Rooibos Tea and Health: A Systematic Review of the Evidence from the LastTwo Decades

An expanse of research has investigated the effects of black and green teas in relation to aspects of health. Rooibos tea, also known as Red bush is derived from the South African Cape fynbos plant, Aspalathus linearis, and is caffeine free, naturally sweet and abundant in polyphenols. Evidence related to the health aspects of drinking Rooibos tea is advancing, but does not appear to have been collated. Therefore, we aimed to examine the health effects of Rooibos tea through a systematic review of the literature. A PUBMED search was undertaken (2000 up to June 2020) for human and laboratory studies investigating the efficacy of Rooibos in relation to health. Seven human studies and 49 laboratory studies were identified. Overall Rooibos tea consumption seems to benefit the lipid and redox profiles of those at risk of cardiovascular disease. It also appears to possess other promising ‘general’ effects on glycaemic control, bone, liver, cognitive and respiratory health. Ongoing research using standardised interventions is now needed to help formulate congruent conclusions that are relevant to public health.


Introduction
The pattern of health and disease is changing-continued shifts in longevity mean that multimorbidity and 'disease clusters' are now on the rise [1]. It is well appreciated that lifestyle e.g. alterations in diet, physical activity and avoiding smoking can improve outcomes for medical conditions such as cardiovascular disease [2]. There is already an extensive body of evidence showing that drinking two to three cups of tea daily could be beneficial for health, including reduced risk of cardiac death, coronary artery disease, stroke, type 2 diabetes mellitus and total mortality [3]. Beneficial inter-relationships have also been observed for several cancers, cognitive, skeletal and maternal health [3]. Most of this research has focused on green, black and oolong tea [4].
"Rooibos" is Afrikaans for "red bush" [5]. It is prepared from unfermented and fermented plant material from the Cape fynbos plant, Aspalathus linearis [6,7]. In South Africa, the proportion of black tea drinkers has declined between 2011 and 2015, from 58.6% to 51.5% whilst the percentage of Rooibos consumers has risen from 29.4% in 2011 to 30.9% in 2015. 8 The demand for Rooibos tea is also extending further afield with South Africa exporting Rooibos tea to more than 30 countries [8].
Such shifts in consumption habits are being attributed to rising awareness and interest in the health properties of Rooibos tea [8]. provided six cups of fermented, traditional Rooibos daily (one tea bag in 200ml with an infusion time of 5 minutes) for 6 weeks [21].

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Three studies reported positive findings in relation to aspects of health [21,22,25]. One study used ex vivo samples from atopic adults showing that extracts prepared from both fermented and unfermented Rooibos inhibited basophil activation, an effect that was stronger using the extract of fermented Rooibos [22]. These findings are aligned with that from earlier laboratory models [26] and indicate that Rooibos appears to possess anti-allergic effects by inhibiting antigen-and calcium ionophore-stimulated degranulation. One of the largest trials conducted on 40 participants observed significant increases in plasma total polyphenol levels, reductions in markers of lipid peroxidation, improvements in lipid profiles (low-density lipoprotein declined and high-density lipoprotein increased) and redox status [21]. Effects were observed after drinking six cups of fermented (traditional) Rooibos tea daily for 6 weeks [21]. Another trial looking into underpinning mechanisms showed that freshly prepared Rooibos tea (made with 10g tea in 400ml boiled water for 10min using a tea filter) significantly inhibited angiotensin-converting enzyme (ACE) activity, 30 and 60 minutes after ingestion, indicating possible cardiovascular effects via the inhibition of ACE activity [25].
Two human studies focused on aspects of metabolite absorption and bioavailability [14,20]. In a human crossover study where 500 ml unfermented Rooibos tea was ingested aspalathin (a dihydrochalcone C-glucoside) was found to be particularly bioavailable [20]. An earlier bioavailability trial comprised of 10 adults drinking a similar quantity of Rooibos tea also showed that most metabolites were absorbed either via the small or large intestine [14]. In two trials involving Rooibos tea ingestion, one did not show any significant effects on renal stone formation [23] and the other showed that rooibos tea and plain water similarly rehydrated 23 athletes [24].
Other research has focused on aspects of metabolic health. Some work has found associations between unfermented/green Rooibos extract and improvements in fasting blood glucose levels using type 2 diabetic mice [40]. Similarly, In vitro work showed that Rooibos extract high in aspalathin had a sustained glucose lowering effect [41]. Other work indicated that aspalathin or nothofagin flavonoids inhibit glucose-mediated vascular hyperpermeability and inflammation [42]. A further seven studies showed that Rooibos could improve insulin resistance and have antidiabetic potential [43][44][45][46][47][48][49].
With regard to potential mechanisms aspalathin found in Rooibos stimulated glucose uptake in muscle tissues and insulin secretion from pancreatic beta-cells in a type 2 diabetes mouse model [48]. Work by Ulicna O, et al. (2006) found that the antioxidant compounds in Rooibos tea prevented oxidative stress concluding that it could be a suitable adjunctive therapy for diabetic vascular conditions [49]. One study using a cell model found that a fermented Rooibos infusion prepared at 'cup-of-tea' strength and the soluble matter of the infusion Increasingly, rooibos has been studied in human populations, mainly for its antioxidant and cardio protective properties [14,20,21]. Alongside this, over the years a growing body of laboratory and mechanistic studies have investigated how rooibos tea could impact on health. Given the gaining popularity of rooibos tea, the current publication collates evidence from human and laboratory studies, published over the last two decades. Such a review does not appear to have been undertaken previously. The present review focuses on rooibos tea which has been gaining popularity both in its native province the Western Cape of South Africa and worldwide in recent years [8].

Methods
The National Centre for Biotechnology Information (NCBI) search engine (PubMed) was used to extract relevant publications. Two search phases were undertaken. In Phase 1 English-language human studies published between January 2000 (month start) and June 2020 (month start) were screened. Publications were included if they used Rooibos tea or RTE and studied a named health outcome. Studies were excluded if they were conducted with children due to potential ethical issues, focused on chemical profiling, used a multi-intervention or did not specify a distinctive health outcome. External health conditions such as skin healing were also excluded.
The search terms "Rooibos", "Red bush" or "Aspalathus linearis" were used. ED and TB identified the scientific publications. In Phase 1 the database search was restricted to human studies. In Phase 2 the search was restricted to laboratory studies the same search terms were applied. Data extracted from each human study included: (1) Study (author, year, location and reference number), (2) Subjects (age, gender, number), (3) Study design (type), (4) Tea intervention (type) (5) Intervention type (dosage) and (6) Main findings. Data extracted from each laboratory/mechanistic study included: (1) Study (author, year and reference number), (2) Study Design, (3) Intervention (type), (4) Main outcome and (5) Main findings.

Results
In Phase 1 using the applied search terms 47 human studies were identified. Of these, three were review papers, three had methods that were unclear or not fully reported, five were laboratory studies and 29 irrelevant as they did not investigate health outcomes. Subsequently, after these exclusions seven human studies were included in the final review. Of the human studies three were conducted in South Africa [21][22][23], one in the USA [24], one in Italy [14], one in Sweden [25] and one in Germany [20].
In Phase 2 91 laboratory studies were first identified. Two of these were excluded due to them being multi-interventions, two focused on external (skin) conditions, four were review papers, seven had methods that were unclear or not fully reported and 27 were irrelevant as they did not measure health outcomes. This resulted in 49 laboratory studies being included in the main paper. The algorithm of qualifying papers is shown in figure 1.

Human Studies
Seven human studies were identified using Rooibos tea preparations (Table 1) [14,[20][21][22][23][24][25]. Sample sizes ranged from eight to 40 subjects. Interventions also varied between studies. Most studies used Rooibos infusions that had been seeped for approximately 10 minutes in hot water infusions [20,23,25]. One study provided Rooibos tea (two 125 ml cups each made with two tea bags, low mineral content water and a brewing time of 5 min at 90 o C) for 30 days [23]. Other research  (Table 1) 49 studies included (  inhibited adipogenesis, also implying a potential role in obesity prevention [50]. These findings imply a potential role in glycaemic regulation.

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Other research suggests beneficial effects on aspects of bone health [51,52], hepatoprotection [31,43,44,53], allergic response [26] and immune function [54,55]. Rooibos tea has been found to improve osteoblast activity [52] while fermented Rooibos was found to inhibit osteoclasts and associated gene expression [51]. Some work has discovered that Rooibos tea could help to stabilise the liver from injury [12]. In another study Rooibos extract eased induced liver injury by suppressing oxidative stress and the formation of pro-inflammatory cytokines [31]. A laboratory model showed that Rooibos tea acted as a 'hepatoprotector' showing histological regression of liver cirrhosis and steatosis in an experimental model of liver cirrhosis [53].

Discussion
Overall there appears to be a growing body of evidence relating to Rooibos tea and various biological health outcomes. The largest body of evidence is currently derived from laboratory and mechanistic studies although human research is emerging. Research focusing on cardiovascular health looks particularly promising. The trial conducted by Marnewick JL, et al. (2011) [21] was well conducted showing that drinking six cups of fermented, traditional rooibos daily significantly improved the lipid profile and redox status which is relevant to adults at risk for developing cardiovascular disease. Elsewhere other review findings also conclude that Rooibos appears to have preventative and complementary therapeutic benefits in the context of cardiovascular disease [63].
The antioxidant properties of Rooibos tea and its ability to sequester oxidative stress are also prominent in the research [27,34,49,64]. The polyphenols aspalathin (present at >5 mg/l) and nothofagin (present at <1 mg/L) found in Rooibos tea have been attributed to some of its health benefits [65,66]. Antioxidants such as these suppress oxidative stress in the body which has been implicated in the pathophysiology of certain diseases including Alzheimer's disease [67]. Several laboratory studies concentrated on aspects of sperm function [36][37][38][39]. Fermented Rooibos, in particular, was found to improve several dimensions including sperm concentration, viability and motility [38]. These findings imply that Rooibos consumption could have a role to play in supporting male fertility, namely by sequestering oxidative damage by improving antioxidant defence mechanisms and subsequently improving the sperm quality and function [39]. Human randomised controlled trials are now needed to explore this.
Fermented Rooibos extracts have been found to inhibit human basophil activation [22]  The overall metabolite levels excreted were 82 and 352 nmol-accounting for 0.09 and 0.22% of the flavonoids in the fermented and unfermented drinksrespectively. Most of the aspalathin metabolites were excreted within 5 h of tea consumption-implying absorption in the small intestine. Aspalathin activated Adipoq and modulated the expression of Pparγ and Srebf1/2-decreasing inflammation via Il6/Jak2 pathway-which increased expression of Bcl2 preventing myocardium apoptosis.

Rooibos tea
Oxidative stress Rooibos tea appears to (i) reverse the increase in stress-related metabolites (ii) prevent lipid peroxidation-(iii) restore stress-induced protein degradation-(iv) regulate glutathione metabolism and (v) modulate changes in the activities of antioxidant enzymes.  Fermented Rooibos-'green' Rooibos-Chinese green tea-Rooibos supplement-green tea supplement or water.

Sperm function
Both Rooibos extracts offered a measure of protection against induced oxidative damage by increasing the antioxidant defence mechanisms improving the sperm quality and function.

Hypoglycaemic potential
In vivo the extract sustained a glucose lowering effect comparable to metformin over a 6h period after administration (25mg/kg body weight (BW)) to STZ-induced diabetic rats.

Bronchodilatorantispasmodic
The bronchodilator-antispasmodic and blood pressure lowering effects of Rooibos tea appear to be mediated predominantly through K (ATP) channel activation with the selective bronchodilatory effect. Open Access Journal effects [26]. This research suggests promise for conditions such as allergic rhinitis [22] but further investigations are needed. Additionally, its effects on bone health [51,52], cognitive health [56,57] and potential hepatoprotective effects [12,31,53] warrant further exploration in the form of human trials. Recently, a review of experimental evidence [68] concluded that blood glucose levels were significantly lower in diabetic rodent models treated with rooibos extracts providing phenolic compounds. Other work suggests that Rooibos extracts and its constituent C-glucosyl flavonoids, and Z-2-(β-D-glucopyranosyloxy)-3-phenylpropenoic acid may elicit positive effects on cellular oxidative stress, inflammation and transcription factors that control genes regulating glucose and lipid metabolism helping to attenuate features of metabolic disease [69].

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In the present review human subjects tended to drink Rooibos beverages after they had been infused for approximately 10 minutes [20,23,25]. Recently, from a consumer stance the ideal 'optimal cup' of Rooibos tea has been defined as one that had been seeped for 10 minutes or longer [70]. Unfortunately, recent research shows that only 15.9% of respondents consumed an 'optimal cup' of Rooibos in an amount that enables its potential cardioprotective health benefits to be provided (defined as a minimum of 4 to 6 cups per day) [70]. Interestingly, compared with cold and regular-brewed Rooibos infusions of unfermented/green and red Rooibos prepared by boiling have the highest antioxidant capacities and total polyphenol profiles [9]. Such findings suggest that more information needs to be imparted to consumers' regarding preparation methods and the potential health benefits that drinking Rooibos tea could bring.
With regard to limitations and future research directions greater uniformity is needed in forthcoming studies. For example, some studies used fermented and others unfermented Rooibos extracts or infusions the fermented form appeared to have stronger impact on some health effects [38,51]. Others used Rooibos tea and Rooibos 'extracts' or unfermented/green Rooibos. Production seasons and grade quality could also alter the phenolic profiles and antioxidant capacity of Rooibos samples and should be considered [66]. Future trials should also better encompass how findings could be translated into public health messages. For example, Marnewick JL, et al. (2011) [21] asked volunteers to drinks six cups Rooibos tea daily for 6-weeks to study its cardiovascular effects. Similar studies are now needed for other health outcomes such as bone health, liver or brain (cognitive) function.
Considering such evidence Rooibos tea, like teas from Camellia sinensis (e.g. black, green and oolong tea) could be regarded as a 'general health' tea, potentially providing an array of benefits to human health and wellbeing [56] Rooibos tea is naturally slightly sweet [16] thus does not require the addition of extra sugar or artificial sweeteners. It is also caffeine free with a favourable phenolic composition [9,10]. (Table 2 Rooibos tea showed histological regression of steatosis and cirrhosis in the liver tissue with a significant inhibition of the increase of liver tissue concentrations of malondialdehyde-triacylglycerols and cholesterol.

Rooibos tea extract Immune function
Rooibos tea extract may facilitate the antigenspecific antibody production through selective augmentation of IL-2 generation both in vitro and in vivo.

Conclusions
In conclusion previous health research has tended to focus on black or green tea. Rooibos is widely consumed in South Africa but is gaining popularity globally. This has been attributed to the fact that it is caffeine free, naturally sweet and abundant in polyphenols with potent antioxidant properties. Now, a growing body of evidence from 7 human studies and 49 laboratory studies suggests that Rooibos could be regarded as a 'general' health tea. Evidence for cardioprotective effects (especially lipid profile) looks promising, particularly as a potential adjunctive therapy. It is suggested that future research now builds on the other potential aspects of health including glycaemic, bone, liver and cognitive wellness enhancing effects that also appear to be emerging.

Disclosure
The views expressed are those of the authors alone and personnel from the UK TEA & INFUSIONS ASSOCIATION (UKTIA) had no role in writing this review.

Conflicts of Interest
The authors declare no conflicts of interest.