A 10-year study of more than 500,000 American adults found that those who were just slightly overweight in their 50s were 20 to 40 per cent more likely to die in the next 10 years.
Another study involving more than 1 million Korean adults, also being published in The New England Journal of Medicine this week, produced similar results.
The studies were aimed at helping resolve a long debate over whether the millions of people who are not obese but are nevertheless considered overweight are at significant risk.
Sceptics remained unconvinced, saying the analysis was flawed and would alarm people unnecessarily.
"I think they are just adding to the obesity hysteria," said Glenn Gaesser, of the University of Virginia. "They are presenting the data in a way that paints overweight and obesity in the worst possible light."