Garlic (Allium sativum) is an ancient civilised plant, originated from the Asian continent between the Mediterranean and China over 600 years ago. Humans use garlic as a medicinal herb in food as well as to relieve from pain and physical and emotional stress. Currently, people are looking for alternative natural medicine. The utilisation of herbs and remedies has brought astounding outcomes to people, and garlic has many medical applications. Garlic products are used as medicine in several ways in day-to-day life activities. Chronic diseases can be treated with very popular health foods associated with natural plant origin. Currently, chronic diseases are the major threat to human health and economic growth, which estimates US$47 trillion for the five major chronic diseases which includes mental illness, diabetes, cancer, chronic respiratory diseases and cardiovascular diseases. The abundance of chemical compounds such as sulphur in garlic has proven its beneficial effects against various diseases. Garlic has the possible positive effect in treating fungal skin infections such as jock itch, ringworm or athlete’s foot.
People consume a capsule of garlic worldwide as it has an antimicrobial effect and can also cure high cholesterol, thrombosis, hyperlipidaemia and Alzheimer’s diseases. Moreover, in India it is used as a galactagogue to enhance lactation in humans. It contains antioxidants that help to destroy the free radicals. Many people use crushed garlic mixed with coconut oil to prevent hair loss and it can promote the regeneration of hair growth. Garlic has been consumed in multiple aspects, as cooked or raw. Garlic has been included in food dishes, which include soups, chutney, salads, fish, and meat curry. The plants of allium have powerful antioxidants, sulphur and phenolic compounds, which serve as the most attractive quality in the food industry. The chemical compounds in garlic have proven its beneficial effects against various diseases including infections, snake bites, hypertension, blood fibrinolytic activity andhyperlipidaemia. Many clinical studies revealed the post effectiveness of garlic towards lowering the cholesterol formation rate, inhibition of enzymes affecting lipid synthesis and angiotensin-converting enzymes. Garlic minimizes the parameters like the low-density lipoprotein (LDL) oxidation rate and reduces the platelet aggregation resulting in decreased blood pressure as it controls the cardiac arrest of a heart patient [1
]. The black fresh garlic can raise the immune response with reduced side effects under high temperatures and humidity. Black garlic (BG) was widely used against the treatment of diabetes through the proliferation of monocytes and granulocytes and reduces lymphocytes production rates. BG has the potential beneficial effects on allergic disorders [2
]. For many years it is used to treat the diseases like the common cold, illnesses, chronic respiratory diseases, sexually transmitted diseases, wound infection, malaria, cough, mental illness, lung tuberculosis, kidney diseases, liver diseases, asthma, chronic respiratory diseases, and cardiovascular diseases [3
]. It has the ability to kill parasites, bacteria, and fungi as well as to protect the liver. Diallyl trisulfide (DATS) from garlic has unique medicinal therapeutic uses, as it inhibits the proliferation of cancerous cell growth through apoptosis and cell cycle arrest [4
]. It provides cardiovascular protection, which lowers the cholesterol and blood pressure level. It gives protection against atherosclerosis and helps to decrease the levels of serum glucose, triglycerides, and uric acid as well as insulin resistance and reduces cytokine levels [5
Generally, garlic can enhance the immune system and exhibits anticancer and chemopreventive activities. It contains active antioxidants involved in cardioprotective and neuroprotective actions. Many preclinical animal model studies showed that the compounds of organosulphur from garlic can inhibit the growth of transplanted as well as spontaneous cancers without any adverse side effects. In addition to this, garlic can greatly act as antioxidant, apoptotic, anti-inflammatory, immunomodulatory, hepatoprotective, anti-invasive and chemopreventive agents [6
]. Garlic has proven to exhibit antiviral action against Influenza A and B, cytomegalovirus, rhinovirus, HIV, herpes simplex virus 1 and 2, and rotavirus [2
]. The organosulphur compounds from garlic demonstrate an antimicrobial effect. Even though numerous medicinal plants exist, garlic has a specific antimicrobial property to protect the host from pathogens [9
]. Garlic is more effective with fewer side effects compared to available antibiotics in the market. Though several technical advancements have been achieved in the field of medicine, several diseases remain incurable and fatal for the human race.
Hence, identification and understanding of molecular mechanisms of various diseases are essential to develop novel treatment strategies. In this connection, we have reviewed the therapeutic potential of garlic and its compounds in various fatal disorders. In the review, we focus on various water-soluble and oil-soluble sulphur based phytochemical compounds present in garlic. These sulphur compounds have a key role to play on the pharmaceutical mechanism in treating health conditions. Here, we have highlighted the mechanism of action of various sulphur ingredients in treating cancer, cardiovascular diseases, obesity, immune disorders and obesity. The review will revisit the interesting studies reported on garlic phytochemicals and their pharmaceutical activity.
2. Types of Active Compounds
The garlic bulb contains approximately 65% water, 28% carbohydrate, 2% protein, 1.2% amino acids, 1.5% fibres, fatty acids, sulphur compounds, phenols and various other vitamins, iodine, chlorine and various minerals (Figure 1
]. The cell cytoplasm of garlic contains S-2-propenyl-l
-cysteine S-oxide (alliin). When the garlic was squashed, a catalytic enzyme of alliinase will be released from the vacuole of the cell, which reacts with alliin to form a compound called allicin [13
The dry weight of garlic contains fructose, sulphur, protein, fibres and amino acids of lysine, arginine, histidine, aspartic acid, threonine, glutamine, proline, glycine, cysteine, alanine, valine, methionine, isoleucine, leucine, tryptophan, and phenylalanine. In addition, it contains calcium, protein, iron, zinc, germanium, niacin, riboflavin, carbohydrates, ascorbic acids, folic acids, iodine, pyridoxine, fat, sodium, potassium, saponins, selenium, phosphorous, Vitamin A, Vitamin C, B complex Vitamins, phenols, and manganese. Allyl mercaptan, dipropyl disulphide and allyl methyl sulphide are the water-insoluble components present in garlic. The intact garlic bulbs contain a high amount of γ-glutamylcysteine. Sulphur compounds have a wide range of pharmacological properties and play a key role as an active pharmaceutical component. Alliin, S-allylcysteine sulphoxide, S-allylcysteine and S-allymercapto cysteine are the water-soluble sulphur compounds whereas allicin, diallyl trisulphide, dipropyl sulphide and diallyl disulphide are the oil-soluble sulphur compounds present in garlic [15
4. Garlic Oil, Powder and Its Biological Uses
Garlic powder is mainly used as a flavouring agent for condiments and processed foods. Garlic cloves are sliced or crushed, dried, and ground into powder. The composition of garlic powder is the same as that of raw garlic; however, the proportion and amount of various constituents differ significantly, whereby, the average content of alliin present in garlic is 0.8% but the raw garlic contains around 3.7 mg/gm of alliin. Crushed garlic bulb contains antimicrobial components like allicin and thiosulphate that act against Burkholderia cepacia
, a life-threatening human pathogen [42
]. In addition to fresh garlic, there are mainly four types of commercially available garlic products prepared by a variety of processing methods, including aged garlic extract, garlic oil, garlic oil macerate dehydrated and garlic powder. The medicinal uses of garlic available in the market are in the form of pills, oil and powder. Garlic oil used for therapeutic purposes is widely obtained by the steam-distillation process. Steam-distilled garlic oil contains allylmethyl, diallyl and dimethyl mono to hexa sulfide (Figure 2
). The water-soluble commodities are removed from the oil. Each glove of garlic can have 0.20–0.5% of oil and contains numerous sulphide groups of DAS and DATS. The water-soluble compounds like allicin were completely removed from the garlic oil. Oil macerate products are produced usingraw garlic cloves ground in vegetable oil and are packaged in soft gel capsules. This formation includes allicin-decomposed components like sulphides, ajoene, dithiins, and residual amounts of alliin and other constituents in garlic [43
Herbs are nature’s endowments to us. The usage of garlic in cultural and traditional settings had a wide range of medicinal value, which may differ from concepts accepted in current western medicine. Though garlic consumption is highly beneficial, long-term and large trials are necessary to evaluate the serious adverse effects it may pose. Consumption of raw garlic directly causes the sensation of burning in the mouth and throat or stomach, heartburn, diarrhoea, nausea and vomiting. Consumption of garlic products may cause the reddening of the gastric mucous layer and also has a chance of having hyphaemia leading to permanent vision loss. Sometimes it might increase the risk of bleeding including nosebleeds and bleeding gums. In some individuals, consumption of excess garlic may also cause GERD (gastroesophageal reflux disease) and loss of appetite. Overdose of garlic may lead to kidney haematoma and had a chance of getting an autoimmune disorder of pemphigus. The safe dose of garlic consumption should be tested particularly in pregnant or breastfeeding women and also in young children. During the second and third trimesters of pregnancy, it is advisable to take a moderate level of garlic cloves to lower blood pressure and maintain good development of the baby, however consuming large quantities of garlic during pregnancy may lead to some adverse effect such as blood thinning and miscarriage. The U.S Food and Drug Administration, certified garlic as “generally recognized as safe” (GRAS) for consuming garlic as a flavouring agent as well as food including during lactation. Some reports stated that working with garlic could cause side effects like asthma and possible allergic reactions. may lead to miscarriages. However, it is advisable to consult the well-trained practitioner for using the herbal supplements.