Wednesday May 18 2011
Previous studies have found no link between coffee and prostate cancer
“Men who consume a lot of coffee cut their risks of lethal prostate cancer,” reported The Independent. It said a study has found that men who drank six or more cups a day reduced their risk of getting the cancer by 20% and their risk of developing fatal prostate cancer by 60%.
The study followed nearly 50,000 men in the US for over 20 years to test whether coffee intake was associated with the risk of prostate cancer. Compared to men who did not drink coffee, men who drank six or more cups of coffee a day had a slightly lower overall risk of developing the cancer and a substantially lower risk of developing lethal cancer. The findings applied to both caffeinated and decaffeinated coffee.
This is good-quality research, but several limitations mean that its findings need to be interpreted with caution. One limitation is that the study relied on men to recall how much coffee they had drunk over the previous year and this information was only updated every four years. This raises the possibility of errors in the results, and an inaccurate picture of the men’s coffee consumption.
Consuming large amounts of caffeine may have negative effects on health, and has been linked to heart palpitations. Men should not increase their coffee intake based on this research.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health in the US. It was funded by the National Cancer Institute, the American Institute for Cancer Research and the Prostate Cancer Foundation, all in the US. The study was published in the peer-reviewed Journal of the National Cancer Institute.
The research was reported accurately but uncritically in the Daily Express and The Independent. The BBC and the Daily Mail delivered more balanced reports, including comments by independent experts who pointed out that other studies had failed to find the same association and that heavy caffeine intake is associated with other health problems. The Daily Express also warned that excess caffeine could increase the risk of a heart attack.
What kind of research was this?
This prospective cohort study followed 47,911 men over 20 years. The researchers investigated whether coffee consumption was associated with the risk of developing prostate cancer, and specifically the aggressive form of the disease. The researchers pointed out that coffee contains many biologically active compounds and has been found to have effects on levels of the hormone insulin and sex hormones. It is also a potent antioxidant.
The researchers tested their theory that coffee consumption may be associated with a lower risk of advanced prostate cancer. They said their theory was based on observations that the links between levels of insulin, antioxidants and sex hormones are stronger for advanced disease than for prostate cancer overall. Previous studies of coffee consumption and prostate cancer have found no association. However, the researchers said that these studies were limited as they were small in size, only a narrow range of coffee intake was examined, and the studies did not focus on advanced disease.
Cohort studies, such as this one, in which large groups of people can be followed for long periods, are useful in providing information about associations between lifestyle factors (such as coffee consumption) and the risk of specific disorders. However, they cannot prove cause and effect.
What did the research involve?
The researchers based their analysis on a large study of over 50,000 male health professionals in the US, which began in 1986. The men, who were 40–75 years old at the start of the study, completed a questionnaire about their health and lifestyle when they enrolled. They then answered regular follow-up questionnaires to update this information.
For this study, the researchers included men who had completed a food-frequency questionnaire (FFQ) with questions on over 130 food items at the start of the study. They excluded men whose energy intakes were “implausible”, those who had left more than 70 food items blank and those who already reported a diagnosis of cancer. That left 47,911 men, who were followed to see which of them developed prostate cancer, the cancers that had spread and which men had died.
The men updated the dietary information they had given at the start of the study every four years, in 1990, 1994, 1998 and 2002. They were asked how often they had consumed a specified portion size of each item over the previous years, with nine responses ranging from “never or less than once a month” to “six or more times a day”. The questionnaire also asked the men about their intake of decaffeinated and regular coffee. The researchers said the reports of coffee intake on the FFQ were validated in a study looking at two-week diet records. They said they used the four-yearly reports of coffee intake to work out the average intake for the subsequent four-year period.
The researchers identified diagnoses of prostate cancer initially by self-reports from the men themselves or their relatives and then confirmed these by checking medical records and pathology reports. Deaths were ascertained through reports from family members and the National Death Index. The underlying cause of death was decided based on data such as medical records, death certificates and other formal information sources.
The researchers looked at how many men developed prostate cancer overall. They also separately examined data for men who developed aggressive prostate cancer before the end of the study. They defined this as lethal, advanced or high-grade cancer. Advanced cancers were those that had spread beyond the prostate. Lethal cancers were advanced cancers that caused death or had spread to the bone. Cancers were also classified as high grade or low grade using standard scoring (called Gleason scores), although this was not available for all men with prostate cancer.
Standard statistical methods were used to look for any link between coffee consumption and the overall risk of prostate cancer and the risk of aggressive cancer. The analyses were repeated for regular and decaffeinated coffee separately and for caffeine intake. The researchers also took into account when the men developed cancer, as the introduction of PSA screening in the US increased the early diagnoses of prostate cancer. To do this, they also arranged their results by time period into the pre-PSA (1986-1994) and PSA screening eras (1994-2006), to account for PSA screening as a factor that might affect the results (confounder). They also adjusted their results for other potential confounders, such as smoking, obesity, physical activity, diabetes, family history of prostate cancer, other dietary information and alcohol intake.
What were the basic results?
During the 20 years of follow-up, 5,035 of the 47,911 men were confirmed to have developed prostate cancer. Of these, 642 cancers were lethal, 896 were advanced and 3,221 were non-advanced.
At the beginning of the study in 1986, two-thirds of the men drank at least one cup of coffee a day and 5% reported drinking six or more cups a day.
The main findings of the study were as follows:
- Compared with non-coffee-drinkers, men who consumed six or more cups of coffee a day had an 18% lower risk of overall prostate cancer (relative risk [RR] 0.82, 95% confidence interval [CI] 0.68–0.98) and a 60% lower risk of lethal prostate cancer (RR 0.40, 95% CI 0.22–0.75).
- The lower risk of lethal prostate cancer was similar for regular and decaffeinated coffee (RR 0.94, 95% CI 0.88–1.01 for regular coffee and RR 0.91, 95% CI 0.83–1.00, P = 0.05 for decaffeinated coffee, for each one-cup-a-day increase).
- The age-adjusted prostate cancer incidence rates for men who had the highest (six or more cups of coffee a day) and lowest (no coffee) consumption were 425 and 519 total prostate cancers respectively per 100,000 person-years, and 34 and 79 lethal prostate cancers respectively per 100,000 person-years.
- No association was found between coffee consumption and low-grade prostate cancers.
How did the researchers interpret the results?
The researchers say the strong association they found between coffee consumption and lower risk of lethal and advanced cancers appears to be related to non-caffeine components of coffee and is biologically plausible. Coffee contains biological compounds that improve glucose metabolism, have anti-inflammatory and antioxidant effects and affect sex hormone levels, all of which play a role in prostate cancer progression.
This large study has several strengths including its size, long follow-up period and the wide range of coffee intakes reported by participants. However, it also has limitations and the findings, although significant, need to be interpreted with some caution.
- The study relied on men to recall and self-report their diet and coffee intake, which may have led to inaccuracies.
- Coffee intake was only assessed every four years, so any fluctuations between these assessments were not included in the analysis.
- The researchers did not have access to the men’s coffee intake in earlier periods of life, which may also have had an effect.
- It is possible that “reverse causation” was involved. For example, men in the early stages of prostate cancer may have cut back on their coffee intake because of urinary symptoms.
- Although the researchers took into account potential factors that could have affected their results, it is still possible that other factors played a role in the risk of developing prostate cancer.
- The participants in this study were all defined as health professionals. It is unclear if the results would apply to men from all socioeconomic backgrounds.
- It is possible that cancers that were diagnosed near the end of the study were misclassified as non-advanced.
Further research on this topic is required before it can be known for certain if coffee consumption lowers the risk of aggressive prostate cancer. The BBC reported Yinka Ebo, senior health information officer at Cancer Research UK, as saying:
"There's no need for men to start drinking gallons of coffee in an attempt to lower their prostate cancer risk.
"A number of other studies looking at coffee and prostate cancer have found that drinking coffee does not affect the risk of the disease, and this study only found a lower risk of advanced prostate cancer in men who drank more than six cups a day.
"We would need to see these results repeated in other large studies before we can be sure whether coffee consumption affects the risk of prostate cancer."