A body mass index classified as overweight does not necessarily mean that someone is unhealthy
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Being “overweight” may not be so bad for your health. The latest evidence comes from a large study where people who were classified as overweight, but not obese, had a slightly lower mortality rate than those who were at a so-called ideal weight – suggesting that the threshold at which individuals are classified as overweight has also been established low.
It is undisputed that being very overweight is bad for people’s health, but it is unclear from when the health risks start. Doctors usually advise people to lose weight if they have a high body mass index (BMI), which is a person’s weight in kilograms divided by the square of their height in meters.
In most countries, a healthy weight is defined as a BMI between 18.5 and 24.9. A BMI between 25 and 29.9 is classified as overweight and 30 and above is obese. These thresholds became medical orthodoxy after being cited in a report of the World Health Organization in 1997.
Previous research caused a stir when it was found that people who are slightly above the “healthy” threshold of 25 actually have a slightly lower mortality rate than those who are slimmer. But many of these studies are quite old, done when people in the US were generally slimmer and their participants weren’t ethnically diverse, says Ayush Visaria at Rutgers Institute of Health in New Brunswick, New Jersey.
To address these issues, his team analyzed data from a more recent study, which began in 1999, and tracked the survival of about 500,000 ethnically diverse American adults of known height and weight, for up to 20 years.
A BMI between 25 and 27.4 had a 5 percent lower risk of death during this period than a BMI within the healthy weight category of 22.5 to 24.9. A slightly higher BMI, from 27.5 to 29.9, seemed even better, linked to a 7 percent lower risk of death.
One criticism of this kind of research is that the apparent benefit of being overweight may be an artifact caused by people who lose weight while sick being more likely to die.
But in the new study, the pattern was seen even when people who died within two years of participating in the study were excluded from the numbers.
Visaria says it would be premature to conclude that having a BMI currently classified as overweight is better than falling into the healthy weight category, because population studies like this one can have biases that skew the results. “It is not yet clear to us whether this is really interpretable,” he says. “A more appropriate message is that BMI in general is simply 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 show that being classified as slightly overweight carried a lower risk of death, says the current thresholds for overweight and obesity are arbitrary. “Nature doesn’t organize itself with nice, neat numbers — these are clearly number preferences when you have numbers like 25 and 30,” she says.
The American Medical Association informed doctors in June BMI should not be used in isolation to assess people’s weightbut should be considered alongside other factors such as waist circumference and other health measures.
Roy Taylor at the University of Newcastle in the UK says BMI was developed as a way of assessing the health of populations and should not be used to provide health advice to individuals.
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