The research, published in the journal BMJ Open, lays the ground for future studies to determine whether there is a causal relationship between coffee consumption and reduced prostate cancer risk.
Cancer and coffee
According to the American Cancer Society, approximately one in eight men will receive a prostate cancer diagnosis.
Prostate cancer is the second leading cause of cancer death in the United States. Researchers estimate that prostate cancer is the most commonly diagnosed cancer in men in more than half the countries in the world for which there are data.
The top geographical areas for diagnoses are Australia and New Zealand, Europe, and North America, while the top geographical areas for death from prostate cancer are the Caribbean and Southern, Middle, and Western Africa.
Researchers suggest that coffee may have a protective effect against some types of cancer. This is significant because coffee is very popular around the world — if even a small protective benefit can be demonstrated, then it may have a big overall effect on the health of populations.
The current research on whether coffee has a protective effect has been mixed. However, recent studies have found a link between coffee consumption and reduced risk of prostate cancer, including in Sweden, the U.S., the United Kingdom, and Japan.
In the present study, the researchers summarized the findings of previous studies that explored the possible relationship between coffee consumption and prostate cancer.
The researchers wanted to see whether they could identify any overall trends across the individual studies. They focused on cohort studies or case-control studies that were nested within a cohort.
This means that all the studies that were included had prospectively collected information about coffee drinking, cutting down the possibility of recall bias. For example, men with prostate cancer might recall their coffee-drinking differently from men without prostate cancer.
The participants were followed up to determine who developed prostate cancer and who did not. The nested case-control studies only included people in the original cohort, thereby reducing selection bias.
Without this, there could be a danger of, for example, selecting participants who did not drink coffee because they belonged to a health club and were, therefore, fitter and less likely to develop prostate cancer.
Two researchers independently reviewed the relevant literature that is available. After discussing their reviews, they decided to include 16 studies in their meta-analysis. The studies collectively involved 1,081,586 men. Of them, 57,732 developed prostate cancer.
In the studies, the participants self-reported their coffee consumption. As many as 15 of the studies compared the highest level of coffee consumption with the lowest level.
However, the amount of coffee consumption these categories represented varied from two to nine cups for the highest category, to none to one for the lowest.
After analyzing the data across the studies, the researchers found that the highest category of coffee consumption was associated with a 9% decrease in the risk of prostate cancer when compared with the lowest category.
Each single cup of coffee was associated with a 1% decrease in prostate cancer risk.
More specifically, the researchers found that localized prostate cancer — cancer that has not spread to another part of a person’s body — was 7% less likely to develop in the highest category of coffee consumption compared with the lowest.
For advanced prostate cancer, the highest category of coffee consumption was associated with a 12% reduction in risk compared with the lowest. For fatal prostate cancer, the highest category was associated with a 16% reduction in risk compared with the lowest.
The study did not look at what may account for this association. However, the researchers propose a number of biological mechanisms.
Dr. Kefeng Wang, of China Medical University in Shenyang, and his co-authors explain:
“Coffee improves glucose metabolism, decreases concentrations of plasma insulin and insulin-like growth factor-1, has anti-inflammatory and antioxidant effects, and affects sex hormone levels, all of which may play roles in the initiation, development, and progression of prostate cancer.”
The meta-analysis had some limitations. For example, there may be other factors that were not accounted for in the original studies that may play a role in the perceived relationship between coffee consumption and reduced prostate cancer risk.
Furthermore, since study participants self-reported their coffee consumption, they may have inaccurately described their coffee intake and therefore been assigned to the wrong category, potentially affecting the results.
However, as only cohort studies were included, the likelihood that this would distort the results is minimal, since this information was collected before any of the men’s prostate cancer was diagnosed.
The researchers call for further research to confirm their findings and to attempt to demonstrate a causal relationship between coffee consumption and prostate cancer risk.