Epidemiological studies on the association between coffee intake, arguably a herbal remedy, and cancer risk have yielded inconsistent results. To summarize and appraise the quality of the current evidence, researchers conducted an umbrella review of existing findings from meta-analyses of observational studies.
They searched PubMed, Embase, Web of Science and the Cochrane database to obtain systematic reviews and meta-analyses of associations between coffee intake and cancer incidence. For each association, they estimated the summary effect size using the fixed- and random-effects model, the 95% confidence interval, and the 95% prediction interval. We also assessed heterogeneity, evidence of small-study effects, and excess significance bias.
Twenty-eight individual meta-analyses including 36 summary associations for 26 cancer sites were retrieved for this umbrella review. A total of 17 meta-analyses were significant at P ≤ 0.05 in the random-effects model. For the highest versus lowest categories, 4 of 26 associations had a more stringent P value (P ≤ 10− 6). Associations for five cancers were significant in dose-response analyses. Most studies (69%) showed low heterogeneity (I2 ≤ 50%). Three and six associations had evidence of excessive significance bias and publication bias, respectively. Coffee intake was inversely related to the risk of liver cancer and endometrial cancer and was characterized by dose-response relationships. There were no substantial changes when the researchers restricted analyses to a meta-analysis of cohort studies.
The authors concluded that there is highly suggestive evidence for an inverse association between coffee intake and risk of liver and endometrial cancer. Further research is needed to provide more robust evidence for cancer at other sites.
This is an interesting analysis that begs many questions. Let me just make four brief points:
- Correlation is not causation! Epidemiological studies throw up all sorts of associations that are too often mistaken as causal relationships. The question of whether coffee causes a decrease in the risk of certain cancers is as yet unanswered. The authors mention dose relationships which would, of course, increase the likelihood of a causal effect. Yet, they do not prove it.
- Another argument that would strengthen the possibility of a causal effect would be a plausible mechanism of action. However, the biological mechanism of how coffee might affect the risk remains unclear. Coffee contains a range of biologically active chemicals, including caffeine and phenolic compounds. In so-called alternative medicine (SCAM), coffee is also claimed to be a ‘detox‘ remedy. Yet it is unclear how exactly they might reduce the risk.
- The studies were all about the oral consumption of coffee. None considered anal application, like in Gerson therapy.
- The only way to find out whether coffee does, in fact, reduce the risk of certain cancers is to conduct prospective controlled clinical trials. Such studies are, however, not easy to conduct, particularly if designed such that their findings are truly reliable.
So, the answer to the question DOES COFFEE CONSUMPTION PREVENT CANCER? will remain unanswered for some time, I am afraid. Meanwhile, I suggest we enjoy our coffee per oral (and avoid it per anal).