“Very hot tea and coffee linked to raised oesophagus cancer,” reads the headline in The Times today. The newspaper reported on findings from a study in northern Iran, which found that drinking tea at 70ºC or more increased the risk of cancer eight-fold compared to drinking warm or lukewarm tea (65ºC or less). Drinking it at 65-69ºC doubled the risk.
There are a number of points to consider when interpreting this study. It did not actually measure the temperature of tea drunk by the people with oesophageal cancer, rather it asked whether they drank their tea “very hot”, “hot”, or “warm or lukewarm”. These categories may mean different things to different people, and so it is difficult to identify the exact temperatures with which the risks are associated. Also, the researchers did not look at hot beverages other than tea, and so the results may not apply to these.
As the researchers say, studies have shown that the preferred tea temperature in the UK is 56-60ºC.
Where did the story come from?
The research was carried out by Dr Farhad Islami and colleagues from Tehran University of Medical Sciences in Iran, and other research centres in the UK, France, Sweden, the US and Iran. The work was funded by the Digestive Disease Research Center of Tehran University of Medical Sciences, the National Cancer Institute, and the International Agency for Research on Cancer. The study was published in the peer-reviewed British Medical Journal .
What kind of scientific study was this?
This case-control study looked at the effects of drinking tea at different temperatures on the risk of the most common form of oesophageal cancer (oesophageal squamous cell carcinoma). The study also had a cross-sectional part, which looked at the temperature that people in the Golestan province in Iran drank their tea.
In Europe and the US, the main risk factors for oesophageal cancer are tobacco and alcohol consumption, and the disease is more common in men than women. In Golestan, however, the risk of oesophageal cancer is high, and is found in equal levels in women and men, even though smoking and alcohol consumption is uncommon.
Suggested reasons for this include low intake of fruit and vegetables, low socioeconomic status, opium use, and drinking very hot tea. The researchers were particularly interested in looking at the effects of drinking very hot tea, as this risk factor is widespread in the area, it starts at an early age, and it continues throughout life, and affects both men and women.
The researchers enrolled 300 people with oesophageal squamous cell carcinoma, which had been confirmed by microscopic analysis (cases) from Golestan province. For each case, the researchers used data from an annual health census to identify potential controls who were of the same gender, age and place of residence, but did not have oesophageal cancer. From this list, two controls were randomly selected and asked to participate. If they did not agree, alternative controls were randomly selected. Using this process, the researchers managed to enrol 571 controls.
Cases and controls were interviewed by researchers using a standard questionnaire, which asked about personal characteristics and factors that might affect oesophageal cancer risk. They asked participants about their usual tea drinking habits; cases were asked for their habits before they developed symptoms of their cancer. This included whether they drank black or green tea, how often, and the volume of cups used (based on photos of five different-sized cups and mugs commonly used in the region). Those who drank tea were asked how hot it was when they drank it (very hot, hot, warm, or lukewarm), and how long they waited to drink their tea after it was poured. Data on tea temperature was available for 99% of participants, and the amount of tea was available for 89% of cases, and 67% of controls.
The researchers compared the cases and controls to see if they drank their tea at different temperatures. The analyses took into account factors that could affect results (potential confounders), such as vegetable intake and tobacco use (including cigarettes, pipes, hookah pipe, and chewing substances containing tobacco). They also took into account opium use, ethnicity, and indicators of socioeconomic status, such as education, car ownership and length of residence in rural areas.
For the second part of their study, they assessed tea drinking habits in 48,582 healthy adults (age 40-75 years) from the same province. These participants were asked the same questions about tea drinking as the participants in the case-control study.
In addition, the researchers also measured the temperature of tea drunk by the participants. To do this, the researchers made a cup of tea for each participant, and measured its temperature. Once the temperature dropped to 75ºC, they asked the participant to sip the tea and say whether it was the temperature at which they usually drank their tea. If not, the tea was allowed to cool to 70ºC, and the participants were asked to try the tea again. This process was repeated with 5ºC temperature drops until the participant’s usual tea temperature was reached. They then compared results obtained on this test with the temperatures the participants had reported that they drank their tea, to see how well they matched.
What were the results of the study?
In the cross-sectional part of their study looking at the temperature at which people in the Golestan region drank their tea, the researchers found that almost all people (97%) surveyed in the Golestan region drank black tea regularly, and 6% drank green tea. On average, they drank more than a litre a day. When looking at tea temperatures, they found that 22% of people drank their tea at temperatures over 65ºC, 38.9% drank it at 60-64ºC, and 39% drank it at less than 60ºC.
Statistical tests showed a moderate agreement between the measured temperature and how hot the participants reported drinking their tea (very hot, hot, warm or lukewarm). There was slightly less agreement between the measured temperature and how long the participants reported waiting between pouring and drinking their tea.
In the case-control study, cases had slightly lower levels of education than controls, were less likely to own a car, and were more likely to use tobacco, opium or both. More cases drank their tea hot or very hot than controls. Of the cases, 21% reported drinking their tea very hot, 36% hot, and 43% warm or lukewarm. Among the controls, 3% reported drinking their tea very hot, 27% hot, and 69% warm or lukewarm.
After taking into account potential confounders, drinking very hot tea was associated with an eight-fold increase in the odds of getting oesophageal cancer, and hot tea with two-fold increased odds, compared with drinking warm or lukewarm tea. Similarly, people who drank their tea less than two minutes after pouring were almost 5.5 times more likely to develop oesophageal cancer compared with drinking tea four minutes after it was poured. Those who waited two to three minutes increased their odds by about 2.5 times.
What interpretations did the researchers draw from these results?
The researchers concluded that “drinking hot tea…was strongly associated with a higher risk of oesophageal cancer”. They say that “Nonetheless…..the nature and strength of the association needs to be established in prospective studies”.
What does the NHS Knowledge Service make of this study?
There are a number of points to consider when interpreting this study:
- As with all studies of this kind, it is possible that there are differences between cases and controls other than tea drinking, which have influenced the risk of developing oesophageal cancer. For example, the use of tobacco was higher in the cases than controls, and this could be having an effect. Although, the researchers did take into account tobacco use in their analyses, they only categorised the participants as using or not using, which does not account for the amount used or the duration of smoking. This and other unknown and unmeasured factors may still be having an effect.
- In this type of study, questions have to be asked about previous exposure (in this case temperature of tea drinking), and this may lead to inaccuracies. This can be a particular problem if people with oesophageal cancer remembered their tea drinking differently to the controls, which might happen if they thought that their tea drinking was likely to have contributed to their cancer.
However, the researchers thought this was unlikely because they did not discuss their study hypothesis with participants, and they found no difference in results between rural and urban areas, nor between those with and without formal education (those from urban areas or with formal education might be more likely to be aware of a possible link between hot tea and the risk of oesophageal cancer).
Also, because the questionnaire asked about “usual” tea drinking habits before symptoms began, this may not adequately capture lifetime tea drinking habits, or drinking habits before the development of cancer (as the cancer may have existed for a while before symptom onset). * The case-control study was relatively small and, ideally, the results would be confirmed in larger prospective cohort studies, as acknowledged by the authors. * In the case-control study, participants were simply asked whether they usually drank tea very hot, hot, warm or lukewarm. It is difficult to say exactly what temperatures these categories represent, as they may mean different things to different people. * The study was carried out in Iran, and the results may not be representative of what would be found in other parts of the world, nor among ethnic backgrounds that differed from the study population. * This study only looked at tea drinking, therefore results may not be representative of the effects of other hot beverages.
This study suggests that it may be better to leave your tea to cool for a while rather than drinking it scalding hot. However, it is worth noting that the researchers say that studies have found that in the UK, a temperature of 56-60ºC is the average preferred tea temperature, while their study found that most people in the Golestan region drank their tea at above 60ºC.
Smoking and alcohol consumption are the main risk factors for oesophageal cancer in Europe, and people who want to reduce their risk should stop smoking and reduce their alcohol intake.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Times, 27 March 2009
Daily Express, 27 March 2009
The Daily Telegraph, 27 March 2009
Daily Mirror, 27 March 2009
BBC News, 27 March 2009
Metro, 27 March 2009
Daily Mail, 27 March 2009
Links to the science
BMJ 2009; 338: b929