by Errina Tendean 

We all know that everyone has their sweet-tooth cravings, whether mild or extreme. For certain people, they like everything to be sweet, their food or even their life. However, sometimes, too much sweetness can kill you, when consume excessively. There is a natural sweetener which is found in a plant and since about 2015 has become popular both as a plant and for its use as a sweetener. Stevia rebaudiana (Bertoni) Bertoni is currently chosen by consumers as a natural sweetening agent since people are now aware of the disadvantageous effects of artificial sweetening agents such as aspartame, saccharin, sucralose, sorbitol and xylitol. Stevia rebaudiana leaves have been used previously by Paraguayan people as a sweetener and medicine, and notably as an oral contraceptive. This article will review and analyse the available literature on Stevia rebaudiana in terms of pharmacological evidence, their chemical constituents, toxicology and safety issues. 

Introduction 

Stevia rebaudiana, also known as Eupatorium rebaudianum (Daisies Family – Asteraceae) is a herbaceous perennial (Yadav, Singh, Dhyani and Ahuja, 2011) which is native to South America, particularly Paraguay (Yadav et al., 2011). Stevioside is a molecule acting as a natural sweetener which is present in the stems and leaves of Stevia. In the early 1970s it was first used as a substitute for sugar by a Japanese consortium (Madan et al., 2010). Stevioside is a diterpene glycoside sweeter than sucrose (the common sugar) (Yadav et al., 2011). Stevia rebaudiana products exist as simple powders, as liquids or powdered extracts (Madan et al., 2010). The extract of S. rebaudiana has a non-caloric property and several reported therapeutic activities such as anti-hypersensitive (Chan et al., 1998), anti-hyperglycaemic (Jeppesen, Gregersen, Alstrup and Hermansen, 2002) and anti-cancerous (Jeppesen et al., 2002) have been demonstrated in different animal models. 

Figure 1 Stevia rebaudiana

Geographical distribution 

The genus Stevia, particularly Stevia rebaudiana, is originally from Paraguay. The sweet characteristic of Stevia has been utilised by the Paraguayan Indians for many centuries (Madan et al., 2010). Stevia rebaudiana is currently cultivated widely in South America and across Asia including Japan, Korea and China. In the beginning of 1970s, Stevia was already grown in Indonesia using seeds from Japan (Goenadi, 1983). It is also cultivated on a smaller scale in 

Canada, Mexico, United States of America, some European countries as well as Israel (Brandle and Rosa, 1992). 

Morphology 

Stevia rebaudiana has green leaves which grow in pairs (Singh and Rao, 2005). The flower heads of Stevia rebaudiana are usually white with sprouts that are initially tender and eventually harden (Fig. 2a) (Goettemoeller and Ching, 1999). The Root system of Stevia rebaudiana is superficial and with a few lateral roots (Yadav et al., 2011). The seeds of Stevia are generally small, brown and black in colour and when the seeds mature turn into black coloured fruits (achenes) which are distributed with the help of persistent pappus bristles as in case of dandelion (Fig. 2b) (Goettemoeller and Ching, 1999). 

Figure 2 (a & b): Stevia rebaudiana plant and seeds of stevia plant

Chemical constituents 

Eight diterpene glycosides with their sweetening characteristics have been identified in Stevia leaves (Yadav et al., 2011). At the initial stage, these diterpene glycosides are synthesised using the same biochemical pathway as in case of gibberellic acid, a vital plant hormone (Singh and Rao, 2005). The four primary sweeteners are dulcoside-A, rebaudioside-A, rebaudioside-C, and, most importantly, stevioside (Yadav et al., 2011). The sweet taste of these compounds is stronger than sucrose. In terms of quantities the two principal glycosides in the Stevia leaves are rebaudioside-A (2 to 4% of the dry weight) and stevioside ( 5-10% ) (Sekaran, Giridhar and Ravishankar, 2007). These two compounds are known to be the sweetest molecules among the other diterpene glycosides. 

Moreover, Stevia leaves also contain minor glycosides, for instance, dulcoside-C, rebaudioside-B, rebaudioside-D, rebaudioside-E, rebaudioside-F and steviolbioside, and other compounds such as cinnamic acid, phenylpropanoids, flavonoid glycosides, coumarins and essential oils (Matsui et al., 1996; Megeji et al., 2005; Sekaran, Giridhar and Ravishankar, 2007; Woelwer-Rieck , 2012.). 

Description of Stevioside and Rebaudioside A 

Both rebaudioside A and stevioside are the major steviol glycosides which are found in the leaves of Stevia rebaudiana. Their sweet taste is based on the structure of the diterpenoid part of the molecule The majority of the steviol glycoside products currently sold contain either rebaudioside-A (Fig.3) or stevioside (Fig.4). (Prakash et al., 2008). Both stevioside and rebaudioside A are structurally similar to each other, with a slight difference in their molecular structure where rebaudioside-A contain one more sugar moiety in comparison to stevioside (Celaya and Kolb, 2016). 

Furthermore, it is known that stevioside produces a liquorice aftertaste, whereas rebaudioside- A does not have a liquorice aftertaste (Goyal, Samsher and Goyal, 2009). The purification and filtration procedure of Stevia extract will influence the characteristic, quality, taste and concentration of the extract. Rebaudioside-A can be purified to a high degree – 99% (Goyal, Samsher and Goyal, 2009). Rebaudioside-A is costly to obtained compared to stevioside. 

 Figure 3: Molecular structure of Rebaudioside A

Figure 4: Molecular structure of Stevioside

Historical and Modern usage of Stevia 

Traditionally, in Paraguay the dried leaves of Stevia were used as a sweetener for medicines and teas, and they used to chew these leaves to fulfil their ‘sweet tooth’ (Goyal, Samsher and Goyal, 2009). Stevia leaves were used frequently in drinks with no known adverse effects. However, Stevia dried leaves could not be used in cooking as these leaves may leave a residue in drinks and can also give a green colour stain, and moreover, the dried leaves may also give unpleasant smell (Tsanava, Sardzhveladze and Kharebava, 1991). Therefore, a suitable process of removing the unpleasant aroma from Stevia dried leaves are necessary in order to remove other constituents (Tsanava, Sardzhveladze and Kharebava, 1991). 

Since artificial sweeteners were forbidden to be used in Japan, Stevia has been used as a natural sweetener substitute in beverages and food (Tateo, 1990). The Japanese have been conducting further clinical studies and discovered that Stevia is safe to be used. In addition, the crude form of Stevia is very sweet. For this reason, Stevia is used as an alternative sweetener to be utilised among phenylketonuria (PKU) and diabetics patients, and among individuals who are planning to lose weight by preventing their sugar intake in their diet as both stevioside and rebaudioside A contain no calories (Genus Jan, 2002). 

Furthermore, Stevia is known to be save for use by diabetics since it does not affect the level of blood sugar. Moreover, Stevia also does not produce an effect on the renal or neurological system when compared to artificial sweeteners (Melis, 1995)(Goyal, Samsher and Goyal, 2009). 

Stevia has anti-microbial and anti-fungal properties and Stevia can be used as herbal medicinal products for diabetic individuals and can be used in personal care products, for instance toothpastes and mouthwashes. Stevia leaf as teas are claimed to be useful for stomach-ache (Goyal, Samsher and Goyal, 2009). 

Pharmacological evidence 

• Carbohydrate metabolism

Preclinical studies have implied an advantageous effect of Stevia rebaudiana leaf extracts for controlling hypertension and hyperglycaemia. Von Schmeling and colleagues (1977) reported that Stevia leaf extracts could improve the glucose tolerance among alloxan-diabetic rabbits (von Schmeling, Carvalho and Espinoza, 1977). A study by Suzuki and co-workers (1977) also reported that powdered Stevia reduced blood sugar as well as the levels of hepatic glycogen among rats after four weeks of treatment (Suzuki, Kasai, Sumiiiara and Sugisawa, 1977; cf also the study by Hossain et al., 2011)). An intervention study described that consumption of Stevia extract among human volunteers showed a 35% drop in the blood sugar level eight hours after their consumption (Oviedo, Franciani and Moreno, 1979; Huxtable, 2002). 

These studies overall suggested that consumption of Stevia could affect the carbohydrate metabolism positively by lowering the blood sugar level among diabetic and non-diabetic individuals and this outcome has been demonstrated in animal studies as well. 

• Cardiovascular effects

The consumption of Stevia leaf extracts can lead to a reduction in the heart rate as well as the arterial blood pressure among humans (Boeckh and Humboldt, 1981). Melis (1995) reported that after 40-60 days of treatment aqueous extracts of Stevia fed to the rats produced hypotensive effects (Melis, 1995). In addition, a treatment using stevioside can reduce the content of lipid plaques. This treatment could also lead to a reduction in the lipid accumulation, and thereby lowering the blood cholesterol as well as reducing ox-LDL build up in the plaque (Geeraert et al., 2010). 

• Reproduction

Stevia was used by Paraguayan women as an oral contraceptives (Kinghorn, 2002). This usage has become important in the discussion about whether Stevia extracts have an effect on the reproductive system and this has led to numerous investigations in rats and other animals to examine the extracts’ effects on fertility. 

Planas and Kuc (1968) reported that an aqueous extract of Stevia leaf caused a reduction in the fertility among female rats (Planas and Kuc, 1968). Akashi and Yokoyama (1975) reported that there were no fertility effects observed among the rats or the rabbits fed an aqueous extract of Stevia leaves (Akashi and Yokoyama, 1975). Other researchers such as Oliveira-Filho and colleagues (1989) and Melis (1999) found that by using semi-purified or crude Stevia leaves extract, there was a reduction in the mass of male rats’ reproductive organ. No detailed microscopic or macroscopic lesions were visible (Oliveira-Filho, Uehara, Minetti and Valle, 1989; Melis, 1999). However, several researches such as Aze et al (1991), Usami et al (1995) and Toyoda et al (1997) showed that both purified rebaudioside A or stevioside did not cause side effects on female or male reproductive organs of rats (Aze et al., 1991; Usami et al., 1995; Toyoda et al., 1997). 

However, for years this had posed uncertainty regarding the long-term Stevia consumption and whether it would produce adverse effects on reproductive system of both male and female. Therefore, to be on the safe side, for pregnancy women it is recommended to avoid using Stevia overall. However, in general Stevia products are is today considered to be safe. 

• Effects on renal system

Kidney swelling is an initial indication of diabetes because of an upsurge in the capillary diameter and length and was associated with the extent of glycaemic control (Melis, 1995, 1999; Seyer-Hansen, 1977). Stevia supplementation led to a reduction of rat kidney’s weight (Shivanna, Naika, Khanum and Kaul, 2013). In conclusion, there are currently very limited pharmacological data on Stevia’s effects on the renal system, however, it is generally agreed that Stevia is safe to be consumed. 

As a note of caution, the majority of these studies were conducted in animals and it is beyond the scope of this essay to assess the quality of this studies, for example, in terms of whether therapeutically relevant doses were used. 

Toxicological studies 

The main constituents responsible for its sweetening effects in Stevia rebaudiana are stevioside and rebaudioside-A. Based on Mizushina et al., (2005), stevioside does not cause bladder cancer. Stevioside when given as a single (acute) dose did not show lethality to rodents within 14 days following administration (Aze et al., 1991). Furthermore, the same study also suggested that there is no signs of histopathological or toxicity or morphological alterations were observed in those tested rodents. 

In addition, Maki et al (2008) did a 4-week study in 100 patients with normal to low-normal blood pressure. Rebaudioside-A even at an extremely high dose of 1000 mg/day and volunteer did not significantly change diastolic blood pressure, heart rate, systolic blood pressure, mean arterial pressure or a 24-hour ambulatory response of blood pressure when contrasted with a placebo study (Maki et al., 2008). Rebaudioside A does not change resting blood pressure of glucose homeostasis among patients suffering from diabetes mellitus type 2 (Maki et al. 2008). In this study, the patients were given 1000 mg/day of rebaudioside A for 16 weeks instead of 4 weeks (Maki et al., 2008). The researchers discovered that there was no hypoglycaemia among the patients receiving rebaudioside A when compared with the placebo group. 

Rebaudioside A is closely related to stevioside which exhibit low level of toxicity. Acute administration of rebaudioside A to male mice at a dosage of 2 g.kg by weight showed no toxic results (Carakostas, Curry, Boileau and Brusick, 2008). Furthermore, no side effects were detected after rebaudioside A administration to both female and male rats at dosages of nearly 4,645 and 4,161 mg/kg bw/day in female and male rats correspondingly (Carakostas et al., 2008). Therefore, this concludes that both stevioside and rebaudioside A show little or no toxicity effects to the consumer. 

Safety of Stevia sweetness 

Uses of Stevia as a sweetener has been long known in South America and spread its popularity to Japan with no side effects have been reported. It had been reported that a 5% Stevia leaf extract solution had a strong impact in the fertility of both female and male rats causing them to be infertile (Planas and Kuc, 1968). This has resulted in a range of studies assessing this outcome. In essence the results could not be reproduced (= negative results) indicating that Stevia leaf extract did not induce infertility (Sincholle and Marcorelles, 1989; Yodyingyuad and Bunyawong, 1991). Pezzuto and colleagues (1985) suggested that steviol in metabolically activated form is mutagenic and this result has been verified by Matsui and co-workers (1996) (Pezzuto et al., 1985; Matsui et al., 1996). It is suggested that Stevia as sweeteners, leaf extract and stevioside, is safe to be consumed for human (Kinghorn and Soejarto, 1985; Kinghorn, 1992). However, the metabolically activated steviol form which is mutagenic has not been known thus far and moreover, it is unknown that if such steviol activation may occur in human (Matsui et al., 1996). 

In addition, Stevia leaves products are consumed as sugar alternatives by many people and thus, researchers were interested to conduct a study if both rebaudioside A and stevioside may have potential side effects on dental caries under long-term use. Animal studies were conducted by using rats where these rats were given a diet containing 0.5% rebaudioside A or 0.5% stevioside for 5 weeks (Das, Das and Murphy, 1992). This study showed that both the compounds did not show a potential risk of dental caries. Studies done by Suttajit et al. (1993) and Klongpanichpak et al. (1997) reported that both stevioside and rebaudioside A did not cause any mutagenicity (Suttajit, Vinitketkaummuen and Meevatee, 1993; Klongpanichpak, Temcharoen and Toskulkao, 1997). Both stevioside and rebaudioside A are non-cariogenic (Ferrazzano, Cantile, Alcidi and Coda, 2016), non-mutagenic (Gupta, Purwar, Sundaram and Rai, 2013) as well as non-toxic (Yadav et al., 2011). Therefore, Stevia is now generally considered to be save and widely used in beverages and food products. 

Conclusion 

The leaves from Stevia rebaudiana have been used widely as a sugar substitute due to its natural sweetening properties with non-caloric characteristic. This plant species is increasingly popular in both developing and developed countries in terms of crop production. Stevia rebaudiana is non-mutagenic, non-toxic, and non-cariogenic, as mentioned earlier. Several studies have shown that Stevia is safe for long-term usage. However, one of the major problem of cultivating Stevia in large-scale is that there is a lack of quality in planting (Yadav et al., 2011). The quantity and quality of stevioside and rebaudioside A is strongly depending on the quality of the seed. Therefore, further investigations on the development of seed varieties have been implemented to produce Stevia seeds with better quality and a higher quantity of the major active constituents (stevioside and rebaudioside A), better germination and their wide-range of adaptability to various climatic conditions (Yadav et al., 2011). 

Overall, high quality Stevia products can be used safely as a natural sweetening agent. 

Errina Tendean, MSc Medicinal Natural Products and Phytochemistry 2016/2017 (student), Research Cluster “Biodiversity and Medicines” / Centre for Pharmacognosy and Phytotherapy, UCL School of Pharmacy, Univ. London, 29-39 Brunswick Sq., London WC1N 1AX. 

Disclaimer 

In this essay, we do not intend to advise or recommend herbs for medicinal or health use. This information is 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). The information provided should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. 

Image credits 

Figure 1- Stevia rebaudiana plant image obtained from Gardensonline.com.au 

Figure 2a- Illustration of Stevia rebaudiana original image obtained from https://sweetgreenfields.com/our-story/about- Stevia /. Image editing and illustration: own work 

Figure 2b- Stevia seeds image obtained from energianet.gr 

Figure 3- Molecular structure of Rebaudioside A obtained from https://en.wikipedia.org/wiki/Rebaudioside_A#/media/File:Rebaudioside_A.svg 

Figure 4- Molecular structure of stevioside obtained from https://en.wikipedia.org/wiki/Steviol_glycoside#/media/File:Stevioside.svg 

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