The Therapeutic Benefits of Essential Oils

Since ancient times, essential oils are recognized for their medicinal value and they are very interesting and powerful natural plant products. They continue to be of paramount importance until the present day. Essential oils have been used as perfumes, flavors for foods and beverages, or to heal both body and mind for thousands of years (Baris et al., 2006; Margaris et al., 1982; Tisserand, 1997; Wei & Shibamoto 2010). Record findings in Mesopotamia, China, India, Persia and ancient Egypt show their uses for many treatments in various forms. For example, in the ancient Egypt, the population extracted oils by infusion. Later; Greeks and Romans used distillation and thus gave aromatic plants an additional value. With the advent of Islamic civilization, extraction techniques have been further refined. In the era of the Renaissance, Europeans have taken over the task and with the development of science the composition and the nature of essential oils have been well established and studied (Burt, 2004; Peeyush et al., 2011; Steven, 2010; Suaib et al., 2007). Nowadays, peppermint, lavender, geranium, eucalyptus, rose, bergamot, sandalwood and chamomile essential oils are the most frequently traded ones.


Introduction
Since ancient times, essential oils are recognized for their medicinal value and they are very interesting and powerful natural plant products. They continue to be of paramount importance until the present day. Essential oils have been used as perfumes, flavors for foods and beverages, or to heal both body and mind for thousands of years (Baris et al., 2006;Margaris et al., 1982;Tisserand, 1997;Wei & Shibamoto 2010). Record findings in Mesopotamia, China, India, Persia and ancient Egypt show their uses for many treatments in various forms. For example, in the ancient Egypt, the population extracted oils by infusion. Later; Greeks and Romans used distillation and thus gave aromatic plants an additional value. With the advent of Islamic civilization, extraction techniques have been further refined. In the era of the Renaissance, Europeans have taken over the task and with the development of science the composition and the nature of essential oils have been well established and studied (Burt, 2004;Peeyush et al., 2011;Steven, 2010;Suaib et al., 2007). Nowadays, peppermint, lavender, geranium, eucalyptus, rose, bergamot, sandalwood and chamomile essential oils are the most frequently traded ones.

Definition and localization of essential oils
Essential oils (also called volatile or ethereal oils, because they evaporate when exposed to heat in contrast to fixed oils) are odorous and volatile compounds found only in 10% of the plant kingdom and are stored in plants in special brittle secretory structures, such as glands, secretory hairs, secretory ducts, secretory cavities or resin ducts (Ahmadi et al., 2002;Bezić et al., 2009;Ciccarelli et al., 2008;Gershenzon et al., 1994;Liolios et al., 2010;Morone-Fortunato et al., 2010;Sangwan et al., 2001;Wagner et al., 1996). The total essential oil content of plants is generally very low and rarely exceeds 1% (Bowles, 2003), but in some cases, for example clove (Syzygium aromaticum) and nutmeg (Myristica fragrans), it reaches more than 10%. Essential oils are hydrophobic, are soluble in alcohol, non polar or weakly polar solvents, waxes and oils, but only slightly soluble in water and most are colourless or pale yellow, with exception of the blue essential oil of chamomile (Matricaria chamomilla) and most are liquid and of lower density than water (sassafras, vetiver, cinnamon and clove essential oils being exceptions) (Gupta et al., 2010;Martín et al., 2010). Due to their molecular structures (presence of olefenic double bonds and functional groups such as hydroxyl, aldehyde, ester); essential oils are readily oxidizable by light, heat and air (Skold et al., 2006;Skold et al., 2008). Some examples of oxidations are illustrated in figure 1.

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Extraction of essential oils
Oils contained within plant cells are liberated through heat and pressure from various parts of the plant matter; for example, the leaves, flowers, fruit, grass, roots, wood, bark, gums and blossom. The extraction of essential oils from plant material can be achieved by various methods, of which hydro-distillation, steam and steam/water distillation are the most common method of extraction (Bowles, 2003;Margaris et al., 1982;Surburg & Panten, 2006). Other methods include solvent extraction, aqueous infusion, cold or hot pressing, effleurage,

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The Therapeutic Benefits of Essential Oils The major volatile constituents are hydrocarbons (e.g. pinene, limonene, bisabolene), alcohols (e.g. linalol, santalol), acids (e.g. benzoic acid, geranic acid), aldehydes (e.g. citral), cyclic aldehydes (e.g. cuminal), ketones (e.g. camphor), lactones (e.g. bergaptene), phenols (e.g. eugenol), phenolic ethers (e.g. anethole), oxides (e.g. 1,8 cineole) and esters (e.g. geranyl acetate) (Deans, 1992). All these compounds may be classified into two main categories: terpenoids and phenylpropanoids Griffin et al., 1999;Lis-Balchin, 1997;Sangwan et al., 2001) or also into hydrocarbons and oxygenated compounds (Akhila, 2006;Halm, 2008;Hunter, 2009;Margaris et al. 1982;Pourmortazavi and Hajimirsadeghi, 2007;. This latter classification seems less complex, and for the current book chapter, we have adopted it. The fragrance and chemical composition of essential oils can vary according to the geo-climatic location and growing conditions (soil type, climate, altitude and amount of water available), season (for example before or after flowering), and time of day when harvesting is achieved, etc Deans et al., 1992;Margaris et al., 1982;Pengelly, 2004;Sangwan et al., 2001). In addition, there is another important factor that influences the chemical composition of essential oils, namely the genetic composition of the plant. Therefore, all these biotope factors (genetic and epigenetic) influence the biochemical synthesis of essential oils in a given plant. Thus, the same species of plant can produce a similar essential oil, however with different chemical composition, resulting in different therapeutic activities. These variations in chemical composition led to the notion of chemotypes. The chemotype is generally defined as a distinct population within the same species (plant or microorganism) that produces different chemical profiles for a particular class of secondary metabolites. Some examples of various chemotypes are given in

Trade of essential oils
The knowledge of composition of essential oils and their therapeutic properties have contributed to the development of their cultivation and markets. Although only 100 species are well known for their essential oils, there are over 2000 plant species distributed over 60 families such as Lamiaceae, Umbelliferae and Compositae which can biosynthesize essential oils. They are about 3,000 essential oils, out of which approximately 300 are commercially important and are traded in the world market (Baylac and Racine, 2003;Burt, 2004;Delamare et al., 2007;Sivropoulou et al., 1995;1996;. Essential oils constitute a major group of agro-based industrial products and they find applications in various types of industries, such as food products, drinks, perfumes, pharmaceuticals and cosmetics (Anwar et al., 2009a;2009b;Burt, 2004;Celiktas et al., 2007;Hammer et al., 2008;Hay & Svoboda, 1993;Hussain et al., 2008;Teixeira da Silva, 2004).
The world production and consumption of essential oils is increasing very fast (Lawless, 1995). Despite their high costs (due to the large quantity of plant material required), essential oil production has been increasing. The estimates of world production of essential oils vary from 40,000 to 60,000 tonnes per annum and represent a market of approximately 700 million US $ (Verlet, 1994).
The predominately produced essential oils for industry purposes are from orange, cornmint, eucalyptus, citronella, peppermint, and lemon (Hunter, 2009) but the more commonly domestically used ones include lavender, chamomile, peppermint, tea tree oil, eucalyptus, geranium, jasmine, rose, lemon, orange, rosemary, frankincense, and sandalwood. The countries that dominate the essential oils market worldwide are Brazil, China, USA, Indonesia, India and Mexico. The major consumers are the USA, EU (especially Germany, United Kingdom and France) and Japan.

Bioavailability of essential oils
The term bioavailability, one of the principal pharmacokinetic properties of drugs, is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation and can be used for a specific function and/or stored. By definition, when a drug is administered intravenously, its bioavailability is 100%. However, when a drug is administered via other routes (such as oral), it has to pass absorption and metabolic barriers, before it reaches the general circulation system, and its bioavailability is prone to decrease (due to gastro-intestinal metabolism, incomplete absorption or first-pass metabolism). Bioavailability is measured by pharmacokinetic analysis of blood samples taken from the systemic circulation and reflects the fraction of the drug reaching the systemic circulation. If a compound is poorly absorbed or extensively metabolised beforehand, only a limited fraction of the dose administered will reach the systemic circulation. Thus, in order to achieve a high bioavailability, the compound must be of sufficiently high absorption and of low renal clearance (measurement of the renal or other organ excretion ability).
Various factors can affect bioavailability such as biochemical, physiological, physicochemical interactions; habitual mix of the diet; individual characteristics (life-stage and life-style) as well as the genotype. In the case of essential oils, the comprehension of their bioavailability by studying their absorption, distribution, metabolism and excretion in www.intechopen.com The Therapeutic Benefits of Essential Oils 159 the human body is necessary. Unfortunately, there exists only limited data on the bioavailability of essential oils, and most studies are based on animal models.
All findings confirm that most essential oils are rapidly absorbed after dermal, oral, or pulmonary administration and cross the blood-brain barrier and interact with receptors in the central nervous system, and then affect relevant biological functions such as relaxation, sleep, digestion etc. .....
Most essential oil components are metabolized and either eliminated by the kidneys in the form of polar compounds following limited phase I enzyme metabolism by conjugation with glucuronate or sulfate, or exhaled via the lungs as CO 2 . For example, after oral administration of (-)-menthol, 35% of the original menthol content was excreted renally as menthol glucuronide (Bronaugh et al., 1990;Buchbauer, 1993;Hotchkiss et al., 1990;Jirovetz et al., 1992;Kohlert et al., 2000).
The same happens with thymol, carvacrol, limonene and eugenol. After their oral administration, sulphate and glucuronide forms have been detected in urine and in plasma, respectively Guénette et al., 2007;Michiels et al., 2008). The fast metabolism and short half-life of active compounds has led to the belief that there is a minimum risk of accumulation in body tissues (Kohlert et al., 2002).

Therapeutic benefits of essential oils
The feeding with aromatic herbs, spices and some dietary supplements can supply the body with essential oils. There are a lot of specific dietary sources of essential oils, such as example orange and citrus peel, caraway, dill; cherry, spearmint, caraway, spearmint, black pepper and lemongrass. Thus, human exposure to essential oils through the diet or environment is widespread. However, only little information is available on the estimation of essential oil intake. In most cases, essential oils can be absorbed from the food matrix or as pure products and cross the blood brain barrier easily. This later property is due to the lipophilic character of volatile compounds and their small size.
The action of essential oils begins by entering the human body via three possible different ways including direct absorption through inhalation, ingestion or diffusion through the skin tissue.

Absorption through the skin
Essential oil compounds are fat soluble, and thus they have the ability to permeate the membranes of the skin before being captured by the micro-circulation and drained into the systemic circulation, which reaches all targets organs (Adorjan & Buchbauer, 2010; Baser & Buchbauer, 2010).

Inhalation
Another way by which essential oils enter the body is inhalation. Due to their volatility, they can be inhaled easily through the respiratory tract and lungs, which can distribute them into the bloodstream (Margaris et al., 1982;Moss et al, 2003). In general, the respiratory tract offers the most rapid way of entry followed by the dermal pathway.

Ingestion
Oral ingestion of essential oils needs attention due to the potential toxicity of some oils. Ingested essential oil compounds and/or their metabolites may then be absorbed and delivered to the rest of the body by the bloodstream and then distributed to parts of the body. Once essential oil molecules are in body, they interrelate with physiological functions by three distinct modes of action: -Biochemical (pharmacological): Interacting in the bloodstream and interacting chemically with hormones and enzymes such as farnesene. -Physiological: By acting (for example phytohormones) on specific physiological function. For example, the essential oil of fennel contains a form of estrogen-like compounds that may be effective for female problems such as lactation and menstruation. -Psychological: by inhalation, the olfactory area of the brain (limbic system) undergoes an action triggered by the essential oil molecules and then, chemical and neurotransmitter messengers provide changes in the mental and emotional behavior of the person (Buchbauer, 1993;Johnson, 2011;Shibamoto et al, 2010). Lavender and lemon essential oils are examples for their sedative and relaxant properties.
Biological activity of essential oils may be due to one of the compounds or due to the entire mixture. In the following, we present effects of different classes of compounds present in essential oils together with their major properties and we give some examples of essential oils and their potential therapeutic activities.

Esters
Esters are sweet smelling and give a pleasant smell to the oils and are very commonly found in a large number of essential oils. They include for example, linalyl acetate, geraniol acetate, eugenol acetate and bornyl acetate (Figure 3). Esters are anti-inflammatory, spasmolytic, sedative, and antifungal (Table 2).

Oxides
Oxides or cyclic ethers are the strongest odorants, and by far the most known oxide is 1,8cineole, as it is the most omnipresent one in essential oils. Other examples of oxides are bisabolone oxide, linalool oxide, sclareol oxide and ascaridole (Figure 4). Their therapeutic benefits are expectorant and stimulant of nervous system (Table 2).

Lactones
Lactones are of relatively high molecular weight and are usually found in pressed oils. Some examples of lactones are nepetalactone, bergaptene, costuslactone, dihydronepetalactone, alantrolactone, epinepetalactone, aesculatine, citroptene, and psoralen ( Figure 5). They may be used for antipyretic, sedative and hypotensive purposes, but their contraindication is allergy, especially such involving the skin (Table 2).

Alcohols
In addition to their pleasant fragrance, alcohols are the most therapeutically beneficial of essential oil components with no reported contraindications. They are antimicrobial, antiseptic, tonifying, balancing and spasmolytic ( Table 2). Examples of essential oil alcohols are linalol, menthol, borneol, santalol, nerol, citronellol and geraniol ( Figure 6).

Phenols
These aromatic components are among the most reactive, potentially toxic and irritant, especially for the skin and the mucous membranes. Their properties are similar to alcohols but more pronounced. They possess antimicrobial, rubefacient properties, stimulate the immune and nervous systems and may reduce cholesterol (Table 2). Phenols are often found as crystals at room temperature, and the most common ones are thymol, eugenol, carvacrol and chavicol (Figure 7).

Aldehydes
Aldehydes are common essential oil components that are unstable and oxidize easily. Many aldehydes are mucous membrane irritants and are skin sensitizers. They have characteristically sweet, pleasant fruity odors and are found in some of our most well known culinary herbs such as cumin and cinnamon. Therapeutically, certain aldehydes have been described as: antiviral, antimicrobial, tonic, vasodilators, hypotensive, calming, antipyretic and spasmolytic (Table 2). Common examples of aldehydes in essential oils include citral (geranial and neral), myrtenal, cuminaldehyde, citronellal, cinnamaldehyde and benzaldehyde (Figure 8).

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The Therapeutic Benefits of Essential Oils

Ketones
Ketones are not very common in the majority of essential oils; they are relatively stable molecules and are not particularly important as fragrances or flavor substances. In some cases, ketones are neurotoxic and abortifacients such as camphor and thujone (Gali-Muhtassib et al., 2000) but have some therapeutic effects. They may be mucolytic, cell regenerating; sedative, antiviral, analgesic and digestive (Table 2). Due to their stability, ketones are not easily metabolized by the liver. Common examples of ketones found in essential oils include carvone, menthone, pulegone, fenchone, camphor, thujone and verbenone (Figure 9). Fig. 9. Structures of some ketones commonly encountered in essential oils.
In Table 2; the different classes of these compounds are summarized with their bioactivities based on various biological studies cited in literature.

Mechanism of the biological activities of essential oils
So far, there is no study that can give us a clear idea and be accurate on the mode of action of the essential oils. Given the complexity of their chemical composition, everything suggests that this mode of action is complex, and it is difficult to identify the molecular pathway of action. It is very likely that each of the constituents of the essential oils has its own mechanism of action.

Antibacterial and antifungal action
Because of the variability of amounts and profiles of the components of essential oils, it is likely that their antimicrobial activity is not due to a single mechanism, but to several sites of action at the cellular level. Then, different modes of action are involved in the antimicrobial activity of essential oils.
One of the possibilities for action is the generation of irreversible damage to the membrane of bacterial cells, that induce material losses (cytoplasmic), leakage of ions, loss of energy substrate (glucose, ATP), leading directly to the lysis of bacteria (cytolysis) and therefore to its death. Another possibility of action is inhibition of production of amylase and protease which stop the toxin production, electron flow and result in coagulation of the cell content (Bakkali et al., 2008;Burt 2004;Di Pasqua et al., 2007;Hammer et al., 2008).
Antifungal actions are quite similar to those described for bacteria. However, two additional phenomena inhibiting the action of yeast are worth mentioning: the establishment of a pH gradient across the cytoplasmic membrane and the blocking of energy production of yeasts which involve the disruption of the bacterial membrane.

Antiviral activity
The complex mixture of essential oils usually shows a higher antiviral activity than individual compounds (due probably to synergism phenomena); with exception ofcaryophyllene which is the most famous antiviral compounds found in many different essential oils from different plant families. Different mechanisms of antiviral activity of different essential oils and their constituents seem to be present. The antiviral activity of the essential oil is principally due to direct virucidal effects (by denaturing viral structural proteins or glycoproteins). Proposed mechanisms suggest that essential oils interfere with the virus envelope by inhibiting specific processes in the viral replication cycle or by masking viral components, which are necessary for adsorption or entry into host cells, thus, they prevent the cell-to-cell virus diffusion (Saddi et al., 2007).

Conclusion
According to literature, we can say that the essential oils and their components have many uses, both in pharmacology and in food. In addition, they are endowed with interesting biological activities and have a therapeutic potential. For example, essential oils exhibit antimicrobial activities, antiviral activities with broad spectrum, and may be useful as natural remedies and it seems that essential oils can be used as a suitable therapy for many pathologies. In the cosmetic and in the food industry, essential oils uses are an integral part, as they may play different roles. Therefore, economic importance of essential oils is indisputable. It appears therefore imperative to preserve our natural, diverse flora and support its protection in order to keep this inexhaustible source of molecules destined for multiple targets.