A recent study from Japan was much hyped in the media earlier this week but as NHS Choices points out, the researchers themselves offered a number of limitations to their study:
The information on illness, green tea and coffee consumption was all self-reported, which introduces the possibility of error. For example, although food frequency questionnaires are an accepted way to assess food intake, there may still be errors in people’s estimations of their consumption.
People’s consumption of green tea and coffee was only measured once, so any changes in consumption over the years was not taken into account by the study. It was not clear when the potential confounders (e.g. smoking) were assessed, and these may also have changed over the course of the study.
The results may have been affected by other factors (confounders). The researchers did adjust for a number of these, including other dietary factors, but others could be having an effect. In particular, although researchers adjusted their results for whether people took medication for high blood pressure or high cholesterol, they did not take account of the existence of these conditions in participants who were not taking medication.
The study took place in Japan so its results may not be applicable to the UK or other countries.
These kinds of studies are endless, one week they demonstrate an effect or correlation the next another team demonstrates the opposite. But, there are no panaceas, no miracle cures or preventatives, too much of any substance will have detrimental effects one way or the other. So, if you enjoy a cup of tea at breakfast and a coffee mid-morning, fine, carry on, if you’re drinking twenty cups of either every day to boost your health, you’re probably on to a loser…