It is uncontroversial that being very heavy is bad for health, but it is unclear at what point health risks begin. Doctors usually ad vise people to lose weight if their Body Mass Index (BMI) is high, which is calculated by dividing a person's weight in kilograms 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 classed as overweight and 30 and above as extremely fat.
Previous research made waves when it found that people whose BMI was somewhat over the "healthy" threshold of 25 may have a slightly lower death rate than slimmer individuals. But many of the studies are fairly old and were done when people were mostly slimmer, and subjects weren't racially diverse, says Aayush Visaria, a researcher in New Jersey.
To address those issues, Visaria tracked the survival of about 500,000 racially diverse US 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 period than a BMI within the healthy category of 22.5 to 24.9. A slightly higher BMI, of 27.5 to 29.9, seemed even better, linked with a 7 percent lower risk of death.
One criticism is that the apparent benefit of being overweight could be a misconception, as people who lose weight due to illness are more likely to die. However, in the new research, the pattern persisted even if people who died within two years of entering the study were ruled out from the figures.
Visaria says it's too early to conclude that having an "overweight" BMI outperforms being in the healthy category, because population studies may be prejudiced and lead to misinterpretation of the results. "We aren't certain if this is truly interpretable," he says. "A more appropriate message is that BMI isn't a good indicator of death risk—other factors, like body fat distribution, also play a role."
