaluated by Mendoza-Lopez et al. (2018). The major constituent in corn silk reported was fiber among the carbohydrates ranged from 39 to 53% out of which insoluble fiber percentage was high (36–52%) in comparison to soluble fiber (> 3%). Although the amount of soluble fiber content in corn silk is relatively low, it is sufficient to possess beneficial properties (Hasanudin et al. 2012). Although largely dependent on the method of drying and variety used, the indicative proximate composition of dried corn silk (variety: Zheng Dan 958) reported by Wang et al. (2011) was found to be 9.65–10.4% of moisture, 9.42–17.6% of protein, 0.29–4.74% of fat, 1.2–3.91% of ash, 7.34% of dietary fibre and 65.5–74.3% of carbohydrates.

Bioactive constituents

The difference in antioxidant content represents that the total phenolic and flavonoid content varies according to maturity. The difference in solvents during the analysis also plays a critical role. Corn silk is enriched with numerous amounts of phytoconstituents and the major compounds are summarised in Table 3. Corn silk shows high polyphenolic content as shown by Sarepoua et al. (2015) in their study of purple maize grown in Mexico. Ten different corn hybrids including five purple waxy corns, two super sweet corns and three white waxy corns at different growth stages (R1 harvested at the silking stage; R4 harvested at an immature stage and R6 harvested at physiological maturity stage were evaluated for phytochemical constituents. The total phenolic content increased from 26.5 to 206.8 µg GAE/g of dry samples from R1 to R6 stage as shown in Table 2. The purple waxy corn showed the highest percentage of total phenols among all the other hybrid varieties. They suggested that collecting samples at silking and immaturity reproductive stage gives the best results if considered for phytochemical constituents. In a similar study, Zilic et al. (2016) assessed four Serbian hybrid corn silk varieties (ZP Exp, ZP 555, ZP 341 and ZP 366) for their phytochemical compounds and compared them with different medicinal herbs and concluded that the corn silk at an immature stage (R1stage) has highest total phenolic and flavonoid content which is suitable for its incorporation in food or tea preparation as compared to the dough stage (R4 stage). The polyphenols show a hypoglycemic response that leads to reducing blood glucose levels (Zhang et al. 2016). Immature and corn silk at baby corn and sweet corn stage grew in Malaysia was studied by Rahman and Rosli (2014) for the difference in their antioxidant potential and they reported that the immature corn silk showed the highest value for moisture, protein and lipid than matured silk but the ash content, total fiber and total carbohydrate was reported to be high in matured corn silk than immature silk.

Table 3.

Bioactive phytochemical components and nutritive composition of corn silk

Class of phytochemicals	Phytochemical components	Amount present	Functions possessed by phytochemical components	References
Polyphenols	Tannins, flavonoids, saponins, alkaloids, cardiac glycosides, steroids, anthocyanins, allantoins, hesperidin and resins, antioxidant	Total phenolics- 8101.6 ± 73.5 to 10,160.8 ± 250 mg CGAE/100 g Flavonoid- 5565 ± 40.9 to 6478.3 ± 409.9 mg CE/100 g Anthocyanins- 192.9 ± 0.3 mg CGE/100 g Proanthocyanidins- 69.4 ± 6.6	Antioxidant, anti-inflammatory, prebiotic property and act as vasodilator	Zilic et al. (2016)
Phenolic acids	Vanillic acid, Para-aminobenzoic acid (PABA), chlorogenic acid, protocatechuic acid, caffeic acid, maizenic acid, hydroxycinnamic acid ester, ferulic acid and 3-O-caffeoylquinic acid	3-O-caffeoylquinic acid- 22.6 ± 0.9 to 49.9 ± 1.4 mg/100 g	Inhibit oxidative damage diseases including coronary heart diseases, cancer, stroke. Antibacterial activity, anti-inflammatory, anti-allergic properties	Zilic et al. (2016)
Flavonoids	Catechin, protocatechin, quercitin, rutin, 3, 4, 5, 7-hydroxy flavones and isoflavones, cardiac glycosides, Maysin derivatives, methoxymaysin derviative	Rutin- 0.1398 mg/L Quercitin- 0.11 mg/L Maysin derivatives- 1.1 ± 3.6 (593 m/z) Methozymaysin derivative- 1.2 ± 0.1 to 4.9 ± 0.3	The antioxidative capacity of flavones exhibits property against prevention of cancer and coronary heart diseases, osteoporosis, neurodegenerative diseases and postmenopausal bone loss	Ismael et al. (2017)
Carotenoids	Β-carotene and zeaxanthin	Total carotenoid count = 11.3 mg CGE/100 g dw (ethanolic extract)	Treat cardiovascular diseases, cancer, eye related disorders, protects skin and show high antioxidant power and reduces oxidative stress	Laeliocattleya et al. (2014)
Sterols	Stigmasterol and beta-sitosterol	10.5886 mg/g 963.86 ± 198.39 to 1783.37 ± 57.70	Regulation of membrane permeability and fluidity, control metabolic process, substrate for synthesis of secondary metabolites and precursor in cellular and developmental process	Haslina et al. (2017)
Tannins	Gallotannins and phlobatannins	–	Improves cardiovascular health, inhibit atherogenesis, treat ischemia, reduce platelet aggregation and lipid peroxidation	Hu et al., (2010)
Vi9(tamins	Vitamin C, E and K	–	Reduces oxidative stress, helps in collagen synthesis, prevents cancer, sepsis and neurodegenerative diseases, prevents blood clotting and oxidation of LDL	Rahman and Rosli (2014)
Minerals	Sodium, Potassium, Calcium, Magnesium, Copper, Iron, Manganese, Zinc	Sodium- 12.5 ± 0.5 to 28.9 ± 0.3 mg/100 g Potassium- 1359.9 ± 46.6 to 1832.6 ± 37.5 mg/100 g Calcium- 0.1465 mg/g Magnesium- 0.1602 mg/g Copper- 0.0072 mg/g Iron- 0.0198 mg/g Manganese- 0.0187 mg/g Zinc- 0.0136 mg/g		Zilic et al. (2016)
Sugars	Fructose (Dried corn silk) Glucose (Dried corn silk) Sucrose (Dried corn silk)	14.20 ± 0.12 22.20 ± 1.10 4.40 ± 0.20		Rosli and Rahman (2015)
Miscellaneous compounds	Polysaccharides (galactan), terpenoids, apigenin, anthraquinones, xanthoproteins	–

Aires and Carvalho (2016) evaluated the polyphenols and antioxidant capacity of corn silk by ultrasound extraction method and mentioned the presence of ferulic acid, chlorogenic acid, caffeic acid, apigenin and pelargonidin out of which apigenin is responsible for inhibiting the cancer growth in the pancreas, regulating lipids and glucose levels and ameliorate vascular dysfunction in type 2 diabetic population (He et al. 2015) and pelargonidin and related isomers helps in reducing postprandial inflammation and protect against insulin sensitivity. Six samples of three Malaysian corn silk varieties were evaluated for phytochemical constituents (Big Fruit, Supersweet and Bi-color) at young corn ears and the corn silk stage. Bio-colour (BioCo) corn silk showed the highest polyphenolic content as compared to the other two varieties and for flavonoid content, corn silk of the Supersweet variety showed the highest value in comparison to other samples. The difference in the values might be due to genetic variations and good exposure of sunlight to these varieties (Ho et al. 2016). Three local varieties of Indonesian corn silk (powdered) varieties (Bisma, Arjuna and Srikandi Putih) were studied for their chemical and phytochemical properties and the result showed that Bisma possessed the highest content of protein and fat. The difference in the proximate values was due to genetic variation, growing conditions, soil fertility, harvesting time, storage conditions and production process. The highest moisture content was shown by Arjuna and Srikandi Putih had a high content of carbohydrate and ash content. The difference in the proximate content is due to the accumulation of hydrocarbons at a stage of maturity which affects lipid composition and the change in protein content is due to the biosynthesis and functioning of amino acids during the growth process. Bisma had the highest polyphenolic and flavonoid content than the other two varieties of dried corn silk (Haslina et al. 2017).

The medicinal and pharmaceutical importance of plants is usually recognised by their antioxidant potential. Corn silk shows diversity in the phytochemical content and has high antioxidant activity, including free radical scavenging activity, ferric reducing antioxidant power, ABTS radical scavenging activity and xanthine oxidase as shown in Table 4. Hu et al. (2010) has suggested extracts of corn silk improve the antioxidant status of organs by stimulating the activity of the antioxidant enzymes.

Table 4.

Antioxidant activity of corn silk

Type of Antioxidant activity (AA)	Extracting solvent	% inhibition	Reference
Free Radical Scavenging Activity (FRSA)	Water Ethyl acetate Ethanol Methanol	195.21 ± 7.48 411.69 ± 9.57 68–84 143.55 ± 4.67	Rosli and Rahman (2015) Sarepoua et al. (2013)
ABTS	Water Ethyl acetate Methanol BHT/Trolox	829.00 ± 94.26 2870 ± 112.30 349.07 ± 15.15 131.77 ± 16.25	Rosli and Rahman (2015)
Ferric reducing Antioxidant Power (FRAP)	Water Methanol Ethanol Ethyl acetate	35.01 56.41 38.90–65.46 27.21%	Nurhanan & Rosli (2012) Ho et al. (2016)
Xanthine Oxidase (XOD)	Water Methanol Ethyl acetate	1174 ± 150.13 261.41 ± 12.55 412.17 ± 12.97	Rosli and Rahman (2015)

Based on traditional remedies, the Chinese have been using CS as an oral anti-diabetic agent for decades (Guo et al. 2009). However, there were some studies related to the mechanism of underlying hypoglycaemic activity of corn silk, but data regarding its anti-diabetic and anti-hyperglycemic activities are very limited. The presence of diterpenes consisting of carnosic acid and carnosol acting glitazones makes it suitable for treating diabetes (Rao et al. 2006). The impact of extracts of corn silk on liver markers and plasma glucose of rabbits was studied by Olaniyan and Fadare (2014). Fifteen rabbits were divided into three groups and each group was fed with no extract (control), aqueous and methanolic extract respectively. The methanolic and aqueous extract of corn silk significantly reduced the blood glucose levels as compared to the control group. They reported that the lowering of blood glucose levels is due to the presence of phytochemicals like flavonoids, alkaloids, tannins and saponins in corn silk.

The flavonoids of corn silk were studied for anti-diabetic, anti-oxidant and anti-hyperlipidemic effects by Zhang et al. (2015). The streptozotocin-induced diabetic mice were fed with crude flavonoids extracted from corn silk. The total dose of 160 mg/kg of body weight of streptozotocin was mixed in 0.1 mol/L cold citrate buffer of pH 4.2 was given in the abdominal cavity of the mice and the level of blood glucose was recorded every sixth day. The different groups included non-diabetic control (NC), non-diabetic corn silk flavonoids high dose group (CS), diabetic control group (DC), diabetic dimethylbiguanide group (PC), diabetic corn silk flavonoids low dose group (LD), diabetic corn silk flavonoids medium-dose group (MD) and diabetic corn silk high dose group (HD). The observation stated that the blood glucose levels were reduced in PC, MD and HD groups as compared to the control groups which shows that the ingestion of corn silk flavonoids (300 mg/kg and 500 mg/kg) can possess blood glucose-lowering properties in the diabetic population. The insulin resistance in hyperglycaemic patients causes impaired GLUT4 mechanism which further leads to poor uptake of glucose by the cells (Alam et al. 2016). Corn silk proved to help increase the uptake of glucose by showing peripheral action. The in vitro study to check the effect of corn silk on glucose uptake by isolated rat hemi-diaphragm was conducted by Ghada et al. (2014). Adults and healthy rats were divided into groups of six rats each and were killed by decapitation. Four sets of experiments were performed as a control (tyrode solution and 2% of glucose), insulin (0.25 U/ml), methanolic corn silk extract (200 mg/ml) and a mixture of insulin and corn silk extract. The result depicted that corn silk extract showed the highest and enhanced uptake of glucose (109 mg/g/30 min) by isolated rat hemi-diaphragm significantly (p < 0.001) in comparison with insulin (49 mg/g/30 min).

Due to its high flavonoid activity, the antimicrobial activity of corn silk in comparison with gentamicin was studied by Nessa et al. (2012). The investigation of the antimicrobial activities of different solvents of flavonoids present in corn silk was pursued and compared with the activities of standard antibiotic gentamicin. The extracts prepared included petroleum ether (PECS), chloroform (CECS) and methanol (MECS) of corn silk and testes on twelve pathogenic bacteria, including Salmonella typhi, Enterobacteraerogenes, Escherichia coli and one yeast Candida albicans for antimicrobial activity. For testing antimicrobial activity, two isolated flavonoid glycosides (2.0 mg/mL) were used. The agar hole-plate diffusion method showed that PECS, MECS and flavonoids showed antimicrobial activity against eleven bacteria (except Escherichia coli) out of twelve bacteria. A significant (p < 0.05) difference with higher sensitivity was reported against several bacteria than gentamicin.

Effect of growth on bioactive compounds of corn silk

The growth of the corn plant can be divided into two categories viz. Vegetative (V stages) and reproductive (R stages). The vegetative stages comprise of the corn emergence (VE stage/sprouting) under the soil surface followed by various stages like V1 (the first leaf emerged and leaf collar is visible), V2 (two leaves after 7 to 10 days of emergence), V3 (after 10 to 20 days of emergence), V4-V6 (initiation of uppermost ear and tassel and kernel row numbers are visible), V7-V9 (8 leaves are formed), V10 (10 leaves are formed) and V11-V15 (completion of kernel row determination). The reproductive stage of corn silk only emerges post pollination (9 or 10 weeks after the emergence of corn) after the VT (tassel emergence) stage begins. R1 is the primary silking stage when the silk is visible outside the husk. Pollen shreds over the silk to fertilize the ovules and further forms the kernel. R2 to R6 are the grain fill stage where R2 is the blister stage, appears after 10–14 days of silking, R3 (after 18 to 22 days of silking), R4 also known as dough stage (after 24 to 28 days of silking), the inner fluid begins to thicken due to starch accumulation, R5 known as dent stage (after 35 to 42 days after silking), each kernel have a dent over it and lastly R6, when kernels continue to gain weight until the formation of black layer (after 55 to 65 days of silking) (Sun et al. 2018). The bioactive compounds during these reproductive stages change from their immature to mature state as depicted in Fig. 1, representing R1 and R6 as two distinctly different stages in the silking cycle of corn.

Fig. 1.

Effect of growth on compositive value of corn silk. Source: [Rahman and Rosli (2014)]

The maturity of corn silk affects the amount of antioxidant content and a significant decrease is found as the crop matures^. The mature corn silk showed the highest value for Fe (4.50 µg/g), Na (266.67 µg/g), K (35,671.67 µg/g) and Mn (35.57 µg/g) when compared to immature corn silk. For the antioxidant activity, ABTS free radical scavenging results showed that immature silk extracts of ethanol, ethyl acetate and water have a higher percentage of inhibition in comparison to mature silk. The ethanolic extract of immature (EC₅₀ = 0.478 mg/ml) and matured silk (EC₅₀ = 0.799 mg/ml) possessed the strongest antioxidant activity.

Though the variations in the nutritive and phytochemical constituents are dependent on genotype and maturity stage, the silk irrespective of maturity and immaturity state has the potential for the development of functional and nutraceutical foods.

The antioxidant activity of corn silk

The antioxidant activity of corn silk includes free radical scavenging activity, ABTS, ferric reducing antioxidant power and xanthine oxidase which helps in the treatment of non-communicable diseases like diabetes, cancer, obesity and cardiovascular diseases. antioxidant activity of corn silk as shown in Table 4 depicts the different antioxidant activity with different solvents. The hypoglycemic effect of corn silk flavonoids on alloxan-induced diabetic mice and the result included a reduction in the level of blood glucose due to the presence of flavonoids and also the significant reduction was observed in body weight of the diabetic mice. The high antioxidant activity of corn silk in combination with Binahong leaves improves the function of β-cells and hence provides hypoglycemic response (Sukandar et al. 2013).

The gamma-irradiated corn silk was investigated in male albino rats for its hypoglycemic and hypolipidemic properties. A significant increment was observed in total phenolic content and total antioxidant activity as a result of irradiation. The damage effects were reduced by increasing the level of insulin, glutathione content, high-density lipoprotein and the activity of superoxidase dismutase and catalase. A significant decrease was observed in the glucose level and malondialdehyde (Hamza et al. 2013).

Advanced Glycation End products (AGEs) are produced by non-enzymatic glycosylation of proteins which is responsible for creating abnormalities in cells and tissues. AGE helps in improving vascular permeability by adhering to the specific macrophage receptor which further causes free radical production and endothelial dysfunction (Khan et al. 2012). They are the cause of various diseases including aging and diabetes. The plant derivatives are known for inhibiting non-enzymatic glycation and the formation of the AGEs. The corn silk contains high amount of flavonoids and total phenols which shows a similar property as studied by Farsi et al. (2008). Thirteen modern maize inbreds and one landrace were examined for in vitro inhibition of non-enzymatic glycation of bovine serum albumin. The highest genotype (CO441) activity displayed an IC₅₀ of 9.5 µg/mL in comparison with aminoguanidine which is a potent inhibitor of glycation. The corn silk possessed this property because of the presence of total phenols. They concluded that high phenolic maize inbreds act as a potent component for the development of natural AGE inhibitors which further prevents and treat diabetic-related complications and degenerative effects related to ageing. The presence of alkaloids is reported by Limmatvapirat et al. (2020) in the ethanolic extract of baby corn silk.

Corn silk polysaccharides were also well known for their anti-diabetic property and Zhao et al. (2012) has utilised the corn silk polysaccharides (POCS) extracted by distilled water and analysed them for anti-diabetic properties on streptozotocin-induced diabetic rats. The result showed that 100–500 mg/kg body weight of POCS significantly decreases the blood glucose level compared to the control group.

Corn silk extract for 12 weeks to Orlistat induced obese mice were studied by Ahmed et al. (2016) and the results revealed a significant decline (p < 0.05) in the glucose level, insulin resistance value, serum insulin levels and the body mass index of HCD fed group with aqueous or methanolic extract of corn silk.

Mechanism of the hypoglycaemic response of corn silk

Corn silk possesses high hypoglycaemic properties as per evidence as summarized in Table 5. The corn silk in the form of extracts exhibits hypoglycaemic response not only by increasing the insulin level and recovering injured beta cells but also increasing glycogen and inhibiting gluconeogenesis (Guo et al. 2009). The details regarding the mechanism of corn silk are shown in Fig. 2.

Table 5.

Hypoglycemic property of corn silk extracts

S. No	Methods	Results	References
1	Ethanolic extracts of 52 herbs including corn silk were taken and evaluated for peroxisome proliferator-activated receptors (PPAR)	Corn silk exhibited α and γ type of PPAR which plays vital role in glucose homeostasis	Rau et al. (2006)
2	Inhibition of non-enzymatic glycation and formation of advanced glycation end products (AGEs) by maize inbreds	The active maize genotype (CO441) with IC50 of 9.5 µg/ml showed more efficiency than aminoguanidine which is potent inhibitor of glycation	Farsi et al. (2008)
3	Alloxan induced hyperglycemic mice are treated orally with corn silk extract for 45 days	Reduced blood glucose and glycohemoglobin (HbA1c) levels. Increased insulin secretion	Guo et al. (2009)
4	Comparison of antimicrobial activities of isolated flavonoid corn silk extracts with gentamycin against 12 pathogenic bacteria	Extracts and isolated flavonoids of corn silk showed significant higher sensitivity towards the pathogenic bacteria than gentamycin	Nessa et al. (2012)
5	Streptozotocin induced diabetic rat treated with 100-500 mg/kg body weight of polysaccharides of corn silk extract	Reduction of blood glucose levels	Zhao et al. (2012)
6	Crude corn silk polysaccharides were obtained first and further the different derivatives were prepared like sulfated derivative S-CSPS, acetylated derivative A-CSPS and carboxylmethylated derivative C-CSPS	Carboxylmethylated polysaccharide possess higher antioxidant and α-amylase inhibitory abilities	Chen et al. (2013)
7	Methanolic extract of corn silk used to study the glucose uptake by isolated rat hemi-diaphragm	Significant increase (p < 0.001) in glucose uptake (109 ± 1) was observed by isolated rat hemi-diaphragm after the addition of corn silk extract when compared with control (12 ± 1) and insulin (49 ± 8) mg/g/30 min	Ghada et al. (2014)
8	Aqueous and methanolic extract of corn silk were studied on three experimental groups of 5 rabbits each with one group as control	Significant decrease observed in mean value of glucose in the rabbit given with methanolic extract as compared to aqueous extract	Olaniyan and Fadare (2014)
9	Streptozotocin induced diabetic rat treated with 300-500 mg/kg body weight with crude flavonoids extracted from corn silk	Ameliorated blood glucose levels	Zhang et al. (2015)
10	Animals fed with high cholesterol diet to induce obesity and then treated with corn silk extract for 6 weeks	Anti-obesity property of corn silk revealed attributed to its hypolipedemic, hypoglycemic and anti-inflammatory properties	Ahmed et al. (2016)
11	Four different corn silk fractions were developed from ethanolic crude extract of corn silk	Ethyl acetate fraction (YMS-EA) showed reduction in reactive oxygen species and increased glucose-stimulated insulin secretion	Chang et al. (2016)
12	Methanolic extract of cooked, dried and pulverized stigma was assessed in alloxan induced diabetic albino mice	Decrease in blood glucose level was observed after 72 h and 96 h with the doses of 500 and 750 mg/kg body weight	Sani (2016)
13	Corn silk extract was reacted in different concentrations with α-amylase and with pre-incubated α-glucosidase	Modulates blood glucose levels by inhibiting α-amylase and α-glucosidase activity	Sabiu et al. (2016)
14	Streptozotocin induced diabetic mice treated with polysaccharides extracted from corn silk	Decreases serum insulin and blood glucose levels. Inhibits α-amylase activity	Pan et al. (2017)
15	Anti-diabetic inhibitory potential of corn silk against enzyme causing diabetes mellitus (α-glucosidase, β-glucoamylase, maltase glucoamylase were studied	Methanolic and hexane crude extracts reported to be potent inhibitor of α-glucosidase with an IC50 range of 31.6 ± 0.4 µg/mL to 35.7 ± 0.6 µg/mL	Adewole et al. (2018)
16	Intraperitoneal injection of freshly prepared Streptozotocin was given to male wistar rats to induce hyperglycemia and further given with ethanolic extract of corn silk for 35 days	The comparison of commercial drugs and natural extracts for inhibition of glycosylation of haemoglobin was observed and it was found corn silk exhibits more inhibition as compare to insulin	Vemuri et al. (2018)
17	Ethanolic extract of corn silk was used to investigate the antioxidant capacity and inhibition property of α-amylase and α-glucosidase	Ethyl acetate and butanol fraction showed the highest antioxidant activity and reducing power and significant inhibition of α-amylase and α-glucosidase was also observed	Wang and Zhao (2019)

Fig. 2.

Anti-diabetic mechanism and their responsible phytoconstituents of corn silk [Sources: Sani (2016); Chen et al. (2013); Vemuri et al. (2018); Farsi et al. (2008); Pan et al. (2017); Ahmed et al. (2016); Chang et al. (2016)]

Corn silk increases glucose uptake, improves glucose tolerance and increases hepatic glycogen by inhibiting glucokinase

Phenolic compounds are responsible for enhancing the glucose uptake with the help of mediators, the insulin-signalling pathways and also reduce the intestinal absorption of glucose, regenerate β-cells and act on adipose cells which further stimulate insulin activity. In vitro study of peripheral uptake of glucose was estimated by Ghada et al. (2014) where they analysed the effect of the methanolic extract of corn silk on glucose uptake by isolated rat hemi-diaphragm and reported the enhancement in glucose uptake (100 ± 1 mg/g/30 min) in comparison to insulin (49 ± 8 mg/g/30 min). They reported that the methanolic extract of corn silk significantly enhanced the uptake of glucose and was found to be more effective than insulin. Similarly, another hypoglycaemic effect of corn silk is shown by increasing the hepatic glycogen level as depicted by Guo et al. (2009). The blood glucose concentration in mammals is maintained by glycogen stored in the liver also known as hepatic glycogen. Corn silk extract has been reported to increase the liver glycogen in alloxan-induced mice by 17.0 ± 4.2 mg/g tissue in comparison to saline treated mice 14.2 ± 3.4 mg/g. They also reported a significant (p < 0.05) decline in blood glucose level after the administration of corn silk extract (11.5 ± 2.1) when compared with Xiaoke pill (Chinese hypoglycaemic drug) (13.4 ± 3.0). Corn silk inhibits glucokinase activity, hence, discouraging glycolysis and promoting glycogen formation (Guo et al. 2009).

Hypoglycaemic effects of corn silk polysaccharides were studied in streptozotocin (STZ) induced diabetes mice and different crude polysaccharides were prepared based on molecular weight (PCS1, PCS2 and PCS 3) (Pan et al. 2017). They analysed antioxidant activity, α-amylase inhibitory activity and physico-chemical properties. They reported that PCS2 with total neutral polysaccharide content of 50.1 ± 4.1% showed the highest DPPH scavenging activity and α-amylase inhibitory activity in comparison to PCS1 and PCS3. The insulin levels also decreased significantly (p < 0.01) in STZ induced diabetic mice, which might contribute to the increment in insulin sensitivity. They stated that polysaccharide of corn silk of molecular weight of 45.5 kDa has potential for the development of functional or nutraceutical food for treating Type 2 diabetes mellitus.

Corn silk possesses anti-oxidative and anti-glycation effect

The anti-oxidative and anti-diabetic efficacy of Zea mays (corn silk) along with Artocarpus heterophyllus (Raw Jackfruit), Syzygium cumini (Black plum), and Shilajit (Black asphaltum) in vitro and in vivo models were studied by Vemuri et al. (2018). They compared the phytoextracts with commercial diabetic drugs like Aminoguanidine, Insulin and Glibenclamide for controlling Streptozotocin-induced hyperglycaemia and AGEs. The results showed that corn silk extract showed 96% of anti-oxidant capacity, 85% of superoxide radical scavenging and 42% of hydroxyl radical scavenging activity. They observed that corn silk (50 µg/ml) along with the other phytoextracts decreased the glycation of haemoglobin when compared with diabetic drugs. Glycation products like methylglyoxal which are responsible for the formation of AGEs along with oxidative stress lead to cell toxicity and inhibition of these compounds could help in eliminating the metabolic-related disorders.

The antioxidant potential of corn silk was examined by Chang et al. (2016) against protein glycation and oxidative stress for the protection of β-cells. Four different fractions were prepared from an ethanolic crude extract of corn silk by thin-layer chromatography. Free radical scavenging, cell-based viability test and glycation assay were studied to report the best fraction of corn silk. At last, the β-cell function was assessed by β-cell gene expression and acute insulin secretion test. The result reported that ethyl acetate fraction (YMS-EA) was the most potent fraction as it decreases the cell viability, inhibited the cell proliferation and under hyperglycemic conditions, YMS-EA significantly reduced reactive oxygen species (ROS) levels, improved mRNA expression of insulin, glucokinase and pancreatic and duodenal homeobox (PDX-1) and enhanced glucose-stimulated insulin secretion. The dual properties of reducing oxidative stress and protein glycation are due to the presence of bioactive compounds apigenin and luteolin.

Corn silk increases insulin secretion by β-cells of the pancreas and promotes recovery of damaged pancreatic β-cells

The depletion in the secretion of insulin leads to a high level of glucose in the blood (hyperglycemia) and makes people susceptible to diabetes^. Type 1 diabetes is associated with autoimmune destruction of pancreatic β-cell mass, moreover, type 2 results from the combination of a reduction in pancreatic β-cell mass and increased insulin resistance. The cause of destruction to the pancreatic β-cell includes chemical reactions and genetic manipulation (Marrif et al. 2016).

The presence of cardiac glycoside, terpenoids, flavonoids, anthraquinones, steroids and alkaloids in corn silk imparted hypoglycaemic properties as stated by Sani (2016) in their study of cooked corn silk on alloxan-induced diabetes in albino mice. Corn silks were cooked first and then were macerated with methanol and 250 mg, 500 mg and 750 mg/kg body weight were given to albino mice induced with 150 mg/kg body weight of alloxan monohydrate mixed in 0.9% physiological saline. The results observed showed that the methanolic extract of doses 500 and 750 mg/kg body weight showed the highest anti-diabetic property by reducing the blood glucose level at 72 h and 96 h to 6.5 and 5.4 mmol/L respectively when compared with the control group administered with glibenclamide. The phytochemical screening showed the presence of saponins, steroids, flavonoids, terpenoids, alkaloids, anthraquinones which holds hypoglycemic properties and stimulate insulin secretion. They concluded that the methanolic extract of the cooked corn silk showed dose-dependent action and helped in the survival of beta cells to stimulate more insulin.

The corn silk aqueous extract also significantly decreased blood glucose levels in hyperglycemic mice induced with Streptozotocin (STZ). The mechanism for the action of anti-diabetic effect includes healing of pancreatic β-cells, increment in insulin levels, decrease in glycohaemoglobin (HbA1c) and regeneration of damaged β-cells (Parle and Dhamijia, 2013).

Corn silk inhibits the α-amylase activity and α-glucosidase activity

Alpha amylases are the calcium metalloenzymes responsible for the cleavage of large starch molecules into smaller molecules of sugar, where the role of insulin comes into action. However, with the excess of alpha-amylase activity, the blood glucose level rises which results in hyperglycaemia (Agarwal and Gupta, 2017). Similarly, alpha-glucosidase helps in the hydrolysis of oligosaccharides into glucose and other monosaccharides. The inhibitory effect of both of these enzymes produces an anti-hyperglycemic effect by decreasing extent and rate of glucose absorption. In diabetic people, these enzymes are given as oral-hypoglycemic drugs for reducing the high blood glucose levels (Rege and Chowdhary, 2014). Chen et al. (2013) studied the chemical modification in water-soluble corn silk polysaccharides (CSPS) to attain their sulfated, acetylated and carboxymethylated derivatives. The isolated CSPS were fractionated on DEAE-52 cellulose column and four fractions, including CSPS water elution fraction (CSPS-W), CSPS 0.1 M NaCl (CSPS-1), CSPS 0.2 M NaCl (CSPS-2) and CSPS 0.5 M NaCl (CSPS-3) were extracted. The fraction with high antioxidant activity named by N-CSPS was used for further study. Chemical modification in NCSPS was made to obtain sulphated (S-CSPS), acetylated (A-CSPS) and carboxymethylated derivative (C-CSPS). The antioxidant activity and α-amylase inhibitory activity was studied and the results showed that the carboxymethylated derivative (C-CSPS) showed the highest antioxidant and α-amylase inhibitory effect when compared with IC₅₀ values in comparison to other derivatives, which showed that polysaccharides of corn silk could slow down the starch digestion of food resulting in the delayed synthesis of free glucose by inhibiting the α-amylase enzyme and thus giving hypoglycaemic effect. The inhibitory, as well as antioxidant capacity of corn silk on diabetes mellitus and diabetic nephropathy, was investigated by Wang and Zhao (2011) where an ethanolic extract of corn silk was fractionated to prepare petroleum ether fraction (PCS), n-butanol fraction (BCS), water fraction (WCS) and petroleum ether fraction (PCS). The extracted result showed the highest total phenolic content (TPC) and total flavonoid content (TFC) values possessed by ECS and BCS fractions and has also displayed the strongest scavenging activity against DPPH and hydroxyl radicals in comparison to corn silk and other fractions. The inhibitory properties of α-amylase and α-glucosidase in enzymatic assays were given by ECS and BCS as anti-hyperglycemic effect and in the BSA-glucose model, these fractions also inhibited the advanced glycation end products (AGEs). The result showed that ECS showed 156.2 ± 12.5 µg/mL and BCS 164.8 ± 10.4 µg/mL alpha-amylase inhibitory activity. For the α-glucosidase inhibitory analysis, ECS and BCS showed the highest values with the IC₅₀ values as 151.2 ± 9.0 and 170.8 µg/mL respectively. The anti-diabetic nephropathy activity assay reported the significant inhibition in the production of Col IV, IL-6 and FN in high-glucose stimulated mesangial cells at 200 μg/mL.

The anti-diabetic potential of corn silk extracts was also studied for drug properties of bioactive compounds by Adewole et al. (2018). Hexane and methanol extract was prepared of corn silk by GC–MS method after drying. The enzymatic activity such as maltase-glucoamylase, alpha-glucosidase, beta-glucosidase inhibition was studied and the result portrayed that the hexane extract of corn silk was the most potent inhibitor of α-glucosidase with an IC₅₀ value of 31.5 ± 0.4 µg/mL. The calculated value in the hexane extract was positively associated with thymol and mannitol present in corn silk, which reduces the blood glucose levels. Sabiu et al. (2016) investigated corn silk extract for α-amylase and α- glucosidase inhibitory potential. The result showed that inhibitory activity was concentration-dependent with respective to the half-maximal inhibitory concentration (IC₅₀) values of 5.89 and 0.93 mg/ml. The inhibitory activity achieved due to the presence of phenols, flavonoids, saponins, alkaloids and phytosterols, which further leads to a reduction in the starch hydrolysis and increased palliated glucose range and revealed the hypoglycaemic candidature of corn silk.

Corn silk stimulates the activation of Glitazones

Glitazones (Thiazolidinediones) prescribed for the treatment of type 2 diabetes are peroxisome proliferator-activated receptors (PPAR) responsible for glucose and lipid homeostasis. Types include PPARα (Fibrates), PPARγ (glitazones) and PPARδ which function as lipid sensors. After activation, PPARγ stimulates the cellular uptake and storage of free fatty acids and glucose in the body tissues. This further increased cellular glucose uptake, which is advantageous for type 2 diabetes mellitus (Vemuri et al. 2018). Herbal extracts of 52 herbs including Capsicum frutescens (chilli) Urtica dioica (stinging nettle) and corn silk were screened for the activation of human PPAR of which corn silk was found to be one of the most active herbs in terms of drug extract ratio among others. The presence of PPARα and PPARγ in corn silk was reported by Rau et al. (2006).

Toxicity associated with corn silk

Previous studies have claimed the pharmacological properties of corn silk as being natural and traditionally used medicine. Users do not doubt the safety of corn silk, but the research associated with corn silk toxicity and safety limits is still lacking (Hasanudin et al. 2012). Some of the studies have also reported toxicity associated with corn silk. In vivo study in Wistar rats (male and female) has confirmed that corn silk is non-toxic and no histopathological and side effects were reported at a concentration of 8.0% (w/w) when consumed for 90 days. They stated that mean daily intake of approximately 9.354 and 10.308 g/day/kg of body weight for males and females respectively (Wang et al. 2011). Corn silk extract was analysed for the level of acute and subacute toxicity in mice. Corn silk consisting of high levels of maysin was administered at a dose of 0 to 2000 mg/kg of body weight recorded for the period of 14 weeks. After 4 week intervals, body weight, organ weight, water and food consumption were analysed along with urine and serum concentrations. They reported that no significant change was observed in all the parameters and the histopathological examination also showed no abnormal change after administering 500 mg/kg of corn silk extract for subacute toxicity study (Ha et al. 2018).

The subchronic toxicity of corn silk was also assessed in Wistar rats (males and females) at concentrations of 0.5, 2.0 and 8% (w/w) for 90 days. The parameters tested included overall health, body weight, haematology, organ weight, food consumption, gross microscopic appearance of tissues and blood chemistry were compared between control and test groups. The result showed that no side effects were observed and corn silk could be used as a no-observed-adverse-effect-level (NOAEL) and support the safety of corn silk for humans (Wang et al. 2011).

Corn silk extract was tested for acute toxicity, as well and the aqueous extract was orally administered to rats at a dose of 5 g/kg of body weight. The signs of acute toxicity, behavioural changes and mortality were analysed. For the sub-acute toxicity, 500, 1000 and 2000 mg/kg body weight of corn silk extract was administered to rats for 28 days and serum chemistry, lipid profile, haematology, histopathology of the liver and kidney were studied. The result of acute toxicity reported no adverse effect at the doses up to 5 g/kg of body weight, but in contrast, for the sub-acute study, a significant increase (p < 0.05) in triglycerides, low-density lipoprotein and very low-density lipoprotein were observed and the values for high-density lipoprotein significantly decreased. The levels of AST and ALT increased significantly and degenerative changes were observed in the liver at 2000 mg/kg of body weight (Ikpeazu et al. 2018).

Recommendations for the use of corn silk

As suggested by the previous studies, the non-toxic dose of corn silk extract consisting of high levels of maysin was more than 500 mg/kg of body weight in safer limits. The result obtained by Wang et al. (2011) suggested approximately 9.354 and 10.308 g/day/kg of body weight for males and females respectively. For the long term treatment period, toxic and adverse effects were shown at the doses of 1000 and 2000 mg/kg. Therefore, the corn silk extracts could be used ≥ 1000 mg/kg could be hepatotoxic so the lower ranges should be used for therapeutic uses (Ikpeazu et al. 2018).

Conclusion

Herbal modulation of chornic metabolic disorders are considered as a better way to regulate them. Corn silk is identified as a potential herb owing to its phytochemical constituents that exhibit hypoglycemic properties. Corn silk posseses diversfied mechanisms of modulating the glycaemic response. The corn silk polysaccharides and phenols have shown anti-diabetic potential comparable to the various other herbal sources of importance as per claims. Studies also claim no toxic effects of corn silk upon consumption and thus, is considered safe for any nutraceutical and function food use. The incorporation of corn silk could be a great formula in development of functional foods and can reduce the use of oral hypoglycaemic drugs and insulin pills.

Abbreviations

IDF

International diabetes federation

MEA

Middle east & Africa

APEJ

Asia pacific excluding Japan

GAE

Gallic acid equivalent

ABTS

(2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)

CS

Corn silk

NC

Non-diabetic control

DC

Diabetic control

LD

Low dose

HD

High dose

MD

Medium dose

PD

Dimethylbiguanide

GLUT4

Glucose transporter type 4

PECS

Petroleum ether corn silk

CECS

Chloroform corn silk

MECS

Methanol corn silk

AGE

Advanced glycation end PRODUCTS

HFD-STZ

High fat diet streptozotocin

POCS

Corn silk polysaccharides

DPPH

2,2-Diphenyl-1-picrylhydrazyl.

YMS-EA

Ethyl acetate fraction

ROS

Reactive oxygen species

PDX-1

Pancreatic and duodenal homeobox

PCS

Petroleum ether fraction

BCS

n-Butanol fraction

TPC

Total phenolic content

GC-MS

Gas chromatography mass spectrometry

PPAR

Peroxisome proliferator-activated receptors

NOAEL

No-observed-adverse-effect-level

AST

Aspartate aminotransferase

ALT

Alanine transaminase

Author’s contributions

PR planned and supervised the manuscript, JS conceptualized and developed the manuscript, SKur co-supervised, reviewed and illustrated the manuscript and VN co-supervised, interpreted the data and proof read the manuscript.

Funding

The authors declare no funding supports was acquired from any source for this work.

Availability of data and material

All the required data is given in the manuscript.

Code availability

Not applicable.

Declarations

Conflict of interest

The authors declare that there are no conflicts of interest.

Ethical approval

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Consent for publication

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