Coffee and mouth cancer
Previous studies have had mixed findings on the benefits of coffe.
The Daily Express reported that “one cup of coffee a day could halve the risk of dangerous cancers affecting the mouth and gullet”. It said that a Japanese study found that just one cup a day gave drinkers a reduced chance of getting tumours compared with those who hardly ever drank it. The researchers believe that it could “minimise” some of the risks from alcohol and tobacco, the main causes of mouth and oesophageal cancers.
This well-conducted research followed more than 40,000 people for over 13 years to see which of them got cancer of the mouth and oesophagus. The accumulated evidence from this study and other studies quoted by the researchers seems to indicate that some component in coffee does have a protective effect, at least in Japan.
However, this needs to be put into perspective. The study found that 157 people in the study developed these specific cancers, which is a rate of about four in every 1,000. Knowing this figure – the absolute rate of cancer – is important in this type of study because the apparently large relative reduction in risk of developing these cancers (in this case 49%) is equivalent to only a few people per 1,000 getting possible protection.
As the researchers confirm, the best advice to help reduce the risk of developing these cancers is to reduce or stop drinking alcohol and to stop smoking.
Where did the story come from? Dr Toru Naganuma and colleagues from the Department of Public Health and Forensic Medicine at the Tohoku University School of Medicine in Japan carried out the research. The work was funded by grants from the Japanese Ministry of Health, Labour and Welfare. The study was published in the American Journal of Epidemiology, a peer-reviewed medical journal.
What kind of scientific study was this?This was an analysis of data from a prospective cohort study known as The Miyagi Cohort Study.
The researchers were interested in investigating the link between coffee consumption and the risk of oral, pharyngeal and oesophageal cancers (together known as mouth and oesophagus cancer). They say that previous case-control studies had suggested that caffeine provides some protection against these cancers, but with inconsistent results. The researchers wanted to see if this was also true in better-designed, prospective studies. In addition, alcohol and tobacco both increase risk, but a high intake of fruit and vegetable may decrease the risk. So the researchers were also interested in how these risk factors interact with one another.
In this large study, all 51,921 residents (25,279 men and 26,642 women) aged between 40–64 years old and living in 14 out of 62 geographical areas in northeastern Japan, were enrolled on April 1 1990. From June through August 1990 they completed questionnaires on various health habits. Usable questionnaires were returned by 47,605 residents (22,836 men and 24,769 women) – a high response rate of 91.7%.
In the 1990 questionnaire, the researchers asked the participants about 36 types of food and four drinks, including coffee. They grouped the responses to the coffee questions into five groups: people who never drank coffee; people who occasionally drank coffee; people who drank one to two cups of coffee per day; three to four cups per day; and five or more cups per day. The researchers did not ask about the type of coffee used, the method of brewing, or the temperature of the beverage. The volume of a typical cup of coffee was estimated to be 150ml.
These patient details were then linked to corresponding data from the Miyagi Prefecture Cancer Registry, one of the oldest and most accurate population-based cancer registries in Japan. By doing this, the researchers were able to find out who had died from cancer, and the type of cancer they had died from.
Recognised statistical techniques were then used to assess the significance of the associations found, which took into account (adjusted for) all the other cancer risk factors that had been collected. They adjusted for age, sex, body mass index, alcohol consumption, cigarette smoking, consumption of vegetables and fruits, and green tea consumption. Because only a small number of people developed new cancers, the researchers chose to combine all people who drank one or more cups of coffee into a single group.
During the study period, 2,207 subjects (1,051 men and 1,156 women: 5.7% of the total) were not followed-up, mainly because they moved out of the area.
What were the results of the study? Over the 13.6-year study period, there were 157 cases of mouth and oesophagus cancer. These occurred mostly in men (135 men and 22 women). The risk of developing mouth and oesophagus cancers was ‘inversely associated’ with coffee consumption, meaning that people who drank more coffee had a lower risk of these cancers.
The researchers report the adjusted hazard ratio (HR) of these cancers, which measure the strength of this association when adjusted for other risk factors. People who drank one or more cups of coffee per day reduced their risk by around half compared with those who did not drink coffee at all (HR 0.51, 95% confidence interval 0.33 to 0.77). This was a statistically significant reduction.
This inverse association was consistent regardless of sex or cancer site, and it was present whether or not the person drank or smoked at the beginning of the study.
What interpretations did the researchers draw from these results? The researchers conclude that “coffee consumption was associated with a lower risk of oral, pharyngeal and oesophageal cancers, even in the group at high risk of these cancers”.
What does the NHS Knowledge Service make of this study? This is a well-conducted piece of research. In their write-up, the researchers make points about the interpretation of their results:
- They describe further the inconsistency in the other studies that have been published on the subject. They say that of the 12 published case-control studies, four also supported an inverse association; two actually showed an increased cancer risk (especially for hot coffee); and the other six showed no association. Two cohort studies had similar contradictory findings, with one smaller study finding no association with coffee, while the other showed an inverse association. They have explanations for why these differences occurred, and they maintain that theirs was the largest and longest running study and is likely to be the least biased because they adjusted for other risk factors.
The fact that the reduced risk for these cancers was seen in the groups thought to be at high risk, such as smokers and drinkers, supports the claim that coffee has an independent, separate effect from these other risk factors. Observational studies of this type can never completely eliminate the possibility of bias, and it is still possible that coffee drinkers were healthier in ways that were not measured by the researchers. For example, they may have been more physically active. - The characteristics of the volunteers at the start of the study were subtly different. Subjects with higher coffee consumption tended to be younger and less overweight. Coffee drinking was also associated with higher rates of smoking, lower vegetable consumption and lower green tea consumption by both men and women. All these factors were adjusted for in the analysis, but it is unclear whether their effect was fully removed by the adjustments.
- This study was carried out in Japan where the methods of brewing coffee, the components of coffee and the other dietary influences on cancer, may be different to the UK.
The incidence of these types of cancer is relatively low. This means that any difference between the groups can appear large when the hazard ratio is quoted. In this case, reducing the risk of developing this disease by 49% might seem impressive. However, it is equivalent to a reduction of a few people per 1,000 in this uncommon group of cancers.
The accumulated evidence from this study and the other studies quoted by these researchers seems to indicate that some component in coffee does have a protective effect, at least in Japan. More studies will be required to determine what this component might be, and whether the apparent protective effect occurs in countries with other dietary patterns.
As the researchers confirm, the best advice to help reduce the risk of developing these cancers is to reduce or stop drinking alcohol and to stop smoking.
