Having an ‘overweight’ BMI may not lead to an earlier death

Being “overweight” may not be so bad for your health after all. The latest evidence comes from a large study where people who were classed as overweight, but not obese, had a slightly lower rate of death than people with a supposedly ideal weight – hinting that the threshold at which individuals are classed as overweight has been set too low.

It is uncontroversial that being very heavy is bad for people’s health, but it is unclear at what point health risks begin. Doctors usually advise people to lose weight if they have a high body mass index (BMI), which is someone’s weight in kilograms divided by the square of their height in metres.

In most countries, a healthy weight is defined as a BMI between 18.5 and 24.9. Having a BMI between 25 and 29.9 is classed as overweight and 30 and above is obese. These thresholds became the medical orthodoxy after being cited in a report from the World Health Organization in 1997.

Previous research made waves when it found that people who are somewhat over the “healthy” threshold of 25 may actually have a slightly lower rate of death than those who are slimmer. But many of these studies are fairly old, done when US people were generally slimmer and their participants weren’t ethnically diverse, says Aayush Visaria at Rutgers Institute for Health in New Brunswick, New Jersey.

To address those issues, his team analysed data from a more recent study, which began in 1999, and tracked the survival of about 500,000 ethnically diverse US adults of known height and weight, for up to 20 years.

Having a BMI between 25 and 27.4 carried a 5 per cent lower risk of death in this time period than a BMI within the healthy weight category of 22.5 to 24.9. A slightly higher BMI, of 27.5 to 29.9, seemed even better, linked with a 7 per cent lower risk of death.

One criticism of this kind of study is that the apparent benefit of being overweight could be an artefact caused by people who lose weight when they are ill being more likely to die.

But in the new research, the pattern was seen even if people who died within two years of entering the study were excluded from the figures.

Visaria says it would be premature to conclude that having a BMI currently classed as overweight is better than being in the healthy weight category, because population studies such as this one can have biases that distort the results. “We are not clear that this is truly interpretable yet,” he says. “A more appropriate message is that BMI overall is just not a good indicator of mortality risk – other factors such as body fat distribution also play an important role.”

Katherine Flegal at Stanford University in California, who was one of the first researchers to demonstrate that being classed as being somewhat overweight carried a lower risk of death, says the current thresholds for overweight and obese are arbitrary. “Nature does not organise itself with nice, neat numbers – these are clearly digit preferences when you have numbers like 25 and 30,” she says.

In June, the American Medical Association advised doctors that BMI shouldn’t be used in isolation to assess people’s weight, but should be considered alongside other factors such as waist circumference and other measures of health.

Roy Taylor at Newcastle University in the UK says BMI was developed as a way to assess the health of populations and shouldn’t be used to give health advice to individuals.

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