It sounds like the plot for a murder mystery: a silent killer claiming huge numbers of victims each year and evading detection with false trails and red herrings. Yet it could be the basis of a major advance in public health, with implications for us all. And the biggest clue to the villain of the piece may have been staring many of us in the face - quite literally - every morning.
For years we have been told that heart disease, strokes and even some kinds of cancer are a direct result of an unhealthy diet, specifically one rich in fat. This has led to international health organisations recommending diets that have led many of us to consume more carbohydrate-rich food like bread and potatoes. Yet at the same time, it has been clear for years that there is still something wrong with our diet. The most obvious sign is the global obesity epidemic, the cause of which is increasingly acknowledged to be more complex than just a combination of greed and sloth.
Less well-known but more telling is the mounting evidence against the widespread perception that carbs are better than fat. Since the 1970s, studies of the impact of diet on coronary heart disease (CHD) have repeatedly produced unexpected results which fail to fit in with the official view of what is good and bad in our diet. A comprehensive review published earlier this year by a team led by Dr Sonia Anand of Hamilton General Hospital in Canada found only weak evidence for a direct link between CHD and such supposed killers as saturated fat and meat. In contrast, the researchers found strong evidence of a link between CHD and certain types of carbohydrates, such as those found in white bread and sugar.
As Professor Frank Hu of Harvard School of Public Health put it in a review of the current state of play on dietary advice, published last month: "Cumulative evidence indicates that types of fats and carbohydrates are more important than total amounts in determining risk of CHD". It is perhaps surprising, even shocking, that even after all this time much of the hard scientific evidence backing official dietary advice is at best ambiguous or simply absent. But according to Professor Philippe Hujoel of the University of Washington in Seattle, a huge body of evidence is still being overlooked. He believes the principal culprit in the pandemic of diet-related disease could have been identified decades ago, had researchers not been blindsided by the focus on dietary fat. And the key clue lies in the link between life-threatening conditions like CHD and oral health.
At first sight, the idea that the state of our teeth and gums might predict our risk of having a heart attack seems to make little sense. In fact, the evidence has been building up for years. Over half a century ago, anthropologists and others noted that ethnic groups ranging from aborigines to Zulus who made the transition to a western lifestyle faced a substantial increase in risk of CHD and similar diseases - along with a decline in dental health.
Correlation does not imply causation, least of all in the complex relationship between diet and disease. Certainly those who see fat as the principal dietary cause of CHD could easily dismiss th link with dietary health as mere coincidence. Yet Professor Hujoel believes there is good reason for taking it seriously - based on the simple fact that our teeth and gums have evolved to enable us to eat. As such, they should be able to cope with a healthy diet. In contrast, if our diet causes damage to our teeth and gums, it may be a warning sign that what we are consuming is not good for us in other ways.
It is an argument that leads to a prediction: that the "sugary" foods even children know are bad for their teeth may have other serious health effects. And that's exactly what researchers are now finding in studies pointing to a link between so-called high glycemic index carbohydrates and CHD. Reviewing the evidence for this link in the current issue of Critical Reviews in Oral Biology and Medicine, Professor Hujoel shows that the carbohydrate-CHD connection explains the otherwise perplexing correlations between dental health and a range of major diseases. Around 20 years ago, researchers suggested that the missing link between the two might be certain types of oral bacteria. As a theory, it had the merit of being easily tested: eliminate the bacterium, and the risk of later ill-health should also decline. When the studies were carried out, however, they failed to show any benefit.
Prof Hujoel points out that the suggestion that the real culprit was certain types of carbohydrate dates back at least 50 years, to the work of two British medical researchers, Thomas Cleave and John Yudkin. By the early 1980s, however, their work has been largely forgotten because of the growing belief that artery-clogging fat was the major dietary threat to health. According to Prof Hujoel, this in turn led dental health experts to miss the significance of the fact that they were repairing tooth and gum damage caused by sugary food. He believes oral health acts as an early warning system for a diet likely to cause life-threatening diseases in later life.
If so, it suggests that the success of dental health campaigns such as fluoridation of water may have a darker side. By helping to combat the tooth-rotting effect of sugary food, such measures may have concealed the much greater threat to health posed by high glycemic index carbohydrates - one that may be causing the deaths of millions of people every year. After decades of controversy, claim and counter-claim about the health effects of our diet, it is surely time scientists got to the bottom of this mystery once and for all.
Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England