Pharmacognosy and Health Benefits of Garlic

Introduction

Garlic has been used medicinally even in ancient Egypt. It was earlier used to treat conditions such as leprosy. It later gained popularity for relieving scurvy, ear aches, flatulence etc. Garlic is commonly known as stinking rose, poor man’s treacle, nectar of the god, lehsun (in Hindi), ajo (in Spanish) etc. Some of these names have arised due to the pungency of garlic obtained due to the presence of allicin which is formed on crushing or injuring garlic. Apart from its medicinal uses it is used far and wide as a spice or condiment. The sulphur compounds present in garlic are mainly responsible for the pharmacological activities of the drug. Garlic is called as the nature’s wonder drug because it has various beneficial pharmacological properties. Garlic prevents blood clotting, lowers cholesterol and blood sugar as well as boosts immunity. Garlic also has antimicrobial, antioxidant, chemoprotective properties.

Garlic

Biological Source Bulbs of Allium sativum
Family Liliaceae
Geographical Source Garlic is native to central Asia and is also cultivated in Europe, Africa, USA, India etc. It has been a staple food in the Mediterranean region for a long time.
Morphology/Macroscopical Characters
  • The bulb grows at the base of a perennial plant with a erect flowering stem that grows 2-3 ft long.
  • The bulb is made up of several outer thin protective sheaths covering the inner sheath. The inner sheath covers the swollen leaves called as cloves. The mature bulb has around 8 or more cloves in each bulb.
  • The cloves have no symmetry except for a few present in the center.

Organoleptic propertiesThe bulbs are pinkish-white in color and are odoriferous. The size of the bulb varies between 1.5- 2.5 cm.

Microscopic Characters
  • The bulbs are covered by an outer scale. The outer scale is made up of an epidermis which encloses a mesophyll (devoid of chlorophyll), a ground tissue and below it is a layer of lower epidermal cells.
  • The dry scales also contain about 2-3 layers of rectangular cells.
  • The rectangular cells may have many rhomboid crystals of calcium oxalate.
  • The epidermal cells contain parenchymatous cells connected to several rectangular cells and vascular bundles made up of alternating xylem and phloem.
  • The epidermal cells contain thick, pitted walls.
  • The lower epidermis consists of smaller rectangular – cubical cells.
Chemistry
  • Garlic bulbs are made up of numerous minerals, vitamins, carbohydrates, amino acids, volatile oils and other trace elements.
  • Amongst all the members of the Allium species, garlic is said to have to the highest sulphur content.
  • Volatile oils are present in about 0.1-0.5% concentration in  garlic. These  constitute of sulphur containing compounds like diallyldisulphide, diallyltrisulphide, methylallyltrisulphide, allyl propyl disulphide, alliin, ajoene etc.
  • When the garlic clove is crushed, alliin (S-allyl-l-cysteine sulfoxide) by the action of the enzyme allinase gets converted to 2-propene-2-sulfenic acid which in turn dimerizes to allicin (diallyl thiosulfinate).
  • Allicin is responsible for the pungent odor of crushed garlic and also for some of its pharmacological activities.
  • The other sulphur compounds alliin, allicin, diallyldisulphide etc are also responsible for the pharmacological activities of garlic.
  • Vitamins like Vit B1, Vit A, Vit C etc;
    17 amino acids including 8 essential amino acids; and
    Minerals like phosphorus, calcium, magnesium, potassium,iron, selenium, germanium etc are present.

Structures of Allin, Allicin and Ajoene

Structures of Allin, Allicin and Ajoene

Chemical Tests
Adulterants/Allied drugs/ Substitutes _
Uses It has been proved that garlic has beneficial effects on cardiovascular system, gastrointestinal system and as antimicrobial agent. Below described are the detailed uses of garlic:

  • In Hypercholesterolemia– Garlic has said to lower cholesterol levels. The proposed mechanism for this is that the diallyl disulphides and diallyl trisulphides present in garlic oil interfere with the factors normally responsible for lipid synthesis. Garlic can reduce the activity of the thiol group in the enzymes in the body. Garlic oil can also carry out oxidation of NADPH. Thus both the above activites interfere with normal lipid synthesis and blood lipid levels are reduced. These thiol containing enzymes are HMG-CoA reductase and coenzyme A which are essential enzymes for cholesterol biosynthesis.
    Other proposed mechanisms for reducing lipid levels is increased loss of bile salts in the feces. It has also been suggested that mobilization of these lipids into circulation can also reduce lipid levels.
  • As Antithrombotic agentAjoene ( 4,5,9-trithiadodeca-1,6,11-triene-9-oxide) present in garlic is considered to inhibit platelet aggregation and is the most potent antithrombotic component of garlic. Methylallyltrisulphide present also acts as an antithrombotic agent. The suggested mechanism for this is the interference with thromboxane synthesis.
    It also shows fibrinolytic properties thereby helping in clot degradation.
  • Antimicrobial properties– Garlic shows antimicrobial activity against various pathogens such as bacteria including resistant types, fungi, virus etc. It is active against both gram positive and gram negative bacteria and strains of Mycobacterium. These effects are seen due to ajoene present in garlic. Allicin also shows the antimicrobial properties by inhibiting thiol containing enzymes thereby affecting protein, DNA, RNA synthesis.
    Due to the antifungal properties, garlic has been proposed for the treatment of oral and vaginal candidiasis.
    Garlic is active against various viruses such as herpes simplex virus type I and II, Infulenza B virus, parainfluenza virus, cytomegalovirus, human rhinovirus type 2 etc.
  • Chemoprotective properties– Animal studies have shown that garlic has positive effects against hepatoxins. The reaction of allicin in garlic with the sulfhydryl groups contribute to the inhibitory effect. Sulfhydryl groups concentration is high in rapidly dividing cells.
  • Immunity– Immunity is increased due to a number of factors on consumption of garlic. Selenium and germanium present in garlic are said to be responsible for immunologic activities. Enhanced phagocytosis, increased killer cell activity, lymphocyte proliferation, increased production of cytokines and reduction of immune suppresion are the suggested mechanisms via which garlic increases immunity.
  • Antioxidant propertiesAllicin present in garlic is responsible for the increase of catalase and glutathione peroxidase enzymes which are two important antioxidant  enzymes in the body. The other sulphur compounds may also show potential antioxidant properties by inhibiting lipid peroxidation in the liver and preventing a reaction which is considered to be one of the main features of aging in liver cells.

Other Proposed Uses:

  • Blood pressure– Garlic component allicin is said to reduce blood pressure. Proposed mechanism for this is said to be the effects of allicin on the elastic properties of the blood vessels (vasodilation) through the nitric oxide system.
  • Glycemic effects/ Lowers blood glucose levels– Garlic has been proposed to reduce blood glucose levels due to the antioxidant S-allyl cysteine sulphoxide present in it. It Increases serum insulin levels by increase in insulin secretion from beta cells as demonstrated in mice.
  • Gastrointestinal disorders– Garlic may be used in managing hypermotile intestinal disorders. It has also been suggested that it can be used in treating H.pylori infections.
Other Notes (life cycle, extraction etc.) _
Adverse reactions For people allergic to garlic, exposure to it can cause contact dermatitis. On repeated exposure it can also induce asthmatic reactions. Rare symptoms such as wheezing, coughing, difficulty in breathing, running/itching nose, chest tightness may appear. Gastrointestinal symptoms such as burning mouth, mouth and throat ulcers, irritable bowel, diarrhea, nausea etc can also occur.

References:

  1. Louise Tenney MH, Garlic: Nature’s Amazing Nutritional and Medicinal Wonder Food. Woodland Publishing (1996).
  2. Sigma Aldrich – Garlic Plan Profiler. http://www.sigmaaldrich.com/life-science/nutrition-research/learning-center/plant-profiler/allium-sativum.html (accessed on 20th January, 2014).
  3. Pendbhaje, N. S.; Narang, A. P.; Pathan, S. M.; Raotole, S. A.; Pattewar, S. V.  Ethnopharmacoloy, Pharmacognosy and Phytochemical Profile of Allium Sativum L.: A Review. Pharmacologyonline 2: 845-853 (2011).
  4. Trease and Evans’ Pharmacognosy, 16e (Evans, Trease and Evans Pharmacognosy)
    Elsevier: New York, 2009.
  5. Ara DerMarderosian, et. al. The Review of Natural Products 4th Edition.
  6. Kokate, C. K.; Gokhale, S. B.; Purohit, A. P. A textbook of Pharmacognosy, 29th ed.; Nirali Prakashan: Pune, 2009.

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