For the first time in history, the number of obese people around the world now is greater than the number of undernourished, according to a new analysis of population data from a prestigious medical journal.

While the rates of obesity have leveled off in some countries where it's already a public health concern, other places are seeing their obesity rates increase, helping to prop up the global average, according to a study published in The Lancet late Thursday.

"Over the past four decades, we have transitioned from a world in which underweight prevalence was more than double that of obesity, to one in which more people are obese than underweight, both globally and in all regions except parts of sub-Saharan Africa and Asia," the researchers wrote in their study. Indeed, parts of South Asia and Central and East Africa are among the places where excessively low bodyweight remains a public health concern. But by 2025, one-fifth of adults around the world are forecast to be obese, including 40 percent of American adults and one-third of adults in the United Kingdom. A team of researchers from two schools in the U.K. — Imperial College, London, and the School of Social and Community Medicine, in Bristol — performed an analysis of body mass index data from around the world since 1975. The body mass index is a formula that uses weight and height to determine whether someone is underweight, obese or somewhere in between.

"If you want to go back really far in time, BMI has the most data," said Majid Ezzati, one of the paper's co-authors, and a professor at the Imperial College, London School of Public Health. Ezzati acknowledged that some experts question the utility of BMI, but "we really wanted to know where the beginning of this problem was." He said his team is working on calculating other measures for use in future research. Interestingly, the correlation between the wealth of a country and its obesity rate is becoming "weaker and weaker," Ezzati told CNBC. "The problem is not worse in high-income countries than in middle-income countries."

The highest average obesity rates are found in Middle Eastern and North African countries, a few Caribbean countries, and Pacific Island nations.