"Some portion of those thin people are actually sick, and sick people tend to die sooner," said Donald Berry, a biostatistician at the University of Texas MD Anderson Cancer Center in Houston.
The problems created by the study's inclusion of smokers and people with pre-existing illness "cannot be ignored," said Susan Gapstur, vice president of epidemiology for the American Cancer Society.
A third critic, Dr. Walter Willett of the Harvard School of Public Health, was blunter: "This is an even greater pile of rubbish" than the 2005 study, he said. Willett and others have done research since the 2005 study that found higher death risks from being overweight or obese.
Flegal defended her work. She noted that she used standard categories for weight classes. She said statistical adjustments were made for smokers, who were included to give a more real-world sample. She also said study participants were not in hospitals or hospices, making it unlikely that large numbers of sick people skewed the results.
"We still have to learn about obesity, including how best to measure it," Flegal's boss, CDC Director Dr. Thomas Frieden, said in a written statement. "However, it's clear that being obese is not healthy - it increases the risk of diabetes, heart disease, cancer, and many other health problems. Small, sustainable increases in physical activity and improvements in nutrition can lead to significant health improvements."
Obesity info: http://www.cdc.gov/obesity/data/trends.html
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