Garden thyme (Thymus vulgaris L.) an aromatic shrub from the Mint family (Lamiaceae), has been used for centuries not only for culinary purposes (as a spice), but also for a wide range of therapeutic properties. Health benefits including the treatment of a cough and gastrointestinal upsets have been investigated in some detail. The shrub exhibits scientifically proven antiseptic and antioxidant properties. Thyme also may be responsible for some adverse effects, mainly dermatological and allergic side effects. The aim of this review is to provide an insight into the ethnopharmacological uses, evidence-based data on pharmacological benefits and the possible side effects or interactions of garden thyme.
Garden thyme, botanically named as Thymus vulgaris L., is one of the most popular medicinal plants cultivated for its valuable uses for both culinary and medicinal purposes. It is also commonly known as summer thyme or English thyme. The shrub is a member of the mint family (Lamiaceae). It is therefore related to peppermint (Mentha piperita L.), sage (Salvia officinalis L.), and rosemary (Rosmarinus officinalis L.). The genus Thymus includes over 400 species. Related species may be used as substitutes to garden thyme and include Spanish thyme (Thymus zygis L.), wild thyme or mother-of-thyme (Thymus serpyllum L.) and larger wild thyme (Thymus pulegioides L.)(Stahl-Biskup, & Sáez, 2003). The name ‘Thymus’ has been linked to several possible etymological roots. The most common are ancient Greek words including thymiama meaning incense (Sertel, Eichhorn, Plinkert, & Efferth, 2017), perfume (thyo) and strength or courage (thymos) (Stahl-Biskup, & Sáez, 2003).
Garden thyme is a small aromatic evergreen shrub that is native to the Mediterranean countries such as Spain and Morocco. It is however cultivated across Europe due to its well-known benefits.
Historical and Modern Uses
Garden thyme has been a popular medicinal plant for many centuries. Egyptians are said to have used it for the preservation of deceased human bodies. The Romans and the Greeks have used it as a sign of courage, soldiers bathed in thyme preparations before heading out to battles to boost bravery. The Romans also used this shrub as a flavouring in foods such as cheese. The Greeks associated the shrub with elegance; it was used as incense in temples, during massages and as an aphrodisiac (Ambrose, Manickavasagan, & Naik, 2016). During the Middle Ages, the Europeans used this shrub for the treatment of a cough, as a gastrointestinal remedy and for the prevention of nightmares. Traditionally, apart from culinary purposes, the British herbalists have also used the shrub, thyme, as an antiseptic and for the treatment of coughs and respiratory conditions (Boon, & Smith, 2004). It has been used continuously as an important medicinal and aromatic plant throughout the Mediterranean, e.g. on Cyprus (Lardos and Heinrich 2013).
Garden thyme (Figures 1 and 2 above) is a small aromatic shrub with numerous hairy stems from a single woody stem with fibrous roots. The shrub is usually up to 8 inches or 20cm high. The plant has short, pointed, broad greenish-grey leaves of about 6mm long, and 0.5mm to 2mm broad. The leaves are usually oblong-oval or lanceolate in shape with recurved margins and flowers around the times of early summer (Peter, 2004). It has pink to lilac flowers growing in clusters which are very appealing to bees. The shrub also has a distinct spicy aromatic smell and a pleasant taste (Breverton,2011). It grows in the Medicinal Herb Spiral at the Mecklenburgh Square Garden.
Medicinal Plant Parts Used
The aerial parts of the shrub which consists of the leaves and the flowers are used as dried shrubs or as a source of volatile oils (Boon, & Smith, 2004).
Garden thyme is ingested or used topically in the forms of dried leaves, infusions, tinctures, oleoresins, syrups or the essential oil obtained through distillation is used. (Boon, & Smith, 2004)
Cultivating Garden Thyme
Thyme is grown from seeds or from cuttings of fast growing (green) shoots. The shrub desires light and dry calcareous alkaline soil. It is advisable to add agricultural lime to acidic soils of pH less than 5.5 during propagation to increase the alkalinity of the soil for optimum growth. Garden thyme grows in versatile climates. The ideal annual temperature for optimum growth is preferably between 7˚C to 20 ˚C. Temperature is particularly important for the maintenance of the quality of thyme after harvest. The best temperature for the harvest of garden thyme was reported to be 0 ˚C, this provides a shelf life of 3 to 4 weeks. At 5 ˚C the shelf life is expected to be two to three weeks. Therefore, to store thyme for a longer shelf life, the cooling of fresh thyme is recommended. Nitrogen fertilizers have been reported to improve the yield of garden thyme. Reports have associated the cultivation of thyme with fungal infections, leaf and root diseases, and spider mites (Peter, 2004).
Contemporary, Commercial Uses
Today, thyme is commercially grown for attaining thyme oil, thyme extracts, dried thyme leaves and oleoresins. Studies have shown the shrub contains properties for relieving flatulence, an antiseptic, it is used orally and topically for parasitic and fungal infections. It is also a sputum (a mixture of saliva and mucus) enhancer which aids in treating and preventing coughs. The shrub acts as a muscle relaxant used to treat gastrointestinal discomfort, including menstrual cramps. (Baranauskienė, Venskutonis, Viškelis, & Dambrauskienė, 2003) (Boon, & Smith, 2004).
The Chemistry of Thyme
Garden thyme is mainly made up of volatile oils (about 1 to 2.5% of the total dried thyme), phenylpropanoids (eugenol, caffeic acid, rosmarinic acid), triterpenes (ursolic and oleanolic acid derivatives) and flavonoids (Boon, & Smith, 2004). The main constituents responsible for the shrub’s therapeutic properties are the volatile oil (thymol and carvacrol are main active components of the oil) and flavonoids (Boon, & Smith, 2004).
Volatile Oil of Garden Thyme
The volatile oil, also known as the essential oil, is responsible for the spicy aroma of the thyme plant. On hot sunny days, traces of the volatile oil can penetrate through cuticles of the leaves, in large fields, where the spicy aromatic smell can be perceived from a far distance (Peter, 2004).
The volatile oil content of thyme is affected by several factors including climate, geographic origin, the genetics, harvesting time, the soil type and use of fertilizers where the species is growing (Torras, Grau, López, & de las Heras, 2007) (Baranauskienė, Venskutonis, Viškelis, & Dambrauskienė, 2003). It is very important to understand that within the same species, plants can produce essential oils that are of different chemical compositions and properties. This phenomenon is defined as the chemotypes of a species (Torras, Grau, López, & de las Heras, 2007). Garden thyme, for instance, has seven reported chemotypes. The commercial thyme is mainly of the ‘thymol chemotype’.
The total volatile oil content of any given thyme is expected to range from 5 to 25 ml/kg (Bruneton,1999). The oil used medicinally may be white or red in colour. The red oil is made by redistilling the white oil. It is important to be conscious that in 2004 Boon, & Smith considered the red oil commonly to be adulterated.
Thyme volatile oil contains a wide range of compounds, the main constituents are terpenoidic phenols of which in the thymol chemotype about 72% is thymol and 5.7% is carvacrol. Linalool, a-terpineol, 1,8-cineole, and borneol are also found in relatively significant amounts (Lee, Umano, Shibamoto & Lee, 2005). The selection of a good extraction process for obtaining optimum volatile oil is important. The most suitable and widely used method is the steam distillation. It has shown to be more efficient, ensuring the stability of compounds and providing higher quality and yield of essential oils in comparison to other techniques, such as the use of organic solvents for extraction. A study was conducted to determine a favourable extraction technique using steam distillation in a Neoclevenger system and extraction by organic solvents. The steam distillation produced high quality volatile oil, while, the organic solvent method provided a mixture of the volatile oil and other undesirable constituents. However, the analysis of antioxidant potential showed that the extraction technique does not affect the antioxidant capacity of the volatile oil (Grigore, Paraschiv, Colceru-Mihul, Bubueanu, Draghici & Ichim,2010)
Pharmacological Aspects of Garden Thyme
Thyme and thymol have been shown to inhibit a wide range of bacteria (Imelouane, Amhamdi, Wathelet, Ankit, Khedid & El Bachiri, 2009; Basch E, Ulbricht, Hammerness, Bevins, & Sollars 2004) including bacteria which have developed resistance to antibiotics (Bruneton,1999). Examples of anti-bacterial studies have shown thyme to inhibit pathogenic organisms such as Salmonella, Escherichia coli, which are common disease-causing bacteria found in food, and Heliobacter pylori (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004), a type of bacteria found in the stomach known to be associated with gastric ulcers. Thymol and carvacrol found in the volatile oil are known to be responsible for this effect. Bactericidal activity is highest for thymol and carvacrol chemotypes, however, all chemotypes are active (Bruneton,1999). Fungal and yeast growth and activity have also been shown to be inhibited or affected by thymol (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004). Thymol is found in toothpaste and mouthwashes such as in, Listerine, a popular mouthwash, as an active ingredient (Vlachojannis, Al-Ahmad, Hellwig & Chrubasik, 2016).
Antispasmodic Effects and Antitussive Effects
Thymol in thyme oil has shown to be a muscle relaxant in-vivo and in-vitro. Studies carried out on the intestine’s smooth muscle and tracheas of animal models have shown positive results. It has also shown to have anti-inflammatory properties (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004).
The human body constantly creates oxidative stress, this may lead to many diseases. Antioxidants help in the prevention of these diseases. The antioxidant property of thyme is important in both the medicinal and non-medicinal context (prevention of food spoilage). Several studies carried out in-vivo and in-vitro have reported the potency of the oil as an antioxidant (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004) (Peter, 2004). Research has also shown thyme, particularly its active constituent thymol, to have potential pharmacological activities of restoring the structure and function of damaged tissues such as in wound healing (Riella, Marinho, Santos, Pereira-Filho, Cardoso, Albuquerque-Junior, & Thomazzi, 2012) and for the treatment of epilepsy (Sancheti, Shaikh, Chaudhari R, Somani, Patil, Jain & Sathaye, 2014). However, the therapeutic and preventive relevance of these effects remains poorly understood.
Garden thyme has also shown in-vivo qualities of detoxifying the liver after alcohol damages. A study carried out in mice using water extracts of thyme showed positive results of detoxifying the liver, including the phenolic antioxidant activity of compounds which may protect the liver of harm from alcohol (Shati & Elsaid,2009). It is important to remember that subspecies may differ in medicinal action, mainly due to the different proportions of constituents and such information is often not or only poorly reported in such studies.
There are several clinical studies which have been carried out on thyme, however, these studies are mainly limited for the treatment of respiratory diseases. There are no recent clinical studies on this medicinal plant. (Kohlert et al., 2002) (Gruenwald, Graubaum & Busch, 2005, Edwards et al 2015). A placebo – controlled study conducted with 771 German general physicians, where each doctor gave one group of five people a herbal drug known as Bronchipretâ, a herbal tablet containing extracts of thyme and primula, and another group a placebo. A complete data of 7783 sets were collected and analysed, results suggested Bronchipretâ is a safe and effective herbal medicine for the treatment of acute bronchitis in adults and children (Ernst, März & Sieder, 1997). Another study carried out in 2004 on 62 patients between the ages of 16 and 89, suggested that thyme alleviates cough with good efficacy and tolerability (Büechi, Vögelin, von Eiff, Ramos & Melzer, 2005).
Orally, thyme should be taken at a dose of 1g to 4g of dried drug (e.g. as an infusion) for adults and 0.5g to 1.0g for children. Tinctures for oral use are prepared at a ratio 1:5 in 45% alcohol and 2ml to 6ml should be taken three times a day. Oral tinctures of 1:10 in 70% alcohol should be taken as 40 drops up to three times daily. It is important to ensure oils are diluted in suitable carrier oils before use (Boon & Smith, 2004).
Cautions, Risks, and Possible Interactions
Although thyme has been shown to be beneficial for many conditions and safe for culinary purposes, it is important to weigh and understand the risks of using the medicinal plant. Thymol which is a phenol has been reported to be highly toxic at high doses. According to the U.S. National Library of Medicine, ingestion of up to 1g in adults or 50 to 500mg in children may be lethal (TOXNET, 2017). Studies have shown the ingestion of certain quantities of phenol by human beings results in symptoms of gastrointestinal illnesses, abdominal pain, nausea, vomiting and diarrhoea (Buyukleyla, & Rencuzogullari, 2009). It is suggested that not more than 10 grams a day of dried thyme with 0.03% phenol should be ingested to prevent toxicity (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004). . Side effects associated with thyme at exceeded doses include nausea, vomiting, headache, sweating, convulsions, digestive unease, dizziness, coma and respiratory collapse.
If an individual is allergic to other members of the mint family, it is advisable to refrain from using thyme.
Ingestion of the essential oil should be avoided since reports have shown links with nausea and respiratory arrest (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004). Studies have also reported garden thyme to be a moderate skin irritant and a strong mucous membrane irritant (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004) (Boon & Smith, 2004). Most of the reported adverse effects are dermatologic or allergic reactions (Basch, Ulbricht, Hammerness, Bevins, & Sollars, 2004). An essential oil has been shown to cause mutations and damages to genetic materials (DNA). A study carried out on thymol concluded the compound has genotoxic effects in vitro (in cell lines); therefore posing a potential risk of damaging genetic materials in humans (Buyukleyla, & Rencuzogullari, 2009). Scientific data on the safety of thyme for pregnant and lactating women is unavailable. Garden thyme has, however, been reputed as a shrub that increases blood flow to the uterus and the pelvis. It is, therefore, important for pregnant women to be cautious of using the shrub for medicinal purposes, especially in high doses (Boon & Smith, 2004) and such a use is not recommendable due to a lack of experience with this use.
These adverse effects are likely to be different between individuals as every individual will metabolize garden thyme differently.
Garden thyme has shown a vast range of uses spanning many historical periods. Several studies have been carried out in the laboratory, although limited clinically, to demonstrate the therapeutic properties of this medicinal plant, notably for the treatment of respiratory diseases. Although the shrub is safe for culinary purposes, the toxicity and risks especially at higher doses associated with the shrub are important to weigh before using it for medicinal purposes. If used as a spice or as a medicine at normal doses it is generally considered to be safe, but thymol continues to be of toxicological concern (see above). For medical uses, it is advised to attain medical advice from a medical practitioner before usage and to stop usage as soon as any adverse effect is noticed. The inexpensive and widely used shrub has a very promising future in the field of medicine.
Disclaimer: In this essay, we do not advise or recommend herbs for medicinal or health use. This information is intended for educational purposes only and should not be considered as a recommendation or an endorsement of any particular medical or health treatment. The use of any such product should be based on the appropriate advice of a health care professional or based on the information available in the patient information leaflets (i.e. for THR products).
© Halima Mohammed, MSc. Student (2016-2017), Biodiversity and Medicines, Center of Pharmacognosy and Phytotherapy. UCL School of Pharmacy, University of London,29-39 Brunswick Square, London, WC1N 1AX.
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