Photographs by Michael Freeman

During the 7th Century CE, Tibet’s power and influence rose suddenly and brought the Tibetan kingdom into contact with a drink that would soon become central to its people’s diet: tea (Camellia sinensis, Theaceae).¹ The Tibetan medicine system values tea for providing the body with dangs-ma (essential nutrients) that nourish the vital and physical energies, and in turn create the body’s structure and maintain well-being. Tea was first received in Tibet by Buddhist lamas and aristocrats, before eventually becoming a drink of the common people. By the end of the Tang Dynasty (618 - 907 CE), Tibetans and nomadic groups north and west of the Chinese border had adopted the practice of drinking tea.² Today, such tea is often referred to as pu-erh tea.

However, because of the extreme altitudes and temperatures of the Tibetan territory, pu-erh tea has remained an imported item from tropical and sub-tropical areas. Starting in the 7th century, a network of caravan routes collectively known as the Southwest Silk Road (Xi’nan Sichouzhilu) or Tea Horse Road (Chama Dao) were carved through forests and mountains from China’s Yunnan and Sichuan provinces to Tibet, Nepal, India, and Burma in order to facilitate the exchange of tea and other natural resources beyond their native habitats.³ Some of these caravan routes revitalized more ancient migration, trade, and military routes. The Southwest Silk Road became one of the most important trade routes of the ancient world including for promoting well-being through the exchange of medicinal natural resources. Over the course of up to 3,000 kilometers, tea oxidized and fermented during the trade journey as it interacted with temperature fluctuations, moisture, and microorganisms. The flavor and health benefits of tea transformed to the rich and probiotic characteristics that are unique to oxidized and post-fermented pu-erh tea.

Pu-erh (pu’er) tea refers to processed leaves and buds from the broad-leaf variety of the tea plant (C. sinensis var. assamica) native to the Upper Mekong River Region.⁴ This area is the motherland of the tea plant and covers parts of the Yunnan Province⁵ and neighboring areas of China, Laos, Vietnam, Myanmar, and India. Numerous indigenous groups in the native tea-growing area have produced and consumed pu-erh for centuries as a tonic, beverage, and food for its well-being and stimulant properties. Among these socio-linguistic groups are the Bulang (Blang), Wa, Akha (Hani), Lahu, Yao, Hmong (Miao), Jinuo, De’ang, and Dai. The Pu people—who are ancestors of the Bulang, Wa, and De’ang—are considered the first cultivators of the tea plant and have a cultivation history of over 1,700 years.^6,7

Pu-erh tea is thought to have developed from a medicine that was sourced from forests to a food and beverage from managed landscapes. The earliest known documentation of tea is in the 3^rd and 4^th centuries, when the poet Liu Kun mentioned the medicinal use of tea in the Jiangsu Province of China.² Tea consumption was also mentioned during this period in the Sichuan Province where it was taken in the form of gruel. Buddhist monks helped disseminate the practice of drinking pu-erh and other teas throughout China, southeast Asia, and Japan. They cultivated tea plants in their monastery gardens and used it as an aid for extended meditation. Monks also prepared tea as a tonic to treat a range of health conditions of local communities and strengthen, balance, and cleanse the body. As a traditional healing product, pu-erh is valued to strengthen the immune system, improve circulation, balance the body’s hot and cold levels, detoxify blood, treat rheumatism and stones, remedy headaches, and reduce swelling and soft tissue. Pu-erh has several mental well-being health claims such as invigorating the mind and relieving stress. It is also prized for providing nutrition, aiding digestion, and weight control. Additionally, pu-erh is consumed as a social beverage and for cultural continuity.⁴

Pu-erh Production

The leaves of pu-erh are sourced from tea plants growing in montane forests, indigenous agro-ecosystems, and terrace plantations in the native tea-growing area.⁸ Tea plants in forests and indigenous agro-ecosystems such as agro-forests (known as “ancient teagardens” in Yunnan) grow as trees of several meters tall in plant species-rich environments without the use of chemical fertilizers, pesticides, and herbicides. The abundant biodiversity and cultural practices involved in tea production by multiple socio-linguistic groups in this area contribute to diverse terroir and to a product with wide-ranging organoleptic properties.4 Alternatively, tea plants in terrace plantations are usually pruned to waist-high shrubs in monoculture systems that rely on chemical input for management of pests, disease, and fertilization.

Pu-erh may be processed as a green or post-fermented black tea of either loose or compressed leaves. Following harvest, leaves are processed into a loose green tea (san cha or “scattered tea”) that is the raw material, pressed green pu-erh (sheng bing or “raw cake”), aged pressed green pu-erh (lao bing or “old cake”), and pressed black pu-erh (shu bing or “cooked cake”). Similar to some other artisanprocessed green teas, harvested leaves are withered and pan-fried to remove moisture and deactivate enzymes responsible for oxidation.⁹ Leaves are then hand-rolled to disrupt cell walls to further remove moisture and shape the final product. The heated and rolled leaves are spread to dry on bamboo mats to prevent degradation and to capture the “taste of the sun” (tai yang wei). Unlike other green teas, the heat-processing step is less complete for green pu-erh, and consequently pu-erh has a distinct oxidation profile with age.⁴ For ease of transport and storage, loose pu-erh leaves have traditionally been compressed as bricks, cakes, logs, or various nest and gourd shapes in bamboo or stone molds.

As pu-erh journeyed on the backs of human porters, mules, horses, and yaks from the teamountains of Yunnan to the Tibetan Plateau along the Southwest Silk Road, it oxidized and fermented through interactions with moisture and temperature fluctuations and its flavor gradually transformed. The characteristics of such weathered pu-erh have come to be highly valued. Connoisseurs attempt to optimally age pu-erh by storing pressed green pu-erh in clay jars, bamboo wrapping and baskets, caves, and underground pits.⁴ Weathered pu-erh can also be mimicked by microbial food-processing technology (hou fa jiao, “post- fermentation,” “cooking,” or “ripening”). This controlled post-fermentation involves heap-fermenting loose green pu-erh for several hours to days as it interacts with fungi, yeasts, and bacteria. Leaves may intentionally be inoculated with selected microorganisms such as Aspergillus spp. (Trichocomaceae).¹⁰

Tea Trade on the Southwest Silk Road

Tea cultivation spread where climatic conditions allowed, while the practice of drinking tea reached far beyond.¹ Tibet, Mongolia, and other neighboring areas of China’s north and western frontiers generated significant demand for tea for its stimulant, nutritional, and medicinal properties, and thus drove its production and trade. However, tea was only one side of the trade equation. The other was China’s continuing search for warhorses for mobility and protection against invaders, and to maintain control over the empire. Tea and horses became inseparable as China used tea as a political tool for the procurement of warhorses, the formation of alliances, the negotiation of treaties, the security of its borders, and the creation of an empire. Historian Rossabi describes how China’s dynastic courts exchanged tea to acquire warhorses from neighboring territories to the north and west.¹¹ These horses had to be paid for, and by the middle of the 1^st century CE, when China lost its monopoly on silk, it turned to procuring warhorses through the trade of tea. The state promoted the exchange of warhorses for tea, and under the Song Dynasty (960 – 1279 CE) took control over the trade via the Tea Tax Bureau and the Tea and Horse Trading Office. Merchants became prominent in the exchange of tea as political relations between China and its neighbors shifted.

Tea developed as a key commodity during the Ming Dynasty (1368-1744 CE) on the Southwest Silk Road.³ Two principal roads of the Southwest Silk Road begin in the tea mountains of southern Yunnan and the area of Sichuan around Ya’an. A network of smaller trails connect to these 2 principal roads leading towards Tibet, meeting at the trading town of Mangkham near the Mekong, and continuing west to Lhasa. Abundant natural resources were exchanged on the various branches of the Southwest Silk Road over the course of 2 millennia, including precious metals and stones, herbs, tea, spices, fruit, nuts, grain, tobacco, opium, incense, dyes, shell, silk, cotton, horses, lumber, butter, and wildlife products. The Southwest Silk Road historically linked to the Maritime Silk Road and the Northern Overland Silk Route, creating an immense and integrated network between the East and the West. Tea reaching India from Yunnan further made its way on a trade network extending through Eurasia. The earliest textual record of the Southwest Silk Road is found in the Han envoy Zhang Qian’s exploration in the western regions in the late 2nd century BCE, where he mentioned a trade route other than the Silk Road connecting southwest China with India.³

The trade of pu-erh on the Southwest Silk Road continued into the 20th century, but declined naturally as horses ceased to have a major military use, and as roads were paved for more efficient transport. During the middle of the 20th century there was one last burst of activity, as the road and the horse caravans became the commodity lifeline for western China, which was cut off by the Japanese armies advancing from the east. The Chinese communist collective era that followed 1949 brought final closure to the trade.¹

Consumption of Pu-erh in the Tibetan Plateau

Tibetans initially drank tea as a healing tonic and acquired a yearning for its well-being and stimulant properties when tea became more attainable during the Song Dynasty. Tea enhanced their livelihoods in a harsh environment and supplemented the nutritional deficiencies of a high-fat dairy diet. The extreme altitudes of the Tibetan plateau simultaneously create oxidative stress for life and limit the cultivation of fruits and vegetables.¹² Tibetan inhabitants have adapted to these severe environmental conditions through their agropastoralist livelihood strategies, which came to include the consumption of bod ja, or butter tea, as a dietary staple.¹² Tibetans traditionally prepare butter tea with ghee (clarified butter) from the di (female yak) and salt and consume it along with tsampa (roasted barley flour; Hordeum vulgare, Poaceae). Butter tea plays a multi-functional role in livelihoods of the Tibetan territory as a ritual object, stimulant, and source of nutrition and medicine. It is regarded to meet the nutritional needs often fulfilled by the consumption of meat and produce.

Farming and grazing at high altitudes of the Tibetan Plateau are strenuous tasks that cause imbalances in the body’s constitution from cold winds, wet ground, and vigorous physical activity. Joint pain and blood circulation diseases are especially associated with strenuous grazing and agricultural work that require excessive effort because of the low productivity of the land.¹⁴ Pu-erh tea prepared with butter thus became a staple food and the ultimate energy drink for Tibetans, as described by the writer Frederick Spencer Chapman in 1940: “The leaves are boiled for several hours, then the infusion is poured into a section of hollow bamboo, where it is churned up with a plunger, together with a handful of salt, a pinch of soda, and a good lump of butter—usually rancid. The result is a purplish liquid of unusual taste for tea, but as soup excellent.”¹³

Preparation of butter tea involves brewing approximately 8 grams of pu-erh in 80mL of water and then churning the hot infusion with 4-8 tablespoons of butter and several pinches of salt. The tea leaves are most often infused several times. The number of infusions, brewing duration, leaf and butter amount, and water temperature used in preparing butter tea varies with production variables of tea and drinker preferences. In some Tibetan communities, butter tea infusions are distributed in a hierarchical order. The most elder or senior drinkers are offered the first serving and the youngest or least senior drinkers receive the last infusions. Studies have found that the method of preparing tea through multiple infusions results in variation of the phytochemical profile of tea.⁴

Tibetans may consume 2-4 bowls of butter tea at each of their daily meals and around 6-16 cups per day.¹⁴ After several bowls of butter tea at a single sitting, they mix and eat a small amount remaining in their cups with finely-ground roasted barley. Butter tea has become so esteemed by Tibetan communities that it is incorporated into Buddhist ceremonies as a sacred offering and represents hospitality. Tibetan communities continue to consume butter tea, and for some individuals, particularly the elder generations, this traditional practice persists for all daily meals.¹⁴

Pu-erh Tea in Tibetan Medicine and Health

Butter tea in the Tibetan medicine system is valued for providing the body with dangs-ma (essential nutrients) that nourish the vital and physical energies.¹⁴ Tibetan medicine has deep roots in Buddhist philosophy. It was brought to Tibet—concurrently with Buddhism—from India, where it derived from the Ayurvedic system of healing.¹⁵ The Buddha spread medical knowledge in his manifestation as the Medicine Buddha. Monks, physician-saints, and lama-doctors carried on this tradition in Tibet and modified it to reflect the habitat and resources of their surrounding landscape, including shamanic pre-Buddhist practices. They continued to develop the system with additional study and exchange with surrounding communities in India, Nepal, Yunnan, Sichuan, and Mongolia. The Dalai Lamas, who emerged as spiritual leaders in Tibet by the 16^th century, supported the study and spread of medical healing knowledge through personal study and by creating medical schools. The first medical schools were created in monasteries serviced by monks and lama-doctors. These are regarded as the beginning of the public-health system in Tibet.¹⁶

Health and well-being in Tibetan medicine are perceived as the balance and interaction of the body’s 7 energies, 3 humors, and 3 excrements. Disease is perceived as the pathological manifestation resulting from excess or weakness in the body’s 7 energies, 3 humors, and 3 excrements. Such an imbalanced state can result from both mental and physiological processes, including physical and spiritual. Disease in Tibetan medicine is classified as either hot or cold. Disease is also classified as (1) dependent disease caused by past karma; (2) imaginary disease caused by demons; (3) absolute disease of the present life; and (4) ostensible disease. Absolute disease of the present life may be caused by improper diet and behavioral patterns.¹⁵

At the physiological level, essential nutrients from ingested food and beverage are regarded as the most influential of all the 7 substances that may create imbalance of the body’s energies, humors, and excrements. Tibetan medicine regards nutrition from food and beverage to be ultimately the source of the blood, muscle, fat, bone, marrow, and regenerative fluid making up the body. Thus, the careful selection of foods and their combination is crucial for maintaining well-being and preventing illness. The Tibetan medical text The Four Tantras contains 3 chapters that address dietary principles for health maintenance and healing. Tibetan medical practice emphasizes the combination of foods and warns against certain food combinations that are regarded as poisonous. Each food strongly influences the different humors, and may negate the good qualities of each other, or the combined effect may be deleterious to the body. Excess of particular foods is believed to cause blockage of the vital channels of the body that create circulatory problems. A balanced and moderate diet is viewed as essential in keeping open the body’s channels and varies based on an individual’s typology.¹⁵

According to Tibetan medicine, the consumption of pu-erh—in synergy with other traditional foods—creates a perfect balance in the body and strengthens the body’s blood, muscle, fat, bone, marrow, and regenerative fluid, and creates energy. It was found especially advisable for bile disorders caused by physical exercise, injury, hot and sour diet, indigestion, anger, and evil spirits. The combination of butter and tea is regarded to give greater mind-body balance than either item individually. While the indigenous Tibetan territory has hundreds of endemic medicinal plants of its own, pu-erh tea provided the Tibetan diet with what was believed to be an ideal balance. It did not contain the potency of many indigenous plants that would disrupt the balance of the body if consumed frequently. When tea was unavailable or inaccessible, Tibetans had no recourse but to substitute tea with local plants.¹⁴

Various health claims of the components of butter tea and roasted barley flour, or bod ja and tsampa, have been supported by laboratory studies. Yak butter contains approximately 80% fat, of which 2.5% is classified as conjugated linoleic acids. These compounds improve bone mineralization activity and have pharmacologically been shown to have anti-carcinogenic and antidiabetic properties. Salt is a dietary mineral crucial for human life for its role in regulating water balance. It is valued locally to counterbalance the diuretic properties of tea. Barley is a rich source of fiber, vitamins, and minerals including magnesium, phosphorus, potassium, and selenium, plus the carotenoids lutein and zeaxanthin.

Like other teas, pu-erh is a source of dietary polyphenols. These phytochemicals are inversely associated with the incidence of chronic disease.^17,18 The antioxidant attributes from the catechin compounds in tea are regarded to provide an adaptive strategy to cope and buffer the stress of high altitude, and also aided in the digestion of fat. Caffeine, the key central nervous system stimulant methylxanthine in tea, further provides energy for grazing, farming, meditating, trekking, pilgrimage, and other daily activities. During the processing of black teas, catechins undergo oxidation to form the oxidized high molecular weight components bisflavanol, theaflavin, and thearubigin.⁹ Other compounds in tea include the methylxanthines theobromine and theophylline, amino acids, and their derivatives, such as theanine, plus proanthocyanidins, gallic acid, quinic esters of gallic, coumaric and caffeic acids, and free sugars.²¹ Variations of these phytochemicals depend on genetic, environmental, processing, storage, and preparation factors. Several in vitro and animal studies have supported the notion that tea has a range of health-protective effects including anti-oxidative, anti-inflammatory, neuroprotective, anticancer, immune-enhancing, anti-microbial, antiviral, antidiabetic, anti-obesity, and anti-atherosclerotic activities.^14,17,18

The processing of black pu-erhs results in a difference in phytochemical profile from other black tea types because of the deactivation of leaf enzymes during the heat application step of pu-erh and because of the associated microorganisms involved in fermentation. The microorganisms in black pu-erh oxidize polyphenols more completely than the enzymatic oxidation of other black teas and create fermentation-derived compounds known as statins.^19,20 Statins are a group of hydroxymethylglutaryl-coenzyme A reductase inhibitors which reduce low-density lipoprotein cholesterol levels in humans and prevent cardiovascular disease. Prescription synthetic statin drugs are widely sold by the pharmaceutical industry. Black pu-erh teas are natural fermentation-derived sources of statins that are found in other natural products such as oyster mushrooms (Pleurotus ostreatus, Tricholomataceae) and red yeast rice (Monascus purpureus, Monascaceae). The level of statins in black pu-erhs increases with fermentation and varies depending on the microorganism strains involved in the fermentation process.²⁰

The weathered and post-fermented pu-erhs traditionally consumed by Tibetan communities have probiotic health claims that make them distinct from other tea types because of their associated microorganisms. Probiotics are live organisms that can have a beneficial effect on the host’s health by restoring the balance of microflora of the gastrointestinal, respiratory, and urinary tracts, and displacing potential pathogens via competitive exclusion or production of antimicrobial agents.^22,23 Studies have demonstrated probiotics to be effective in treating diarrhea, upper respiratory tract infections, atopic eczema, and some inflammatory conditions.^23,24 However, investigations on the health-related properties of post-fermented pu-erh tea are sparse and warrant further study, particularly since teas, as traditional conventional foods, do not undergo the testing and approval process required for pharmaceutical drugs. Studies are needed to understand the efficacy of different microorganism strains associated with pu-erh teas and how this varies depending on single versus multiple strains and the synergy of microorganisms with other tea constituents.

Pu-erh Tea and the Southwest Silk Road at Present

Today, Pu-erh tea and the Southwest Silk Road have new and very different lives.¹ The market for pu-erh tea boomed in the early years of this century, reaching a peak in 2007, with active buyers—particularly from Hong Kong, Taiwan, and Guangzhou—demanding pu-erh from indigenous agro-ecosystems without the use of chemical input. As the market peaked, aged individual bing (the discus-shaped compressed pu-erh cake) from an agro-forest was fetching as much as 300,000 RMB (roughly $44,000 US dollars) at auction. The pu-erh market resembled that of fine wine for investment, attracting the less scrupulous dealers, with practices such as false labeling and inferior product masquerading as the best. The bubble burst, leaving pu-erh with a damaged reputation.⁸ However, demand for high-quality pu-erh from indigenous agro-forests remains strong, and its unique characteristics keep it prominent in the overall spread of teas internationally.

The traditional links between tea mountains of Yunnan and Sichuan and tea consumers in the indigenous Tibetan territory were broken for the most part after 1949 during China’s Collective Era as land and resources were nationalized.¹ The quality of tea arriving in Tibetan communities has declined in many cases since the caravans stopped running and the urban and global demand for highquality pu-erhs redirected these resources to those willing to pay the highest price premiums. Tibetan communities now generally rely on low-quality pu-erh sold at local village stores by outside entrepreneurs. This is likely manifesting itself in a decline in the health-related properties of pu-erh, exacerbated by the increased use of pesticides, hormones, fluoride, chemical fertilizers, and herbicides from poor production practices of terrace plantations.

Butter-tea diets of the Tibetan Plateau are transitioning because of political and socio-economic influences. The traditional Buddhist medical and philosophical frameworks that encouraged the consumption of tea for well-being are weakening in the face of exogenous influences. The consumption of butter tea is giving way among the younger generations to soft drinks, instant noodles, and refined rice in an echo of globalization trends elsewhere. Some elders maintain that these new foods are unable to provide the rlung (vital energy) and dangs-ma (essential nutrients) provided by butter-tea diets. As the Southwest Silk Road, or Tea Horse Road, acquires an historical presence, it is easy to forget its vital and practical former role of maintaining community health.

Ethnobotanist Selena Ahmed studied tea and culture in the mountains of Yunnan for 4 years for her doctoral study at The New York Botanical Garden and City University of New York. Selena is currently a National Institutes of Health TEACRS (Training in Education and Critical Research Skills) post-doctoral fellow at the Antioxidants Research Lab at the Jean Meyer USDA Human Nutrition Research Center on Aging at Tufts University. Her research seeks to understand the role of phytochemicals from plant foods in promoting health and reducing risk of chronic disease.

Award-winning photographer and author Michael Freeman has made a specialty of documentary reportage on Asia over the last 3 decades, for the Smithsonian Magazine, Time-Life, the Sunday Times Magazine, and GEO, among many others. He has produced more than 30 books on Asian subjects as diverse as the ancient Cambodian temple complex at Angkor Wat, other sacred places, contemporary Chinese design, and ethnic minorities. He lives in London.

References

1. Freeman M, Ahmed S. Tea Horse Road: China’s Ancient Road to Tibet. Bangkok: Thailand: River Books Ltd.; 2011.
2. Ceresa M. Diffusion of tea-drinking habit in pre-Tang and early Tang period. Asiatica Venetiana. 1996;1:19–25.
3. Yang B. 2004. Horses, silver and cowries: Yunnan in global perspective. Journal of World History. 2004;15(3):281-322.
4. Ahmed S, Unachukwu U, Stepp JR, Peters CM, Chunlin L, Kennelly E. Pu-erh tea tasting in Yunnan, China: Correlation of drinkers’ perceptions to phytochemistry. J Ethnopharmacol. 2010;132:176-185.
5. Ming T, Zhang W. The evolution and distribution of genus Camellia. Acta Botanica Yunnanica. 1996;18:1-13.
6. Huang G. An Overview of Pu’er Tea Culture. Kunming, China: Yunnan Min Zu Chu Ban She; 2005.
7. Xiao PG, Li ZY. Botanical classification of tea plants. In: Zhen, Y.S. (Eds). Tea: Bioactivity and Therapeutic Potential, pp. 17-34. Medicinal and Aromatic Plants—Industrial Profiles, London, UK: Taylor and Francis; 2002.
8. Ahmed S, Stepp JR, Toleno R, Peters CM. Increased market integration, value, and ecological knowledge of tea agro-forests in the Akha highlands of Southwest China. Ecology and Society. 2010;15(4):27.
9. Zhen YS. Tea: Bioactivity and Therapeutic Potential. Medicinal and Aromatic Plants—Industrial Profiles, London, UK: Taylor and Francis; 2002.
10. Ku K, Kim J, Park H, Liu K, Lee C. Application of metabolomics in the analysis of manufacturing type of pu-erh tea and composition changes with different post-fermentation year. Journal Agric and Food Chemistry. 2010;58:345-352.
11. Rossabi M. The tea and horse trade with Inner Asia during the Ming. Journal of Asian History. 1970; 4(2)136-168.
12. Owen P. Tibetan foods and medicines: antioxidants as mediators of high altitude nutritional physiology. In: Pieroni, A.; Price, L.L. Eating and Healing: Traditional Food As Medicine. CRC Press; 2006.
13. Chapman FS. Helvellyn to Himalaya. London, UK: Harper and Brothers; 1940.
14. Ahmed S. Biodiversity and Ethnography of Tea Management Systems in Yunnan, China. Dissertation submitted to the Graduate Center of the City University of New York; 2011.
15. Khangkar D. Tibetan Medicine: The Buddhist Way of Healing. Torrance: Heian International Inc.; 2000.
16. Khangkar D. Journey into the Mystery of Tibetan Medicine. New Delhi, Yarlung Publications; 1990.
17. Bolling B, Chen C-Y, Blumberg J. Tea and health: preventive and therapeutic usefulness in the elderly? Curr Opin Clin Nutr Metab Care. 2009; 12:42–48.
18. Iwasaki M, Inoue M, Sasazuki S, Sawada N, Yamaji T, Shimazu T, et al. Green tea drinking and subsequent risk of breast cancer in a populationbased cohort of Japanese women. Breast Cancer Res. 12(5):R88.
19. Xie G, Ye M, Wang Y, Ni Y, Su M, Huang H, et al. Characterization of pu-erh tea using chemical and metabolic profiling approaches. Journal Agric and Food Chemistry. 2009; 57:3046-3054.
20. Jeng K, Chen C, Fang Y, Hou R, Chen Y. Effect of microbial fermentation on content of statin, GABA, and polyphenols in pu-erh tea. Journal Agric and Food Chemistry. 2007; 55:8787–8792.
21. Del Rio D, Stewart AJ, Mullen W, Burns J, Lean ME, Brighenti F, Crozier A. HPLC-MS analysis of phenolic compounds and purine alkaloids in green and black tea. Journal Agric and Food Chemistry. 2004; 19,52(10):2807-15.
22. Chapman CMC, Gibson GR, Rowland I. Health benefits of probiotics: are mixtures more effective than single strains? Eur J Nutr. 2011;50:1–17.
23. Rowland IR, Capurso L, Collins K, Cummings J, Delzenne N, Goulet O, et al. Consensus on probiotic science. Gut Microbes. 2010; 1(6):436-439.
24. McFarland LV. Meta-analysis of probiotics for the prevention of traveler’s diarrhoea. Travel Med Infect Dis. 2007; 5(2):97–105.