The United States is the fattest nation in the capitalist world, according a report from the Organization for Economic Cooperation and Development.

Also, health economists predict that by 2020, at least three out of four Americans will be overweight. Experts said that's because Americans have changed their lifestyle. They don't have the time to prepare healthy meals, so they are eating out more often.

"Food is much cheaper than in the past, in particular food that is not particularly healthy, and people are changing their lifestyles. They have less time to prepare meals and are eating out more in restaurants," said Sassi, a former London School of Economics lecturer who worked on the report for three years.

People are also not getting as much exercise as they did in the past. That has caused the obesity rate to increase to nearly 70 percent in the U.S. this year from 50 percent in 1980, according to the report.

In 10 years, a full 75 percent of Americans will be overweight, making it "the fattest country in the OECD," the report stated.