WASHINGTON — The percentage of Americans who were obese in 2011 (26.1%) was down from 2010 (26.6%) and 2009 (26.5%), according to a new Gallup-Healthways Well-Being Index. And for the first time in three years the percentage of Americans who were normal weight (36.1%) was nearly the same as those who were overweight (35.9%).

Gallup, which has tracked Americans’ weight status since 2008, based its findings on more than 300,000 surveys of American adults. The weight categories are based on body mass index scores, with B.M.I. values of 30 or above classified as “obese,” 25 to 29.9 as “overweight,” 18.5 to 24.9 as “normal weight,” and 18.4 or less as “underweight.”

The survey found the percentage of Americans who were obese in all major demographic and socio-economic groups either declined slightly or stayed the same in 2011 compared with 2010, but blacks, low-income Americans and those aged 45 to 64 were still the most likely to be obese. Young adults and high-income Americans remained among the least likely to be obese, the Gallup survey noted.

“The slight drop in America’s obesity rate is a positive reversal of what was previously a negative trend,” Gallup said. “The cost of obesity is so high that even this small improvement has the potential to save the American economy a significant amount of money.

“A December 2010 analysis by the Society of Actuaries estimates that the total cost of obesity to the U.S. economy has climbed as high as $270 billion. Gallup’s own analysis finds that obesity and related chronic health issues cost businesses alone upward of $150 billion annually. But with more than one in four adults still obese, the nation has a long way to go to achieve lasting change.”