"Different ethnic groups, especially Native Americans, African Americans and people from South Asia seem to tolerate obesity less and are more prone to obesity-related comorbidities," said Schauer. "The association might be stronger in those groups."
Others believe the association would be similar to the one seen in whites, and the authors wrote, "Similar analyses are underway in other populations."
The study also found an association between being underweight and a higher risk for mortality.
"With a BMI below normal, we see a rise in mortality. People who have cancer could lose body mass," said Schauer. "It could be an indicator they have another chronic illness."
Despite the study's weak points, obesity experts say the fact that it's very large makes it statistically powerful. They also say the study drives home a long-held belief.
"This study is an updated version of what we've had for many years," said Atkinson. "Back in the 60s and 70s, there were [results] that looked just like this."
"The association between BMI and premature mortality has been known for quite a while, but it strengthens the whole argument," said Schauer.
"It shows the lowest mortality in the BMI range between 20 and 25. This coincides with the guidelines we currently have when we assess someone's body weight," said Kushner. "It's the healthy weight range."