Toothbrushing and cardiovascular disease

All health journalists, repeat after me:

• Correlation does not equal causation
• Correlation does not equal causation
• Correlation does not equal causation
• Correlation does not equal causation
• Correlation does not equal causation
• etc

I am reminded of this by today's story of toothbrushing and cardiovascular disease, which has been atrociously badly reported in the media. The research study, published in the BMJ by de Oliveira et al, was actually very well written. They looked at almost 12,000 men and women from Scotland, asked them how often they brushed their teeth, and followed them up to see who developed cardiovascular disease. They found a strong association between a lower frequency of toothbrushing and an increased risk of cardiovascular disease. This risk diminished, but was still present and statistically significant, after adjusting for some important confounding variables, such as socioeconomic status, smoking, and physical activity. As you would expect, those who brushed their teeth less than once a day had generally less healthy lifestyles, so adjusting for those confounding variables was important.

de Oliveira et al correctly concluded that they had observed a significant association, but that they did not have enough evidence to claim the effect was causal, partly because their adjustment for confounding variables may not have been thorough enough to capture all the effects of confounding, and partly because of some other limitations of their research that they were careful to acknowledge. All in all, it was a good paper, describing a carefully conducted piece of research, reporting everything we need to know about it, and drawing appropriate conclusions.

But then we come to the way it was reported in the popular media.

Oh dear.

Here are some of the headlines: "Brushing teeth 'halts' heart disease" (BBC News website), "Brush teeth twice a day 'to prevent heart disease'" (Telegraph), and "Clean your teeth twice a day to keep a heart attack at bay" (Daily Mail). Even Radio 4's Today programme, normally a bastion of high-quality journalism in a world of ever more universal crapness, reported the story by saying "Brushing teeth 'stops heart disease'".

No. It doesn't.

Brushing teeth does not prevent heart disease (well, it might, but we don't have enough evidence to say that it does). Regular toothbrushing is associated with a lower risk of heart disease, but that does not mean it actually prevents it. It's shocking how lazy some of those journalists are, given that all they had to do was read de Oliveira's paper to be told that the association was not necessarily causal.

On the plus side, this does lend extra support to my theory of fictitious quotation marks, which from now on I shall be calling fictation marks. Note that 3 of the 4 headlines above enclosed the incorrect bit within quotation marks.