Study results show that drinking large amounts of hot yerba mate is linked to the increased risk of cancer of the upper digestive tract, mainly cancer of the oesophagus. The risk of cancer of the oesophagus associated with drinking alcohol and smoking cigarettes is further increased by the drinking of hot yerba mate. The majority of hypotheses identify the high temperature of the drink as the causative element. It is likely that frequent drinking of large amounts of hot drinks causes damage to the lining/mucosa of the oesophagus and makes the absorption of carcinogenic substances in the tobacco smoke or yerba mate easier. It is therefore recommended that yerba mate be drunk not hot but cold, as terere, or warm and that the drinking of hot mate does not come together with smoking tobacco or drinking alcohol. Smoking tobacco and drinking excessive amounts of alcohol alone poses more risk of the cancer than drinking hot mate.
Several epidemiological studies carried out mainly in South America point to a link between drinking mate and an increased risk of cancer of the oral cavity, pharynx and oesophagus.1 The majority of researchers associate the increased risk of cancer only with the drinking of hot mate and not with cold mate. The studies also found a strong link between the increase in the risk and the amount of tobacco and alcohol consumed. Some studies clearly showed synergic action between mate, alcohol and tobacco. In other words hot mate increased the risk of oesophageal cancer created by tobacco and alcohol exposure.2,3,4
Most epidemiological studies share similar methodology. The tests are clinical and hospital based and they involve similar interviews with patients questioned about the main risk factors. The control group is made up of patients suffering from diseases not linked to these risk factors. Studies involving population-based control groups would be more appropriate from the point of view of methodology but the logistics of such studies might be complex.4
The hot temperatures of the drink and/or chemical substances present in yerba mate are identified in the epidemiological tests as the factors increasing cancer risk. The higher risk of oesophageal cancer associated with the drinking of hot mate and the absence of such an association with cold mate together supports the hypothesis that thermal damage is the cause of the increased risk.4,2 This hypothesis agrees with the results of studies in Iran, in the area of Gorgan on the Caspian Sea, which has the world’s highest incidence of oesophageal cancer. Inhabitants of this region drink large amounts of very hot black tea. People living in the nearby area of Shasavar where there is far less cancer of the oesophagus drink their tea at lower temperatures. People of Gorgan drink on average 1180 ml of black tea daily. 62% of adults drink it at a temperature of at least 65 °C. Similar reports of increased numbers of patients with cancer of the oesophagus have been received from various other parts of the world where very hot beverages or meals are consumed.5,6,7
It is possible that high temperature is the sole factor causing damage to the mucus and thus increasing the risk, but it is equally possible that a higher temperature accelerates metabolic reactions including the activities of the carcinogenic tobacco and alcohol.4
The thermal damage caused by hot drinks has been difficult to test because there have been no objective studies of the precise temperature at which mate is drunk. The temperature has been described by drinkers as ‘warm’, ‘hot’ or ‘very hot’ which is difficult to translate into temperatures measured by thermometers. 4
The temperature of the drink is not the only important factor. The amount of the consumed hot drink is also important. Consuming large amounts of hot drinks increases the risk of oesophageal cancer. Small amounts do not have such effect. This is likely to explain why the increased risk is linked to drinking yerba mate, tea and coffee with milk. It is not linked to the consumption of black coffee which is drunk in much smaller amounts.8
A study carried out on 1400 adults living in the south-Brazilian town of Pelolas helps to understand the custom of drinking yerba mate. One third of those tested drank yerba mate daily. In this group an average of 1800 ml of yerba mate per day was consumed and the average temperature of the drink was 69.5 °C. Two thirds of those tested shared the drink with others. People who had never attended any schools drank the highest amount of mate with an average of 2491 ml per day. Smokers and people living in the villages drank mate more often than others. Men and people who drank alcohol tended to drink mate at higher temperatures.9 The study shows that drinking large amounts of hot mate is popular in Brazil. It is most popular amongst those with little education, coming from lower socio-economical groups, consuming large amounts of alcohol, addicted to tobacco and suffering from nutritional deficiencies and bad oral hygiene. All these factors increase the risk of cancer of the oesophagus.4
Studies in Uruguay have shown that there is a link between bladder cancer and drinking yerba mate among smokers but a less clear link between the cancer and non-smokers who drank yerba mate.10
Epidemiological studies carried out in Argentine have shown that drinking yerba mate leads to an increased risk of bladder cancer but only amongst smokers.11
An earlier study in Argentina found no connection between intake of yerba mate and cancer of the bladder. However, the results of that study did show that the main factors increasing the risk of the cancer were cigarette smoking and coffee drinking.12 Other studies have likewise confirmed the possible link between coffee drinking and bladder cancer.13
Yerba mate contains polycyclic aromatic hydrocarbons (PAH) which might have carcinogenic properties and this supports the hypothesis that beside the drinking temperature the carcinogenic substances in the leaves might lead to an increased risk of cancer. PAH are emitted during the burning of conifers and cigarette smoking, they are also present in car exhaust fumes. PAH are also found in, for example, meat cooked at high temperatures such as grilled meat, smoked fish, coffee and tea.
High levels of PAH in yerba mate are mainly caused by the use of smoke from burning wood to dry the mate leaves.14 This potential threat could be eradicated by changing the yerba mate production process.15 Smokeless (sin humo) technology using hot air has been employed to dry mate leaves and such yerba mate is already available commercially.
A 2008 study tested the presence of the 21 PAHs in 8 different brands of yerba mate and green tea. Yerba mate had a PAH concentration between 536 and 2906 ng/g dry leaves and green tea had a concentration of 266 ng/g. 1 g of mate dry leaves had between 8.03 and 53.3 ng of benzo[a]pyrene whilst green tea had none. In 12 tested infusions from mate leaves (brewed for 5 minutes in water of 80 °C) 37% of all measured PAHs was released (1092 out of 2.906 ng) and 50% benzo[a]pyrene (25,1 out of 50 ng). Results obtained from yerba mate brewed in hot (80 °C) and in cold (5 °C) water were similar. During the first brewing only 6% of all the PAHs was released (182 out of 2.906 ng) and 9% of benzo[a]pyrene (4.39 out of 50 ng).16 Studies of black tea showed similar results in relation to PAHs released (from 3 to 8%).17
Earlier tests quoted higher contents of PAH in yerba mate: 542 ng/g benzo[a]pyrene in roasted leaves and 225 ng/g in green leaves.18 The Ruschenburg study also showed a high content of benzo[a]pyrene (24-461 ng/g) in 8 commercial brands of yerba mate leaves.19Table 1. PAH concentration in yerba mate, green tea and black tea (ng/g dry leaves).
|Green tea16||Green tea18||Black tea18||Black tea**|
The PAHs contents in yerba mate are higher than in most green or black teas but lower than in lapsang souchong tea. Lapsang souchong is produced with the use of conifer wood smoke in order to ensure its specific aroma..
Some articles suggest that yerba mate extracts have a harmful effect on lymphocytes.
A study in 2000 tested the effect of an aqueous extract of yerba mate on the bacterial cells. The results showed the mutagenic and genotoxic effects of specific concentrates of yerba mate on bacterial cells of Escherichia coli and Salmonella typhimurium. Some similar effects were observed in relation to human lymphocytes.22
The authors of a different in vitro study using human lymphocytes carried out in 2009 showed that cytotoxic effects of the extract on the cells were most likely caused by caffeine and therefore were not peculiar to yerba mate.23Paweł Garski - Is yerba mate harmful and carcinogenic? YerbaMateInfo.com (publication date: 2011-02-03)