After all, being “overweight” may not be so bad for your health. The latest evidence comes from a large study in which people classified as overweight, but not obese, had a slightly lower death rate than people of supposedly ideal weight, suggesting that the threshold at which they are classified to overweight people has also been established. low.
It is indisputable that being very heavy is bad for people’s health, but it is not clear at what point the health risks begin. Doctors generally 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. Having a BMI between 25 and 29.9 is classified as overweight and 30 or higher is obese. These thresholds became medical orthodoxy after being cited in a World Health Organization report in 1997.
Previous research caused a sensation when it found that people who are just over the “healthy” threshold of 25 may actually have a slightly lower death rate than thinner people. But many of these studies are quite old, conducted when Americans were generally thinner and their participants were not ethnically diverse, he says. Aayush Visaria at rutgers New Brunswick Institute of Health, New Jersey.
To address those issues, his team analyzed data from a more recent study, which began in 1999, and tracked the survival of some 500,000 ethnically diverse American adults of known height and weight for up to 20 years.
Having a BMI between 25 and 27.4 carried a 5 percent 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, from 27.5 to 29.9, seemed even better, linked with a 7 percent lower risk of death.
One criticism of this type of study is that the apparent benefit of being overweight could be an artifact caused by people losing weight when sick and 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 numbers.
Visaria says it would be premature to conclude that having a BMI currently classified as overweight is better than being in the healthy weight category, because population studies like this one can have biases that distort the results. “It’s still not clear to us that this is really interpretable,” she says. “A more appropriate message is that BMI in general is 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 somewhat overweight carried a lower risk of death, says that the current thresholds for overweight and obesity are arbitrary. “Nature does not organize itself with neat, orderly numbers; these are clearly digit preferences when you have numbers like 25 and 30,” she says.
In June, the American Medical Association advised doctors to BMI should not be used in isolation to assess people’s weightbut it must be considered along with other factors such as waist circumference and other measures of health.
roy taylor at the University of Newcastle in the UK says that BMI was developed as a way of assessing the health of populations and should not be used to give people health advice.