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The Mystery of the Toothless Bearded Hag


Posted on 30 May 2010

hag3.jpgFlogging toothpaste may be a dull business, but for once eyes must surely have shone brighter than teeth in the marketing department at Colgate this week. A gift of a study, published in the BMJ last Thursday, linked poor toothbrushing to heart disease. The media predictably flipped the message, with headlines certain to fix a smile on even the most jaded of Colgate lips. Auntie exhorted us to ‘Brush teeth to halt heart disease’, while the Daily Mail directed ‘Clean your teeth twice a day to keep a heart attack at bay’. The ping was at last back in the Colgate ring of confidence, for who needs advertising, when sparkling headlines (351 of them, according to google) say it all?

The research is not without interest. It kicks off with what we might call intuition bias, by echoing the old saw about an apple a day. And what could be more likely than scabby teeth means dodgy tickers, as plaque ridden teeth mirror plaque filled arteries (Dr No kids you not – one Mail commenter equates dental plaque with arterial plaque – and advises using floss to remove it!). But what about the science?

On the face of it, the study appears sound enough, and ticks most of the right boxes – apart from, as we shall see, a crucial one. It is population based, and involved large numbers of subjects. It is ‘prospective’, in that the analysis starts with the risk factors, and moves forward in time to see who develops disease, even if, somewhat confusingly, the study was done retrospectively – in other words, after the event - making it in fact a so-called retrospective, or historical, cohort study. Such studies differ from true prospective studies in that, instead of actively following up subjects over time, they passively make use of data that has already been collected; and so are reliant not on their own fastidious data collection and follow-up, but on the past exertions of others, which may – or may not – have been sufficiently rigorous.

The subjects came from the Scottish Health Survey, a comprehensive occasional survey that collects a wide variety of demographic, risk factor and health related McData from a cross-section of Scots people. The outcomes – ‘cardiovascular disease events’ – were identified using the lugubriously named Information Services Division, a Scottish outfit that collects, amongst other things, identifiable patient data including main diagnosis on hospital admissions, and official records of deaths. Having identifiable data allowed the researchers to ‘link’ the SHS data to the outcome data – and so analyse how risk factors affected outcome. So far, so (reasonably) good (ISD hospital data is considered to be reasonably robust).

The analysis was done using Cox’s proportional hazards model, a widely used but – we are entering dark territory here – poorly understood statistical method that allows analysis of survival data even when the final outcome is not known for all subjects, based on multiple risk factors. We may think of it as a specialised form of multiple regression, that allows us to tease out the extent to which numerous risk factors act and interact to affect survival. The study’s headline result can be summed up simply enough: compared to those who brushed their teeth twice a day, those who brushed less than once a day were 1.7 times (95% CI 1.3 to 2.3) more likely to experience a cardiovascular disease event, even when allowing for the influence of a large number of other risk factors (age, sex, socio-economic group, smoking, physical activity, and visits to dentist, BMI, family history of cardiovascular disease, hypertension and diabetes) – and the 1.7 (compared to 1) gives rise to the ‘70% increased chance of heart disease’ widely reported in the press.

Now, at this point, we could embark on a detailed assessment of the rigour of the study and its methods. We could ask, for example, do we know that self-reported tooth-brushing behaviour – a so-called proxy – actually relates accurately to oral hygiene (in fact, research suggests it does). We could ask about outcome identification (recall it relied on inpatient statistics and deaths – what about patients solely in primary care?). We could even assess the viability of the Cox proportional hazards model (and probably end up with a thumping migraine) – but we need do none of this, because the study contains one big black hole – the mystery of what happened to the toothless hags. As long as that black hole remains, the study is flawed.

It goes like this. The SHS collects data on tens of thousands of people. This particular study, reasonably enough (cardiovascular disease mainly affects older people), decided to limit subjects to those aged 35 and over, giving a starting pool (calculated from figures in the paper) of 16144. From that pool, 204 were excluded because of missing demographic data (fair enough) and 386 because of pre-existing heart disease (fair enough). And then – a staggering, whopping 3685 – nearly one quarter of subjects - were excluded because they were edentulous (that’s no teeth to you and me). One in four subjects, so to speak, bit the dust, or at least would have done, had they any teeth with which to bite the dust.

This creates a black hole that we know very little about at the heart of the study – apart from, that is, a throwaway line that describes this group as more likely to be older, female, and smokers – a bunch, in other words, of toothless hags.

Now the problem is this: we simply do not know what happened to the toothless hags, or what the effect of their exclusion from the analysis had on the results. Let us suppose, for example (not unreasonable, because they are older, and female) that they are in fact survivors who did not suffer cardiovascular ‘events’. We might also suppose that a good number of them did not major on oral hygiene (presumably they were edentulous for a reason) and so many – we may suppose – belong to the poor oral hygiene group. Yet they have been excluded from the analysis, in effect weighting those who were analysed, in that poor oral hygiene group, towards a more male group, who are more likely to suffer a cardiovascular event. We are in effect selecting subjects for inclusion in the poor hygiene group who are more likely to have an adverse event – a form of error called selection bias, which - surprise surprise - can falsely elevate, and so over-estimate, the calculated risk.

All of this is conjecture, but reasonable conjecture; the problem is we simply do not know. The researchers provide no rationale (and Dr No cannot realistically see one) for excluding the edentulous from the analysis; and yet they were excluded. Dr No wonders why. And so it is that this apparently whiter than white study has a black hole at its heart – the mystery of the toothless – and quite possibly bearded, we just don’t know - hag.

6 comments:

I have a friend, 'Freddie', who had a heart by pass some 30 years ago and has not had any problems. Freddie told me some time ago about cleaning one's teeth twice a day! That's a Fact.

However with my growing scepticism of any medical research or statistics I am wondering if there is some spin going on here and whether or not it is all Fiction!

Have you noticed the state of the Nation's teeth lately?
I have, and I have noticed that more and more people are looking like they haven't seen a Dentist for years! Could it be 'Gentle Persuasion' to make the UK public look after their teeth 'cos it's damned hard to find an NHS Dentist these days!

How does this data work I wonder with capped teeth?

Toms Anguish

OMG, thought for a terrible moment this post was about The Witch Doctor.

It is possible, the toothless hags, depending when they became toothless, had as a consequence less oral problems and accordingly less cardiovascular problems than their dentulous peers.

Now this is important to know because, if it is the case, then the recommendations may be to have ritual "extraction totalis" on coming of age to protect against cardiovascular disease events.

Colgate would be displeased about that.

The Steradent manufacturers would be full of glee.

Eh, My Black Cat?

I remember reading something about this years ago, where bacteria that had gathered in the teeth broke off and caused problems with the heart. With animals, you're meant to brush their teeth to prevent problems like this. Having said that, this does smack a bit of post hoc ergo propter hoc. Someone from a poorer background may have bad dental health because they cant afford a dentist and also have heart problems because their diet is poor, not because of their teeth. They need a control group with fewer variables in it.

The BIG problem with this study is the deliberate but unexplained exclusion of almost a quarter of the subjects. For those more familiar with clinical trials (eg RCTs), this exclusion is a bit like failing to do an intention-to-treat analysis (hey, guys, let's just analyse the patients that got better!). Here, being an epidemiological study, the focus is of course primarily on adverse outcomes (epidemiologists study disease…), and so it could be a case of 'hey, guys, let's just analyse the ones that got worse!'. The fact it was a retrospective cohort study somewhat increases the chances of this (lots of data = lots of data dredging/fishing expeditions - unlike a truly prospective study, where the research question is defined in advance).

The black hole (cavity?) in this study, as DN said, means we simply do not know what happened to the toothless hags. DN is inclined to think that they were edentulous because of overwhelming dental/gum disease ie, for some time at least, they belonged to a very poor (indeed, overwhelming so) oral hygiene group, no doubt with stratospheric inflammatory markers. Then, at some point (we don't know how much of their life was spent glowing in the dark with inflammatory markers, and how much as toothless hags), they became edentulous (and so presumably less likely to glow in the dark). It is all speculation: we just don't know.

There really is no excuse for not including the edentulous group in the study, either as part of the rare/never toothbrushing group (which on the face of it is reasonable - difficult to brush your teeth if you haven't got any!) or, probably more informatively, as a separate group in their own right. There’s certainly enough of them…

Meanwhile, DN can't help wondering if there isn't a whiff of Gin Lane about the dental/periodontal disease and heart disease link (the great unwashed get what they deserve). Of course all the inflammatory markers research is most intriguing, but so far as DN is aware, the link has not been proved (even the current study only talks about a ‘possible link’); and we must always remember not to mistake the finger pointing at the moon for the moon.

The 'motivation' for publishing this study in its current form is probably on benevolent view no more sinister than any other public health study that encourages (presumed) healthy behaviour. According to the competing interests declarations, all three authors are whiter than white. The apparent lead author is a Professor and Honorary Consultant in Dental Public Health, and it is not unreasonable to assume that consultants in dental public health want to encourage good dental public health, ie regular toothbrushing, which is of course the take home message from this study.

Julie – DN agrees with your point, but begs to differ on the solution, in that if – for example – one considers (poor) diet may be part of the picture, then you need to include a variable for diet ie more variables. But practically, there are two key problems here: firstly teasing out confounding (blaming ‘x’, when in fact the true villain is ‘y’, which happens to mirror ‘x’ eg low income and poor diet) – multiple regression methods go some way towards doing this, but can easily become unwieldy - and secondly, diet is notoriously difficult to include as a variable. You could, for example, record how many of the five-a-day fruit and veg people eat – but how well would that act as a marker for overall diet?

A cynical witch might say they are going to publish the toothless group separately. Serial research. Two papers instead of one. Good for CV, appraisal, revalidation etc.

It is as well WD is not a cynical witch!

Lately, I started to believe that all researches are done not to find the truth but to promote a specific product. I remember reading a number of researches totally contradicting each other as regarding the usefulness or harm of brushing teeth. Toothless hags issue is not a surprise but another proof of this.


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