The United States, long an exporter of popular culture, may be exporting obesity to Europe, Asia, and Africa thanks to strong fast-food sales led by McDonald’s (MCD).

The World Health Organization says about one billion people worldwide are now obese. Since 1980, the number of obese Europeans has tripled. By some estimates, about half the population of the United Kingdom is overweight or obese.

“The medical community is dumbfounded by the problem,” Dr. Bernadine Healy wrote recently in US News & World Report. “Across the developing world, too, obesity has become a major affliction as people move out of poverty.”

Traditionally, a person more than 20% over the ideal weight has been considered obese. The National Institutes of Health defines obesity as a body mass index, a key for relating weight to height, of 30 or above.

In the US, about 65% of the population is overweight and obesity has doubled since 1980. The US Centers for Disease Control and Prevention reports that in 1960, the average weight of women aged 20 to 29 was 128 pounds. By 2000, the average weight for women in that age group had grown to 157 pounds, a 22.6% increase.

The concern isn’t just aesthetic: Obesity cuts lives short and increases the incidence of diabetes, heart disease, stroke and cancer.

Clearly, there's no single cause for increased obesity worldwide. But it’s curious that the rise of obesity apparently follows the growing popularity of fast food and the increased use of high-fructose corn syrup as a sweetener in soft drinks and other products that are served at many fast food restaurants.

The worldwide success of McDonald’s, the world’s largest restaurant company by sales, may offer some insight into the problem.

The company’s October sales fell 0.1% in the US, only the third time monthly sales totals have declined in 6.5 years.

In general, the recession has helped McDonald’s generate strong sales as consumers looking to stretch a buck trade down to cheaper fare, but tough economic times appear to limit breakfast and lunch sales.

Last year, sales at outlets open at least a year rose 5.3% in the US and 8.2% overall.