The United States is the fattest nation among 33 countries with advanced economies, according to a report out today from an international think tank.

Two-thirds of people in this country are overweight or obese; about a third of adults – more than 72 million – are obese, which is roughly 30 pounds over a healthy weight.

Obesity rates have skyrocketed since the 1980s in almost all the countries where long-term data is available, says the report from the Organization for Economic Cooperation and Development (OECD), which works on policies to promote better economies and quality of life. Countries with the fastest obesity growth rates: the United States, Australia and England.

“Obesity is a growing threat to public health in all the advanced countries throughout the world,” OECD spokesman Matthias Rumpf says. Obesity causes illnesses, reduces life expectancy and increases health care costs, he says.

Obesity increases the risk of heart disease, diabetes, several types of cancer and other diseases. Obesity cost the U.S. an estimated $147 billion in weight-related medical bills in 2008, according to a study by government scientists.

“We have to find the most effective and cost-efficient way to deal with the problem,” Rumpf says. “Countries can learn from each other, and the best and most effective policies can be used in all countries.”