The numbers come from a new report from the Centers for Disease Control and Prevention, using a national survey of about 5,000 people.
National campaigns have been aimed at lowering obesity rates in the US, such as First Lady Michelle Obama`s "Let`s Move" campaign, which offers programming for exercise and healthy eating among youths.
Stacey Snelling, a nutritionist teaching at American University, told the BBC that the rise in obesity rates could be due to the healthy eating campaigns losing their effectiveness.
Over the past 10 years, much of the focus has been on food and nutrition for keeping obesity at bay, but now people were "losing attention" to that message, she said, so perhaps now it was time to focus on exercise.
"Our initial reaction to the obesity epidemic - limited sugary beverages and fried food - that only works to a point," said Ms Snelling.
The consumption of fizzy drinks has fallen in recent years and fast-food chains now offer healthier menus, but these kinds of public health problems are complex, she adds.
Many companies have introduced things like treadmill desks and incentives to exercise with pedometers and staff-wide contests - but not all have taken a stake in employees` health.
"With adults, the corporate environment has not changed as dramatically as schools have," she said. Obesity prevalence was found to be much higher for adult women than adult men - 38% compared to 34.3%.
Study author Cynthia Ogden told the BBC that this study marked the first time in many years that higher women`s obesity rates were statistically significant.
"That`s kind of a new finding," said Ms Ogden. "Now, what`s happened is prevalence in women has gone above what it was in men again."
Non-Hispanic white, non-Hispanic black and Hispanic adults and youth are experiencing higher rates of obesity than their white counterparts, the study found.
Between black men and women, there is a significant difference for rates of obesity - women`s rates soar above men`s at 57% compared to 38%.
Socio-economic forces determine the health status of any group of people, said Ms Snelling.
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