Comment »Posted on Thursday 25 June 2009 at 7:03 pm by Jacob Aron
In Getting It Wrong, Health & Medicine

ResearchBlogging.org

A Canadian study published in the journal Obesity has found that overweight people are 17% more likely to live longer than those of normal weight. In response, the Daily Mail instructed their readers to fatten up, but I would advise against it.

The study looked at data from the Canadian National Population Health Survey, which monitors the health of participants every two years. Using over 11,000 patient records, the researchers were able to track changes in Body Mass Index (BMI) and their relationship with mortality.

BMI is a commonly-used statistic for assessing a person’s body weight. It is calculated by a formula incorporating both height and weight. Normal BMI is considered to be between 18.5 and 25, whilst 25 to 30 is overweight. Outside of this range are underweight and obese.

Unsurprisingly being underweight or obese was found to be bad news when it comes to living longer, although for younger participants aged 25-59 being underweight was not a concern. Whilst we might expect these results, the conclusion that being in the overweight category gives you a slight lifespan advantage requires deeper investigation.

The problem could lie with the way BMI is measured. For the average person BMI is a useful indicator of healthy body weight, but because it doesn’t actually measure total body fat it can be problematic. For particularly athletic or muscular people the formula doesn’t work, because muscle weighs more than fat. Thus, those in the overweight category could actually be fit and healthy with large amounts of muscle tissue – exactly the kind of people we would expect to live longer.

The authors of the study caution against inferring causality as the Daily Mail has done. Getting fatter won’t necessarily help you live longer, and as the researchers point out there is a difference between a long life and a healthy one. Being overweight has been clearly linked with heart disease and diabetes amongst other conditions, so anyone following the Mail’s advice would be putting themselves at risk of developing these afflictions.

There is also the problem of continuous weight gain. Once you start putting it on, it can be hard to stop. The Healthy Survey data shows that a quarter of Canadians who were overweight in 1994/5 had become obese by 2002/3, and obesity will certainly up your chances of an early death.

Even the Mail must realise this, but I guess the sub-editor who wrote the headline didn’t read to the bottom of the article. They report the words of Dr David Haslam, chairman of the National Obesity Forum: “This study shouldn’t be used as an excuse to put on weight.”

Orpana, H., Berthelot, J., Kaplan, M., Feeny, D., McFarland, B., & Ross, N. (2009). BMI and Mortality: Results From a National Longitudinal Study of Canadian Adults Obesity DOI: 10.1038/oby.2009.191


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