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Journal of Public Health in Africa logoLink to Journal of Public Health in Africa
. 2023 Dec 1;14(12):2784. doi: 10.4081/jphia.2023.2784

The health benefits of rooibos tea in humans (aspalathus linearis)-a scoping review

DANIEL AFRIFA 1,2,, LOUISE ENGELBRECHT 1, BERT OP'T EIJNDE 1,2, ELMARIE TERBLANCHE 1
PMCID: PMC10774856  PMID: 38204815

Abstract

Natural remedies in the treatment of health conditions are an appealing option for many individuals. Previous studies reported that fermented and unfermented rooibos tea have considerable anti-inflammatory and antioxidative properties. Most of this knowledge, however, originates from animal and cell culture studies. The aims of this review are to evaluate the existing, but limited, body of knowledge regarding rooibos tea interventions in humans and to identify the gaps in the literature. The PRISMA extension for Scoping Reviews (PRISMA-ScR) guidelines were followed in the collation of this scoping review. Among the databases searched were Google Scholar, PubMed, Cochrane Library, Scopus, and Web of Science. This review comprised 18 publications, with half (50%) of the studies being conducted in South Africa. There were 488 participants in all, ranging in age from six to 83 years, in the investigations. Rooibos tea was either fermented, unfermented, or black in 62% of the studies. Doses ranging from 200 to 1,200 ml were employed. In both healthy and at-risk individuals, rooibos has been shown to enhance lipid profiles, boost antioxidant status, and lower blood glucose levels. The existing findings suggests that rooibos consumption demonstrated to improve lipid profiles, boost antioxidant status, and lower blood glucose levels in both apparently healthy, and individual at-risk individuals or diagnosed of chronic conditions. Thus, it can be presumed that rooibos tea provides some health benefits, yet these findings are based on a limited number of human intervention studies and a small total sample size. Additionally, a variety of rooibos dosages and types of tea in the experiments had inconsistent results that were probably impacted by the amount consumed. Future studies should include a dose-response study in humans, as well as large scaled clinical trials to evaluate the health effects of Rooibos.

Key words: Aspalathus linearis, fermented, unfermented, rooibos, human health

Introduction

Many contemporary medicines have their roots in the long-standing practice of using plants for therapeutic purposes (1). According to the World Health Organization (WHO), approximately 80% of the world's population use herbal products for a variety of ailments. Herbs may be utilized for their purported anti-inflammatory, haemostatic, expectorant, antispasmodic, or immune-boosting properties (1,2). Numerous phytochemicals provide long-term health advantages for humans who consume them, and it can be used to treat a range of human afflictions (1). There has been increased attention on and consumption of herbal products in recent years due to claims that they are allegedly inexpensive and have few, if any, negative effects (1). Teas made from herbs, particularly Rooibos, have become increasingly popular worldwide (3).

Rooibos tea (produced from Aspalathus linearis), is free of caffeine and low in tannins (4), and has attracted a lot of research interest. It contains numerous minerals, especially flavonoids such as dihydrochalcones (aspalathin and nothofagin), phenylpropanoids, flavones, and flavonols (4,5), which may have a variety of positive health effects (6). Infusions made from Rooibos tea contain flavonoids and phenolic acids which have anti-oxidative activity (7) because they play an active role in preventing the formation of Reactive Oxygen Species (ROS) (8). Chronic illnesses including cardiovascular disease and diabetes mellitus can be prevented or treated by reducing highly oxidizing ROS and converting them into less harmful aroxyl radicals (9). In addition to the traditional method of consuming rooibos, a variety of rooibos tea drinks and dietary supplements in the form of capsules and tablets are easily accessible. As a result, it is warranted to promote Rooibos tea consumption and/or the consumption of its bioactive components.

The bioavailability of Rooibos metabolites and their impact on non-communicable lifestyle diseases have been the subject of several studies (11-14). The dosages of various types of Rooibos tea that will have the maximum bioavailability and therapeutic effects in humans are a key subject that has generated a great deal of discussion. Six cups of tea (1,200 ml) each day, according to some researchers (13,14), may have positive effects. However, no human dose-response investigation has been carried out to date. Moreover, due to the scarcity of randomized controlled trials, it is uncertain if Rooibos consumers will experience hepatotoxicity from an overdose, or long-term consumption. Studies by Carrier et al (15) and Reddy et al (16) suggested that liver function may be negatively affected in regular Rooibos consumers.

With the majority of the confirmations coming from animal studies, the dosages that have been linked to advantageous effects in humans have been extrapolated from the various quantities utilized in animal studies. Given the growing interest in Rooibos's potential health benefits and the varied dosages that have been utilized, the main goal of this review is to evaluate the existing, albeit small, body of knowledge regarding Rooibos trials in humans and to offer suggestions for further research. Specifically, we aimed to:

  • identify all studies in humans and summarize the descriptive characteristics of the participants;

  • describe the various dosages (amount of Rooibos) that have been used in human studies;

  • describe the different types of Rooibos tea that have been used in human studies;

  • describe the various flavonoids in Rooibos that are detectable in urine and plasma;

  • describe the effect of the different dosages on health measures.

Materials and methods

Protocol and registration

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) extension for Scoping Reviews (PRISMA-ScR) standards were used for this scoping review (19). To make sure that no scoping review had been registered or was being done to research a related topic, a search was conducted through the International Prospective Register of Systematic Reviews (PROSPERO).

Eligibility criteria

The following criteria was set to assess the eligibility of an article to be included in this scoping review: (a) the article must involve humans; (b) studies must evaluate the effectiveness of Aspalathus linearis in treating a chronic health condition, either alone or in combination with other medications; (c) articles must specify the dosage used; (d) articles must specify the type of Rooibos tea used (i.e., fermented (red), unfermented (green), capsule, or extract); and (e) articles must specify the population used in the study, and (f) the full article must be available and written in English or Afrikaans. Studies were included if the participants were healthy individuals, patients with any chronic condition, and persons who were at an increased risk of developing a chronic condition. In the case of interventional studies, the experimental group must have consumed Rooibos tea, while the comparative group did not consume Rooibos tea or drank a different type of tea.

Data sources and search strategies

Databases including Google Scholar, PUDMED, Cochrane Library, Scopus, and Web of Science were utilized to search for articles. Databases spanning from the very first publications available until December 2022 were searched, to identify all studies conducted in humans. Where information was missing in an article, the authors were contacted personally and asked to provide the detail that was missing.

The primary search of the databases included all studies conducted in humans and consuming Rooibos tea. The search terms included ‘Aspalathus linearis’, ‘rooibos,’ ‘human trials,’ ‘human studies,’ ‘adults,’ and ‘children.’ EndNote version 10 (Clarivate Analytics, Philadelphia, USA) was used to manage the reference list and to remove duplicates.

Study selection

Articles included in the review were independently screened in three different steps and by two authors (AD and LE). First, duplicates were manually deleted. Then, a first selection was made based on the titles and abstracts of the articles. When articles failed to match the predetermined inclusion criteria, they were methodically inspected and eliminated. Additionally omitted were reviews, editorials, congress abstracts, and validation studies. Author disagreements were settled by consensus with a third reviewer (ET).

Data extraction

The first reviewer independently extracted the data from the included publications, and the second reviewer verified it. An already created data collection form was used to extract the data. The reviewers took notes on each article's participant characteristics (age, study location, type of participants, sample size), methods (study design, intervention length, type of Rooibos tea), outcome variables reported and findings. Data were only retrieved from the study arms that satisfied the inclusion criteria when a study included two or more study arms and one of the intervention arms did not.

Study quality assessment

According to Higgins et al (20) and Sterne et al (21), one reviewer (AD) evaluated the quality of the included research, which was then confirmed by the second (LE) and third reviewers (ET). The scale created by Jadad et al (22) was used to evaluate the studies' methodology. Randomization, blinding, and the explanation of withdrawals and dropouts were among the study quality factors that were evaluated. To elicit yes-or-no responses, the items were presented as questions. Depending on how well the methods for creating the randomized sequence and/or the mechanism of double-blinding were explained, points were given for items 1 (randomization) and 2 (blinding). If the trial was randomized and/or double-blind, but no explanation of the methods used to create the sequence of randomization or double-blind conditions was given, one point was awarded in each instance. An extra point was awarded if the method for creating the randomization sequence and/or the method for blinding were disclosed. High-quality studies were those that received at least three out of a possible five points.

Results

Search results and quality assessment

A flowchart of the search results is shown in Fig. 1. The search resulted in 1,270 articles. 580 articles were left after duplicates were eliminated and the titles and abstracts of the remaining articles were examined. Only 18 articles matched the eligibility requirements and were included in this scoping review. The quality of these 18 articles (10-14,23-35) was subsequently evaluated and ranged from poor to excellent. Most of the articles matched one or two of the three criteria for quality assessment and was classified as good quality. Six papers were deemed low quality (11,14,25,26,31,35), while two publications (23,28) were judged as high quality.

Figure 1.

Figure 1.

Flow chart of included studies.

Sample size characteristics

The total number of participants in all investigations was 488, ranging in age from 6 to 83 years. The majority (193, 39.55%) of the participants were men, while 123 (25.20%), 94 (19.26%), and 56 (11.48%) were women, boys, and girls, respectively. The trials lasted from one day to three months. Studies were conducted in South Africa (n=8), Italy (n=2), Germany (n=1), the United Kingdom (n=1), Argentina (n=1), New Zealand (n=1), Sweden (n=1), the United States (n=2), and Japan (n=1). Almost all studies featured participants who appeared to be in good health, except for Hesseling and Joubert (35), Marnewick et al (13,14), Rodgers et al (31), and Munmum et al (28), which included individuals with at-risk factors and those diagnosed with clinical conditions. While the at-risk individuals were defined as those with one or more risk factors for a chronic condition, the seemingly healthy populations comprised individuals who were said to have no diagnosis of any risk factor or chronic condition.

Bioavailability of the flavonoids found in human studies

Four studies (11,12,25,31) reported on the bioavailability of rooibos flavonoids in urine and plasma. The various flavonoids detected in both the urine and plasma samples are shown in Table III. Breiter et al (12) reported that the total concentration of flavonoids was 848±29 µmol in 500 ml with the major flavonoids being aspalathin (636±20 µmol), followed by nothofagin (79±3.1 µmol). However, Stalmach et al (11), reported the total concentration in 500 ml of fermented and unfermented rooibos beverage to be 84±2.9 and 159±6.5 µmol respectively. The concentration of the various flavonoids in urine was found to be 0.100±0.001 (Aspalathin), 0.212±0.002 (Orientin), 0.363±0.007 (Isoorientin), 0.053±0.001 (Isovitexin), 0.081±0.001 (Vitexin), 0.501±0.014 (Hyperoside), 0.015±0.004 (Quercetin), 0.007±0.001 (Luteolin), and 0.004±0.000 (Chrysoeriol) (31). The recovery rates of flavonoids after the administration of rooibos tea were in the range of 0.2% (aspalathin) and 2.3% (vitexin). On average a total of 0.76 nmol of the quantified flavonoids were bioavailable during their maximum concentration in plasma, accounting for 0.26% of the total amount ingested (758 µmol) (12). However, Stalmach et al (11) did not detect any flavonoids in quantifiable amounts after consumption of either fermented or unfermented beverages.

Type of intervention (rooibos type), dosages used, and their outcomes

In general, almost all the studies used three types of rooibos: black, red (fermented), and green (unfermented), except for Chepulis et al (24) and Davies et al (27), which used rooibos extract and a standardized rooibos capsule. The dosages used throughout the studies are listed in Tables I and II and ranged from 200 to 1,200 ml of water (Tables I and II).

Health measures assessed

Several health parameters were measured across various studies included in this scoping review. Health parameters measured in both diagnosed and at-risk individuals, and apparently healthy populations included liver and kidney functions, iron status, inflammatory status (CRP), and physiological parameters (blood pressure, resting heart rate). Health measures assessed only among at-risk individuals and those diagnosed with chronic conditions included total serum immunoglobulin E (IgE), oxidative status, urinary and plasma biomarkers (urinary thiobarbituric acid reactive substances, Urinary N-acetyl-β-D-glucosaminidase and volume, and SS brushite), bone markers (type 1 intact amino-terminal propeptide), melatonin, and emotional and psychological domains. However, antioxidant status, total polyphenol intake, fasting and postprandial serum glucose, fasting insulin, angiotensin-converting enzyme, body mass, plasma osmolarity, blood oxygen saturation, acute mountain saturation, and peak torque extension and flexion were assessed in the apparently healthy population. Tables I and II provide a comprehensive description of the effect of rooibos consumption on the various health measures assessed in the included studies.

Findings from studies

One of the objectives of this scoping review was to report the effects of rooibos consumption on the various outcomes measured among at-risk and apparently healthy populations.

Effect of rooibos on health outcomes among at-risk

population. After acute (short-term) consumption of 500 ml of black rooibos tea among asthmatic and hay fever individuals, Hesseling and Joubert (35) reported no changes in total serum immunoglobulin E (IgE) and the size of skin induration to 12 antigens. However, the skin reaction to four antigens (house dust, grass pollen, dog epithelia, and Aspergillus fumigacus) was larger on the day of treatment with tea than on the control days (P#x003C;0.01 to #x003C;0.05).

Among individuals at risk of coronary heart disease (13), 6-weeks of rooibos tea consumption resulted in significant increases in Aspartate Aminotransferase (AST), Alanine Aminotransferase (ALT), Alkaline Phosphatase (ALP), and creatinine function indicators by 35.9, 45.7, 21.3, and 24.4%, respectively, compared to the control period. Serum unconjugated bilirubin and total bilirubin levels decreased significantly (P#x003C;0.05) by 34 and 24.08%, respectively, while a marked, but non-significant decrease (14.4%) was reported in serum glucose levels compared with the control period. Total cholesterol (TC) decreased by 8.6%, but this change was not statistically significant. However, significantly lower serum triglycerides (TG) (29.4%) and low-density lipoprotein (LDL-C) levels (15.2%) were reported (P≤0.001), while high-density lipoprotein (HDL-C) levels were significantly increased by 33.3% (P≤0.001).

Marnewick et al (13) showed that total polyphenol significantly (P≤0.05) increased by 11.8%. However, the reduced glutathione (tGSH) increased (35.76%) significantly (P≤0.001), while the oxidized glutathione (GSSG) was significantly decreased by 30.53% (P≤0.001), and the ratio of GSH:GSSG increased by 87.47% (P≤0.001). Moreover, the conjugate dienes (CDs) and thiobarbituric acid reactive substances (TBARS) decreased by a significant 34.9 and 54%, respectively. Meanwhile, acute consumption of a single dose of 500 ml fermented rooibos tea (34) increased GSH levels from an initial baseline of 801 µM to reach a peak of 851 µM at 45 min (an increase of 6.24%), after which it returned to the baseline value (P≥0.05) among healthy individuals. The plasma GSSG levels also increased from an initial baseline value of 97 µM to reach a peak of 109 µM at 45 min (an increase of 12.37%) after which it decreased to below the baseline value at 90 to 180 min (P≥0.05) while the GSH:GSSG ratio decreased from a baseline value of 6.23 to 5.81 but increased to reach a peak of 7.98 at 90 min and then remained at this level up to 180 min (P>0.05) (34).

Regarding other clinical parameters, chronic supplementation with rooibos tea three times per day for 3 months among older women diagnosed with osteopenia (28) did not have any significant (P≤0.05) effect on the bone markers procollagen type 1 intact amino-terminal propeptide (total-P1NP) or on P1NP (ng/ml) levels. For type I collagen C-telopeptide (CTX-I, ng/ml), CTX levels were reduced (-38.23±22.63; -29.64%) in the individuals who were supplemented with rooibos. Although CRP was low at baseline, three months of rooibos consumption did not significantly (P≤0.05) affect it. Rooibos supplementation for three months statistically increased (P=0.06) melatonin levels (ng/ml) by 22.97±25.26 (54.21%) from the baseline (42.37±19.40) but did not significantly affect blood pressure levels (systolic and diastolic) or the physical domain scores (impact because of physical problems) compared to the placebo. However, rooibos had a significant (P=0.09) reduction in the emotional/psychological domain scores (impact because of emotional problems) by 2.25±1.63 (5.98%) when compared to placebo (coriander group) (0.57±1.1).

Among kidney stone formers (31), supplementation with rooibos tea for 30 days reduced urinary volume significantly by 17.83% (P=0.016) but did not affect urinary TBARS (µmol/g creatinine), N-acetyl-β-D-glucosaminidase (NAG) (U/g creatinine), or plasma TBARS (nmol/ml).

Table I.

Apparently healthy individuals.

Number of Age of Study Comparative Rooibos Reported Duration of Major
Author (s) participants participants location group(s) Study design type dosage intervention findings (Refs.)
Breet et al 150 children 6-15 years South Black tea Randomized Fermented 50 tea bags 16 weeks ↑ Mean corpuscular (22)
(94 boys; Africa (50 tea bags controlled rooibos in 20 l of volume (MCV),
56 girls) in 20 l of trial watera Serum transferrin
water)a (S-Tfn) and Total
iron binding
capacity (TIBC)
Chepulis et al 10 women Not New 1 green tea Randomized, Rooibos 760 mg of 1 day ↓ Postprandial (23)
specified Zealand (1.0 g) tablet, controlled tea rooibos tea glucose
1.6 Alma trial extract extract
berry (700 mg) (provided to
capsule, participant
1 grape seed in three size
(500 mg) ‘0’ gelatin
capsule, capsules)
6 propolis
tincture
(400 mg)
capsule
Davies et al 32 men 22.2 years South 3 standardized Randomized, Fermented Three 4 weeks ↔ Peak torque (26)
Africa Placebo crossover, rooibos standardized extension (Nm)
capsules study capsule capsule ↑ Peak torque
(equivalent flexion (Nm)
to 6 cups of ↔ total work
fermented extension
rooibos) daily (Joules) and total
work flexion
Davies et al 8 men 46.6±11.2 Argentina No Case study Rooibos Three capsules 14 days ↓ Peripheral blood (25)
years comparator design extract of rooibos oxygen saturation
group extract and symptoms of
(equivalent six acute mountain
cups of sickness
fermented ↑ Resting heart rate
rooibos) daily
Francisco 14 men and 18-35 years South Standardized Randomized Fermented 2 g of rooibos 1 day ↓ Fasting serum (28)
women Africa fat meal crossover rooibos tea tea leaves per glucose
(50.1 g) with design with 100 ml of ↓ Serum total
commercial standar- waterb cholesterol (TC),
sucrose dized fat High Density
beverage (50.1 g) Lipoprotein (HDL)
(Control) meal and Low Density
Lipoprotein (LDL)
but ↑ Triglycerides
(TG)
↓ High sensitive
C-reactive proteins
(hs-CRP)
↓ conjugated dienes
(CDs) and
Thiobarbituric acid-
reactive substances
(TBARS)
Hesseling 30 men 21-34 years South Green tea (5 g Randomized Rooibos 5 g of rooibos 1 day ↓ Ferritin, (10)
et al Africa per 200 ml of controlled tea leaves tea leaves with Transferrin and Iron
water with trial 200 ml of water absorption but
40 ml milk and with 40 ml ↑ Serum iron
20 g cane milk and 20 g
sugar) cane sugar
Boiled water
(200 ml with
40 ml milk and
20 g cane
sugar) groups
Persson et al 20 men and 20-31 years Sweden 10 g of Green Randomized Rooibos 10 g of 1 day ↓ Angiotensin- (29)
women tea leaves in crossover tea Rooibos tea converting enzyme
400 ml of design leaves in 400 (ACE) activity
water and 10 g ml of water ↔ Blood pressure
of black tea (BP), Heart Rate
leaves in 400 (HR) and Nitric
ml of water Oxde (NO)
Utter et al 23 men 19.6±0.3 USA Regular bottled Randomized, Fermented Not specified 1 day ↓ Body mass (31)
years water, and cross-over rooibos tea ↓ Plasma
carbohydrate design Osmolarity (Posm)
(6% or 60 g l-1)
beverage
Villano et al 15 Not Italy Water Randomized Unfer- 1.5 g of rooibos 1 day ↑ Fasting blood (32)
individuals specified crossover mented tea extract glucose and Plasma
study design and powder per 1 l total radical-
fermented of waterb trapping antioxidant
‘ready to potential (TRAP)
drink’
rooibos
teas
Wanjiku 8 men 20-35 years South No comparative Pre-post Fermented 2.5 g rooibos 1 day ↑ Ferric Reducing (33)
Africa group intervention rooibos tea leaves in Antioxidant
study 180 ml boiling Potential (FRAP)
waterb and Oxygen Radical
Absorbance
Capacity (ORAC)
↓ The total
polyphenols
↑ The reduced
Glutathione (GSH)
levels
↑ The plasma
oxidised glutathione
(GSSG)
↓ GSH:GSSG ratio

aEach participant took 200 ml of either black tea or rooibos tea during first and second breaks,bParticipants consumed 500 ml of rooibos tea. FRAP, Ferric reducing antioxidant power; ORAC, Oxygen radical absorbance capacity; GSH, Reduced Glutathione; GSSG, Oxidised Glutathione; GSH:GSSG, Reduced-Oxidised Glutathione ratio; HDL, High Density Lipoproteins; LDL, Low Density Lipoproteins; TG, Triglycerides; BP, Blood Pressure; HR, Heart Rate and NO, Nitric oxide; TRAP, Total radical trapping antioxidant parameter.

Table II.

Individuals with health conditions.

Number of Age of Study Comparative Rooibos Duration of Major
Author (s) participants participants location group(s) Study design type dosages intervention findings (Refs.)
Hesselling 7 adults Not South No comparator Interven- Black 25 g of black 1 day ↔ Total serum (34)
and Joubert (Asthmatic specified Africa group tional cross- rooibos tea rooibos tea immunoglobulin E
or hay fever sectional per 1 l of (IgE)
individuals) study watera
↔ size of skin in-
duration to 12 of
the antigens
↑ The skin
reaction to four
antigens (house
dust, grass pollen,
dog epithelia) and
Aspergillus
fumigacus)
Marnewick 40 men and 30-60 years South 6 cups of water Controlled Fermented 6 cups (one tea 6 weeks ↑ aspartate (13)
et al women Africa clinical trial rooibos tea bag per 200 ml transaminase
(Individuals of freshly (AST), alanine
with two boiled water) transaminase
risk factors daily (ALT), alkaline
of CHD) phosphatase (ALP),
and creatinine
↓ Serum uncon-
jugated bilirubin
and total bilirubin
↓ Serum TG and
LDL-C levels
↑ HDL-C levels
↑ Total polyphenol,
GSH and GSH:
GSSG while their
GSSG ↓
↓ CD and TBARS
Marnewick 40 men and 30-60 years South 6 cups of water Cross over Fermented 6 cups (one tea 6 weeks ↑ Total flavonoids (14)
et al women Africa study design rooibos bag per 200 ↔ serum iron,
Individuals ml of freshly ferritin, transferrin,
with two boiled water) TIBC and % Fe
risk factors daily saturation
of CHD ↔ Hs-C-reactive
protein and
homocysteine
Munmun et al 35 women 36-83 years USA Placebo Randomized, Rooibos 3 tea bags (1 g 3 months ↔ the bone markers (27)
(with (coriander), Controlled tea per tea bag) procollagen type 1
osteopenia) tulsi or oolong trial daily intact amino-
groups terminal propeptide
(total-P1NP) and
P1NP (ng/ml)
levels
↔ CRP
↓ Melatonin levels
↔ Blood pressure
↓ the emotional/
psychological
domain scores when
compared to placebo
(coriander group)
Rodgers et al 28 men 18-26 years South Control group Crossover Freshpak Control group 30 days Control groups: (30)
(8 kidney (Healthy Africa 2 cups (2 tea study Fermented 2 cups (2 tea ↔ The mean
Stone Control) and Japan bags per 125 design Rooibos bags per urinary and plasma
formers and and 30-60 ml of low tea 125 ml of low biomarkers
20 years mineral water mineral water) (Urinary TBARS
apparently (Kidney per cup) of Freshpak (µmol/g creatinine),
healthy stone Japanese Green rooibos tea Urinary N-acetyl-
individuals) formers) tea (JGT) (Control β-D-
(Control group 1) glycosaminidase
group 2) Stone formers (NAG) (U/g
Stone formers: 4 cups each of creatinine) and
Japanese green fresh Pak Plasma TBARS
tea (2 tea bags rooibos tea (nmol/ml)] for
per 125 ml of (4 tea bags oxidative stress
low mineral of per 250 ml Stone formers:
water per cup) of low mineral ↓ Urinary volume
per day) water) per day and SS brushite

a500 ml of black rooibos tea with sugar was taken by participants. AST, Aspartate Aminotransferase; ALT, Alanine Aminotransferase; ALP, Alkaline Phosphatase; GSH, Reduced Glutathione; GSSG, Oxidised Glutathione; GSH:GSSG, Reduced-oxidised Glutathione ratio; TG, Triglycerides; LDL, C-Low density Lipoprotein-cholesterol; HDL-C, High Density Lipoprotein Cholesterol; CD, Conjugated Dienes; TBARS, Thiobarbituric acid reactive substances; P1NP, type 1 intact amino-terminal propeptide; NAG, N-acetyl-β-D-glycosaminidase; CRP, C-reactive protein; TIBC, Total iron binding capacity; IgE, Immunoglobulin E.

Table III.

Bioavailability of rooibos flavonoids found in urine and plasma.

Sample size characteristics Study methods
Number of Age of Study Type of Study
Author(s) participants participants location participants Study design Rooibos type duration Findings (Refs.)
Breiter et al 12 men Ages of 21 to Germany Apparently Randomized 10 g of 5 weeks Four different metabolites (12)
35 years healthy men cross over unfermented of aspalathin (sulphated,
with body trial rooibos tea glucuronidase, methylated,
mass indices leaves of and both glucuronidated
between 20.0 500 ml of and methylated, one
and 27.4 kg/m2 water metabolite of its aglycone
(glucuronidated aglycone),
one metabolite of nothofagin
(glucuronidated) and one
of its aglycone
(glucuronidated aglycone)
were detected
Metabolites identified in
plasma include aspalathin
isoorientin, orientin, and
vitexin
Courts and 6 (two men, Ages of 22–28 UK Apparently Cross- 14 g of green 11 days 3-O-methylated aspalathin (24)
Williamson and four years. They healthy sectional rooibos tea (O-MA) and 3-O-MA
women) used neither individuals study design leaves per 1 l glucuronide excretion
routine nor with a of waterb was detected but 4-O-MA
spontaneous BMI range of was not detected.
medication 19–27 kg/m2 Urinary: Aspalathin
metabolites was detected
in the first sample
(0-2 h) and remained in
the urine of all subjects
for 6 h after oral exposure
to aspalathin
Stalmach et al 10 (five men Not specified Rome Apparently Randomized 500 ml of 4 days Rooibos teas: (11)
and five healthy controlled ‘ready-to-drink’ Eriodictyol-C-glucoside,
women) participants study design rooibos tea luteolin-6-C-glucoside,
produced from luteolin-8-C-glucoside
unfermented aspalathin, quercetin-O-
leaves and rutinoside,
fermented apigenin-8-C-glucoside,
leavesa . apigenin-6-C-glucoside,
quercetin-3-O-rutinoside,
quercetin-3-O-galactoside,
quercetin-3-O-glucoside,
nothofagin, quercetin, luteolin
Urine: Metabolites detected
in urine samples after
ingestion of both fermented
and unfermented rooibos
tea include aspalathin,
nothofagin, eriodictyol-C-
glucoside, luteolin-8-C-
glucoside, luteolin-6-C-
glucoside, apigenin-8-C-
glucoside, apigenin-6-C-
glucoside quercetin-3-O-
galactoside, quercetin-3-O-
glucoside, quercetin-3-O-
rutinoside, quercetin-O-
rutinoside isomer
luteolin, quercetin
Plasma: No flavonoids
or their metabolites
were detected

aAuthors failed to report the exact quantity of rooibos tea leaves used in the preparation of the rooibos tea;b300 ml of rooibos tea was consumed by participants.

Effect of rooibos consumption on health outcomes among apparently healthy individuals

Some of the results reported by Marnewick et al (13) were contrary to those found in healthy individuals, whereas others were similar. Francisco (29) showed that acute supplementation with 500 ml fermented rooibos herbal tea (2% w/v) significantly lowered serum glucose after 2 h (-22%) and 6 h (-18%) of consumption in the treatment group, but did not have an effect on serum insulin. However, compared with the control group, serum blood glucose tended to decrease with acute supplementation of 0.76 g rooibos by 35.5% (P≤0.005) and 32.9% (P≤0.005) using glucose and bread and ham tests, respectively (24). Moreover, serum total cholesterol levels were significantly lower in the treatment group at 2 h (15%; P≤0.0001), 4 h (10%; P≤0.001), and 6 h (6%; P≤0.0001) after consumption. Rooibos consumption also significantly (P≤0.05; P≤0.0001) reduced HDL-C levels at 2 h (4%), 4 h (2%), and 6 h (0.3%) and LDL-C levels at 2 h (18%), 4 h (11%), and 6 h (7%) (29). Triglyceride (TG) levels were also significantly lower (P≤0.05, P≤0.001) after supplementation with rooibos at 4 h (97%) and 6 h (78%). CRP levels were significantly (P≤0.05) lower (8%) at six hours post ingestion (29) among individuals supplemented with rooibos. The conjugated diene (CDs) levels in individuals supplemented with rooibos were significantly (P≤0.001; P≤0.0001) decreased at 2 h (14%) and 4 h (14%) after ingestion of rooibos, while TBARS was significantly (P≤0.001) lowered at 4 h (59%) after the intake of rooibos when compared with that of the control group (29).

When comparing the effects of both fermented and unfermented rooibos tea, Villano et al (33) reported that acute supplementation with fermented rooibos tea increased blood glucose by 32.0% (P≤0.001), whereas unfermented rooibos tea increased blood glucose by 21.6% (P≤0.001) compared with the baseline. However, neither tea affected the TG, total cholesterol, or uric acid levels. The consumption of fermented and unfermented rooibos tea increased by 4.8 and 1.7%, respectively, in plasma total radical-trapping antioxidant potential (TRAP) assay after 30 min, reaching a statistically significant increase after 1 h (6.6%; P≤0.05, fermented; 2.9%; P≤0.01, unfermented) when compared with the baseline and control groups (P≤0.05). However, the TRAP values associated with fermented rooibos tea began to decrease at 2 h (4.9%; P≤0.05 compared to the baseline) and returned to the baseline values after 5 h (+2.2%) (33). The TRAP values for those who consumed unfermented rooibos tea continued to increase at 2 h (+2.7%; P≤0.05) when compared with the control group before returning to the baseline values after 5 h (33). Wanjiku (34) also reported that Ferric reducing antioxidant power assay (FRAP) assay values increased to reach their peak (610 µmol/l) at 45 min (an increase of 6.10%) and then subsequently decreased to 584 µmol/l after 180 min (P≥0.05) after a single dose of 500 ml of fermented rooibos in healthy individuals. ORAC values also increased slightly from a baseline value of 31.4 to 32.4 µmol TE/ml (an increase of 3.18%) 45 min after the ingestion of the rooibos, remaining at this level up to 180 min (P>0.05).

Contradictory results have also been reported for iron status. Acute supplementation with rooibos among adults (10) resulted in lower ferritin (54.0±42.6 µg/ml) levels compared to those in individuals supplemented with ordinary tea (80.9±48.7 µg/ml) and water (68.1±57.9 µg/ml). Nonetheless, these individuals had the highest serum iron levels (22.7±7.6 µmol/l) compared to those taking ordinary tea (21.3±6.2 µmol/l) and water (21.7±4.2 µmol/l) (10). However, Marnewick et al (14) reported no significant changes in iron status indicators (serum iron, ferritin, transferrin, total iron binding capacity (TIBC), and % Fe saturation), C-reactive Protein (CRP), and homocysteine levels among individuals at risk of coronary heart disease after six weeks of chronic supplementation. This was contrary to the results of Breet et al (23), who found increases in serum iron (S-Fe) (1.378 µmol/l; 10.69%), transferrin saturation (TS) (1.17%), and haemoglobin (Hb) (0.29; 2.34%). They also found a decrease in serum ferritin (S-Fer) (0.20 µg/l); 6.70%) among children after chronic supplementation, although it was statistically insignificant after chronic supplementation. Breet et al (23) reported a significant increase (P≤0.001) in the mean corpuscular volume (MCV) (1.66 fL; 2.20%), serum transferrin (S-Tfn) (0.18 g/l; 7.09%), and TIBC (3.92 µmol/l; 6.91%), as well as a significant decrease (P≤0.0001) in mean corpuscular Hb (MCH) (3.101 pg; 11.19%).

Acute supplementation with rooibos tea in young individuals (30) did not cause any significant changes in blood pressure or nitric oxide (NO) levels. However, Angiotensin-converting enzyme activity (ACE) was significantly reduced with the rooibos tea after 30 (P≤0.01) and 60 min (P≤0.05). A significant inhibition of ACE activity was also observed for genotype II 60 min after the intake of rooibos tea (P≤0.05). However, Davies et al (26) reported that the mean peripheral blood oxygen saturation decreased by 15% after three daily supplementations with three capsules of rooibos extract (after breakfast, lunch, and supper). However, resting heart rate increased by 19.12% and acute mountain sickness symptoms decreased by 37.5% compared to 80-90% of the data found in the literature (26).

When used as an ergogenic aid (27), peak torque extension (Nm) increased significantly by 7.85% in Bout 3 (31.6±8.3) when compared with the placebo (29.3±6.1) (P=0.21). Peak torque flexion (Nm) also increased with rooibos intake by 10.80% in Bout 3 (39.0±10.1) when compared with placebo (35.2±7.6) (P=0.08, effect size=0.42) and 10.33% in Bout 4 (36.3±8.9) when compared with placebo (32.9±7.2) (P=0.09, effect size=0.42).

Discussion

The objective of this study was to identify all studies conducted on humans and summarize the descriptive characteristics of the participants. However, it specifically describes the different types of rooibos used and the flavonoids found in them. Moreover, various dosages and concentrations (of rooibos used) and their health outcomes were also evaluated.

First, three types of tea were used: green (unfermented), red (fermented), and black Rooibos teas. Aside from Hesseling and Joubert (35), almost all studies used either green or red Rooibos tea. Since black rooibos tea is no longer commercially produced, green and red rooibos currently dominate the market. Through value addition, green rooibos tea has been produced in other forms, including extracts, cosmetic products, and animal foods (6). Red rooibos tea is produced by subjecting green leaves to natural fermentation. This gives red rooibos a distinct taste and aroma (3). The fermentation process results in the partial oxidation of the polyphenol content and subsequent colour change from natural green to reddish brown. Therefore, green rooibos contains approximately three times higher levels of total phenolic compounds than fermented rooibos (3,6). This was supported by Breiter et al (12), who reported that the free flavonoids luteolin and quercetin found in the green rooibos beverage were higher (159±6.5 µmol) than those in the red rooibos beverage (84±2.9 µmol). The high usage of fermented rooibos in previous studies could be attributed to the taste and aroma and the high solubility of rooibos tea in both cold and warm water.

Second, the review showed that the dosages used ranged from 200 ml (10) to 1,200 ml (13,14). High dosages were found to have a positive impact by improving various health parameters, against which rooibos was assessed during chronic supplementation. However, inconsistencies have been reported with acute supplementation. Low-dosage supplementation was also found to have no effect on chronic supplementation. Based on the data presented in this review, it appears that rooibos may have beneficial health effects in humans. These beneficial effects can be attributed to the various flavonoids found in rooibos. However, most of the flavonoids are absorbed and not bioavailable and quantified in both urine and plasma as reported by Stalmach et al (11), and Breiter et al (12). Interestingly, several flavonoids have been found to affect various chronic health conditions. A systematic review by Abdulai et al (36) reported that vitexin and isovitexin have an influence on several molecular drug points and pathophysiological and metabolic pathways involved in the development and progression of diabetes mellitus, and as such, have a positive effect on patients with diabetes mellitus. Therefore, it was not surprising that Francisco (29) and Chepulis et al (24) reported a reduction in glycaemia among their populations.

Aspalathin and nothofagin are particularly interesting. Stalmach et al (11) and Breiter et al (12) reported that they made up more than 90% of the metabolites found in rooibos. This was similar to the findings of Kazuno et al (37) after determining and quantifying glycosyl flavonoids using liquid chromatography-triple quadrupole mass spectrometry. Orientin and isoorientin are the flavone analogues of aspalathin. The 8-C and 6-C-β-D-glucopyranoside derivatives of luteolin and rutin are the major monomeric flavonoids found in rooibos (38). Because aspalathin targets several key enzymes involved in fatty acid synthesis and oxidation, leading to enhanced glucose and fat metabolism (39,40), it is not surprising that rooibos intake improves glucose concentrations in the blood and improves overall glycaemic control.

Compared to Aspalathin, nothofagin was found in lower quantities (12,38). The flavone analogues of nothofagin found in rooibos include vitexin and isovitexin, which are 8-C- and 6-C-β-D-glucopyranoside derivatives of apigenin, respectively (38). Marques et al (41), reports that supplementation with nothofagin can lead to diuretic, natriuretic, and potassium-sparing effects. Interestingly, the diuretic and reno-protective properties associated with nothofagin have been linked to its ability to improve antioxidative capability and enhance plasma NO bioavailability (41). However, in humans, the consumption of rooibos does not reduce high blood pressure (30). This could be attributed to the period during which supplementation took place. Four weeks could be too short for an effective change in blood pressure to have taken place. Another reason for the lack of improvement in blood pressure can be deduced from the population used. Rooibos could affect blood pressure in hypertensive individuals as was shown in hypertensive animal models but not in apparently healthy individuals.

Moreover, the supplementation period could also affect the effects of rooibos. Acute and chronic supplementation with 500 ml of both red and green rooibos tends to have beneficial effects on both healthy individuals and individuals diagnosed with a high risk of coronary heart disease. However, the magnitude of the effects tended to differ between acute and chronic supplementation. Serum glucose, triglycerides, LDL-C, total cholesterol, conjugated dienes, and TBARS levels were all reduced with both acute and chronic supplementation. Reduced serum glucose levels prevent diseases and reduce the risk of coronary heart disease, stroke, kidney diseases, and vision and nerve problems. There were enhancements in HDL-C levels, oxidation status as oxidized glutathione, and the ratio of reduced and oxidized glutathione. A similar trend was observed for both acute and chronic supplementation. HDL-C and GSH/GSSG decreased with acute supplementation of 500 ml but increased with 1,200 ml of fermented rooibos per day for six weeks while oxidized glutathione increased with acute supplementation but decreased with chronic supplementation. These results suggest that both acute and chronic supplementation with fermented rooibos could be effective in preventing and treating people at a high risk of coronary disease.

Rooibos supplementation did not have any effect on iron status among adults who were at risk of coronary heart disease but tended to affect iron absorption among healthy individuals. High levels of iron have been indicated as a potent risk factor for coronary heart disease, especially acute myocardial infarction (42). Hesseling et al (10) reported low ferritin and high serum iron levels in individuals who were supplemented with rooibos compared with individuals who were supplemented with other teas. However, in children, rooibos had some effect on iron status indicators. It tends to increase serum iron, haemoglobin, transferrin saturation, MCV, transferrin, and total binding capacity, while decreasing serum ferritin and MCH. Therefore, the results indicate that rooibos may not be a good supplement for children, as it may result in iron deficiency. Low ferritin levels and high serum iron reported among adults could also indicate the effect of rooibos tea on iron absorption, leading to iron deficiency. According to Fan (43), polyphenols found in food can affect the absorption of both heme and non-heme iron across cells. Fan (43) indicated a dose-dependent inhibitory effect of polyphenols on heme iron absorption. This explains the unexpected influence of tea consumption on iron status, irrespective of whether an individual consumes a normal diet (43). However, this negative effect on iron absorption could be helpful and beneficial for individuals who are at high risk of iron overload, as tea consumption decreases iron accumulation in thalassemia and hereditary hemochromatosis (43).

Finally, unlike green tea, which has an effect on kidney stone formation (44) and blood pressure (45,46), rooibos supplementation did not affect blood pressure, nitric oxide levels, or kidney stone formation but had an effect on ACE in humans. This was contradictory to the results reported by Webster et al (47), who assessed the effects of first-line FDC drug treatment on lipid levels, myocardial ischemic tolerance, oxidative stress markers, and vascular endothelial function in male Wistar rats. Moreover, 30 days of supplementation with rooibos did not affect urinary TBARS, urinary N-acetyl-β-d-glucosaminidase (NAG), or plasma TBARS in kidney stone formers. Even though plasma TBARS was affected by acute (29) and chronic (13) supplementation, plasma TBARS was not affected among kidney stone farmers.

Conclusions

From the studies reviewed and conducted among humans, it can be concluded that of all the rooibos products available, mostly red and green rooibos, are consumed and that green rooibos contain more flavonoids than red rooibos. However, the dosages used in these human studies tend to yield contradictory results. For example, rooibos consumption of six cups per day (1,200 ml) improved TBARS, reduced blood glucose concentration, and improved insulin sensitivity among healthy individuals and those at risk of coronary heart disease; however, chronic consumption of four cups (500 ml) per day failed to have an impact on blood pressure and TBARS among kidney stone formers. Moreover, the number of human studies and sample sizes used make it very difficult to either confirm or refute the notion that rooibos has a significant effect on humans. Based on the different types of rooibos used and the contradictory results reported in these few human studies, a dose-response study that investigates the dosages that offer the greatest benefits to humans in individuals with both healthy and chronic conditions is warranted. Researchers should also investigate the effect of rooibos consumption in large-scale clinical studies with larger sample sizes.

Funding Statement

Funding: This scoping review was supported by the special research fund (BOF21BL05) of Hasselt University and the personal funds of the authors.

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