Why epidemiological studies don’t ‘prove’ anything

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January is typically a time when individuals seek to adopt healthier habits, and activity and exercise often feature here as part of the plan. While it is often said that exercise is healthy, actually there is not as much evidence for this as you might think.

The best test for any lifestyle factor is to subject people to it, and compare the effects to those achieved in individuals who were not subjected to the same change. If weight loss is the aim, the results of such ‘intervention’ studies show that exercise is not very helpful at all [1-3]. I have previously written about why exercise generally has limited impact on body weight, and what might work better for those seeking to lose weight [4]. Also, weight is only one aspect of health. It may be the case that activity and exercise have some capacity to protect against potential conditions such diabetes and heart disease.

Even here, though, the evidence is not as robust as you might imagine. Conditions like heart disease and type 2 diabetes generally develop over many, many years. This means that intervention studies assessing the role of exercise in the prevention of heart disease may need to go on for years, possibly decades, to show any meaningful effect. Such studies are costly and impractical, and so evidence of this type is scant.

In the absence of appropriate intervention studies, we may look to what is known as ‘epidemiological’ data. In short, such research looks at associations between a factor (in this case, exercise) and disease risk. Such studies generally find that those who are physically active are usually at a reduced risk of conditions such as diabetes and heart disease. Ideally, these observations should be made over a period of time. Such studies are described as ‘prospective’ in nature.

The results of these studies can be weakened, however, by the fact that physically-active individuals may also, compared to sedentary individuals, have other factors in their favour. Active individuals may, for instance, be less likely to smoke and have lower body weight than less active individuals. Such factors may be the real reason for the apparent reduction in risk of ill-health seen in active individuals.

It is important, therefore, in epidemiological studies to take into account these so-called ‘confounding factors’. While the process of controlling for confounding factors is never perfect, it’s generally a damn sight better than not doing it at all.

Confounding factors were considered in a recent study which explored the relationship between physical activity and heart disease in British men and women [5]. The researchers took into account several confounding factors including smoking habits, weight, blood pressure, and the presence of diabetes. Having done this, the results showed that an active lifestyle was associated with a 32 and 49 per cent reduced risk of heart disease in men and women respectively. This study, like others, shows that activity and exercise is associated with protection from heart disease.

Notice the words I have used here: ‘is associated with‘. I’ve used these words because epidemiological studies cannot be used to prove that one thing causes another. For that, we’d require positive results from those rather elusive intervention studies.

Now, bear this in mind now as you know take a look at the title of this study. The suggestion implicit in Leading an active lifestyle substantially lowers the risk of future coronary heart disease in apparently healthy men and women is that the researchers have proven that exercise leads to a reduced risk of heart disease. And that’s simply not the case.

Now, I’ve noticed that lots of news reports seem to suggest that lifestyle factors have ‘proven’ health effects, despite the fact that they are based on epidemiological evidence ” Housework cuts risk of breast cancer” is one recent example that springs to mind. I suppose one could just about forgive the journalists that churn out such headlines without thinking. I don’t find it quite as easy, however, to excuse scientists and researchers who are responsible for the epidemiological studies themselves.

It is beyond me that the authors of the recent epidemiological study on exercise and heart disease do not realise that the title they have come up with simply cannot be justified from a scientific perspective. And what has happened to the ‘peer review’ process here, where other scientists get to comment on and correct inaccuracies in a piece of science?

What may have happened here, of course, is that the title was chosen for maximum impact, at the expense of accuracy. If researchers and scientific journals cannot be relied upon to provide accurate interpretation of whatever evidence they provide, no wonder increasing numbers of individuals are growing sceptical about the ability of the holy church of science discern fact from fiction.


1. Reilly JJ, et al. Physical activity to prevent obesity in young children: cluster randomised controlled trial. BMJ, doi:10.1136/bmj.38979.623773.55 (published 6 October 2006)

2. Votruba SB, The role of exercise in the treatment of obesity. Nutrition. 2000 Mar;16(3):179-88

3. Shaw K, et al Exercise for overweight or obesity. The Cochrane Database of Systematic Reviews. 2006 Issue 4

4. Why exercise is not a cure for obesity and what works better for those seeking to shed weight in the long term

5. Matthijs Boekholdt S, et al. Leading an active lifestyle substantially lowers the risk of future coronary heart disease in apparently healthy men and women. European Journal of Cardiovascular Prevention and Rehabilitation. 2006;13(6):970-976

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