Bioactivity and health effects of garlic essential oil: A review

Abstract Garlic (Allium sativum L.), the underground bulb of the Allium plant in the family Liliaceae, is a common and popular spice that has historically been used to prevent and treat many different diseases such as pain, deafness, diarrhea, tumors, and other healthy problems. Garlic essential oil contains a variety of organosulfur compounds, such as the most representative diallyl disulfides (DADS) and diallyl trisulfides (DATS), which have attracted great interest in medicine, food, and agriculture because of their rich biological activities. This paper reviews the research progress on the composition and bioactivities of garlic essential oil mixtures and the bioactivity of some typical monomeric sulfides in garlic essential oil. The active mechanisms of representative sulfides in garlic essential oil were analyzed, and the applications of garlic essential oil in functional food, food additives, and clinical treatment were discussed. Combined with the current research status, the limitations and development direction of garlic essential oil in the study of molecular mechanism were discussed, which is of great significance to the development of garlic essential oil as a natural and safe alternative medicine for treatment.


| INTRODUC TI ON
At present, natural products as effective therapeutic agents for human diseases are receiving extensive attention, among which plant-derived natural products are the representative (Alinezhad et al., 2013). Plant-derived natural products mainly refer to the secondary metabolites of plants, which widely exist in nature, have extremely rich types, and have many types of effective antibacterial components, such as essential oils, polyphenols, flavonoids, alkaloids, and organic acids. Because of their efficacy and a variety of antibacterial mechanisms to prevent the development of drug resistance. Essential oils are aromatic oily liquids, which are known as complex mixtures of several volatile constituents including sesquiterpenes, monoterpenes, aldehydes, alcohols, esters, and ketones. Research shows that essential oil plays an important role in the process of plants dealing with pests, fungi, and bacteria (Harkat-Madouri et al., 2015). Therefore, the use of plant essential oil for drug development is very important for the prevention and treatment of various diseases caused by pathogenic microorganisms.
Garlic (Allium sativum L.) is an underground bulb of Allium in Liliaceae. It originated in Central Asia and the Mediterranean and was introduced into Japan and South Asia in the 9th century. And now that it has been widely cultivated all over the world. Garlic is a famous dual-use plant for food and medicine. It has been widely used pharmacological activity, garlic has become popular for applications in chemoprevention and chemotherapy research. For example, in the field of food, previous studies reported that nano-phytosomes prepared from garlic essential oil can be used as food preservatives (Nazari et al., 2019). Garlic contains more than 33 different organic sulfur compounds, which are responsible for its specific flavor and pharmacological effects (De Greef et al., 2020). Previous studies have shown that 100 g of fresh garlic contains 4.4 g of protein, 0.2 g of fat, 23 g of carbohydrate, 0.7 g of crude fiber, and 1.3 g of ash (Zhang, Bai, et al., 2020;Zhang, Liu, et al., 2020). At present, many bioactive compounds have been found in garlic essential oil. In addition to organic sulfides such as DADS and DATS, garlic essential oil is also very rich in steroidal saponins, essential amino acids, phenolic compounds, saponin ligands, and other non-sulfur compounds (Amagase, 2006).
The representative volatiles in garlic essential oil are allyl sulfides such as DADS and DATS. At present, there is little review on the bioactivities of garlic essential oil. Therefore, this article mainly introduces the components, properties, and related applications of garlic essential oil. In addition, this article also systematically reviewed the antibacterial, insecticidal, antioxidant, and other biological activities of garlic essential oil and its effects on human health, such as antitumor, hypoglycemic, and anti-inflammatory ( Figure 1).

| CHEMI C AL COMP OS ITI ON OF G ARLI C E SS ENTIAL OIL AND ITS B I OSYNTHE TI C PATHWAY
Garlic essential oil contains more than 95% sulfides and mostly allyl polysulfides, of which the five most important allyl sulfides are diallyl sulfide (1.9%-9.5%), diallyl disulfide (20.8%-27.9%), diallyl trisulfide (16.8%-33.4%), allyl methyl disulfide (4.4%-8.3%), and allyl methyl trisulfide (14.5%-19.2%) (Satyal et al., 2017). It was found that a large amount of γ-glutamylcysteine in garlic bulbs was oxidized to an inactive derivate-alliin by the action of alliinase released from the vesicles during garlic crushing, which was further metabolized to the highly active thiosulfite diallyl ester and finally formed allicin. Allicin can be rapidly broken down into many sulfur-containing compounds, including DADS and DATS. Until now, there are about 68 different compounds have been identified in garlic essential oil (Table 1).
Among the essential oils of garlic, thiosulfate is a more representative class of compounds. Thiosulfate exists in all Allium plants.
These molecules are mainly converted from s-alkyl-l-cysteines-oxide in the cytoplasm. The latter generates sulfite through a catalytic reaction of a C-S lyase in the vesicles. These sulfites are usually highly reactive intermediates, which can immediately generate thiosulfite through condensation reaction in the cell (Alinezhad et al., 2013). Thiosulfates are very unstable compounds that undergo further rearrangements, resulting in a wide variety of derived sulfur compounds, which involve further transformations and remain biologically active. The most representative substances among sulfosuccinates are allyl compounds, which are also the most dominant in garlic essential oils.

| Antimicrobial activity
Since ancient times, people have been studying natural methods to prevent and treat diseases. Garlic has attracted extensive attention because of its beneficial properties and less side effects (Misharina et al., 2009).
Many plant essential oils have broad-spectrum antibacterial, antifungal and antiviral activities. In addition, plant essential oil also showed strong growth inhibition for drug-resistant strains that were F I G U R E 1 Biological activity of garlic essential oil and its effect on human health. TA B L E 1 Compounds identified in garlic essential oil.
Previous studies have shown that different solvent extracts of garlic essential oil (distilled water, methanol, and ethanol) can inhibit a variety of plant pathogens and fungi under different pH conditions (Chen et al., 2018). The water extract had the best antibacterial effect, and the antibacterial effect was the best under strong acid conditions (pH 3.0). Through scanning electron microscope observation, garlic essential oil extract may cause microbial cell death by destroying the structural integrity of the cell membrane, thus exerting its antibacterial effect. Studies have shown that the allyl sulfides in garlic essential oil exert their antibacterial effects by reacting with the sulfhydryl groups of many important metabolism-related enzymes in bacteria (Kyung, 2012). The action of allicin is mostly non-specific as it is found to inhibit urease, papain, amylase, and alcohol dehydrogenase. In the food industry, studies have found that chitosan films by incorporating garlic essential oil showed stronger growth inhibition than that of ordinary chitosan films against the food pathogens such as Escherichia coli, Staphylococcus aureus, Salmonella typhimurium, Listeria monocytogenes, and Bacillus cereus (Pranoto et al., 2015).
Further studies revealed that the allyl sulfide in garlic essential oil could regulate the quorum sensing signal molecules and extracellular virulence factors of Pseudomonas aeruginosa in vivo and in vitro by suppressing the expression of las, rhl, and pqs (Cady et al., 2012;Persson et al., 2005).
The antibacterial activity of garlic essential oil is mainly derived from its abundant allyl sulfide and its derivatives, and the number of sulfur atoms determines the strength of its antibacterial ability (Casella et al., 2012;Tsao & Yin, 2001). In addition, it has been shown that allyl sulfide in garlic essential oil can exert its antibacterial activity by modulating genes related to bacterial metabolism, membrane transport system and secretion system. For example, in Campylobacter jejuni, allyl sulfide can downregulate 14 genes related to bacterial cellular homeostasis and oxidative stress transport proteins, thus reducing the ability of bacteria to adapt to adverse environments and the ability to adapt to adverse environments (Tang et al., 2021).
It has been found that DATS in garlic essential oil can regulate genes related to the membrane transport system and secretion system in bacterial pathogens. For example, in C. jejuni cells, DATS reduces the ability of bacteria to resist environmental stress by downregulating 14 genes related to environmental balance in bacterial cells. In addition, the amount of DATS in this study can also significantly affect the integrity of bacterial cell structure. Generally, treatment with high concentration of DATS can cause significant morphological changes such as cell deformation, dissolution, and cell membrane damage (Tang et al., 2021).

F I G U R E 2
Mode of action of essential oils in bacteria.

| Antioxidant activity
It is well known that free radicals play an indispensable role in biological activities. Many endogenous and exogenous antioxidants have been successfully applied to treat cellular damage induced by NO. Although some synthetic antioxidants, such as butylhydroxyanisole and butylhydroxytoluene, are available, these synthetic antioxidants are accompanied by a range of side effects. Therefore, the recent trend in the control and treatment of diseases favors the use of natural compounds instead of synthetic ones to scavenge free radicals. The allyl sulfide that is rich in garlic essential oil has been shown to exert antioxidant effects through its ability to scavenge reactive oxygen species (ROS) (Borek, 2001). It can inhibit lipid peroxidation, reduce ischemic/reperfusion damage, and reduce oxidative stress, thereby protecting DNA from free radical-induced damage and mutations (Kim, Chun, et al., 2011;Kim, Park, et al., 2011;Shaarawy et al., 2009).
Garlic essential oil is usually obtained by distillation of It has been found that garlic essential oil can inhibit the lipid peroxidation induced by Fe NTA and reduce the level of glutathione and increase the production of H 2 O 2 . Garlic essential oil can also improve Fe-NTA-mediated renal injury by inhibiting the activities of antioxidant enzymes such as CAT, glutathione peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase, and glutathione S-transferase. In addition, gallic essential oil can ameliorate diabetes-induced oxidative stress in the rat liver model and normalize oxidative stress in alloxan-induced diabetic rates (Iqbal & Athar, 1998;Saad Abdultawab & Ayuob, 2013).
The joint regulation of garlic essential oil and fish oil against oxidation and drug metabolism system also has been studied. in liver and blood (Mirunalini et al., 2004).
Previous studies have shown that after allicin in garlic essential oil enters cells, it interacts with reducing glutathione to form S-allyl mercaptoglutathione, which has a long-term antioxidant effect (Horev-Azaria et al., 2008;Rabinkov et al., 2000). Furthermore, allicin can also exert its antioxidant effect by positively regulating the KEAP1-NRF2 regulatory pathway and reducing the transcription of cytoprotective genes (Hong et al., 2019;Li et al., 2012;Yang et al., 2020).
The antioxidant properties of garlic essential oil also have many applications in terms of drug side effects. For example, cyclophosphamide (CYP) is an alkylated anticancer drug with excellent effects. However, due to its off-target multiple organ toxicity, its clinical use usually produces certain side effects, such as testicular injury (Mohammadi et al., 2013). The results showed that garlic essential oil significantly attenuated the reduction of testicular SOD, CAT, and glutathione peroxidase (GPx) activities induced by CYP. Furthermore, histopathology has confirmed that CYP can reduce the level of glutathione, significantly increase malondialdehyde (MDA), and significantly reduce the levels of serum testosterone, follicle-stimulating hormone, and luteinizing hormone (Nafees et al., 2011). In contrast, garlic essential oil attenuates biochemical changes in the testes, increases hormone levels, reduces tissue damage, and protects the testes from CYP toxicity through its antioxidant properties. Zhang et al. did similar research. They found that garlic essential oil can inhibit Nnitrosodiethylamine-induced oxidative stress in rats by enhancing the levels of intracellular SOD, glutathione reductase, glutathione peroxidase, and glutathione-S-transferase, so as to achieve its antioxidant effect (Zhang, Zeng, et al., 2012).

| Insecticidal activity
The growth of mosquito pests, structural pests, and economic pests in the world can seriously affect the growth of crops (Singh & Pandey, 2018). In addition, some parasites also pose a great threat to human health. Due to the shortcomings of the use of synthetic pesticides, the development of new biological pesticides has always been an important problem in the modern agricultural production system (Abdelgaleil et al., 2020 (Zhang, Bai, et al., 2020;Zhang, Liu, et al., 2020).
Sidiropoulou et al. found that garlic essential oil has a certain inhibitory effect on Eimeria in bovine kidney cells in vitro. At the same time, in vivo studies show that garlic essential oil can significantly reduce the number of intestinal bacteria and oocyst output. Therefore, it is concluded that garlic essential oil can be used as a significant inhibitor of Eimeria growth and reproduction (Sidiropoulou et al., 2020).
A similar study was carried out by Asghar et al. (2020). Their study evaluated the anti-coccidial effects of garlic essential oil, ginger essential oil, and traditional anti-coccidial drug Amprolium in vivo. The results showed that Amprolium and garlic essential oil had a good control effect on coccidiosis, but ginger essential oil had no clinical effect on quail coccidiosis (Sidiropoulou et al., 2020).
In addition, Kamel et al. studied the immunomodulatory effect of garlic essential oil on Shistosomiasis mansoni in mice at different developmental stages and its effect on enhancing the host immune system. The results showed that garlic essential oil could affect the larval and mature stages of the parasite, and enhanced the immune system of the host, so as to prevent and treat the diseases caused by schistosomiasis (Kamel & El-Shinnawy, 2015).
In addition to its inhibitory effect on pests, studies have shown that allyl disulfide in garlic essential oil can also prevent pests by inhibiting the hatching of insect eggs. Muturi et al. further studied the toxicity of Garlic essential oil and asafoetida essential oil to Culex pipiens Linnaeus and its larvae when used alone or in combination. The results showed that allyl disulfide in the two essential oils showed strong egg killing and larvicidal activities, indicating that it played an important role in the overall toxicity of the two essential oils (Muturi et al., 2018).

| Medical relevance
The health benefits of garlic essential oil in the treatment of various diseases have been widely studied in animals and humans, such as anti-cancer, anti-inflammatory, anti-diabetes, anti-hyperlipidemia, liver protection, anti-fibrinolysis, and anti-platelet aggregation activities, as well as its potential role in the prevention of cardiovascular diseases. The effects of some allyl sulfides in garlic essential oil on cells are listed in Table 2.

| Antitumor activity
Cancer, also known as a malignant tumor, kills up to 7 million people worldwide every year. At present, the methods to treat cancer include surgery, radiotherapy, and chemotherapy, but most of them will lead to many side effects after treatment. Therefore In addition to its strong anticancer activity, garlic essential oil can also be used to inhibit various sequelae after cancer treatment.

Madin-Darby canine kidney cells
The early signaling effects of diallyl sulfide on renal cells loaded with Ca 2+ -sensitive dye fura-2 Diallyl sulfide induced a significant rise in [Ca 2+ ] i in Madin-Darby canine kidney renal tubular cells by stimulating both extracellular Ca 2+ influx and thapsigargin-sensitive intracellular Ca 2+ release (Chen et al., 2009) Kupffer cell in mice Explore whether the protective effect of diallyl sulfide on lipopolysaccharide /Dgalactosamine-induced acute liver injury was associated with the regulation of Kupffer cell activation The protective effects of diallyl sulfide on lipopolysaccharide/D-galactosamine induced acute liver injury may be achieved by inhibiting the activation of the Kupffer cell, thereby suppressing the secretion of various proinflammatory mediators (Li et al., 2019) Rat colonocytes The effects of DADS on histone H4 and H3 acetylation levels in vivo in colonocytes isolated from non-tumoral rat The involvement of histone acetylation in modulation of gene expression by DADS in normal rat colonocytes, which might play a role in its biological effects as well as in its anticarcinogenic properties in vivo DATS has greater stimulatory effects on drug resistance gene expression levels but cytotoxicity in vitro and in vivo is higher for DATS than that of diallyl sulfide and DADS (Lai et al., 2012) Human A549 lung tumor cells The antiproliferative effects of DATS and DADS on cultured human neoplastic (A549) and nonneoplastic (MRC-5) lung cells DATS is extremely effective in retarding the proliferation of cultured human lung tumor cells. Neoplastic cells (A549) were considerably more sensitive than nonneoplastic (MRC5) human lung cells to DATS, the response is obviously dose dependent (Sakamoto et al., 1997) Human melanoma A375 cells and basal cell carcinoma cells

Mechanism of action of diallyl sulfide, DADS and DATS on skin cancer cells DATS increased intracellular ROS generation, induced cytosolic
Ca2þ mobilization, and decreased mitochondrial membrane potential (Wang, Qin, et al., 2010; Human hepatoma (HepG2) cells Determine whether the mechanism of biological action of garlic-derived sulfur compounds in human hepatoma (HepG2) cells can be dependent on the presence of labile sulfane sulfur in their molecules DATS showed the highest biological activity in HepG2 cells. DATS did not affect the activity of sulfurtransferases and lowered sulfane sulfur level in HepG2 cells (Iciek et al., 2011) effects on primary cancer cells, anti-proliferation and cell cycle regulation on mutant cells, induction of apoptosis on tumor cells and anti-invasion during tumor cell proliferation and metastasis. Figure 3 shows the main anti-tumor mechanisms, molecular targets, and signal pathways of the active components of garlic essential oil.

| Anti-inflammatory activity
Inflammation is a kind of defense response of the body to stimulation, which is characterized by redness, swelling, heat, pain and dysfunction.
Usually, inflammation is beneficial and an automatic defense response of the human body, but sometimes, inflammation is also harmful, such as attacks on human tissues and inflammation in transparent tissues.
Similarly, excessive or persistent inflammation can lead to asthma, conjunctivitis, arthritis, and other diseases. At present, natural products have attracted researchers' interest because of their excellent antiinflammatory activity and almost no side effects.
Many previous studies have proved that the good antiinflammatory activity of garlic essential oil is also achieved through its antioxidant effect. Its main mechanism is to inhibit the enzymes involved in the production of inflammatory F I G U R E 3 (a) Major antineoplastic mechanisms of active ingredients of garlic essential oil. (b) Major molecular targets, and signaling pathways of active ingredients of garlic essential oil.
prostaglandins and thromboxanes, inhibit the activation of NFkB and the expression of pro-inflammatory cytokines and inducible NO synthase (Sener et al., 2007). In inflammatory bowel diseases, garlic essential oil exerts anti-inflammatory effects through dysregulation of IL-10 and reduction of IL-12, preventing IL-12 from binding to its receptors on T and NK cells and, thus, inhibiting IFNγ production (Hodge et al., 2002). In addition, some sulfides in garlic essential oil exerted their anti-inflammatory effects by inhibiting the production of NO, prostaglandin E2, and the expression of the pro-inflammatory cytokines tumor necrosis factor, interleukin-1b, and interleukin-6 in lipopolysaccharide-activated macrophages (Lee et al., 2012).
The anti-inflammatory effect and potential molecular mechanism of DADS in garlic essential oil on acute pancreatitis and related lung injury in mice have also been studied (Mathan Kumar & Tamizhselvi, 2020). The researchers found that DADS can significantly reduce the level of related enzymes in mouse pancreatic cells, reduce the production of H 2 S, affect tumor necrosis factor TNFα, and inhibit the infiltration of macrophages and neutrophils. In addition, DADS reduced caerulein-induced I-κB degradation and subsequent translocation of NF-κB in the pancreas and lung.
Similarly, garlic essential oil is also used to treat peptic ulcers due to its antioxidant properties. Previous studies have found that intracellular glutathione can protect gastric mucosal cells from ethanolinduced damage in vivo and in vitro (Mutoh et al., 1990). In a study

| Anti-diabetic and anti-hyperlipidemic activity
Diabetes mellitus is a metabolic disorder caused by insufficient insulin secretion and insulin antagonism, characterized by persistent hyperglycemia, which eventually leads to specific complications.
According to Ryan et al., about one-third of people with diabetes choose to use alternative medications, which they believe are effective, and garlic is the most commonly used of these alternatives (Ryan et al., 2001). However, there are few studies related to the hypoglycemic effect of the active ingredients in garlic essential oil and some researches are inconsistent. In the 1970s, Jain et al.
studied the hypoglycemic effect of aqueous extracts of garlic as well as several different organic solvent extracts on normal and alloxan-induced diabetic rabbits (Jain et al., 1973). They found all of the garlic preparations had acute hypoglycemic effects, with the action of the ether extract being competitive with that of tolbutamide, a drug used to treat diabetes. Meanwhile, Duncan et al. found in diabetic patients, garlic essential oil can improve the symptoms of hyperglycemia (Duncan, 1999). In addition, the study found that S-allylcysteine sulfoxide, the precursor of DADS in garlic essential oil, also has hypoglycemic effect, and the mode of action is similar to that of glibenclamide, a traditional hypoglycemic drug (Sheela & Augusti, 2002).
In the skeletal muscle cells of streptozotocin-induced diabetes rats, the content of glucose transporter-4 and glucose transport rate were lower, and the activity of glycogen synthase was lower, which further reduced the glucose utilization stimulated by insulin and lactate production in the cells (Muñoz et al., 1996;Oku et al., 2000).
The DATS in garlic essential oil improves glycemic control in diabetic rats through increased insulin secretion and increased insulin sensitivity (Liu et al., 2005).
In addition, it has been shown that the DADS in garlic essential oil can improve the level of phosphodiesterase 5, a negative regulator of cyclic guanosine monophosphate(cGMP), which was upregulated in adipose tissues of high-fat/high-sucrose (HF/HS) diet-fed mice (Liu et al., 2006). Also, DADS is thought to suppress the HF/HS diet-induced upregulation of fatty acid synthesis-related enzymes including sterol regulatory element-binding protein-1, fatty acid synthase, and stearoyl-CoA desaturase-1 (Bae et al., 2018).
Previous research found that garlic essential oil significantly enhanced SOD activity in liver and kidney tissue homogenates of streptozotocin-induced diabetic rats, and garlic essential oil may effectively normalize the impaired antioxidants status in streptozotocininduced diabetes (Anwar & Meki, 2003). Garlic essential oil may be used in delaying the complicated effects of diabetes like retinopathy, nephropathy, and neuropathy due to an imbalance between free radicals and antioxidant systems. In similar studies, Liu et al. found that garlic essential oil can improve the glucose utilization stimulated by insulin in vivo or in vitro, so as to synthesize glycogen in skeletal muscle, and then play its role in the treatment of streptozotocin-induced diabetes . The possible reason is that garlic essential oil increased glutathione peroxidase activity and decreased nitrate level in skeletal muscle cells of diabetes rats. and garlic essential oil has a strong inhibitory effect on the negative regulator of cyclic guanosine monophosphate (phosphodiesterase) (Bae et al., 2017). In a follow-up experiment, they found an increase in the content of cyclic guanosine acid negative regulator phosphodiesterase in the adipose tissue of mice fed an HF/HS diet, which in turn was ameliorated by DADS, which is the major organosulfur compound in garlic essential oil. They also found that the combination of green tea extract and DADS attenuated HF/HS diet-induced increase in hepatic fat and triglyceride accumulation. Finally, they concluded DADS in garlic volatile oil enhanced the anti-obesity effect of green tea extract while inhibiting sterol regulatory elementbinding protein-1 and activating peroxisome proliferator-activated receptors (Bae et al., 2018).
In addition to DADS, DATS is another major component of garlic essential oil. Diabetic rats were fed with garlic essential oil (100 mg/ kg body weight), diallyl three thioethers (40 mg/kg body weight), and corn oil every other day for three consecutive days. The effects of garlic essential oil and diallyl three thioether on blood glucose control in streptozotocin-induced diabetic rats were studied (Liu et al., 2005). The results showed that garlic essential oil and DATS improved glycemic control in diabetic rats by increasing insulin secretion and insulin sensitivity.

| The hepatoprotective effects
At present, the research on the liver protection of garlic essential oil mainly focuses on allyl sulfide. DADS and DATS are the main active components in garlic essential oil. It is reported that both of them have certain liver and kidney protective activities (Avato et al., 2000). In terms of hepatocyte toxicity, traditional therapeutic drugs acetaminophen (AAP) have certain side effects (Latchoumycandane et al., 2007). Excess AAP leads to excessive production of NAPQI, which in turn depletes GSH levels in hepatocytes, leading to subsequent hepatocyte death (Zanger & Schwab, 2013  . Furthermore, Somade OT et al. found that the protective effect of DADS in garlic essential oil against trichloromethane (CHCl 3 )induced nephrotoxicity (Somade et al., 2018).

| TOXICIT Y S TUD IE S ON G ARLI C E SS ENTIAL OIL
Garlic has been widely used in food and medicine since ancient times. Therefore, it is generally recognized that garlic, as a common food, is harmless to the human body (Brodribb, 2018). At present, there are not many in-depth studies on the toxicity of garlic essential oil. Previous studies have found that the use of excessive garlic essential oil will stimulate the eye mucosa, and then cause certain damage to the normal function of the eyes. In serious cases, it may cause adverse consequences such as blurred vision and dry eyes.
In addition, excessive consumption of garlic essential oil is likely to damage the gastrointestinal function of the human body. This is because excessive consumption of garlic essential oil will damage the red blood cells in the human body and cause strong stimulation to human gastric mucosa. If the consumption is not controlled in time, anemia or various gastrointestinal diseases will be caused.
It is found that the LD 50 of DADS in female and male mice is 130 mg/kg and 140 mg/kg, respectively (Amagase, 2006). DADS in garlic essential oil can adversely affect the signal pathways related to the growth and proliferation of nerve cells in the hippocampus.
DADS does not affect the life activities of mature nerve cells but can affect neural progenitor cells (Ji et al., 2013).
In addition, it has been shown that some of the volatile components of garlic essential oil may cause some acute toxicity, such as stomach ulcers, nausea, and vomiting (de Muniz et al., 2021).
Excessive intake of garlic oil may also cause a number of chronic diseases, such as renal hematoma. Excessive intake of garlic oil can also increase the risk of miscarriage in pregnant women. There are limited studies on the toxicity of garlic essential oil. Previous studies have speculated that the fat-soluble sulfur-containing compounds in garlic essential oil are more toxic than the water-soluble fraction, but specific evidence from experiments is still needed to verify this conclusion (Rahman, 2007).

| CON CLUS IONS
Garlic essential oil has been reported as a frequently used natural product and has several potential health benefits. In this review, composition, biological activity, effect on human health, and application in the medical field of garlic essential oil are comprehensively summarized. Pharmacological findings give some help to the industry of functional food ingredients and formulation of new drugs. In animal experiments and clinical trials, it has been proved that garlic essential oil may be considered as a new drug to prevent cancer, oxidative damage, and inflammation, protect the liver, and reduce blood sugar.
Meanwhile, garlic essential oil can inhibit or alleviate the side effects induced by some existing drug treatments. Furthermore, garlic essential oil can be used to overcome environmental pollution caused by microorganisms, control agricultural pests, and produce bioactive natural products.
Although great progress has been made in the research of garlic essential oil, there are still some problems to be studied and solved in the molecular mechanism of biological activity. For example, there are few studies on anti-neuroinflammation and the mechanism of garlic essential oil. According to previous reports, garlic essential oil has shown effective therapeutic options against several diseases like cancer, diabetes, inflammation, and hepatoprotective activity. To develop ideal therapeutic drugs, it is necessary to study the pharmacology of garlic essential oil in vivo and in vitro. Future research should explore the toxicity, clinical therapeutic effect, and clinical side effects of garlic essential oil and clarify its active mechanism at the molecular, cellular, and gene levels.

ACK N OWLED G M ENTS
None declared.

FU N D I N G I N FO R M ATI O N
The authors did not receive support from any organization for the submitted work.

CO N FLI C T O F I NTE R E S T S TATE M E NT
None declared.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no data sets were generated or analyzed during the current study.

E TH I C S S TATEM ENT
Not applicable.