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Rooibos Tea: Antioxidant and Antimutagenic Properties

by Laurie Erickson

Antioxidants are hot topics in the health news these days, and an herbal tea called rooibos (pronounced ROY-boss) is becoming popular partly because it is being marketed as a healthy beverage with high levels of antioxidants. The rooibos plant (Aspalathus linearis (Burm. f.) Dahlgren, Fabaceae) is a South African flowering shrub used to make a mild-tasting tea that has no caffeine, very little tannin, and significant amounts of polyphenol antioxidants. Although the tea is new to many Americans, it has been made in the Cedarberg mountain region of South Africa for generations. Distributors are promoting the tea for numerous health benefits, citing recent studies that show some antioxidants found in rooibos tea may protect against cancer, heart disease, and stroke. What’s the evidence for these claims?

A Note on Tea Terminology

In the strict sense, the word tea has been reserved for infusions made from leaves of the evergreen shrubCamellia sinensis (L.) Kuntze, Theaceae, while infusions made from herbs such as rooibos have been calledtisanes. Over time, however, the common use of the word tea has been extended to include herbal infusions, and this relaxed usage is followed here. Rooibos is often referred to as red tea because it makes a vibrant red-colored tea, which can be confusing because black tea and hibiscus herbal tea are also sometimes called red tea.

Historical Background

More than 300 years ago, indigenous inhabitants of the mountainous regions of South Africa’s Western Cape were the first to collect wild rooibos and use it to make tea.10 These people discovered that they could brew a sweet, tasty tea from rooibos leaves and stems that they cut, bruised with wooden hammers, fermented in heaps, and then sun-dried. Botanists first recorded rooibos plants in 1772 when they were introduced to the tea by the Khoi people.10

Rooibos became a cultivated crop by the early 1930s, has been grown commercially since World War II, and now is exported to countries worldwide, including Germany, Japan, the Netherlands, England, Malaysia, South Korea, Poland, China, and the United States.10 In 1999, about 29 percent of South Africa’s total rooibos sales were exported to 31 countries.10 The quantity of rooibos exported in 2000 was two and a half times greater than the quantity exported in 1999, and exports continue to grow.10 The small towns of Clanwilliam and Wupperthal, north of Cape Town in the Cedarberg region, have a long history of rooibos cultivation; these towns are popular tourist stops because of their beautiful rural scenery and their role in the rooibos industry.

Roughly 70 percent of the bulk rooibos that is exported goes through Clanwilliam-based Rooibos Ltd. <www.rooibosltd.co.za>, a partnership of private growers/processors and a cooperative of large and small farmers in the area. The rooibos is sold in a variety of products in Europe, Asia, and, increasingly, America. Other South African companies that market rooibos tea products include Khoisan, Cape Natural Tea Products, and Coetzee & Coetzee. International demand for rooibos has been increasing since trade sanctions against South Africa were lifted following the demise of apartheid in the 1990s. Since 1999, the nonprofit organization Agribusiness in Sustainable Natural African Plant Products (ASNAPP, <www.asnapp.org>) has helped small farmers in and around Wupperthal to introduce sustainable methods of rooibos cultivation that allow them to compete in the world market. ASNAPP is sponsored by the U.S. Agency for International Development, Rutgers University, and Stellenbosch University. Through Stellenbosch University, ASNAPP also helped the farmers at Wupperthal fund construction of a tea court to process rooibos.

Rutgers University provides a quality control program for ASNAPP’s Wupperthal tea program, evaluating parameters such as color, taste, aroma, pH, moisture content, cleanliness, total phenol content, and antioxidant capability for tea samples collected from the industry in general and from all the growers in the Wupperthal tea program.11 Data from their analyses are made available to the farmers and also to prospective buyers via product specification sheets.

The Perishable Products Export Control Board (PPECB) of South Africa ensures that all exported rooibos products pass a phytosanitary inspection and are certified to be free of bacteria and impurities.4,10 In order to pass these health and safety tests, rooibos producers steam pasteurize the tea as the final step before packing. Organic rooibos is also monitored by various international organizations that provide organic certification, such as the German firms Ecocert and Lacon.

Antioxidants in Rooibos

Free radicals (unstable molecules that have lost an electron) can damage the DNA in cells, leading to cancer, and they can oxidize cholesterol, leading to clogged blood vessels, heart attack, and stroke. Antioxidants can bind to free radicals before the free radicals cause harm. Some antioxidants are called polyphenols because these substances contain a phenolic ring in their chemical structure. Polyphenols are common in plants; they act as pigments and sunscreens, as insect attractants and repellants, and as antimicrobials and antioxidants.12,13 The polyphenol group is further divided into subgroups such as flavonoids and phenolic acids. Polyphenols can also be classified as monomeric (molecules containing a single unit) or polymeric (larger molecules containing more than one unit). As described in this section, laboratory studies have found that rooibos tea contains polyphenol antioxidants, including flavonoids and phenolic acids, that are potent free radical scavengers.

Total Antioxidant Capability

To assess the antioxidant capability of rooibos tea as a whole, researchers compared the antioxidant activity of rooibos tea extracts to that of green and black tea extracts with the DPPH radical scavenging assay as well as the beta-carotene bleaching method.47 All the teas showed strong antioxidant activity with both methods. Using the DPPH method, the ranking from highest to lowest antioxidant activity was green tea (90.8 percent inhibition), unfermented rooibos (86.6 percent), fermented rooibos (83.4 percent), and black tea (81.7 percent). Green tea was significantly higher than the others (< 0.05), but the other three teas did not differ from each other significantly with respect to DPPH inhibition. Using the beta-carotene bleaching method, the ranking was green tea, black tea, fermented rooibos, and unfermented rooibos. The relative ranking varies with the type of test because the substance to be tested will have different reactivity to the different oxidizing agents used. These tests only measure the antioxidant capability of substances outside of the body and don’t provide data on whether the antioxidants are absorbed by the body and effective after the food is consumed.

In this study, all the tea extracts were diluted to the same amount of soluble solids rather than to the amounts of solids found in the teas.47 This method allows a comparison of antioxidant capability on a mass equivalent basis, but does not reflect a comparison of the antioxidant strength of equal volume servings of the teas. Although the soluble solid content varies with the method of tea preparation, it usually decreases in the order green tea, black tea, unfermented rooibos, fermented rooibos.47 The percent of soluble solids represented by polyphenols is similar for the four teas and the DPPH antioxidant activity is similar on a mass equivalent basis, so the DPPH antioxidant capability of equal-sized servings will decrease in the order of the soluble solid content.47 Black and green teas have over twice as much soluble solids as rooibos tea when prepared conventionally, so over two 200 ml servings of rooibos tea would need to be consumed to receive the same antioxidant benefit (as measured by DPPH) as one 200 ml serving of black or green tea (or the rooibos would need to be brewed to twice the standard concentration).47 This result agrees with the data given previously for 60 to 80 mg polyphenols for a 150 to 200 ml serving of rooibos tea22 as compared to 128 to 199 mg polyphenols for a 200 ml serving of black tea.23

The studies referenced above show that rooibos tea contains antioxidants that have positive effects when tested as isolated substances and that the tea as a whole has good antioxidant activity in vitro. So, do all these antioxidants in rooibos tea lead to health benefits for tea drinkers?

Rooibos Research in Live Animals and Animal Cells

Laboratory studies have demonstrated potential health benefits of rooibos in vitro (in test tubes) and in vivo (in live animals), but human studies have not been conducted. Much more research is needed, but the studies so far look intriguing.

Fermented vs. Unfermented Rooibos:Another study found that both fermented and unfermented rooibos tea exhibits antimutagenic properties in vitro as measured by the Salmonella typhimurium mutagenicity assay with several different mutagens; the antimutagenic activity was stronger against the metabolically activated mutagens 2-acetylaminofluorene (2-AAF) and aflatoxin B1 (AFB1) than it was against three direct-acting mutagens.51 Further research showed that the fermentation process causes a decrease in the antimutagenic and antioxidant activity of rooibos tea as measured by the Salmonella typhimurium mutagenicity assay (with 2-AAF), the hydrogen donating ability (assessed with DPPH), and the superoxide anion radical scavenging assay.52 The researchers suggest that fermented rooibos may show less antioxidant and antimutagenic activity because it has less polyphenols than unfermented rooibos. One analysis showed that polyphenols represent about 41 percent of the total solid matter in unfermented rooibos tea extract, but only about 30 percent of the total solid matter in fermented rooibos tea extract.51

One of the authors of both these studies is senior research scientist Jeanine Marnewick of the Program on Mycotoxins and Experimental Carcinogenesis (PROMEC) at the Medical Research Council of South Africa. She says, "Rooibos showed protective effects against DNA damage when tested in an in vitro assay as well as in an in vivo animal system." 53 The in vitro studies found unfermented rooibos was generally more protective against DNA damage than fermented rooibos. But Marnewick says her group’s research shows that fermented rooibos has a stronger effect against some mutagens. She says, "Both the fermented and unfermented rooibos showed a significant protection, and we’re busy elucidating the mechanisms."53 She is currently evaluating the protective effect of rooibos on liver, esophageal, colon, and skin cancer induced in live animal models. The studies are in the early phases and she cautions, "Very little is known about the effect of rooibos on cancer development." 53

Joubert also adds a cautionary note, saying that many questions about rooibos still need to be answered.22She says that researchers need to determine which of the antioxidant substances in rooibos tea are absorbed by the body and how much tea is needed to produce a measurable benefit. She also emphasizes that no human studies have been conducted yet.

Whole Foods vs. Isolated Antioxidants: The full benefits of teas are likely to come from a combination of all the antioxidants in them rather than from just one substance. Quite a few studies have found that isolated antioxidants don’t have as positive an anticancer effect as the mixture of antioxidants found in natural food sources; whole apple extracts were better than pure quercetin at inhibiting the growth of cancer cells in vitro,13,54 tomato powder was better than pure lycopene at extending the life of rats with prostrate tumors,13,55 and freeze-dried strawberries exhibited better anticancer properties in animals than did pure ellagic acid.13,56 Also, white and green tea extracts demonstrated better antimutagenic propertiesin vitro than mixtures of nine polyphenols found in the teas (mixed according to their relative proportions in the teas).57 Researchers believe these results indicate that other substances in the whole food products besides the identified antioxidants probably contribute to the total anticancer effect of the food, and that the relative amounts of all these substances could be important. Different teas have different mixtures of antioxidants, and they will protect against different mutagens. Sorting out all of these interactions will take time.

Rooibos Folklore: What’s Proven?

Although rooibos does contain active antioxidants, many of the other health claims made for rooibos tea are not well documented (based only on anecdotal evidence) or are not supported by science. Researchers are still investigating many of these claims to evaluate all the potential benefits of rooibos.

Vitamins And Minerals: Despite some promotional claims that rooibos is a source of vitamin C, Joubert says it is not. "We have tested both the traditional rooibos and green rooibos, and vitamin C was not present," she says.22

With the exceptions of fluoride and copper, the trace amounts of minerals in rooibos are not enough to make the tea a meaningful dietary source of minerals for the average consumer. As shown in Table 5, the nutritional labeling that is given on some packages of rooibos tea and on some websites of distributors4,5indicates that the amounts of iron, potassium, zinc, calcium, and magnesium in a 200 ml serving of rooibos tea are all less than 1 percent of the U.S. reference daily intake (RDI). A 200 ml serving of rooibos provides over 5 percent of the RDI of fluoride for adults and over 7 percent of the RDI for copper (see Table 5). Marc S. Micozzi, M.D., Ph.D., director of the Policy Institute for Integrative Medicine in Bethesda, Maryland, notes that when rooibos is used as a fluid replacement throughout the day, as is done with some athletes in South Africa, it does provide measurable amounts of several minerals and electrolytes.58

 

Colic, Allergies, And Other Ailments:Distributors of rooibos tea often suggest it can help allergies, sleep problems, digestive problems, headache, and other ailments,4,5but these claims have not been verified by scientific research. If the indigenous people of the Cedarberg region used rooibos tea medicinally, that tradition was lost and rooibos was just enjoyed as a good-tasting beverage until the recent interest in its health benefits.10 Many of the health claims for rooibos tea began in 1968 when a South African woman, Annekie Theron, found that rooibos tea eased her infant’s colic.10 As the story goes, she found no documentation on the benefits of rooibos and began her own experiments with local babies who had colic and allergies.10 She concluded that rooibos helped these babies, and she published a book in 1970 titled Allergies: An Amazing Discovery. Since then, she patented a rooibos extract that is now used in cosmetic products, and she started her own line of health and cosmetic products.10

Today, South African physicians recommend rooibos for infant colic.59 South Africans also use it to calm digestive upset in adults, to help induce sound sleep, and topically to sooth eczema, skin allergies, and diaper rash.59 Not enough research has been done to know if these folk remedies really are effective or to identify the substances in the tea that might be responsible for any observable benefits. Joubert says the tea does seem to help infant colic, but no formal studies have been done.22

Immune Function: An in vitro and in vivo study showed that rooibos might enhance immune function, but very little research has been done on this topic.60 One study found that a polysaccharide in rooibos leaves may have antiviral activity against the HIV virus, but the polysaccharide had to be chemically extracted from the leaves and is not found in tea made by steeping the leaves in water.61 There’s no evidence that rooibos tea fights the HIV virus.

Zero Caffeine And Low Tannin: Several other health advantages of rooibos tea that are often mentioned are its zero caffeine content and its low tannin content. Because rooibos is naturally caffeine-free, it does not have to be subjected to a decaffeination process and, therefore, does not lose any of its polyphenol content (as occurs when green and black teas are decaffeinated). The zero caffeine content also means rooibos can be enjoyed by those who want to avoid the stimulating effects of caffeine and can be consumed in quantity by those who want to use it as a fluid replacement.

Rooibos only has about 4.4 percent tannin content,51 which means that it does not have the astringent taste associated with C. sinensis and will not become bitter even after long steeping times. Rooibos tea can be a good alternative to C. sinensis for people who prefer the milder taste of a less astringent herbal tea or for those who have digestive problems with tannin-rich beverages. And as Micozzi observes, some people can receive a higher total antioxidant intake from rooibos than from green or black tea because the low tannin content and caffeine-free nature of rooibos allow it to be consumed in larger quantities.58

Iron Absorption: Other disadvantages have been attributed to tannins; they can bind to non-heme iron (iron from non-meat sources), reducing iron absorption, and they can decrease the metabolism and utilization of proteins.62-69 Black and green teas reduce the amount of non-heme iron absorbed by the body when the tea is consumed at the same time as the iron source.62-66 These effects do not cause problems for most people, but they can cause problems for people who have nutritionally marginal diets or low intake of heme iron sources (meats).69

Other polyphenol-rich beverages besides C. sinensis teas can also inhibit iron absorption. One study found that the inhibition of iron was 79 to 94 percent for black tea, 84 percent for peppermint tea, 73 percent for hot cocoa, and 47 percent for tea of chamomile (Matricaria recutita L., Asteraceae).62 The teas still inhibited iron absorption to the same degree even if milk was added to them. Some of these beverages contain only low levels of tannins, but other polyphenols in foods and beverages can also reduce iron absorption.62,64 The ability of polyphenols to chelate prooxidant metal ions might provide some antioxidant protection, but it can also be a disadvantage by decreasing absorption of necessary dietary minerals such as iron.64

The low tannin content of rooibos is sometimes used to infer that rooibos tea won’t inhibit iron absorption, but that conclusion is not automatic since rooibos is rich in other polyphenols that might decrease iron absorption. In one small study, three groups of 10 young healthy men were given an oral dose of iron, followed by rooibos tea, C. sinensis tea, or plain water.71 Iron absorption was measured to be 7.25 percent for rooibos tea, 1.70 percent for C. sinensis tea, and 9.34 percent for plain water. The result for C. sinensiswas significant (P < .0001), but the data for rooibos did not reach statistical significance (that is, the data for rooibos were not good enough to determine whether this result can be generalized to the whole population or whether the result was just chance). More studies are needed to better document the effect of rooibos on iron absorption, but this study implies that rooibos might not inhibit iron absorption nearly as much as C. sinensis tea.

The Bottom Line

Rooibos tea has become popular because of its fruity, sweet taste and its caffeine-free, low tannin, antioxidant-rich status. Although more research is needed, rooibos appears to be safe and free of side effects. The antioxidants present in rooibos may help protect against free radical damage that can lead to cancer, heart attack, and stroke. Unfermented (green) rooibos has a higher amount of polyphenols than traditional fermented rooibos and generally demonstrates higher antioxidant and antimutagenic capabilitiesin vitro. Future research should reveal whether the antioxidant benefits of rooibos observed in vitro and in animals translates into health benefits for humans. 

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